All About AHUs [1, 1 ed.]

Table of contents :
Selecting the Right Air Quanity
Constant Volume or Variable Air Volume System
Selecting the Proper Fan Static Pressure
Choosing the Proper Cooling Coil
Coating of Coils
Horizontal or Vertical AHU
Blow-Through or Draw-Through Coil
Fan Types
Plug Fans
Face and Bypass Dampers
The Electric Motor and its Drive
Flat Belts
Variable Frequency Drives
Selecting Air Filters
Use of Heat Recovery Wheels
Use of Heat Pipes
Effective Mixing of Fresh and Return Air
Construction of the Casing
EN Standard for Mechanical Performance & Classification
EN Standard for Rating and Performance of AHUs
Condensation on AHU Panels
How Cooling Capacity is Controlled
Coil Piping
Space Planning
Unit Positioning
Condensation Removal
Intermediate Drain Pans
Flexible Connections
Acoustic Treatment of the AHU Room
Testing and Balancing for Good Performance
Give the AHU the Respect it Deserves
Installation & Maintenance Manual
Start-up and Performance Problems
Calculating Cooling or Heating Coil Performance
Checking Actual Cooling Capacity
Analysing Problems with Air Distribution Systems
Fan Laws, Airflow Formulas used in Air Filtration, Useful Conversions
Fan Connections

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Website.: www.ishrae.in

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Preface The publishing of this booklet is due to the initiative of Delhi Chapter of ISHRAE, the founder chapter of Indian Society of Heating, Refrigerating and Air Conditioning Engineers. An air handling unit is the heart of the air distribution system in an air conditioning plant. Proper selection of the AHU will ensure satisfactory performance of the air conditioning system and maintenance of the specified indoor air conditions of temperature, humidity, air cleanliness and air movement. Written in a simple, easy-to-understand style with many diagrams and photos, it will hopefully make it easier for the design engineer and installation supervisor to properly select and instal the air handling unit. There is nothing original in the contents of this booklet. All the material has been collected from various sources such as sales catalogs and technical bulletins issued by some manufacturers of air handling units and trade magazines from USA and UK. My thanks to Rakesh Aggarwal, group engineering head, of Caryaire Equipments India Pvt. Ltd., who has been of great help in collecting such material. No attempt has been made to cover the entire theory and practise of various subjects that are involved in the design and selection of the major components of the AHU such as the heat transfer coils and fans. Several excellent books are available with complete chapters devoted to Psychrometry and Fan Design and the booklet provides a list of such publications which the reader, interested in a deeper study of AHUs, can refer to. Readers are requested to send in their comments so that future updates can be improved.

Hiru M. Jhangiani Author, All About AHUs Mumbai January 2010

While all possible care has been taken in compiling this booklet, ISHRAE cannot accept responsibility for errors and indemnify themselves against any claim that may arise out of the use of any of the recommendations made herein.

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No portion of this booklet may be reproduced or copied, in part or in full, or circulated in any manner without permission from ISHRAE.

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Contents Selecting the Right Air Quanity........................................................................................................ 3 Constant Volume or Variable Air Volume System........................................................................... 3 Selecting the Proper Fan Static Pressure........................................................................................... 3 Choosing the Proper Cooling Coil................................................................................................... 3 Coating of Coils................................................................................................................................. 4 Horizontal or Vertical AHU.............................................................................................................. 4 Blow-Through or Draw-Through Coil............................................................................................... 5 Fan Types........................................................................................................................................... 5 Plug Fans............................................................................................................................................ 5 Face and Bypass Dampers.................................................................................................................. 6 The Electric Motor and its Drive....................................................................................................... 6 Flat Belts............................................................................................................................................ 7 Variable Frequency Drives................................................................................................................. 7 Selecting Air Filters............................................................................................................................ 8 Use of Heat Recovery Wheels........................................................................................................... 9 Use of Heat Pipes............................................................................................................................ 10 Effective Mixing of Fresh and Return Air....................................................................................... 10 Construction of the Casing............................................................................................................. 11 EN Standard for Mechanical Performance & Classification.......................................................... 12 EN Standard for Rating and Performance of AHUs...................................................................... 12 Condensation on AHU Panels........................................................................................................ 12 How Cooling Capacity is Controlled...............................................................................................17 Coil Piping........................................................................................................................................17 Space Planning ............................................................................................................................... 18 Unit Positioning............................................................................................................................... 18 Condensation Removal................................................................................................................... 18 Intermediate Drain Pans.................................................................................................................. 20 Flexible Connections....................................................................................................................... 20 Acoustic Treatment of the AHU Room.......................................................................................... 20 Testing and Balancing for Good Performance................................................................................ 20 Give the AHU the Respect it Deserves........................................................................................... 21 Installation & Maintenance Manual............................................................................................... 21 Start-up and Performance Problems................................................................................................ 21 Calculating Cooling or Heating Coil Performance........................................................................ 22 Checking Actual Cooling Capacity................................................................................................. 23 Analysing Problems with Air Distribution Systems ....................................................................... 24 Fan Laws, Airflow Formulas used in Air Filtration, Useful Conversions...................................... 26 CMYK

Fan Connections............................................................................................................................. 27

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All About AHUs “All About AHUs” provides basic information to the HVAC engineer in selecting, specifying, installing, maintaining and balancing Air Handling Units. An Air Handling Unit is designed to provide the function of moving air with a choice of facilities, as required, to achieve air mixing, air volume control, filtration, cooling, dehumidifying, heating, humidifying, space pressurisation, energy recovery and noise as well as vibration control. The basic components of an Air Handling Unit are a supply fan, a fan motor, a water cooling coil or a direct expansion refrigerant coil, filters, a mixing box, dampers, control systems and a casing. A return fan, heating coils, precooling coils and a humidifier may also be included, depending on the application. Next to the packaged chiller or condensing unit the AHU is the most important part of an air conditioning system. At times even more important, since through the air distribution ductwork the unit is directly connected to the conditioned space and the user who hears, breathes, works and lives with the air around him or her. The quality of air delivered by the AHU affects occupants of the building in more ways than one. Many engineers tend to oversimplify the selection of the air handling unit and its installation. Many think of the AHU as just a “dubba” or box, which it was, when it was crudely fabricated from steel sheets around a blower and coil, by any steel fabrication shop in India. With the advent of the double skin AHU using aluminium profiles, pre-insulated panels and plasticised steel skin, the AHU has assumed its rightful pride of place in an HVAC system and considerable thought and skill now goes into its design and construction. This booklet explains the important items that the consultant or design engineer should keep in mind when selecting or specifying an Air Handling Unit. The construction engineer will also find some useful hints for a better installation.

Selecting the Right Air Quantity The basic relationship between the space heat load and the conditioned air required to satisfy this load can be expressed as : Q = 1.1 × CFM × DT Where Q, in Btuh, is the sensible load of a conditioned space at a given time that must be met by supplying a certain volume of conditioned air at a temperature that is below, for cooling, or above, for heating, the space temperature. 1.1 is a conversion All About AHUs

factor for air at sea level and DT is the temperature difference between room air and supply air.

Constant Volume or Variable Air Volume System

The above equation expresses the cooling or heating load of a conditioned space as a function of the supply air and the temperature difference DT. From this it can be concluded that air handling systems can be categorised into two major groups. One group will keep the supply air volume constant and rely on varying the temperature difference to meet the varying load requirements in the conditioned spaces. The second group will keep the temperature difference constant and vary the supply air volume to satisfy the load variation. These two major groups of air handling systems are commonly referred to as Constant Volume (CV) and Variable Air Volume (VAV) systems, respectively.

Selecting the Proper Fan Static Presssure

Manufacturers of AHU’s generally furnish data of their standard units showing, for each model, supply fan size, coil size, volume flow rate, air velocity at fan outlet, fan static pressure, revolutions per minute of fan impeller and brake horsepower input to fan shaft. The fan total pressure can be obtained by adding fan velocity pressure to the fan static pressure. Velocity pressure, in inches WG, can be calculated by the equation : 2 Velocity pressure = (outlet velocity) 4005 where outlet velocity is expressed in fpm. An evaluation must be made by the system designer of the external total pressure drop in the duct distribution system including silencers, duct heaters and air outlets as well as the internal pressure loss of the air handling unit including dampers, filters and cooling/heating coils to ensure that the selected fan total meets the system total pressure. See Figure 1 showing pressure drop of various internal components of the AHU.

Choosing the Proper Cooling Coil

Having calculated the dehumidified or coil cfm, the velocity across the face area of the cooling coil is selected depending on the consultant’s specifications or the design engineers choice. The most commonly used coil face velocity is 2.57 m/s (500 fpm) but higher velocities between 550 and 600 fpm are selected by some designers for a more economical coil selection. At these higher velocities installation of mist eliminators after the cooling coil is recommended in order to prevent water carry over to the duct distribution system. See Figure 2. Velocities above 600 fpm 3

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Introduction

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or catalog selection procedure. Cooling coils can be either DX (Direct expansion of refrigerant) or CHW (Chilled water) and water coils can be either standard with brazed headers and return bends or internally cleanable. See Figures 3 and 4. Heating coils can be either steam (usually low pressure under 5 psig) or hot water. At times both a cooling and heating coil are installed in series for humidity control or for separate cooling and heating requirements.

Coating of Coils

Pressure Drop

Cooling coils are available with a “Hydrophilic” coating on the aluminum fin surface to reduce the surface tension of the condensate water droplets and help in producing a more evenly dispersed wetted surface. Tests have shown as much as 30% reduction in air pressure drop across a hydrophilic coil as opposed to an untreated coil. Having the condensate as a uniform thin film on the fin surface, instead of water droplets, also helps in optimizing heat transfer. Another coating, now available on coils is “Heresite”, which helps to protect the coil against corrosion in salty atmospheres, commonly prevailing along our long coastline or against fumes in or around chemical and fertilizer plants.

Horizontal or Vertical AHU

Pressure drop summary

Figure 1 : Air pressure drop of the various AHU components

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are not recommended. A higher face velocity results in a smaller coil, a higher heat transfer coefficient, a greater pressure drop across the coil and filter and a smaller fan room. While a lower coil face velocity has a larger coil, a lower heat transfer coefficient and a smaller pressure drop. A good system designer will bear all this in mind in selecting the AHU. The number of rows deep of the cooling and heating coils is best determined from the coil manufacturers computer program 4

In a horizontal unit, the supply fan, coils and filters are installed at the same level, as shown in Figure 5. Horizontal units need more floor space for installation and most such units are installed inside a fan room or AHU room. At times, small horizontal units are suspended from the ceiling inside the ceiling plenum. In these cases fan noise and vibration must be carefully controlled if the unit is adjacent to the conditioned space. In a vertical unit, the supply fan is not installed at the same level as the coils and filters but at a higher level as shown in Figure 6. Vertical units require less floor space. They are usually smaller, so that the height of the coil section plus the fan section and the height of the ductwork that crosses over the AHU uner the ceiling is less than the headroom (the height from the floor to the ceiling or the beam of the AHU room). * American Society of Heating, Refrigerating and Air Conditioning Engineers All About AHUs

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Figure 2 : Mist eliminator made of PVC by Munters India Ltd.

Figure 4 : Internally cleanable water coil with cleaning plugs

Blow–Through or Draw–Through Coil

Fan Types

The terms “blow–through” and “draw–through” are self explanatory regarding the location of the fan in relation to the coil. A “draw–through” arrangement, as shown in Figure 5 provides the simplest approach to even air flow across the face of the coil. ASHRAE* considers the air flow to be uniform when velocity measurements across the coil vary no more than 20 percent. Draw–through units are the most widely used AHUs. Putting the coil in the “blow–through” position (i.e. downstream of the fan) usually requires the installation of baffles or diffuser plates between the fan discharge and the coil. This introduces additional presssure drops and requires a higher fan hp. If a baffle plate is not installed the velocity across the face of the coil will not be uniform and can cause problems with water carryover at the centre of the coil and affect overall performance.

Fans or blowers provide the energy to move the air through ducts and the other accessories that form the air side of an HVAC system such as grilles, diffusers, air filters, humidifiers, dampers etc. Two types of centrifugal fans are commonly used in air handling units, the forward curved type and the airfoil blade or backward inclined type, see Figure 8. The latter type is more efficient but the forward curved type is generally smaller for a given duty and lower in first cost and hence more commonly used. The fan RPM is selected in order to meet the required system total pressure loss, that is the external pressure loss plus the pressure loss within the AHU. An exploded view of a backward-curved centrifugal fan is shown in Figure 9.

All About AHUs

Also referred to as unhoused centrifugal fans or cabinet fans, these are centrifugal fans with no outer casing or scroll. The impeller is generally directly mounted on the driving motor shaft and the entire assembly mounted in a cabinet or a plenum, which is pressurised by the operation of the fan. This permits supply air ducts coming from any direction to be directly connected to

Figure 5 : A horizontal draw-through AHU 5

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Figure 3 : DX(Direct Expansion) coil with liquid distributor at inlet

Plug Fans

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Figure 8 : Schematic view of the construction of (a) backward-curved fan and (b) forward-curved fan

Figure 6 : A vertical draw-through AHU

the casing, thus saving space and cost. Such fans are often used in large capacity roof top packaged units and in “clean” rooms. See Figure 10.

Face and Bypass Dampers

In regions with high humidity, the face and bypass damper is used as a method of humidity control. External face and bypass dampers are placed immediately upstream of the primary cooling coil. The space relative humidity problems occur at part-load conditions, assuming that the cooling coil is properly sized for full-load conditions in the space. The loss of space humidity control occurs as a result of the changing sensible heat ratio (SHR) during part loads. In humid climates, the sensible loads decrease more than the latent loads at part-load conditions. The result is a sensible heat ratio that decreases, or on the psychrometric chart, the SHR line becomes steeper. If the cooling coil is being controlled by a constant dry

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Figure 7 : A special AHU with a heat recovery wheel, supply fan and exhaust fan. See page 7 for explanation of a heat recovery wheel. 6

bulb temperature, the space relative humidity will increase. By using a face and bypass damper, Figure 11, and operating the cooling coil at full capacity, the space relative humidity can be controlled. A percentage of the supply airflow is bypassed around the cooling coil while the balance of the air passing through the cooling coil is cooled to a significantly lower dry bulb/wet bulb condition, lowering the relative humidity. The results of mixing the air off the cooling coil with the bypass air are supply air conditions that will allow for a high latent cooling load, therefore, maintaining control of the space relative humidity. Consider the specific job conditions before applying face and bypass for humidity control. This application works best with a small percentage of outside air.

The Electric Motor and its Drive

The blower motor is mounted inside the unit leading to quieter operation. An inspection door in the AHU provides ready access to the motor and the V - belt drive for maintenance. In this space-age most electric motors are extremely reliable in operation and seldom fail. In the rare event of a motor failure, a replacement motor, if available as a spare, can be installed in an hour or two at the most. If this delay cannot be tolerated for any reason, the system designer may insist on a standby

Figure 9 : Exploded view of a backward-curved centrifugal fan All About AHUs

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motor installed within the unit and ready to be switched on at immediate notice. Such standby motors can be incorporated in AHUs at the manufactures’ design stage, see Figure 12. An “airflow” switch can also be installed at the fan outlet to indicate “no airflow” to the operator. Blowers are driven by an electric motor either direct, (generally smaller motors 1 hp and below) see Figure 13, or by a V - belt drive using a standard 50 Hz 1440 rpm motor. See Figure 14. In case of a V-belt drive it is advisable to specify a minimum of two belts, so that in case one belt breaks or is loose, the second belt can ensure fan operation until the maintenance crew rectifies the defect. The motor sheave or pulley can be either of a fixed pitch or a variable pitch in case the fan speed needs to be changed to balance the blower with air distribution system characteristics. Most system designers build in a large margin of safety in the fan static pressure calculations and upon operation, the system static pressure turns out to be lower than the design, leading to a larger air supply than required and necessitating a reduced fan

Figure 11 : Face and bypass dampers All About AHUs

Figure 12 : Typical layout of blower with normal running and standby motors at opposite ends

speed. The adjustable pitch pulley, see Figure 15, can come in handy, under these circumstances, while the fixed pitch pulley would have to be replaced with a smaller diameter, causing delays in system balancing. Estimated belt drive losses are shown in Figure 16 and will help in proper motor selection.

Flat Belts

Flat belts are also available as an option to V-belt drives. Today’s high-efficiency flat belts with a high tensile strength result in more compact and cheaper drives that need less maintenance for retensioning or shortening of the belt.

Variable Frequency Drives

VFD or variable frequency drive is a system for controlling the speed of an AC motor by controlling the frequency of electric power supplied to the motor. Significant energy can be saved by using a VFD to achieve variable-air volume control. Changes in static pressure drop across HEPA air filters commonly used in air conditioning systems for pharma plants, hospital operating theatres and “clean” rooms, can also be offset by

Figure 13 : A direct driven blower with motor 7

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Figure 10 : A plug fan installed in a plenum

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simply regulating the fan speed with the help of a VFD.

S.No.

Dust Spot Efficiency ASHRAE 52.1

Arrestance ASHRAE 52.1

1.

Less than 20%

60-80%

MERV 1-4

2.

Less than 20%

80-90%

3.

20-30%

90-94%

Selecting Air Filters

MERV* Rating EN779+ ASHRAE 52.2

EUROVENT 4/9

In India, known as:

G2

EU2

Coarse Filters

MERV 5

G3

EU3

Pre Filter, 90% @ 20 microns

MERV 6

G4

EU4

Pre Filter, 90% @ 10 microns The air handling unit 4. 30-35% 90-94% MERV7 G4 EU4 Pre Filter, 90% @ 10 microns is expected not only to 5. 40-55% 95-98% MERV 8-9 F5 EU5 Fine Filter, 99% @ 5 microns cool and dehumidfy the 6. 60-80% 96-99% MERV 10-12 F6 EU6 Fine Filter, 99% @5 microns. air but also to remove 7. 80-90% 98-99% MERV 13 F7 EU7 Super Fine Filter, 99% @3 microns. impurities suspended 8. 90-95% 99% MERV 14 F8 EU8 Super Fine Filter, 99.5% @3 microns. in the air with the help Extra Fine Filter >75% @0.3 microns. of efficient air filters. 9. 95% + 99%+ MERV 15 F9 EU9 90-95% @ 1 micron. System designers and 10. 95% DOP NA MERV 16 H10 EU10 AHU specifiers must not Not made as a standard 11. 98% DOP NA MERV 16 H11 EU11 overlook this important H12 EU12 HEPA FILTER 99.97%@ 0.3µ aspect of an AHU. 12. NA NA 99.99%@0.3µ H13 EU13 HEPA FILTER Regardless of its 13. 99.999%@ 0.3µ NA NA H14 EU14 HEPA FILTER source, an airborne con14. 99.9995%@0.12µ NA NA U15 EU 15 ULPA FILTER. Not made as a standard taminant can be either * Minimum Efficiency Reporting Value + One of two different European Standards an aerosol or a gas. An Table 1: Classification of filters. aerosol is a suspension of solid or liquid particles in the air. The size of an aerosol is usutend to stay suspended in the air or settle very slowly. Airborne ally measured in microns. A micron is one millionth of a meter dust particles less than 0.1 micron behave like gases and have or 1/25,400 inch. Under the International System Units (SI) no rate of fall. Those in the range of 0.1 to 1.0 microns have “micron” is being replaced by “micrometer”. However “micron” negligible settling velocities while those in the range of 1.0 to 10 is currently the more popular term with the filter industry. The microns have constant and appreciable settling rates but are kept abbreviation for this unit of measurement is m. in suspension by air currents. Particles larger than 10 microns Figure 17 helps visualise the size of a micron by relating this will normally settle out of the atmosphere. dimension to the size of human air and other objects. Currently all major filter manufacturers in the USA rate their There are different aerosols, depending on their source of performance in terms of ASHRAE 52.1 and 52.2. Performance generation - dusts, fumes, fogs, mists and smokes. Dusts are solid values remain unchanged for the same filter tested by either aerosols generated from the reduction of larger solid materials. method. This method requires both an atmospheric dust spot For example a drill creates dust while drilling holes in a rock. discoloration test which defines the "Efficiency" and a synthetic Larger dust particles settle rapidly. Smaller dust particles dust weight test which defines "Arrestance". Filters with average

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Figure 14 : A belt driven blower 8

Figure 15 : An adjustable pitch motor pulley for small variations in blower speed All About AHUs

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Figure 17 : Size in microns of certain small particles

Figure 16 : Estimated belt drive losses

tube manometer will also serve this purpose. See Figure 19. A record should be maintained of the “clean” filter pressure drop and the maximum tolerable “dirty” filter drop and the cleaning or replacement cycle determined accordingly.

All About AHUs

9

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efficiencies below 20% or above 98% are not considered suitable Use of Heat Recovery Wheels for the atmospheric dust spot test. Filters below 20% efficiency Wherever a high percentage of outside air is introduced into are tested by the “Arrestance" procedure and above 98% by the an air conditioned area for improved ventilation and reduced "DOP" procedure. See typical filter selection guide Figure 18 by pollution, a Heat Recovery Wheel is used as an integral part of the a US manufacturer. AHU for heat recovery and energy conservation. See Figure 20. Table 1 shows a classification of filters comparing both The Heat Recovery Wheel can recover both sensible ASHRAE and European Standard Ratings (EN 779 and (temperature) and latent (moisture) energy. As the wheel or rotor Eurovent 4/9). slowly rotates (approximately 20 rpm) between the outdoor and Selecting the proper efficiency, the right quality, and return air streams, the warm outdoor air is cooled by the return maintaining the filters in a clean condition, all have a bearing on air before it is exhausted and simultaneously the high moisture keeping the air conditioned space clean, as dust free as possible and preventing ugly, dark smudge marks around supply air ceiling diffusers. To prevent smudging and discolouration of building interiors where smoking is allowed, medium efficiency filters of dust spot efficiency greater than 65 percent are preferable. A prefilter can extend the service life of the bag filter. Velocity across the filter face area is generally the same as across the cooling coil though lower velocities, as recommended by the filter manufacturer, may by used, space permitting. The only reliable method of determining when filters need cleaning or replacement, if the filters are of the disposable kind, is to instal a gauge across the two sides of the filter on the AHU (air entering side and leaving side) which reads the pressure difference or pressure drop across the filter. Such a gauge is called a “Magnehelic” gauge and is typically manufactured by Dwyer, USA. An inclined Figure 18 : Filter selection guide by particle size

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Vp – Purge/Seal Airflow

Vs – Supply

Ve – Exhaust

Figure 19 : Inclined gauge manometer installed across an air filter and a differential pressure Magnehelic gauge

Figure 21 : Typical fan locations for a Heat Recovery Wheel

content in the outdoor air is reduced by the lower moisture content in the exhaust air. Typical installations involve hospitals, hotels, theatres, auditoriums, laboratories and other high occupancy areas or industrial process areas which produce contaminants. The selection of the Heat Wheel and its performance calculation can be obtained from the manufacturer of the wheel. Typical fan locations are shown in Figure 21.

condenser to re-heat air coming out of the cooling coil. Using a heat pipe, thermal energy can be recovered from warmer air and added to cooler air. In temperate climates, this permits energy saving to be realized through preheating of the outside air. Conversely in hot climates the savings are associated with pre-cooling of the outside air. See Figure 20. Heat pipes can increase an air handler moisture removal capacity by 50% to 100%. Heat pipes not reduce the chiller load by free pre-cooling but also provide free reheating to lower the relative humidity of supply air. As most of today’s primary indoor air quality concerns are humidity related, the health benefits of heat pipes are noticeable.

Use of Heat Pipes

A Heat Pipe is a finned type of heat exchanger with sealed copper tubes. It has two parts—evaporator to pre-cool and

Effective Mixing of Fresh and Return Air

AHUs for air conditioning applications generally include a mixing box where the fresh and return air are supposed to mix evenly before starting their compact or short journey through the filters, cooling and/or heating coils, humidifier and the blower. When fresh air quantities become large and temperature/relative humidity of the fresh air and return air are greatly different,

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Figure 20: Heat Recovery Wheel (top) and Heat Pipe schematic arrangement (below) 10

Figure 22 : Static mixers installed in mixing box All About AHUs

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in the AHU. Figure 22 shows Blender Products (a manufacturer in USA) static mixers installed in a mixing box.

Construction of the Casing

stratification tends to occur in the standard mixing box leading to uneven filtration and cooling or heating of the supply air. In the winter months when the outside air temperature falls below freezing temperature, stratification can cause frozen water coils and tripping of the low temperature limit controller (freezestat). During summer months, the effects of stratification are usually poor mixed air temperature control and increased energy usage. The importance of stratification has also increased as a result of the new concerns over indoor air quality (IAQ) and revised standards for larger quantities of fresh air in buildings where human beings work or gather for entertainment and lectures such as hospitals, hotels, offices, cinema halls, auditoriums, conference/exhibition centres etc. To overcome this problem static air mixers have been designed to fit inside the mixing box of the AHU and can be provided by some AHU manufacturers, if requested. When installed correctly, the velocity profile downstream of the mixer and the pressure drop can be predicted since the product is factory made and installed

Figure 24 : Aluminium extruded profiles of different shapes forming the frame of a typical double skin AHU All About AHUs

Figure 25 : Exposed AHU with protection cover 11

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Figure 23 : A single skin AHU

At one time all factory fabricated air handling units were of “single skin” construction with galvanised steel sheets, suitably reinforced with stiffeners, forming all the side and top panels with a drain pan insulated from the inside, see Figure 23. Stiffeners or heavy gauge steel sheets were used to prevent the units from vibrating and making noise during operation. Panels were insulated from the inside, generally with fibreglass, to prevent condensation and provide acoustic effect to dampen noise. In the course of time this insulation tended to peel off the panels, accumulate dust, get damaged and fibres from the insulation material get mixed with air stream. This was an unsatisfactory state and the need was felt for improved design and construction. As industry progressed, aluminium extrusions became common and European manufacturers first introduced the double skin construction, using aluminium extrusions, see Figure 24, in a frame in which double skin, metal sandwich panels could be bolted. The polyurethane foam insulation injected between two metal panels gave the panels additional rigidity for the same thickness as fibreglass, leading to the use of 0.6mm thickness gi sheets as an industry standard. Prepainted galvanised sheets or plastic coated sheets followed later giving the AHUs an aesthetic look, not imaginable with the early single skin gi panels. Such double skin, aluminium frame, puf insulation and tough plastic coated galvanised steel sheet construction gives AHUs many advantages such as quieter operation, (since the double skin panels muffle the sound of the fan and the motor located inside), clean inside finish of the unit (ideal for hospitals, “clean” rooms, hotels, offices and wherever human beings occupy an air conditioned space) and, of course, better aesthetics. AHUs should preferably be located indoors in air handling unit rooms but can be located outdoors in mild weather with a separate sloping roof over the units to prevent rain water damage. See Figure 25.

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Mechanical Performance and Classification EN 1886:2007 is a European Standard, prepared by the European Committee for Standardization, or CEN, for short, which is part of a series of standards for air handling units used for ventilation and air conditioning of buildings for human occupancy. It considers the mechanical performance of an AHU as a whole and will be supported by a standard for sections and components. The standard specifies test methods, test requirements and classifications for AHUs which are supplying and /or extracting air, via ductwork, for a part or the whole building, except : • fan coil units • units for residential buildings • units mainly for a manufacturing process There are six test criteria for this Standard : mechanical strength, air leakage, filter bypass leakage, thermal transmittance, thermal bridging and acoustic insulation, Except for the last test, all other tests can be carried out by an international test laboratory in India.

Rating and Performance of AHUs

The European Standard EN 13053:2006 covers the rating and performance for air handling units, components and sections and has been prepared by the European Committee for Standardization or CEN, for short. Today, only Eurovent in Europe has the facilities and authority to test and certify the performance of AHUs complying with this Standard.

Condensation on AHU Panels

When an AHU is installed in a room which acts as a return air plenum, the panels and frame (of aluminium profiles) of the AHU are exposed to return air which is almost at the same temperature and relative humidity as the conditioned space. In Thermal Barrier

Thermal Barrier

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Figure 26 : Special aluminium profiles with thermal barrier 12

Figure 27 : Variable inlet vane

such cases, generally no condensation occurs on the AHU panels or profiles. Most AHU’s for comfort application are installed in this manner. However when the return air is ducted to the AHU, as is common in industrial applications such as pharmaceutical plants or when the AHU is drawing in 100% fresh air such as for “fresh-air-treatment” in hotels or hospitals, the space around the AHU is exposed to ambient or outdoor conditions. In such cases, condensation on the AHU panels, in the peripheral area near the aluminium profile framework, can occur, particularly in coastal areas or high humidity locations such as Mumbai, Goa, Trivandrum, Chennai, and Kolkata when ambient air is at 20° to 30°C with relative humidty of 65 to 85% and above. The problem is aggravated when the temperature inside the AHU is 6° to 13°C which is very low and abnormal but can occur if the blower is switched off but chilled water keeps circulating inside the cooling coil. This problem is similar to the condensation occurring on the outside surface of a glass of chilled water in the humid summer months in Mumbai when the glass surface temperature is lower than the dew point of the ambient air. To reduce the severity of this problem, AHUs can be fabricated with special aluminium profiles that have a built-in thermal barrier seperating the inside and outside of the profile. See Figure 26. Also, the panels are manufactured with a thermal barrier between the inner and outer skin. It is commonly perceived that AHUs with thermal break construction will not sweat. It should be realised however that “thermal breaks” do not eliminate condensation on the profiles and panels but merely increase the difference between the surface temperature inside and outside the AHU casing thereby increasing the range of ambient conditions at which All About AHUs

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will require some modification to the control scheme. Since refrigerant flow cannot be controlled as easily as chilled water flow can, by a threeway valve, capacity can be controlled by splitting the coil in two or three sections with each section having an independent expansion and solenoid valve. Alternately the coil can be split across the face area, so that, for example a six row coil can be circuited two rows in one circuit and four rows in a second circuit with each circuit having an independent expansion and solenoid valve. The solenoid valves can be controlled by a two stage room thermostat or a return air thermostat.

Coil Piping

condensation will occur. Please refer any such special application or problems to the AHU manufacturer for his guidance.

How Cooling Capacity is Controlled

There are several ways of controlling the capacity of an air handling unit using a chilled water coil. • The simplest way is to have a bypass damper around the coil, the damper being operated by a damper motor controlled by a return air or room thermostat. When the damper is fully open, a limit switch on the damper motor can actuate an on - off, two way valve on the supply line to the CHW coil thus stopping all water flow through the coil. • A refined version of the above method can substitute a faceand-by pass damper across the coil with a three- way valve to help maintain constant flow through the chiller. • Some designers eliminate the dampers altogether and control water flow through the coil by means of a three way modulating valve only. • A Variable Frequency Drive (VFD) can be used to modulate the fan speed and thus control the fan cfm in response to a return air or space thermostat. This is normally used in VAV (Variable Air Volume) systems . The Variable Inlet Vane (VIV), see Figure 27, control on the fan inlet is another common method used in VAV systems. • Direct expansion coils when used in an air handling unit All About AHUs

Figure 29 : Coil piping with provision for coil pullout 17

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Figure 28: Typical chilled water coil piping with accessories

There are some items that must always be included on coil piping, see Figure 28. One of the most important is a means of accurately determining pressure and temperature differences between the coil inlet and outlet. On coils over 2m2 (20 sq. ft.) install pressure gauges and thermometers on both the supply and return piping. On smaller coils, provide carefully located pressure/temperature taps for use with accurate insertion thermometers or pressure instruments. These instruments provide the most important means of trouble shooting the coil. The pressure gauges will tell you if the coil has developed a blockage and the thermometers with the flow control valves will allow you to calculate the actual capacity of the coil. A strainer on the supply line will collect rust flakes, welding slag and other debris that may be left behind between the pump strainer and the coil. It is a lot easier to clean a strainer

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Figure 30 : A unit mounted on concrete blocks

than it is a coil and the strainers are a cheap insurance to avoid trouble later. When installing the piping to the coil, offset the supply and return risers so that they don’t intrude into the coil-pull space. When the coil has to be pulled the maintenance crew only have to break one joint on the supply and one on the return making their job easier. See Figure 29.

Space Planning

The AHU should be so located that there is enough room to be able to pull out the coil should it ever have to be removed, for cleaning, to repair a leak or just replacement. The ideal arrangement requires room more than twice as long as the coil but unfortunately this much room is seldom available from the architects. Hence the next best thing to do is to position the doors to the AHU room so that the doors are opposite the coil and when opened the space outside the doors can be utilised as the coil-pull space. The only other option involves constructing part of the AHU room wall so that it can be easily dismantled to permit coil pull-out.

avoid damaging stresses on panels and on the frame during the joining of the various sections. A water level should be used to position the unit, adding shims if necessary to facilitate the easy opening of the inspection doors The unit can be installed directly on the floor so long as it can bear its weight and also allow the drain trap to be installed below the unit. If necessary the unit can be raised off the floor by inserting concrete blocks or metal profiles such as channels or I beams. See Figures 30, 31 and 32. Sometimes a concrete house cleaning pad is built under the entire unit with adequate height to accommodate the trap. Anti-vibration mounts are not needed between the unit base and the floor, as the internal moving parts are dynamically isolated from the structure of the unit. For very critical applications, expert advice must be obtained from noise and vibration consultants. It is recommended that enough space be provided around the unit for proper maintenance and replacement of any components at a future date.

Condensation Removal

In most air handling units condensation of water vapour in the air occurs over the surface of the cooling coil. In coastal areas with high humidity this condensation is very heavy and proper care must be taken to drain this water quickly. The drain pans in AHUs are generally sloped to provide positive drainage to the pan outlet avoiding the pools of stagnant water that are prime breeding grounds for bacteria, moulds and algae. Once the water has reached the pan drain outlet, it is upto the planning engineer and the site erection supervisor to dispose

Unit Positioning

The air handling unit must be installed on a level surface to

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Figure 31 : A unit mounted on I beams 18

Figure 32 : A unit mounted on a raised platform All About AHUs

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bacterial growth and condensation on the wall surfaces. A water trap, also called a U trap, must therefore be installed in the condensate drain piping. Without a trap, static pressure blow-Through discharge within the air handler can prevent proper drainage, causing water overflow, air handler flooding and possibly property damage. Figure 29 shows a good example of such a Figure 33 : Condensate U traps and their heights trap. Because condensation drains by gravity, the elevation of it through piping. difference between the bottom of the drain pan and the outlet There are several issues that have to be taken into account of the trap must be sufficient to be certain that no standing water while designing the condensate or drain piping. The drain pan remains in the pan due to the pressure differential between the outlet is typically on the downstream side of the coil on the inside and the outside of the air handler. This also has a bearing suction side of the fan, in the case of “draw–through” units. The on the depth of the U - trap which must be at least 1. 5 times the pressure at this point in the air handler is lower than the ambient pressure in the drain pan to prevent the water seal from being atmospheric pressure and hence outside air would be sucked in broken. See Figure 33 for more details both for a “draw–through” through the drain line. This incoming air stream has sufficient and a “blow–through” air handler. velocity to launch the water droplets forming at the base of the Cleanouts must also be provided to simplify periodical coil into the air. Air flowing through the coil can then spray this cleaning of any biological growth in the piping. See Figure condensate into the fan intake, which can propel the moisture 34. The AHU must be raised sufficiently above floor level to into other parts of the system. The resultant aerosol mist can be provide height and space for installing the water trap and every carried through the ducts and into the conditioned space causing Draw-Through discharge

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Figure 35 : Intermediate drain pans 19

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Figure 34 : Provision for trap cleaning

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Acoustic Treatment of the AHU Room

Figure 36 : A flexible connector

AHU room must have a floor drain that can carry away the condensate water.

Intermediate Drain Pans

On cooling coils that are more than 115cms (45 inches) in height water droplets running down the face of the coil get large enough to be entrained into the air stream even at 2.57m/s (500 fpm). To get the required face area, coils may have to be stacked two or three high. To prevent condensation from being blown into the air stream as it drips from one coil to the next, intermediate drain pans or troughs should be provided. This is illustrated in Figure 35.These intermediate troughs drain down into the main drain pan. ASHRAE recommends that these intermediate drain pans extend downstream of the coil at least half the height of the coil. In practise if you maintain a face velocity below 2.57 m/s (500 fpm), an extension of 30 cms (12 inches) will suffice. Keep in mind that the higher the face velocity the farther the drain pans must extend from the coil face.

Flexible Connections

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A flexible connection is required between the fan outlet and the supply air duct to prevent fan vibrations being transmitted to the ductwork with consequent noise in the conditioned space. The length of the flexible connection should be between 7.5 to 10 cms (3 to 4 inches) minimum for best results. Heavy canvas cloth commonly used will serve the purpose but is not the best material for the job since it becomes wet after use, causing fungus to grow on it, discolor the cloth, generate unpleasant odours and in course of time to tear. Imported synthetic material which retains its flexibility for a long time is used by many companies in their air handling units as a standard and the flexible connection itself is factory provided inside each unit, as shown in Figure 36. 20

When the single skin AHU was the standard in the industry, AHU rooms were often acoustically treated to reduce AHU noise in the air conditioned spaces. Blower motors in such AHUs were usually mounted outside the unit and the motor noise coupled with the belt drive noise made acoustic treatment of the room almost a necessity, since the room was used as a return air plenum. With the advent of the double skin AHU with the motor and drive mounted inside the unit on a floating spring isolated base, a flexible connection on the fan outlet and double- wall, insulated AHU panels the noise is sealed inside the unit and hence acoustic treatment of the AHU room can be safely eliminated in most installations saving a considerable sum of money. Critical applications such as auditoriums, lecture halls, sound studios and other similar applications will however require special advice from acoustic experts about the need for such treatment as well as the installation of duct silencers.

Testing and Balancing for Good Performance

New systems are never really balanced by themselves. In new systems it is possible that only half the amount of air reaches the end outlets and double the required amount flows out from the outlets near the fan resulting in hot and cold areas. Fans may be running too fast or slow, either wasting energy or performing inadequately. Too much or too little outside air may be drawn in resulting either in excessive energy costs or unhealthy air quality conditions. Even if balanced originally, existing systems become imbalanced due to adjustments by occupants, architectural remodelling in the spaces, HVAC system alterations and different use of spaces. In addition: • Balancing can provide a 5 to 10% saving in energy costs. • Balancing improves HVAC performance in terms of comfort, health and occupants work output. • Balancing provides correct air flows at fans and outlets and correct water flows at pumps and coils. • Problems of design, installation, equipment, operation and maintenance are uncovered and corrected in the balancing process. • Balancing reduces maintenance and wasting time “putting out fires” The most common type of HVAC system used is the lowpressure, constant-volume supply system, which is defined as follows: • The total system static pressure does not exceed 500 to All About AHUs

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Give the AHU the Respect it Deserves

625 Pa (2 to 2.5 inches water gauge). • Air velocities do not exceed between 10 and 12 m/s (2000 and 2400 fpm). The systems are constant air volume and variable temperature. To control the temperatures in the spaces, the cooling equipment is cycled on and off or modulated in order to vary air flow and space temperatures. Testing and balancing or TAB as it is commonly referred to, is a subject by itself and several books on the subject are available. One such book is HVAC Testing, Adjusting, and Balancing Manual, third edition, by John Gladstone and W. David Bevirt, published by McGraw-Hill, USA. It is good practise to provide some test openings at various points as shown in Figure 37 and 38 and if provided such openings should be suitably plugged to prevent air leakage and noise

Figure 38 : Measuring system air flow All About AHUs

Installation & Maintenance Manual

Most companies can supply a manual that furnishes valuable information on loading, unloading, temporary storage at site and hints on installation as well as maintenance that will guide you in the proper installation, commissioning and maintenance of your AHU. Feel free to write to the concerned company for a copy.

Start-up and Performance Problems

The Air Handling Unit is very often blamed for problems connected with air distribution system design. A common problem with new systems is excessive air noise caused by too much air. System designers have a tendency to play safe and assume pressure drops in the air duct distribution network in excess of actual. As a result the blower is selected for an external static pressure far in excess of what is required, resulting in excess air supply. To intelligently tackle such problems the supervising engineer must know the Basic Fan Laws which provide the needed calculations to adjust pulley sizes, change rpm, project new static pressures, and determine required horsepower. Each of these laws follows the laws of physics, and their relationships to one another change in set and predictable patterns. Thus : • Rpm varies at an equal proportion to air volume. • System pressures vary as the square of the air volume. • Amperage varies as the cube of the air volume. To determine the new performance of a system at a new CFM, take the following steps : 21

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Figure 37 : Pressure readings with Magnehelic gauges

Most AHUs are designed with care and manufactured with attention to detail. They leave the factory in perfect condition, properly packed when required. What happens after despatch is difficult to control especially during the transportation stage. Unloading and moving into final position or storage, if the building is incomplete, is however very much in the hands of the HVAC contractor and a good contractor will take pains in proper unloading and safe storage, adequately protected from adverse weather. During installation the AHU is often dented by careless workers or deliberately scratched by others who want to leave their mark behind. Very often the top of the unit is used to climb on to and reach piping, insulation or ductwork. Painters tend to leave paint marks all over the unit. The only way to protect the unit is to cover it with thick polythene sheets or tarpaulins. Don’t forget that the AHU can last upto 20 years or more, so why disfigure it before it even gets started on its life journey.

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1. Find the ratio of the existing CFM to the new CFM desired:

Old and new CFM ratio =

CFM new CFM existing 2. To find the new system rpm, multiply step one by the existing rpm: New rpm = CFM new × existing rpm CFM existing 3. To find new system static pressure, multiply step one, squared, by the existing static pressure: ( CFM new )2 existing static New Static pressure = × pressure CFM existing 4. To find new motor amperage, multiply step one, cubed, by the existing motor amperage: ( CFM new )3 × existing motor amps New motor amps = CFM existing The problem of excessive air can now be resolved by measuring the total air quantity and existing fan rpm, referring to the fan curves or rating charts and with their help determining the new fan rpm required to give the design air quantity. If necessary (if an adjustable pitch pulley is used, reduce the pitch diameter to reduce blower speed) the motor pulley diameter may have to be changed. Use the following formula to select the new motor pulley diameter: Fan pulley dia Design motor rpm = Motor pulley dia Design fan rpm When changing fan speeds, ask : • Will the new setting fall within the fan curve of the system? • Will the increased electrical load exceed the capacity of the existing motor? • Will the new noise produced by the new settings be objectionable? If you use these formulae you’ll be able to adjust fan speeds and calculate conditions before having to change motors and drives.

Calculating Cooling or Heating Coil Performance

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There are many situations in which the performance of heating/cooling coils needs to be known at conditions other than those for which they were purchased. One such situation would be that the cooling load has increased – what room temperature can now be maintained? Another situation would be that the occupants are complaining that the building is too hot or too cold – is the coil performing to its full capability, and if so, what can be done to improve the situation? 22

Methods are available to calculate the performance of a coil under any conditions with considerable accuracy. If the flow rates of air and water remain unchanged, calculations are simple enough to be done by an HVAC engineer without expertise in heat transfer. However, most HVAC engineers are not aware of these methods and face considerable difficulties when the need for such calculations arises. Two formulas follow by which the effects of changing air and water temperatures can readily be calculated. Their use is illustrated by three examples.

Formulas

For purely sensible heating or cooling, the following equation should be used: Q1 (Ta1 - Tw1) = (Ta2 - Tw2) (1) Q2 where Q = heating or cooling capacity of coil Ta = temperature of air entering the coil Tw = temperature of water entering the coil Subscripts 1 = conditions at which performance is known 2 = conditions at which performance is needed This formula is applicable if the flow rates of air and water are unchanged and the changes in temperatures from the design conditions are moderate, say 20F or less. When a coil does sensible cooling as well as dehumidification, the total cooling capacity Qt, can be calculated by the following equation : Qt1 (ha1 - hw1) = Qt2 (ha2 - hw2) (2) where ha = enthalpy of air at coil inlet hw = enthalpy of saturated air evaluated at inlet water temperature Subscripts 1 = conditions at which performance is known 2 = conditions at which performance is needed This formula should be used only if both the original and new conditions involve dehumidification.

Example 1

A switchgear room is provided with a recirculating air handling unit that maintains the temperature at 100F. The unit is rated 100,000 Btuh, with chilled water entering at 55 F. New switchgear (all sensible load) is to be installed, adding a load of 10,000 Btuh. Calculate the resulting room temperature using Equation 1: All About AHUs

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Example 2

The conditions are the same as in Example 1, but the room temperature must not exceed 102 F. Calculate the required chilled-water temperature to achieve this, again using Equation 1: 100,000 (100 – 55) = 110,000 (102 – Tw2) Tw2 = 52.5 F Thus a chilled-water supply temperature of 52.5 F is required to maintain a 102 F room temperature. Example 3 An air handling unit was purchased with the following performance rating by the supplier: • Entering air temperature = 95 F db/75F wb • Entering chilled-water temperature = 55 F • Leaving air temperature = 66 F db/58 F wb • Chilled-water flow = 243 gpm • Air flow rate = 20,000 scfm The unit was installed and flow rates balanced to the abovelisted values. The following measurements were then taken: • Entering air temperature = 90 F db/73 F db • Entering chilled-water temperature = 52F • Leaving air temperature = 63 F db/56 F wb Is the coil’s performance better than or inferior to its rated performance? Plotting the rated performance on the psychrometric chart shows that latent cooling is involved, so Equation 2 will be used. • Enthalpy of entering air (he) at 95 F db/75 F wb = 38.7 Btu per lb • Enthalpy of leaving air (hl) at 66 F db/58 F wb = 25.2 Btu per lb • Rated total cooling capacity = 4.5 × cfm (he – hl) = 4.5 × 20,000 (38.7 – 25.2) = 1,215,000 Btuh (the factor of 4.5 represents density of standard air multiplied by 60 to convert cfm to cfh.) The enthalpies at measured inlet and outlet air conditions are 36.8 and 23.9 Btu per lb, respectively. • Measured total cooling capacity = 4.5 × 20,000 (36.8 – 23.9) = 1,161,000 Btuh All About AHUs

• Enthalpy of saturated air at 55 F = 23.2 Btu per lb • Enthalpy of saturated air at 52 F = 21.5 Btu per lb Applying Equation 2, we can now calculate the cooling capacity at the rating conditions from the test conditions as follows: 1,161,000 (36.8 – 21.5) = Qt2 (38.7 – 23.2) Qt2 = 1,179,000 Btuh The rated cooling capacity is 1,215,000 Btuh. Hence, the measured performance is inferior to the rated performance.

Checking Actual Cooling Capacity

Actual cooling capacity is something one can easily calculate once you know the total CFM. Drill two 3/8 in. holes before and after the evaporator cooling coil. Read the entering and exiting web bulb temperature by wrapping a moist gauze sock around the bulb of a thermometer and then reading the wet bulb measurements in the air stream. Plot the two readings on an enthalpy chart. Read enthalpy by following the chart to the upper left; the difference between the two enthalpy values is the enthalpy change (Dnt). Calculate total cooling output by multiplying CFM by the enthalpy change (Dnt) and then multiply the answer by 4.5 (a constant). Total cooling output = CFM × 4.5 × (Dnt)

23

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100,000 (100 – 55) = 110,000 (Ta2 – 55) Ta2 = 104.5 F Thus, the new switchgear will raise the room temperature from 100 to 104.5 F.

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Analysing Problems With Air Distribution Systems I. Problem: Spaces Too Hot or Cold

Possible Causes :

• Too much or too little air being delivered to spaces • Temperature of air delivered to spaces too hot or cold • Thermostat incorrectly set, turned off, malfunctioning, poorly located, miswired, not calibrated • Control dampers incorrectly set or malfunctioning • Control valves incorrectly set or malfunctioning • lnternal heat generation in room more than designed, causing excessive heat gain • Fluid flow volume or temperatures at coils incorrect • Coils dirty or clogged • Terminal boxes not operating properly • Errors in heating or cooling load calculations • Poor zone temperature control built into system, not accommodating shifting sun loads or occupancy • Poor mixture of spaces on the same zone • System imbalanced, too much or too little air in branch ducts and outlets near fan, or too little at end of duct runs • System misdesigned; duct sizing incorrect, pressure calculations off • Occupant changes, HVAC system, architectural changes, internal heat generation.

II. PROBLEM: Too Much Air

Possible Causes :

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• Less resistance in system than design • Ductwork oversized, less ductwork than design, system components’ less resistance than design • Not all the system components installed Access doors open on intake plenum • Coil bypass damper and face damper open • Fan oversized, rpm high, wrong type, system resistance less than design • Over design of heating/cooling load or air volume Windows and doors open in spaces • Control dampers not set properly or malfunctioning • Manual volume dampers open excessively • Terminal units in medium or high pressure systems open, wrong size, wrong controls, set wrong or malfunctioning • Control of dampers, fans, or terminal units not working or incorrectly set • Excessive negative pressure in spaces • Too much direct exhaust air or return air being drawn out of spaces 24

• Grilles and ceiling diffusers not dampered down correctly, too large or wrong type • Filters not installed at all or wrong type Coils wrong size or type • Air density lower than design because it’s warmer or at higher altitude • Imbalance in air distribution system. • Too much air in branches and outlets near fan and not enough in duct runs and outlets • Poor temperature control zoning of system to accommodate changing heating/cooling loads during day, or poor mixture of spaces on the same zone.

III. PROBLEM: Not Enough Air

Possible Causes :

• Supply diffuser and grille dampers closed too much; wrong size or type • Too much resistance in air distribution system • Dirty components, underdesigned ductwork, excessive ductwork, imperfect fittings • Space thermostat incorrectly set, located, calibrated, or malfunctioning • Excessive duct leakage • End caps missing, disconnected ducts, access doors open, excessive leakage at connections • Dirty, blocked off or clogged manual dampers, reheat coils, turning vanes, grilles, ceiling diffusers, fire dampers • Fan undersized, rpm low, wheel installed backwards or wrong type, rotation backwards, wheel and inlet cone misaligned or incorrect gap, cutoff plate missing, manufacturer rating incorrect, air starvation in suction side of fan • Underdesign of heating and cooling load or air volume e Manual volume dampers closed too much • Fire dampers accidentally shut • Control dampers set incorrectly or malfunctioning • Filters dirty, clogged, blocked off, resistance rating too high, wrong type, wrong size • Terminal units in medium or high pressure systems in closed position, wrong size, wrong controls, set wrong or malfunctioning • Excessive positive pressure in spaces, not enough return or exhaust air being drawn out. • Imbalance in air distribution system; excessive dampering, wrong procedures used in balancing, poor design • Poor temperature control zoning • System incorrectly designed All About AHUs

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IV. PROBLEM: Fan Runs, Insufficient Air Flow

VII. PROBLEM: Drafts





Possible Causes :

• Excessive resistance in system • Wheel misaligned, improper wheel overlap and gap with inlet cone can cause a sharp loss of air volume and cycling within the fan • Bad fan discharges can wreak havoc with air quantities, causing statics, throbbing, pulsating, noises and duct wreckage. Poor discharges can be caused by: • Undersized fan • Fan rpm low • Low motor rpm • Wrong ratio of sheaves • Wheel rotating backwards • Wheel loose on shaft • Wrong type wheel • Wheel installed backwards.

• The speed of air is too high when it hits the occupants; not in the 25 to 50 fpm comfort range • The temperature of the air coming in contact with the occupants is too high or low in conjunction with the air speed; should be in 68 to 78 F range • Too much air at diffusers or grilles • Supply diffusers too small, deflection or throw incorrect • Doors or windows open • Horizontal and vertical temperature stratification in spaces; temperatures too high below ceiling or by outside walls and too low near floor or inside walls • Horizontal and vertical air speed stratification and currents; excessive air speeds near occupants.

V. PROBLEM: Fan Does Not Run, No Air Flow





Possible Causes :

• • • • • • • • • •

Broken belts Blown fuses Thermal overloads in starter kicked out No power Bearings froze Motor defective Loose pulleys Wheel housing jammed Motor heater overload kicked out Wheel loose on shaft and spinning.

• Too many rooms on one zone • Room that has thermostat for zone not representative of all spaces on zone • Spaces in zone don’t have homogeneous heating and cooling loads correlating with time and conditions • East. South, West, North solar load zones mixed together on same system • Perimeter and interior spaces mixed on same zone.

VI. PROBLEM: Imbalanced System

Possible Causes :

• Major discrepancies between actual and design cfms. To locate, spot check: • Air Volume at outlets farthest from fan, at end of duct runs, nearest fan and midway in the duct system • Total air flow or static pressure in main duct runs • Total air flow at fan • Suction and discharge pressures.

References 1. Air Handling System Design by Tseng-Yao Sun, McGraw-Hill. 2. Handbook of Air Conditioning and Refrigeration by Shan K. Wang, McGraw-Hill. 3. NAFA Guide to Air Filtration by National Air Filtration Association, All About AHUs

VIII. PROBLEM: Poor Temperature Control in Zones Possible Causes :

IX. MISCELLANEOUS FAN PROBLEMS:

Possible Causes :

• Unbalanced wheels cause vibration, noises and premature wear of bearings and drives; wheels have to be dynamically balanced and weights put on them for balance • Misaligned or incorrectly tensioned belts on drives • Wheel installed backward • A warped shaft caused by heat or in removing a wheel, causes vibrations and premature bearing and drive wear • A shaft at an angle in the fan can be caused by heat or by replacing a wheel, causes vibrations and premature wearing of bearings and drives • Fan cut off plate broken off. Washington D.C., USA. 4. Handbook of HVAC Design by Nils R. Grim & Robert Rosaler, McGraw-Hill. 5. ASHRAE Handbook 1996 HVAC Systems and Equipment by American Society of Heating, Refrigerating and Air Conditioning Engineers, Atlanta, USA. 25

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Possible Causes :

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Fan Laws For the same fan-duct system operating at different speeds, if the difference in air density is negligible, the volume flow rate is proportional to the speed ratio : = rpm2 1. cfm2 cfm1 rpm1 The total pressure increase ratio is equal to the square of the speed ratio : 2. P2 = rpm2 2 P1 rpm1

The fan power ratio is equal to the cube of the speed ratio : 3. Bhp2 = rpm2 3 Bhp1 rpm1 Where subscripts 1 and 2 indicate the original and changed operating conditions.

Airflow Formulas Used in Air Filtration 1) To calculate volume (CFM) from velocity (FPM):

CFM = FPM × Ft.

2

2) To calculate velocity (FPM) from volume (CFM):

FPM = CFM/Ft.

2

3) To calculate velocity (FPM) from velocity pressure (PV): FPM = 4005 PV 4) To c a l c u l a te vo l u m e ( C FM ) f ro m ve l o c i t y pressure(PV): CFM = (4005 PV) × Ft.2



5) To calculate velocity pressure (P V) from volume (CFM): PV = [(CFM / Ft.2) / 4005]2



6) To calculate panel filter face velocity in CFM:

7) To calculate filter media velocity in FPM: FPM = CFM / Media Area (Ft.2) Where : Af = Filter face area CFM = Volume air in cubic feet per minute CFMS = System CFM FPM = Velocity of air in feet per minute PV = Velocity pressure in inches water gauge PV = Total system pressure minus static pressure 4005 = A constant Ft.2 = (1) Face area of filter or filter bank (2) Area of duct cross-section FFV = Filter face velocity Nf = Number of filters

FFV = [CFMS / Nf) / Af]



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Useful Conversions To convert

From

To Multiply by

velocity

fps

m/s

0.30480

velocity

fpm

m/s

0.0050800

velocity

mph

m/s

0.447040

velocity

m/s

fps

3.28084

velocity

m/s

fpm

velocity

m/s

mph

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196.850 2.23694 All About AHUs

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Useful Conversions contd.. To convert

From

To Multiply by

pressure pressure pressure pressure pressure pressure pressure pressure pressure pressure

psi psf ft of water (4°C) in. of water (4°C) in. of Hg(15.6°C) Pa(N/m2) Pa(N/m2) Pa(N/m2) Pa(N/m2) Pa(N/m2)

Pa(N/m2) Pa(N/m2) Pa(N/m2) Pa(N/m2) Pa(N/m2) psi psf ft of water (4°C) in. of water (4°C) in. of Hg (15.6°C)

volume flow rate volume flow rate volume flow rate volume flow rate volume flow rate volume flow rate volume flow rate volume flow rate

cfm cfs gpm gpm m3/s m3/s m3/s L/s

m3/s m3/s m3/s l/s cfm cfs gpm gpm

6894.76 47.8803 2988.98 249.082 3376.85 0.000145038 0.0208854 0.000334562 0.00401474 0.00029613 0.000471947 0.02831685 0.0000630902 0.0630902 2118.88 35.3147 15,850.3 15.8503

Fan Connections

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Fan Outlet Connections

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Fan Inlet Connections