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MASTERCLASS - AIR CONDITIONING TECHNOLOGY

PART 41 FEBRUARY ISSUE

In last months article we commenced a detailed study of Air Handling and therelevant Air Handling Equipment. This month we continue with Part 2 which

considers the design, specification, selection, performance and application of AirHandling Units. It also covers design parameters and specification considerationsrelating to the complete unit and to individual component selection.

AIR HANDLING UNIT SELECTION

The initial selection of an air handling unit and determination of its required duty,size, configuration and arrangement of components needs consideration of thefollowing factors:

Duty

Air volume flow rate and whether constant or variable.

External static pressure requirements of connected system.

Filtration standards.

Heating requirements, including frost protection, preheating, reheating.

Humidification requirements

Cooling/dehumidification requirements

Air mixing, bypass or shut-off arrangements.

Heating/cooling fluids: type, temperatures (flow/return), pressure, flow rates.

Water supply characteristics and drainage arrangements.

Electrical supply characteristics and any limitations.

Motor starting and control arrangements.

System control requirements as they affect number of stages or sections orcircuiting of heating/cooling/humidifying/dehumidifying components.

Casework thermal, air pressure, leakage and break-out noise standards.

Fan/unit sound and vibration: acceptable levels.

Heat/energy recovery provisions.

Duty/standby provisions, duty margins, special safety or operatingrequirements.

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Unit Design

Location: internal/external (weatherproofed).

Configuration: horizontal, vertical, double (or triple) decked, special toaccommodate heat recovery devices, piping or control panels (some

examples are given in Fig.1). Position of components in direction of air flow to achieve performance, e.g.

dehumidification plus reheat.

Number of unit sections for transportation and installation.

Provisions for access for maintenance and component replacement.

Requirements for safety: drive guards, fan inlet/outlet guards, safety grillage,lights, electrical isolation, earthing/bonding, arrangement of door handles /hinges, locks, retaining stays (safety and security), warning / informationlabels.

Baseframe design and height (e.g. to accommodate drainage traps).

COMPONENTS

The air handling unit incorporates a variety of components to condition and controlthe air passing through it:

Air movement (fans)

Air cleanliness (filters)

Air mixing/control (dampers)

Temperature changes (air heaters/coolers)

Moisture content changes (humidifiers / dehumidifiers)

Energy recovery (thermal wheels/recuperators/reclaim coils)

Noise control (attenuators).

The particular components and their relative positions in a given unit depend onsystem design, the particular application, and the changes in the psychrometriccondition of the air to be achieved under varying conditions of load or modes ofoperation.

INSERT FIG 1

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Selection Parameters

The basis of component selection may be governed by the specifiers maximumallowable face velocity or pressure drop requirements, or may be based on themanufacturers experience and recommendations. The following values are typical:

INSERT TABLE 1 SYSTEM PARAMETERS

Fans & Motors

The type of fan used in air handling units is usually the DIDW (double-inlet, double-width) centrifugal type, though the high performance axial-type may be employed forcertain applications. The use of plug fans may show economic benefits in certaincircumstances. Centrifugal fan impellers are normally of either the forward-curvedmultiblade type, typically 36 to 48 blades, and used for a FTP (fan total pressure)requirement up to around 750 Pa, or of the backward-curved-blade type, typically 8-12 blades for a FTP up to 2500 Pa. The total efficiency of the forward-curved type istypically 60-65%, that of the backward-curved 75-85%.

Fan speeds typically range from 300 to 3500 rev/min according to fan type, airvolume flow rate and FTP requirements. Motors range from fractional kW size toover 100 kW. Fan sound power levels range from 70 dB (ref 10-12 W) for very smallfans up to 100-110 dB for fans used in high volume, high FTP applications.

Fan selection involves choosing the most inexpensive combination of size,arrangement and type and class of construction to meet the required duty, whileproviding stable operation at acceptable sound level, efficiency and powerconsumption. This is normally accomplished by using a computer program topresent a range of alternatives and then print an operating characteristic. Fanselection involves consideration of fan type, impeller type, fan size and speed, outletvelocity, absorbed power, efficiency, construction and sound power level, which mustbe considered in conjunction with the following:

Air volume flow rate, m3/s

Fan static pressure FSP, Pa (resistance of system external to unit, and unititself)

FTP, Pa = FSP+ FVP (fan velocity pressure due to outlet velocity)

Entering air condition: temperature, degC; humidity % rh; density kg/m3

Condition of air surrounding unit (may affect motor selection, if external)

Altitude/barometric pressure at plant location (effect on air density)

Type of application or service: mechanical strength of impeller, shaft, bearingsand casing (according to FTP and fan speed)

Type of system and possible changes in duty requirements, e.g. volume andpressure changes in a variable volume system

Space available for motor and drive (if external to unit)

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Fan arrangement, position of outlet, direction of discharge, position of ductconnection

Type of drive (indirect using pulleys, direct using couplings), andarrangement-drive adjustments, provision of standby motors,

Accessories; scroll access doors, drain plugs, type of bearings, drive guards,

vibration isolators, flexible connections, inlet guide vanes

Special features such as spark-proof construction or protective paints orfinishes

Motor requirements: type, output, frame size, enclosure, bearings, electricalsupply, insulation rating, torque, starting/running currents, run-up time (oflarge fans), efficiency and power factor (full and part load), drive type andadjustment, mounting, vibration isolation, cable connections, thermistorprotection

Starter/inverter requirements: type, rating, enclosure, isolation, fusing/protective devices, indicating and interlocking requirements, speed control

Fan sound power level: octave band spectrum, required in-duct/space levels,attenuator requirements (pressure drop, space and cost).

Air Filters

Most air handling units employ standard (600 x 600) or half-size filters (600 x 300)from well-known manufacturers. Filters are generally specified by type, method ofrenewal, and efficiency. Commonly in use are panel, bag and compact type, whichare replaced when a predetermined dust loading (pressure drop is reached).Washable, oil-wetted or automatic filters are very rarely used in air handling units.Panel type filters for pre-filtering duty are typically EU3 or EU4 (Eurovent Grade) and

operate to about 150-170 Pa final (dirty) resistance. Bag or compact filters used assecondary or main filters usually range in grade for commercial/light industrialapplications from EU5 (250 Pa dirty), through EU6 & EU7 (300 Pa) to EU8 (350 Pa).

Special applications such as hospital operating theatres, pharmaceuticalmanufacture or clean rooms generally use a higher grade of special filter of whichthere are numerous types, usually termed HEPA (high efficiency particulate air)filters. Many of these are close to 99% efficiency; but 99%+ types are also available.

The design and effectiveness of filter frames and gaskets is extremely important forHEPA grade filters.

A separate class of filter is available for the adsorption of odours (usually

atmospheric) to protect, for example, the contents of art galleries, museums orlibraries. These are activated carbon type of which a number of formulations areavailable to cater for various contaminants, particularly SO2. A typical specification(M&E TR70) requires 80 kg of activated carbon per 1.0 m3/s of recirculated air to betreated and 120 kg per 1.0 m3/s when treating fresh air. Dwell (contact) timesbetween air and carbon are specified as 0.2 seconds and 0.3 seconds respectively,and maximum air resistance as 100 Pa. The carbon filter requires upstreamprotection by a filter of at least EU5 grade. In an important application (protecting

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national treasures) then the upstream filter would be EU7 and an after (tertiary) filterof EU7 would also be provided.

Filter selection requires consideration of the following:

Air volume flow rate, m3/s, to be handled. (If this is variable, arrangements

may be necessary to prevent pockets on bag filters collapsing at low air flow). Condition of air to be filtered: temperature, humidity, dust

loading/contaminants.

Filter efficiency/arrestance, dust holding capacity, EU grade, HEPA type, etc.. Initial (clean) and final (dirty) air pressure drops, Pa.

Filter duty, e.g. pre-filter, secondary or after-filter. Resistance of filter, filter frames, casings and cell-holding frames, gaskets and

fixing/securing devices to temperature and moisture: also fire properties(normally fireproof or self-extinguishing), resistance to frame/edge air leakage.

Means of filter replacement: side/front withdrawal (or other), access doors/hatches, lights.

Instrumentation: filter pressure drop manometers, gauges, pressure switches,sensors.

Air Control Dampers

Dampers in air handling units may be used for the control of air temperature, flow orpressure, or for air mixing. There are two basic types of multi-leaf air controldamper: the opposed- bladed and the parallel-bladed and their control characteristicsare different. The relationship between the air volume flow rate passing through acontrol damper, the angular opening of its blades, and the change in air pressuredrop depends on the control authority of the damper. For shut-off applications,

dampers should be opposed-blade type. For mixing and bypass applications,dampers should be parallel-bladed. Aerofoil-section blades are to be preferred.

Exhaust discharge dampers in mixing/diverting box applications should be opposed-blade. Return/recirculation air dampers in mixing boxes may require the addition of aresistance grid to achieve the correct pressure drop relationship to fresh/exhaust aircircuits. Dampers fitted to the unit at duct connection locations may be fittedexternally or internally to the unit casework. For units located outdoors, dampersshould be internal type.

Selecting a damper requires consideration of the following factors:

Air volume flow rate, m3/s

Function of damper: shut-off, bypass, mixing, diverting, etc.

System resistance, damper size and resistance and authority.

Pressures at point of operation and allowable air leakage (through the bladeseals and via the blade shaft bearings).

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Height/width ratio, number/depth of blades, frame, frame seals, blade seals,operating mechanism, materials/finish, operating torque and holding torquewhen closed.

Where air conditions are contaminated, eg swimming pools, dampers mayhave epoxy protection applied.

Actuator/motor type, spring-return provision.

Heating & Cooling Coils

Fluid coils (water, steam, refrigerant and glycols) are of the finned-tube type, exceptfrost coils may be bare-tube type. For heating duty, coils are typically of 5/8 inch(16 mm) outside diameter copper tube with continuous plate-type aluminium fins of0.15 mm thickness (0.4 for M&E 100), spaced at from 4 to 12 fpi (fins per inch) (6mm- 2mm fin spacing). For sensible cooling duty, fins may be 0.1 thicknesses, ofaluminium; where cooling is accompanied by dehumidification then fins may bealuminium, copper (0.25 M&E 100), electro-tinned copper or plastic-coatedaluminium. Hot water heating coils are typically 1 or 2 rows deep; 2mm fin spacing,30 Pa air pressure drop, 15 kPa hydraulic resistance. Chilled water coils aretypically 6-8 rows deep, 2.5mm fin spacing, 120 Pa air pressure drop, 20-35 kPahydraulic resistance. To maximise on fin-block (heat transfer) area, cooling coils inparticular may have external headers located in header boxes on the outside of theair handling unit casework. Electric heater batteries, using enclosed tubularelements, may be used for smaller heating loads, typically up to about 30 kW.

Coil selection considerations include:

Air volume flow rate, m3/s

Entering and required leaving air conditions.

Heating/cooling medium available, flow/return temperatures and pressure.

Materials of construction; tubes, fins, headers, casing.

Fin block area, number of rows, fin spacing, division of coil for control.

Air velocity m/s, air-side pressure drop Pa, hydraulic resistance kPa.

For dehumidifying coils: quantity of moisture removed, prevention ofcarryover, use of eliminators and material, construction of drain pans andmaterial, drain positions, trap design (to avoid legionella).

Piping connections: location (should be counterflow), number, size, type,

provision for isolation, draining, venting, coil removal, insulation.

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Humidifiers

Recirculating water-spray type humidifiers are generally not used because ofpotential maintenance problems and risk of legionella. The choice is typicallybetween atomising spray type (non-recirculating), direct-steam injection using a

jacketed manifold, electric steam generator and, ultrasonic type. Of crucial

importance to the success of humidifier operation is the way in which it is mountedinto the air handling unit. Correct positioning of the humidifier to achieve thoroughmixing of air and evaporated moisture without carryover or condensation onto theinner walls of the humidifier chamber or downstream components is essential.

Selection considerations include:

Air volume flow rate, m3/s

Entering air volume, velocity, velocity profile, dry bulb temperature degC, andpercentage saturation (% SAT).

Required moisture addition kg/h and leaving conditions necessary to achievethis.

Unobstructed length downstream of humidifier, typically 1 m minimumrequired.

Method of humidifying: electrical power, steam (pressure) and waterconsumption (water quality, pressure, any special requirement, e.g.,demineralised).

Control and safety requirements.

Energy Recovery Equipment

There are four main types: thermal wheels, run-round coils, recuperators and heatpipes:

Thermal wheels (or rotary heat exchangers) comprise a wheel or drum containing acorrugated material. This is slowly rotated between a fresh or supply airstream andan exhaust airstream. Heat is absorbed from the warmer stream and transferred tothe cooler. Two types of infill material are available -hygroscopic (retaining moisture,and capable of transferring both sensible and latent heat) and non-hygroscopic(transferring sensible heat only). Advantages are high efficiencies and the ability to

transfer total heat. Disadvantages include a high space requirement and thenecessity for the two air streams to be adjacent, usually accomplished using byusing a double-deck air handling unit.

Run-Round Coils are finned water-tube coils located in supply and exhaust airstreams and interconnected by a pumped water (or glycol) pipe work circulatingsystem. This system is seasonally reversible, providing preheating in winter and pre-cooling in summer. The two or more coils can be completely separated and locatedin individual air handling units.

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Static Recuperators (or air-to-air plate heat exchangers) transfer air from oneairstream to another by indirect contact on either side of a metal heat transfersurface -a series of closely spaced parallel plates, usually aluminium. Other metalsand protective coatings are available where the airstream is contaminated orcorrosive. Recuperators require the air streams to be adjacent and this is usually

achieved by using a double-deck air handling unit. A bypass damper system may beused for control purposes.

Heat Pipes are passive heat exchangers that make use of a closed fluid cycle withinan arrangement of sealed vertical or horizontal tubes. When cold air passes overone end of the tubes, the fluid within condenses and heats that air. Warmer air in anadjacent airstream passing over the other end of the tubes is cooled as it evaporatesthe fluid within the tubes. The action is reversible and operates whenever there is atemperature difference between the ends of the tubes. Application is restricted tospecialised projects.

A comparison of the energy recovery equipment types follows:

INSERT TABLE 2 ENERGY RECOVERY EQUIPMENT

Attenuators

Air handling unit manufacturers normally offer a range of cased attenuators, matchedto unit cross-section dimensions and construction/finish. A typical range employsfrom 2 to 8 vertical splitters (according to unit width) contained in modular lengthsections from 640 to 2720 mm, with a wide range of performance. Attenuatordesigns may be customised to meet specific acoustic criteria.

EXAMPLES OF AIR HANDLING UNIT ATTENUATOR INSERTION LOSS dBFrequency Hz

Section Length (mm) 63 125 250 500 1k 2k 4k 8k640 3 5 11 22 28 23 16 111600 5 12 28 42 49 48 33 212720 8 19 44 50 50 50 49 29

Attenuators should not be fitted directly at fan discharge. A properly designed fandiffuser section with a perforated baffle plate is required to spread the air betweenthe attenuator splitters and achieve rated acoustic performance at design pressuredrop. The length of a proper diffuser section increases with unit size/cross-section,

typically ranging from 480 to 1280 mm long.