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Air Conditioning Systems

Classifications
Central and packaged unitary systems Constant air volume (CAV) and variable air volume (VAV) systems: different ways of handling load

variation
– CAV: variable supply air temperature with constant

volume flow to deal with load variation – VAV: variable supply air volume with constant supply air temperature

Classifications
All air, air-water and all-water systems: different medium for offsetting cooling load
– all air system: conditioned air is used – air-water system: both conditioned air and water are

used – all water system: only water is used

Single zone and multi-zone systems Single duct and multiple duct systems

System Selection
Air quality Thermal environmental control Noise Flexibility and reliability Operation and maintenance Space Costs Liquid leakage Aesthetic reasons

Single Zone CAV System
System schematics & control sub-systemscontrol Economizer
Mixing air control Heating coil control Cooling coil control

Single Zone CAV System
Supply air temperature control
Chilled water control valve, air dampers and heating coil control valve operated in sequence

2nd

1st

3rd

Single Zone CAV System
Economizer cycle
– outdoor air (OA) temp T3 > recirculating air (RA) temp T4
EA RA Max. opening OA Min. opening SA

– T3 < T4
EA Fully closed OA Max. opening SA RA

Single Zone CAV System
Economizer cycle
– T3 << T4
EA RA Modulation of opening OA Modulation of opening SA

– T3 <<< T4
EA RA Max. opening OA Min. opening SA

Single Zone CAV System
Psychrometric analysis (same as basic summer and winter cycles) o

m r cc s r m o ru ru s

Fan Coil Unit Systems
Types: primary air fan coil and space ventilation fan coil systems Primary air fan coil system
– outdoor air being treated in primary air handling unit

(PAU) – PA be supplied to mixing plenum of fan coil unit (FCU) or directly to space – normally air-water system

Fan Coil Unit Systems

Fan Coil Unit Systems
Primary air fan coil system psychrometric analysis (summer)
o

Full load Part load
rp mp fcp fc cp ps m r

Fan Coil Unit Systems
Primary air fan coil system (winter heating)
– heating load catered for by warm primary air with

heating in PAU (mild winter) or – heating of supply air in FCU (severe winter)

Fan Coil Unit Systems
Space ventilation fan coil system

– no pre-treatment of outdoor air – all water system

Fan Coil Unit Systems
Psychrometric analysis (same as basic system)
summer Winter heating
o

m r r fc m o fc

Fan Coil Unit Systems
System comparison
Prim Air Fan Coil ary Guaranteed outdoor air Less condensate at FCU Less w leakage problem ater Less noise problem M ductwork ore Space V entilation Fan Coil Outdoor air supply depending on outdoor conditions Increased condensate at FCU M m ore aintenance work M noise problem ore Less ductwork

Fan Coil Unit Systems
Advantages of air-water system
– less ductwork and space – less power required for transport using water – ease of individual control – less expensive

Disadvantages of air-water system
– poor control of indoor environmental conditions – higher relative humidity at part-loads – maintenance problem – free cooling not possible – seasonal changeover problem with 2-pipe system

Fan Coil Unit Systems
Water systems and control
– 2-pipe systems

Fan Coil Unit Systems
Water systems and control (cont’d)
– 4-pipe system

Fan Coil Unit Systems
Changeover and non-changeover 2-pipe systems
– non-changeover 2-pipe system

– warm primary air for heating provision in winter – more suitable for locations with mild winter

Fan Coil Unit Systems
– changeover 2-pipe system
Warm air from PAU to provide heating during mild winter months; hot water being supplied to FCU for heating during severe winter months Problem of seasonal changeover; changeover tolerance built in to avoid frequent changeover

Fan Coil Unit Systems
Control schematics (example)

Fan Coil Unit Systems
Selection of FCU
– total cooling capacity provided by primary air
Vps ρps (hr – hps)

– sensible cooling capacity provided by primary air
Vps ρps Cp (tr – tps)

– total cooling capacity required of FCU
Qt – Vps ρps (hr – hps)

– sensible cooling capacity required of FCU
Qs – Vps ρps Cp (tr – tps)

– other specifying parameters: chilled water supply

temperature, t across the coil, on-coil and off-coil conditions of air, water and air side pressure drop

THE END


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