Building Services. By Dr David Johnston and Alan Newall – licensed under the Creative Commons Attribution – Non-Commercial – Share Alike License http://creativecommons.org/licenses/by-nc-sa/2.5/. ENVIRONMENTAL SCIENCE & SERVICES - LEVEL 1. Air conditioning Alan Newall & Dr David Johnston.
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By Dr David Johnston and Alan Newall– licensed under the Creative Commons Attribution – Non-Commercial – Share Alike License
Alan Newall & Dr David Johnston
The term”air conditioning” is commonly used to describe any system where
refrigeration is included to provide cooling. Cooling may be required to:
A wide range of air conditioning systems are available. The various systems can
be categorised according to their function:
Air conditioning systems do not have to be applied to a whole building. They can
be applied to specific areas of a building in order to provide a healthy and
comfortable environment for the building occupants.
The building can incorporate:
The use of partial or mixed-mode systems instead of a full air conditioning
system can reduce capital installation and operating costs.
Air conditioned buildings have previously consumed 1½ to 2 times as much energy as
an equivalent naturally ventilated building (MoD, 1996), resulting in an increase in
Introducing full air conditioning into a design can often add significantly to the eventual
running costs of the building (CIBSE, 1998).
Air conditioned buildings are more complex and costly to build and maintain.
In the UK, there is a limited set of circumstances in which a building will require air
conditioning. It can be necessary in certain circumstances due to pollution, external
noise and high heat gains (CIBSE, 1998). Climate change is leading to increased
heat gains and peak summertime temperatures, which is in turn increasing the
demand for air conditioned buildings.
In a large number of buildings, full mechanical air conditioning is notalways
necessary and large savings can be made by using mechanical or natural ventilation.
Air conditioning systems are normally specified to prevent overheating of certain
buildings in summer.
Buildings suffer heat gains from a number of sources, predominantly:
These heat gains peak in summer and must be offset by the air conditioning system to
maintain thermal comfort conditions.
The figure below provides useful guidance in assessing the need for full air
conditioning, comfort cooling or mechanical ventilation.
What are your requirements [Source: BRECSU, 1993]
There are three generic types of air conditioning system:
These systems are typically based around a packaged air handling unit(AHU) which
consists of a fan, combinations of heating and cooling coils, filters,humidifiers and
control dampers. They may also include the facility to recycle exhaust air back into the
AHU [Source: Flakt Woods, 2003]
The AHU would be located within a plant room with chillers and boilers located nearby,
which supply hot and cold water to the heater battery and cooling coil.
Constant volume single zone systems
Single duct variable air conditioning system [Source: Chadderton, 1993].
Variable air volume (VAV) system
The VAV system overcomes some of the problems associated with zones which
have different heating or cooling needs.
Fan-powered variable air volume terminal unit installed in a false ceiling
[Source: Chadderton, 1993].
Variable air volume (VAV) system
VAV unit [Source: Barcol-air, 2003]
VAV terminal unit [Source: Hassan, 1996]
The common factor with these systems is that a central AHU is used, with the air
being further heated or cooled to the individual room requirements by additional
heating or cooling coils.
Partially centralised systems generally consist of:
Centralised air systems with re-heat
Fan-coil unit air conditioning system (SDFC) [Source: Chadderton, 1993].
Conventional air conditioning systems provide cooling almost entirely by
convective heat transfer. Cooling can also be provided by a combination of
radiation and convection. This can be achieved using chilled ceilings and/or
Chilled ceiling/beams use chilled or cooled water (normally between 13 to 18°C)
as the cooling medium (CIBSE, 2001). “Free cooling” may be utilised if a suitable
source of groundwater is available.
There are three generic types of chilled ceiling/beams:
Chilled ceiling panels [Source: SAS, 2003]
Chilled beams [Source: SAS, 2003]
Chilled ceilings and beams only provide comfort cooling. Filtration, heating and
humidification are not available.
With active chilled beams ventilation is provided from the central plant, and
filtration may be provided.
With passive chilled beams and ceiling panels, ventilation has to be introduced
separately. This is normally achieved by displacement ventilation.
The avoidance of condensation on the surface of chilled beams and ceilings has
been a major design issue in the UK (CIBSE, 2001).
Chilled ceilings and beams must be controlled to prevent the risk of
condensation, otherwise ‘office rain’ could occur.
This can be achieved by incorporating condensation detection into the chilled
beam control system.
Unitary reversible heat pumps
Calorex heat pumps [Source: Calorex, 2003]
Local systems normally only provide comfort cooling, although filtration and
heating can also be provided. Humidification is not normally available.
They are often used as a refurbishment option.
Local systems may be less effective than centralised plant but can provide
energy savings through reduced distribution losses, simpler heat rejection
equipment, greater control over operating periods, and their ability to be more
readily confined to the areas of greatest need (CIBSE Guide B2), e.g. partial air
Local systems generally consist of:
Through-the-wall packaged units – small scale domestic/commercial application
providing comfort cooling
Single duct packaged unit [Source: Greeno, 1997]
Split packaged units
Multi-split packaged unit [Source: Daikin, 2003]
Variable refrigerant flow units – alternative system to fan coil units.
Variable refrigerant flow system [Source: Daikin, 2003]
Fundamental to the design of airconditioning systems are the following
Refrigeration equipment is used to chill air or water in all mechanical air
conditioning systems. There are two types of system commonly in use:
In the UK, a mechanical vapour compression system is generally used,
although absorption refrigeration systems should be considered where waste
heat is available (e.g. from CHP).
Vapour compression system
Compression refrigerator cycle [Source: McMullan, 1998]
Stage A - Liquid refrigerant evaporates at a low temperature, absorbing heat and providing
Stage B - The pressure of the refrigerant vapour is increased by means of a compressor. The
pressurisation raises the temperature of the refrigerant and requires an input of energy into the
Stage C - The refrigerant vapour condenses. The latent heat released is emitted by the
condenser to the surrounding air.
Stage D - The liquid refrigerant is passed through an expansion valve and its temperature and
pressure drops for a repeat of the cycle.
The efficiency of a vapour compression system is expressed as a Coefficient of
Performance (C.o.P.), where:
C.o.P,=Heat absorbed by the evaporator (cooling provided)
(cooling)Electrical energy used by the compressor
Heat pumps use the vapour compression cycle to provide both heating and cooling for a
building as required.
In winter, when heating is required, the functions of the evaporator and the condenser are
The evaporator absorbs low grade heat from a low temperature source and high grade
heat (at around 40 to 50°C) is released to the building from the condenser.
Heat sources for heat pumps include ambient air, and the ground, where buried coils are
used to extract low grade heat effectively.
Heat pumps perform well with low temperature heating systems, e.g. warm air or
For heating the C.o.P. is defined as:
C.o.P,=Heat supplied to the building
(heating)Electrical energy used by the compressor
C.o.P.’s of around 4 to 5.0 can be achieved using ground source heat pumps, which
provides a significant reduction in environmental impact, relative to conventional heating
Absorption system – used where a source of waste heat is available, e.g. from CHP
Absorption refrigerator cycle [Source: McMullan, 1998]
Stage A - The evaporator coils are placed in the low temperature heat source and heat is
absorbed by the evaporating refrigerant.
Stage B - The temperature of the refrigerant and water solution is increased, separating the
refrigerant from the water.
Stage C - The refrigerant vapour is cooled and condensed to a liquid, releasing heat.
Stage D - The refrigerant is expanded and it evaporates for a repeat of the cycle.
(Waste heat from process/CHP)
There are a number of key issues which need to be addressed when selecting
refrigeration equipment. These are:
BARCOL-AIR (2003) Variable Air Volume Systems (VAV). Available from:<
http://www.barcol-air.co.uk/variable_air-index.htm > [Accessed February 18th 2003].
BRECSU (1993) Selecting Air Conditioning Systems. Good Practice Guide 71. Department of
the Environment, London, HMSO.
CALOREX (2003) Calorex Heat Pumps. Available from:< http://www.calorex.com/ > [Accessed
February 19th 2003].
CHADDERTON, D. (1993) Air Conditioning – A Practical Introduction. E & FN Spon.
CIBSE (2001) Ventilation and Air Conditioning: CIBSE Guide B2. London, Chartered Institute of
Building Services Engineers.
CIBSE (1998) Energy Efficiency in Buildings: CIBSE Guide. London, Chartered Institute of
Building Services Engineers.
DAIKIN (2003) Daikin Industries Ltd. Available from:< http://global.daikin.com/ > [Accessed
February 19th 2003].
FLAKT WOODS (2003) Air Handling Unit EC 2000. Available from:< http://www.flaktwoods.com/
ct=&strModuleID=&strResourceID=> [Accessed February 18th 2003].
GREENO, R. (1997) Building Services, Technology and Design. Harlow, UK, Addison Wesley
HASSAN, G. (1996) Building Services. London, UK, MacMillan Press Ltd.
MoD (1996) Justifying the Provision of Air Conditioning. Defence Works Function Standard
Design and Maintenance Guide 07, Ministry of Defence, London, HMSO.
SAS (2003) SAS International Chilled Ceilings.Available from:< http://www.sasint.co.uk/index.html >
[Accessed February 19th 2003].