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Air Conditioning System
Internal climate
Human comfort
Air quality
Ventilation air
Psychrometric data
Properties of humid air
The thermal duties of the system
Thermal balance
Sequence of air treatment
Air handling sequence
Air intake
Pre-heat
Filtration
Air cooling and humidifying coil
Dehumidification and
humidification
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The thermal duty of the system
Two parameters used to specify the duty of
air conditioning:
1.The dry bulb temperature of the air
2.The humidity of the air
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Air treatment plant deal with:
1. Heat load – heat which causes a temperature change.
2. Latent heat load – heat involved in evaporation and
condensation
(Latent is Latin for hidden, so it is the hidden heat of moisture in
the air)
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Air conditioning system is a ventilation
system with a capacity for cooling,
therefore the plant must provide both
heating and cooling service.
The design values of psychrometric
conditions for both inside and outside the
building, summer and winter conditions,
must be determined
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Recorded meteorological data provide all
aspects of weather conditions through the year
for particular localities, and the internal
psychrometric condition is selected for
optimum thermal comfort.
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Thermal balance
During summer the cooling load borne by the refrigeration
plant must balance the heat gains of the building:
• Thermal transmission through the structure;
• Thermal gain by ventilation air;
• Thermal emission by lighting systems;
• Thermal emission by machinery and equipment (computers,
photocopiers etc)
• Thermal emission by people;
• Thermal gain by solar radiation;
• Latent heat of evaporation involved in change of humidity.
This is relevant to the cooling of ventilation air and to vapour
emission by people
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Sequence of air treatment
o Winter
o External air is cold and has high relative humidity, actual moisture
content relatively low.
Air is heated up its relatively humidity decreases and left untreated air
could be uncomfortable dry(a frequent complaint in centrally
heated houses).
So it is necessary to humidify air.
If water at room temperature is used, energy (latent heat of
evaporation) is required to suspend the water in the air
The air temperature drops (giving energy into water vapour/air
mixture), resulting in possible need to reheat the air.
14. BUILDING | SERVICES | TWO (BLD60503)
Sequence of air treatment
o Summer
o External air warmer than required room temperature, so cooling
necessary
As air cooled –relative humidity increases and more moisture needs
removed.
Air soon reaches its dew-point temperature.
As temperature decreases further, air cannot hold the same
moisture quantity and condensation occurs, removing water from
air.
Process continues until correct moisture content is reached.
At this point air temperature is too low, so re-heating is required
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Basic requirements of an air-conditioning system
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Air heater battery (a typical heat exchanger)
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Filtration
Filters stop solid pollutants found in ventilation air
In larger plants, filters are placed before tempering and re-
heating of coils to protect them from an insulating layer of
dirt.
High efficiency filters (HEPA) are used for hospitals,
photographic processes, printing and food preparation
where clean air is paramount.
But HEPA might exert considerable resistance to air flow (ie
loss of fan static head)
Clean air reduce discolouration of decorations and prevent
dust from entering machinery & other equipment.
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Filtration ‘Throw away’
type – a
blanket of
synthetic fibre,
micro glass
fibre or paper
material.
Discarded and
replaced when
resistance to
air flow is too
great
Used almost
universally
‘Reusable’ type
– a blanket of
synthetic fibre
glass fibre,
formed
polyurethane or
cellular
polyurethane.
Clean and
replace when
resistance to air
flow is too great
Not widely used
any more
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Filtration
• Perforated container with metal labyrinth for passage of air.
• Submerged in a liquid which is inodorous, non-flammable and
non-evaporative.
• When pressure loss reaches very low, filters are immersed in a
cleaning fluid, re-immersed in the viscous fluid, then return to
duty
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Filtration
Electrostatic filters.
Air becomes ionised as it is
influenced by the field within high
voltage conductors.
Downstream it encounters opposite
charged metal plates to which
ionised particles of dust stayed.
Plates may be vacuum cleaned
May be coated with viscous liquid
but will then need special cleaning.
Suitable to remove particles such
as tobacco smoke.
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Filtration
HEPA Filter
Remove most bacteria.
Ultra-violet light sterilisers
can also used.
HEPHe
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Air cooling and dehumidifying coil
Coils constructed of
copper pipes with
aluminium, electro-
tinned, or, copper fins
to increase surface
area of contact.
Cooling to reduce its
temperature and
moisture content.
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Dehumidification and humidification
The spray washer,
humidifier and
dehumidifier system.
Air passes through
deflector plates or
perforated screen to
ensure uniform flow and
prevent backward flow of
moisture
Air leaves the washer
by passing eliminator
plates and scrubber
plates to remove
excess moisture and
dirt that has passed
over from the washer
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Dehumidification and humidification
The capillary
washer, humidifier
and dehumidifier
system.
Air is cleaned efficiently
because the fibre glass
cells provide a similar
service to the eliminator
and scrubber plates.
Dehumidification
capacity varies
according to
varying water
temperature
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Dehumidification and humidification
Steam humidifier system.
Just as dehumidification carried out by
cooling coil, so humidification is via electrode
stream humidifiers
Steam is supplied by heating by direct
electrical resistance heating elements and
discharge the steam through single or multi-
point ‘sparge pipes’.
Since the moisture is steam, with correct
controls of all the moisture is taken up by the
air with little or no carry-over or condensate
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Dehumidification and humidification
Ultrasonic humidifier system.
35. BUILDING | SERVICES | TWO (BLD60503)
Dehumidification and humidification
Ultrasonic humidifier system.
Steam humidifier provides no recirculation water and clean
water .
The principal disadvantage is the heavy electrical loading required,
which can be a problem.
For this reason, ultrasonic humidifiers were developed which can
agitate the water at very high frequency until the water has sufficient
energy to change state and vapourise into the air stream.
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Dehumidification and humidification
Ultrasonic humidifier system.
They consumed considerably less electrical energy but will depress
the air temperature, so re-heating may be necessary.
They produce reasonably clean water vapour and can be
capacity controlled by changing the frequency of agitation.
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Dehumidification and humidification
Ultrasonic humidifier system.
Principal disadvantages:
•Water must be free from small particulars/impurities to allow
discharge through very small agitator nozzles. (Reverse osmosis
often used.)
•The whole agitator appliance must be located into the air
handling plant which makes this system impractical for small
equipment.
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Dehumidification and humidification
Reheater
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Dehumidification and humidification
Reheater
Similar construction to preheater
Heating medium – LTHW, HTHW or Steam
Plain tube heaters and gilled tube heaters are used with
HTHW and steam
To heat air steam to achieve desired psychrometric
conditions after cooling of the air to remove moisture, or
humidification of air by means other than steam injection.
LTHW = Low temperature hot water
HTHW = High temperature hot water
