bs 2 hvac air conditioning 2

BUILDING SERVICES TWO (BLD60503)

HVAC Systems & Building

Prepared by Tan Hee Chai

BUILDING | SERVICES | TWO (BLD60503)

Air Conditioning 2

BUILDING | SERVICES | TWO (BLD60503)Air Con

window unit Air Con Portable unit Air Con Split

unit

BUILDING | SERVICES | TWO (BLD60503)Centralised Air Conditioning

BUILDING | SERVICES | TWO (BLD60503)Centralised Air Conditioning

BUILDING | SERVICES | TWO (BLD60503)

Air Conditioning System Internal climate

Human comfort

Air qualityVentilation air

Psychrometric dataProperties of humid air

The thermal duties of the system Thermal balance

Sequence of air treatment Air handling sequence

Air intakePre-heatFiltrationAir cooling and humidifying coilDehumidification and humidification

BUILDING | SERVICES | TWO (BLD60503)

The thermal duty of the system

Two parameters used to specify the duty of air conditioning:1.The dry bulb temperature of the air2.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)

BUILDING | SERVICES | TWO (BLD60503)

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

BUILDING | SERVICES | TWO (BLD60503)

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.

BUILDING | SERVICES | TWO (BLD60503)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

BUILDING | SERVICES | TWO (BLD60503)

Thermal balance

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Sequence of air treatmento 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.

BUILDING | SERVICES | TWO (BLD60503)

Sequence of air treatmento 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

BUILDING | SERVICES | TWO (BLD60503)

Basic requirements of an air-conditioning system

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Air handling sequence Air intake

BUILDING | SERVICES | TWO (BLD60503)Air handling sequence

BUILDING | SERVICES | TWO (BLD60503)

Air heater battery (a typical heat exchanger)

BUILDING | SERVICES | TWO (BLD60503)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

Air handling unit

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FiltrationAir Handling unit

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Filtration

BUILDING | SERVICES | TWO (BLD60503)Types of Filters

BUILDING | SERVICES | TWO (BLD60503)

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

BUILDING | SERVICES | TWO (BLD60503)Filtration

BUILDING | SERVICES | TWO (BLD60503)

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

BUILDING | SERVICES | TWO (BLD60503)

FiltrationElectrostatic 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 cleanedMay be coated with viscous liquid but will then need special cleaning.Suitable to remove particles such as tobacco smoke.

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FiltrationHEPA FilterRemove most bacteria.Ultra-violet light sterilisers can also used.

HEPHe

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Air cooling and dehumidifying coilCoils 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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Air cooling and dehumidifying coil

BUILDING | SERVICES | TWO (BLD60503)

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

BUILDING | SERVICES | TWO (BLD60503)

Dehumidification and humidificationThe 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 humidifiersSteam 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

BUILDING | SERVICES | TWO (BLD60503)

Dehumidification and humidification Ultrasonic humidifier system.

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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.

BUILDING | SERVICES | TWO (BLD60503)

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.

BUILDING | SERVICES | TWO (BLD60503)

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.

BUILDING | SERVICES | TWO (BLD60503)Dehumidification and humidification

Reheater

BUILDING | SERVICES | TWO (BLD60503)Dehumidification and humidification

ReheaterSimilar construction to preheaterHeating medium – LTHW, HTHW or SteamPlain tube heaters and gilled tube heaters are used with HTHW and steamTo 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 waterHTHW = High temperature hot water

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The End