bs 2 hvac air conditioning 4

BUILDING SERVICES TWO (BLD60503)

HVAC Systems & Building

Prepared by Tan Hee Chai

BUILDING | SERVICES | TWO (BLD60503)

Air Conditioning 4

BUILDING | SERVICES | TWO (BLD60503)Air-Conditioning methods

BUILDING | SERVICES | TWO (BLD60503)Air-conditioning systems

1.The single duct system – central zone (all air, low velocity)


Air-conditioning systems1.The single duct system – central zone (all air, low velocity)Part of building is conditioned by a single duct plant whilst other parts of same building with different psychrometric requirements are served by other systems.Perimeter heating often required to offset cold downdraughts from glazing.Most common use because cheapest of installation.


Air-conditioning systems1.The single duct system – central zone (all air, low velocity)Ventilation fresh air component of supply air – too small to carry thermal load demanded by building, so recirculation of exhaust air necessarySystem relevant to single cell buildings or buildings within which all spaces have equal thermal requirement.


Air-conditioning system2.The multi-zone system – central zone (all air, low velocity)


Air-conditioning system2.The multi-zone system – central zone (all air, low velocity)Building is divided into a number of zones with varying thermal loads.Each zone – served by its own supply duct with supply air at a condition to match the zone’s requirements.Cooling coil and after heater – controlled by each zone thermostat.Requires considerable space to accommodate the zone ducts.


3.The dual duct system – central zone (all air, high velocity)


Air-conditioning systemsPrinciple of mixing box


Air-conditioning systems3.The dual duct system – central zone (all air, high velocity)Air is conditioned for a calculated minimum psychrometric state in a central unit.But final heating or cooling are carried out in two separate ducts.Terminal units – served by both hot and cold air ducts automatic mixing takes place so that

o induction air to the space is in a desired condition ando The discharge of air into space is at constant rate.


Air-conditioning systems3.The dual duct system – central zone (all air, high velocity)Volume control within terminal unit (mixing box) – also serves to balance availability of air to other parts of the system.Mixing boxes are controlled from local thermostats.


Air-conditioning systems3.The dual duct system – central zone (all air, high velocity)Mixing boxes fitted above false ceilings serve:

One discharge terminal in zones or Maybe in form of single units fitted below window sills.

Mixing hoses Reduce incoming air pressure Noise attenuators.


Air-conditioning systemsAdvantage of this system – ‘free cooling’ when outside temperature is about 10 degree Celsius. No refrigeration is required.


4. The induction system (air/water, high velocity)


4. The induction system (air/water, high velocity)- Induction unit



• Induction unit provides heating and cooling to a room

• Ejector nozzles attached to plenum chamber inject primary air into space supply grille

• At same time entrain a large qty of room air

• Room air induced to flow over heating or cooling coils which are automatically controlled by room air sensor.



• Discarding of primary air – expensive through windows etc,

• so either permanently or properly sealed – an exhaust air duct provided - achieve a measure of recirculation

• No recirculation is desired, a relief air duct provided without exhaust fan eg hollow ceiling over corridor comprise a plenum chamber.



• Dehumidifying function (located at central plant) is separated from sensible cooling function

• Plenum chamber – located in the thermal unit.

• High velocity primary air delivered to it at constant rate through damper.



o High in capital & running cost

o Requires large amount of duct space

o System relevant to buildings where spaces have different psychrometric requirements



o Volume regulator in mixing box meet cold air flow match heat gains to space

For space heating:Air volume flow rate always less than for space cooling

Reason: warm air may be delivered with much greater temperature gradient between it and room air



• Distribution of hot and cold water:• Two pipe system – one supply (hot or cold) &

one return o limit controllability of various rooms.o Cheapest to install



•Three pipe system – one hot supply, one cold supply and common return

•Provides independent heating or cooling •but mixed return adds to refrigeration and boiler plant loads.



•Four pipe system – one hot and cold supply with its own return •Full control achieved by separate system•But most expensive to install.


5. The Variable air volume system (VAV)

BUILDING | SERVICES | TWO (BLD60503)5. The Variable air volume system (VAV)



• VAV supplies ventilation air requirement at constant psychrometric condition, ie db temperature 10 c and 50% saturation during summer

• During winter, ventilation air flow rate increase to maintain thermal comfort within spaces.

• As volume decreases throughout building, fan responds with reduced output.



• System can only deal with reductions in sensible heat gain to spaces.

• When heat gains reduce to a value at which constant volume of supply air does not maintain a condition of thermal comfort. Ie spaces too cold- heat energy to be provided.



Common alternatives to satisfy demand above:

•Provision of a system of heat emitters placed around spaces or a warm heating system•Provision of reheaters at VAV terminals•Provision of a hot air duct which blows into mixing boxes when cold air flow has been reduced to its ventilation air flow rate minimum.



• VAV system can deal with different thermal demands in different parts of a building

• Heating elements may be installed in void above ventilating ceilings to meet extraneous requirement.

• Terminal units can be mounted below windows to project air stream upward or Above a suspended ceiling to blow air across it.



• Dropped ceilings over corridors may contains units to supply sidewall grilles.


6. Fan Coil units


6. Fan Coil units

Contain one or more centrifugal fans which induce air to flow from room and deliver it through cooling and/or heating coils back to room.

Ventilation air provides humidity control Exhaust air – egress through natural ventilation

channels Or return air duct if installed some exhaust air

recycled when a central unit installed for tempering.


6. Fan Coil units

Heating & cooling at fan coil units used A four-pipe system comprising a separate

chilled water circuit with separate coils within terminal unit.

A two-pipe systems with common coil within unit

BUILDING | SERVICES | TWO (BLD60503)How Does Radiant Cooling Work?


Chilled Ceiling - Embedded Coils in concrete floor soffit

Laying of PEx pipes in Slab for Radiant Cooling Radiant Cooling through Ceiling embedded pipes.

https://youtu.be/K2Kq0_ngO-8


Chilled ceiling: Proprietary metal pans fixed to pipelines suspended from concrete ceiling

BUILDING | SERVICES | TWO (BLD60503)Ventilated ceilings


The use of terminal water loop heat pump units


Variable refrigerant volume systems

One condensing unit can be connected to several evaporators, each of which are individually controlled.

a VRV system constantly adjusts the amount of refrigerant being sent to each evaporator and takes advantage of existing heat or cool air in the building.

This varying speed allows the system to work only as needed in each area to maintain the comfort level.

BUILDING | SERVICES | TWO (BLD60503)The effects of increased air velocity in the system are:


The End

bs 2 hvac air conditioning 4

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