BS 2 Hvac air conditioning 3

BUILDING SERVICES TWO (BLD60503)

HVAC Systems & Building

Prepared by Tan Hee Chai

BUILDING | SERVICES | TWO (BLD60503)

Air Conditioning 3

Contents Fans Refrigeration Air Conditioning methods Air Conditioning systems

Three (3) common types of fan

Shape of centrifugal fan

Centrifugal fansAlso called ‘radial flow’ fansPerformance is affected by the shape of the blades and shape of the casing.

Centrifugal fans

Fan drives and motorsAC or DC powered

AC uses mains power but noisy due to nature of supply.Speed variation and harmonies easier and cheaper to control using micro processors.

DC motorsQuieter speed control constant but expensive.AC DC rectifier (see right photo)

Fan drive

Axial flow fans Consist tubular casing with

aerofoil section blades arranged at an angle to form helix

Motor contain inside hub or outside casing driven by V belt

Centrifugal fan with forward curved blades develop higher pressure than axial fans

High running speed favours small motor

Latest blades provide quiet operation

Shape of axial flow (screw) fan

Axial flow fan

Bifurcated axial flow fan

Mixed flow fans Combine advantages of

centrifugal & axial fan Air flows over rotor

provides both radial and axial velocity components.

Higher pressure and quieter

Fan characteristics

Performance of fans is analysed and recorded on ‘characteristic chart’ used to select fans for particular duties

Refrigeration

Air conditioning systems for cooling comes with a refrigeration system

Cooling by refrigeration achieved by causing a change of state of a refrigerant between gaseous and liquid phases.

Gases at normal atmospheric temperatures and pressure, boils at low temperature and high pressure

Two principal of refrigeration system

1.The Vapour compression cycle2.The absorption cycle

BUILDING | SERVICES | TWO (BLD60503)Basic vapour compression cycle

BUILDING | SERVICES | TWO (BLD60503)Vapour compression cycle

https://youtu.be/nKZ2DPvvua8

Vapour compression cycle Uses a compound – boil and condense at

different temperatures when subjected to different pressures.

Two devices is used – one compresses the gas. The other allow it to expand plus heat transfer

matrices. One suck heat in (on evaporation) The other to reject it (condenser)

Condenser

BUILDING | SERVICES | TWO (BLD60503)Cooling towers

Cooling towers

BUILDING | SERVICES | TWO (BLD60503)Condenser

Condensers are heat rejectors of the cycle Can be air cooled or water cooled. Water cooled usually cooled by a cooling tower which

continually recirculates water over packing infills panels, increasing surface area for heat transfer.

Make up water to balance water lost in evaporation is made up from the mains approx. 1% of recirculated water volume.

Cooling towers use both air flow and water flow for rejecting heat to atmosphere using latent heat of evaporation and sensible heat transfer of water.

Condenser Recirculating water systems treated using chemicals and/or ultra- violet emitters to reduce bacterial contamination.

Expansion valve

Expansion valve Situated as close as possible to the evaporator. Control refrigerant flow into evaporator, excessive

cause flooding and damage Compressor draws up gas from evaporator- slight

pressure & temperature decrease within felt by bellows.

Bellows exert slight weaker force against the spring, moving the needle to open the valve.

Additional refrigerant is admitted to the evaporator, increasing the pressure, overcoming the force of spring, moving the needle to close the valve.

Expansion valve

The evaporator

Evaporator

The evaporator Cool, high pressure refrigerant liquid is

expanded so that boiling takes place due to pressure drop

Latent heat of evaporator is taken from the water, air or brine in the secondary circuit.

Compressor

Reciprocating Compressor

Rotary Compressor

BUILDING | SERVICES | TWO (BLD60503)Compressor

Reciprocating Compressor

BUILDING | SERVICES | TWO (BLD60503)Compressor

Single or multi stage centrifugal, reciprocating by piston action or rotary to develop a compression of the refrigerant gas

Absorption refrigeration

Absorption refrigerationDessicant-A substance, such as calcium oxide or silica gel, that has a high affinity for water and is used as a drying agent.

BUILDING | SERVICES | TWO (BLD60503)Absorption refrigeration

Absorption cycle utilises property of certain liquids to absorb

gases/vapours Heat energy to separate the refrigerant from

its carrier fluid

Heat rejection – via a condenser Cooling effect – via an evaporator where

refrigerant is vapourised.

Absorption refrigeration Compressor of the vapour compression

cycle – replaced by refrigerant ‘generator’ where two fluids are separated.

Absorber – absorb the refrigerant by carrier liquid.

BUILDING | SERVICES | TWO (BLD60503)Absorption refrigeration

Typical refrigeration/carrier fluid combinations:

Ammonia/Water

Where temperature above 0 degree Celsius Water/Lithium bromide Water/Lithium Chloride

https://youtu.be/0PkOEHMNOLk

The End

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