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Centrifugal fans
Also called ‘radial flow’
fans
Performance is
affected by the shape
of the blades and shape
of the casing.
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Fan drives and motors
AC 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.
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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
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Mixed flow fans
Combine advantages of
centrifugal & axial fan
Air flows over rotor
provides both radial and
axial velocity
components.
Higher pressure and
quieter
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Fan characteristics
Performance of fans is analysed and recorded on
‘characteristic chart’ used to select fans for particular duties
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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
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Two principal of refrigeration system
1.The Vapour compression cycle
2.The absorption cycle
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Vapour compression cycle
https://youtu.be/nKZ2DPvvua8
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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)
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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.
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Condenser
Recirculating water systems treated using chemicals and/or ultra- violet emitters to reduce bacterial contamination.
HEPHe
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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.
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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.
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Compressor
Reciprocating
Compressor
Rotary Compressor
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Compressor
Single or multi stage centrifugal,
reciprocating by piston action or rotary
to develop a compression of the
refrigerant gas
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Absorption refrigeration
Dessicant-A
substance,
such as
calcium oxide
or silica gel,
that has a high
affinity for water
and is used as
a drying agent.
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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.
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Absorption refrigeration
Compressor of the vapour compression
cycle – replaced by refrigerant ‘generator’
where two fluids are separated.
Absorber – absorb the refrigerant by carrier
liquid.
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Absorption refrigeration
Typical refrigeration/carrier fluid
combinations:
Ammonia/Water
Where temperature above 0 degree Celsius
Water/Lithium bromide
Water/Lithium Chloride
