CPD Programme
Heat to high-
will be 3.87 bar absolute.The compressor has
temperature sink
two functions: to pump the refrigerant around
the system; and to raise its pressure. The
compressor capacity determines the flow rate
3
of refrigerant, and should be the same as the
2 Work
evaporation rate in the evaporator, which is a
function of the cooling load on the evaporator.
The compressor discharges high-temperature,
Turbine Compressor
high-pressure refrigerant at point 2 in Figure
3. The temperature reached will depend on
the pressure ratio of the compressor and the
4 1
refrigerant being used. In our example the
discharge temperature will be about 80nullC, but
in some applications this could rise to 130nullC.
The aim of the cycle is to restore the
Heat from low-
refrigerant to low-temperature liquid that can
temperature source
be fed back to the evaporator. This is done
in two stages – condensing and pressure
Figure 1 Components of Carnot cycle
reducing. The highly superheated vapour
enters the condenser at point 2. In our
> cycle is placed within the liquid/vapour moving to the right, effectively reducing the example this is a finned tube heat exchanger
region of the working fluid (refrigerant). cooling effect of the refrigerant. which uses ambient air at 30nullC to cool the
Figure 1: For practical use, the vapour compression refrigerant in the tubing of the condenser.
l 4 – 1 Isothermal heat addition to the cycle is best represented on a Pressure/ Heat is rejected from the refrigerant into
cycle, where the working fluid is Enthalpy diagram, illustrated in Figure 3. the ambient air. The majority of the heat
heated and vapourises from The diagram makes it convenient to obtain rejection is latent condensation from the
a liquid to a vapour at constant properties such as enthalpies, pressures, refrigerant at a temperature above that of
temperature T
1
. This is the process temperatures, specific volumes and the ambient air. This is called the condensing
that cools air or water in a air entropies, all necessary when making cycle temperature and, as a rule of thumb in this
conditioning system. calculations. example, the condensing temperature will be
l 1 – 2 Isentropic compression, where the Taking an air conditioning example where air 15K above the ambient air temperature, or
working fluid is raised in is cooled to 15nullC and the ambient air available 45nullC. For this to take place, the compressor
temperature from T
1
to T
2
. for heat rejection is at 30nullC, the evaporator is needs to raise the pressure of the R134a to
l 2 – 3 Isothermal heat rejection from the an extended surface or finned configuration 11.5 bar absolute. The refrigerant leaving
cycle, where the working fluid is heat exchanger. Refrigerant liquid circulates the condenser at point 3 will be liquid that
cooled and condenses from a through the evaporator tubing and must has been subcooled by about 5K, that is, at
vapour to a liquid, at constant be at a lower temperature than the air, say a temperature of 40nullC and still at 11.5 bar
temperature T
2
. 8nullC. The heat exchanged from the air as it is absolute. Note that about 10 per cent of
l 3 – 4 Isentrop ic Expansion, which cooled vaporises the liquid refrigerant. This the heat rejection is desuperheating of the
requires the use of a turbine. takes place at constant temperature with most refrigerant entering the condenser and has
This cycle is theoretical and provides the refrigerants, so the refrigerant temperature the possibility of being used for heating.
maximum possible efficiency, which is useful stays at the temperature we have specified, The refrigerant is then reduced in pressure
to compare with an actual cycle. In practice, in 8nullC, until all the liquid has evaporated. This through an expansion device from points 3
Figure 2, it is necessary to move point 1 to the evaporating temperature can be changed to 4 in Figure 3, entering the evaporator as
right until it reaches the saturated vapour line by raising or lowering the pressure of the a mixture of liquid and vapour at 8nullC and a
or further to become slightly superheated, in refrigerant. Outside
order to prevent liquid refrigerant entering of refrigeration
the compressor. This means that the initial we would identify
part of the condensation process 2 – 3 will this property as
be taken up with desuperheating, before the saturation K
Atmosphere
condensation occurs at the condensing temperature and
temperature T
2
. relate it to pressure.
The expansion process 3 – 4 requires a The relationship is
Cold room
turbine and for the resulting work to be different for every
T
emperature,
Saturated
used for the compression process. This is fluid used as a
vapour
Saturated
not cost effective at present, so the process is refrigerant, and if
liquid
substituted by a simple throttling or pressure- we use R134a – a
reducing device such as an expansion common refrigerant
Entropy, kJ/kg
•
K
valve (this process is described as constant today – the pressure
enthalpy). However, this results in point 4 in the evaporator Figure 2 Thermodynamic processes
62 CIBSE Journal March 2009
www.cibsejournal.com
CIBSEmar09 pp61-64 cpd.indd 62 4/3/09 14:51:45
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76