CPD Programme
pressure of 3.87 bar absolute. We would expect an evaporating
Note that the expansion process temperature of 1nullC and
results in the formation of 'flash condensing temperature of 45nullC,
gas' and in most air conditioning
Condensation
3 2
resulting in an operating COP
c
. If
applications will amount to the evaporating temperature could
about 20 per cent of the mixture
kPa
be raised by 1K and the condensing
by weight. For the majority temperature reduced by 1K,
of applications the expansion
Expansion
Evaporation
C
ompression
performance of the system will
device is normally a valve,
Pressure,
4 1
increase by between 4 and 8 per
thermostatically or electronically cent – a considerable amount.
operated, which controls the The design and selection of the
refrigerant flow rate to the evaporator and condenser can
evaporator, sensing evaporator play a significant part. These heat
outlet superheat. This is an Enthalpy, kJ/kg exchangers are capacity rated by
important function that ensures the equation
(a)
the evaporator tubing receives the Q = U*A*LMTD kW
optimum amount of refrigerant where
and also protects the compressor 3 2 l Q is the heat transfer rate in
from liquid refrigerant entering Condenser kW
its suction port. Centrifugal l U is the overall heat transfer rate
compressor water chillers use a in W/m
2
K
'flooded' type evaporator where
Expansion
valve
l A is the heat transfer surface
the refrigerant level is controlled,
Compressor
1
area in m
2
as opposed to suction superheat. 4 l LMTD is the log mean
Evaporator temperature difference in K
Efficiency of the Cycle For the evaporator in the
The efficiency of the vapour
(b)
above example, U and A remain
compression cycle is called constant and Q becomes directly
the Coefficient of Performance (COP) and Figure 3 Practical components and Pressure/ proportional to LMTD, which can be taken
strictly speaking should include the subscript
Enthalpy cycle
as (water out – evaporation temperature) for
'c' to denote the cooling mode – COP
c
. The comparative purposes. If we try to operate
maximum efficiency that this cycle can attain requirement for the input data of the EPBD at a lower evaporation temperature, say -1
o
C,
is the Carnot Cycle COP
c
iSBEM assessment calculation. It is defined there is a danger that the water freezes, but
T
e
/(T
c
-T
e
) as the Seasonal Energy Efficiency Ratio we will increase the ∆t across the evaporator.
where T
c
is the Condensing temperature and (SEER) in the Part L non domestic heating, This means we could reduce surface area/
T
e
is the Evaporating temperature, both in cooling and ventilation compliance guide, size/cost, but the penalty is 4 to 8 per cent
absolute temperature or Kelvin. May 2006 section 9. lower efficiency. If we want to increase
For example, a system has an evaporating The major factors that affect the efficiency efficiency, we need to raise the evaporating
temperature of 5nullC and a condensing of a refrigeration system are: temperature, reducing ∆t, but this will
temperature of 45nullC. Using the above l The evaporating temperature require more surface area at the cost of space
equation, the COP
c
is 6.95. In practice, the l The condensing temperature and expense.
actual system COP
c
will be about 50 per cent l The type of refrigerant used You may want to follow up this reading
of that, or 3.5, due to inefficiencies within the l The type of equipment used, such as with the references indicated below.
practical cycle. compressor, evaporator, etc © Terry Welch
In the UK and Europe we express COP
c
as l The system controls
an Energy Ratio, equal to: l Correct maintenance of the system References:
Useful Cooling Duty (kW)/Power Input to The choice of evaporating and condensing Cooling Systems Seminar, CIBSE, Terry Welch
the Compressor (kW) temperatures has the greatest effect on CIBSE Knowledge Series Refrigeration, 2008
If the COP
c
is 4.0, the Refrigeration Unit COP and therefore running costs. A 1K Refrigeration & Air Conditioning, W F Stoecker
will produce 4.0kW of cooling per kW of rise in evaporating temperature or drop in and J W Jones, McGraw-Hill
electrical consumption by the motor. To condensing temperature results in a 2 to
be more realistic, the power consumption 4 per cent increase in COP. Glossary of terms
should include other electrical items such The evaporating and condensing Entropy(s) thermodynamic property (kj/
as condenser/AHU fans and circulating temperatures are determined by the cooling kgK) used to plot a constant
pumps associated with producing the temperature requirement and the heat entropy process such as
cooling capacity. A seasonal COP
c
may also rejection cooling temperature available. For compression
be useful, as annual energy consumption can example, a water chilling application where Enthalpy property that is made up of
be compared with an annual cooling capacity the chilled water temperature needed is 6nullC, heat and internal energy(kJ/
and incorporated into annual energy costs for and air cooled condensers with maximum kg). Only really used as enthalpy
the building or application. In fact, this is a design air dry bulb temperature of 30nullC. differences
www.cibsejournal.com March 2009 CIBSE Journal 63
CIBSEmar09 pp61-64 cpd.indd 63 4/3/09 14:51:45
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