Refrigerant cycles - KHLim i-NET
Refrigerant cycles - KHLim i-NET
Refrigerant cycles - KHLim i-NET
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ICE-E<br />
INFORMATION<br />
PACK<br />
<strong>Refrigerant</strong> <strong>cycles</strong><br />
Fundamental<br />
considerations<br />
about the<br />
thermodynamics<br />
of inverse <strong>cycles</strong><br />
can help in<br />
economic and<br />
technological<br />
choices for<br />
refrigeration<br />
systems<br />
In the e-learning section of the<br />
ICE-E web site, the reader can<br />
achieve basic knowledge about<br />
the refrigeration <strong>cycles</strong>. The<br />
choice of the most suitable<br />
cycle, in terms of energy<br />
consumption reduction must be<br />
supported by several<br />
considerations dealing both<br />
with thermodynamic and<br />
technological aspects. In the<br />
present Info Pack, fundamental<br />
considerations about<br />
thermodynamics are<br />
presented, aiming at<br />
highlighting the consequences<br />
on cycle energetic efficiency.<br />
Regarding technological aspects, the reader<br />
may refer to “<strong>Refrigerant</strong>s”, “Operation and<br />
choice of compressors”, “Heat exchangers”,<br />
“Expansion device” Info Packs.<br />
Back to basics<br />
The purpose of a refrigeration system is to<br />
transfer thermal energy from a lowtemperature<br />
source to a high-temperature<br />
sink. From an energetic point of view, the goal<br />
should be hit utilizing the least amount of<br />
work, i.e. to maximize the Coefficient of<br />
Performance (COP) for a given cooling<br />
capacity and for fixed source and sink<br />
temperatures. More “thermodynamically”<br />
oriented reader could, alternatively, restate the<br />
goal in terms of entropy: the purpose of a<br />
refrigerating system is to transfer entropy from<br />
a low-temperature source to a hightemperature<br />
sink while generating the least<br />
amount of entropy, or stated in another way<br />
the goal is to generate the least amount of<br />
entropy for a given cooling capacity for fixed<br />
source and sink temperatures.<br />
It is well known that the ideal cycle for<br />
achieving this goal (when both the source and<br />
the sink are isothermal) is the Carnot<br />
Figure 1 - Carnot cycle and ideal vapor compression<br />
refrigeration cycle
ICE-E INFO PACK<br />
All the design<br />
strategies of a<br />
vapour<br />
compression<br />
refrigeration<br />
system, are<br />
intended for<br />
reducing the<br />
irreversibilities<br />
linked to<br />
throttling,<br />
compression and<br />
heat transfer.<br />
refrigeration cycle, whose work is depicted by<br />
the area a-b-c-d in figure 1 and whose<br />
cooling capacity is given by the area 1-4-f-g<br />
for figure 1 (taken from Cavallini et al, 2010) .<br />
It is also well known that increasing T L and/or<br />
decreasing T 0 increases the cycle efficiency.<br />
The Carnot refrigeration cycle, however,<br />
cannot be realized via practical hardware.<br />
Therefore, the widely used reference cycle in<br />
practice is based on the so-called ideal vapor<br />
compression refrigeration cycle. The cycle<br />
shown in figure 1 contains two irreversibilities:<br />
(1) isenthalpic expansion ( exp) and<br />
(2) superheating of the compressor discharge<br />
vapor ( sup) to realize a constant-pressure<br />
heat rejection process in the condenser.<br />
In practice, real vapor compression<br />
refrigeration <strong>cycles</strong> include other<br />
irreversibilities, principally among them are:<br />
(3) non-isentropic adiabatic compression,<br />
(4) non-isobaric heat rejection and<br />
(5) non-isobaric heat addition.<br />
Though not shown in the figure, two other<br />
common modifications to the <strong>cycles</strong> are<br />
superheating of the refrigerant at the<br />
evaporator outlet and subcooling of the<br />
refrigerant at the condenser outlet. Finally,<br />
external to the cycle itself, there are large<br />
irreversibilities associated with the heat<br />
transfers to and from the source and sink due<br />
to the finite temperature differences between<br />
the refrigerant and the external heat transfer<br />
media.<br />
All the design strategies of a refrigeration<br />
system (included two-stage<br />
compression/throttling, as depicted in figures<br />
2, 3) are intended for reducing the above<br />
mentioned (1) and (2) irreversibilities.<br />
Accordingly, a detailed analysis of the ideal<br />
vapor compression refrigeration cycle, as<br />
depicted in figure 1, gives cue on how to<br />
reduce energy consumption for any kind of<br />
vapour compression refrigeration cycle.<br />
ù<br />
Evaluating cycle performance<br />
The most common method for evaluating the<br />
overall thermodynamic performance of these<br />
<strong>cycles</strong> is based on a First Law of<br />
Thermodynamics approach, namely,<br />
comparing the Coefficient of Performance<br />
Figure 2. Two stage compression with<br />
intercooler, single throttling vapour<br />
compression cycle (and related T,s<br />
diagram, below).<br />
Figure 3. Two stage compression with<br />
OFT, double throttling vapour compression<br />
cycle (and related T,s diagram, below).
ICE-E INFO PACK<br />
Is a COP of 5<br />
better than a<br />
COP of 10 The<br />
short answer is:<br />
it depends.<br />
(COP) and the Volumetric Cooling Capacity<br />
(VCC). For a given cooling capacity, VCC<br />
gives an indication about the compressor size<br />
to achieve the specified cooling capacity.<br />
The COP is the ratio of the energy<br />
(refrigeration effect) extracted from the low<br />
temperature source (if h is the specific<br />
enthalpy of the refrigerant, q L=h 1 – h 4 is the<br />
energy extracted or the so called refrigeration<br />
effect, referring to figure 1) and the input work<br />
(w comp=h 2-h 1). The volumetric cooling capacity<br />
VCC is the energy extracted from the low<br />
temperature source per unit of refrigerant<br />
volume processed by the compressor.<br />
One limitation to this approach is that the COP<br />
is a function of the operating conditions (the<br />
high-side and low-side temperatures). For<br />
example, is a COP of 5 better than a COP of<br />
10 The short answer is: it depends.<br />
To overcome the mentioned limitation one<br />
should compare the COP and the Carnot’s<br />
cycle COP C (and this can be considered a<br />
Second law of Thermodynamics approach):<br />
= COP/COP C<br />
Another way to consider and quantify the<br />
irreversibilities is to choose an external<br />
reference temperature T 0 (e.g., as the<br />
temperature of the ambient) which can be<br />
used to calculate the exergy losses. For the<br />
example of figure 1, the external reference<br />
temperature has to be chosen as the<br />
temperature of the external cooling medium<br />
(e.g., air) for the condenser. Once this is done,<br />
the specific exergy losses can be calculated<br />
for the four basic processes for the vapor<br />
compression refrigeration cycle. They are<br />
represented by the hatched areas in figure 1<br />
(ideal reference cycle.<br />
It is worth noting that for the ideal vapor<br />
compression refrigeration cycle in figure 1 the<br />
condenser exergy loss reduces to the<br />
superheating loss ( sup), defined by the area e-<br />
b-2, while compression and evaporation are<br />
no-loss processes.<br />
The magnitudes of the exergy losses for nonisobaric,<br />
non-ideal heat transfer processes<br />
and non-isoentropic compression (i.e.<br />
irreversibilities (3), (4), (5), mentioned above)<br />
described above are determined by<br />
component and system designs, and by the<br />
refrigerant. For example, the refrigerant<br />
circuitry in the heat exchangers, the type of<br />
compressor used and its design, and the<br />
system configuration all will influence several<br />
of the exergy losses.<br />
Performance potential: ammonia<br />
as an example<br />
In the following, we consider a largely used<br />
(old) refrigerant in refrigeration applications:<br />
ammonia. We consider an evaporation<br />
temperature T L = -40ºC, while the<br />
condensation is T 0 = 40º. With reference to the<br />
set temperatures, the Carnot cycle COP C is<br />
2.91.<br />
We consider no condenser subcooling or<br />
compressor superheat, and a compressor<br />
isentropic efficiency of 1. When the single<br />
stage compression – single throttling is<br />
considered, = 0.703.<br />
Keeping fixed T L and T 0, we want now to<br />
consider the possibility of installing a twostage<br />
compressor, again ideally with<br />
isoentropic behavior (figure 2). Furthermore,<br />
an ideal intercooler is installed: i.e. it is<br />
possible to lower the temperature of the<br />
ammonia, after the low stage compressor<br />
discharge (point 5, in figure 2) down to the<br />
sink temperature (40 °C), that is a limit<br />
situation achievable theoretically only in an<br />
heat exchanger with infinite heat transfer area<br />
and in perfect counter-current configuration.<br />
The intermediate pressure (i.e. the<br />
intercooling pressure) is set equal to the<br />
square root of the product of condenser an<br />
evaporator saturation pressures.<br />
Single throttling is considered (no condensate<br />
subcooling, no vapour superheating).<br />
In this case superheating losses (see hatched<br />
area indicating sup in figure 1) are reduced,<br />
while throttling losses are the same (see<br />
hatched area indicating exp).<br />
According to previous considerations, we<br />
expect an increase in (or in system COP,<br />
since Carnot cycle COP C, is fixed at 2.91).<br />
It is possible to calculate = 0.730. That is an<br />
increase of less than 4 %, in comparison to<br />
the single stage compression arrangement.<br />
Let’s now evaluate the possibility to implement<br />
the system configuration in figure 3, again with<br />
fixed T L and T 0 and with no condensate<br />
subcooling, no vapour superheating.
ICE-E INFO PACK<br />
Always compare<br />
Carnot cycle<br />
performance and<br />
the performance<br />
of the system<br />
you are going to<br />
evaluate.<br />
You will have<br />
quickly and<br />
easily the first,<br />
objective, clear,<br />
indubitable<br />
number for<br />
starting your<br />
following<br />
technological<br />
and economic<br />
evaluation.<br />
In this case, point 5 temperature can be lower<br />
than ambient temperature, since it is not any<br />
more linked to the external heat sink<br />
temperature, as in the case of the intercooler<br />
in figure 2. This should bring about a reduction<br />
of ( sup). Furthermore, using two-stage<br />
throttling, reduces the relevant hatched area in<br />
figure 1 ( exp).<br />
Accordingly, increases of about 40 % ( =<br />
0.985). Being lower the refrigerant enthalpy at<br />
the evaporator inlet (h 4, in figure 3), also VCC<br />
increases.<br />
Cascade systems<br />
Another possibility for increasing refrigeration<br />
system efficiency when operating with high<br />
temperature lift (between evaporation and<br />
condensation temperatures), is the cascade<br />
system, as in figure 4. The condensation of<br />
the LT cycle occurs by means of the<br />
evaporation of the HT cycle refrigerant.<br />
As outlined in the “<strong>Refrigerant</strong>s” Info Pack,<br />
each refrigerant operates more efficiently in a<br />
particular temperature range. A suitable<br />
refrigerant selection for both the “higher”<br />
temperature cycle (HT) and the “lower”<br />
temperature cycle (LT) allow to have higher<br />
overall Cop, in comparison event to a 2twostage”<br />
compression cycle operated by a single<br />
refrigerant. Typical refrigerants choice is<br />
carbon dioxide for the LT cycle and ammonia<br />
for the HT cycle, but other combinations can<br />
be found (for example R134a or R404A for<br />
HT, instead of ammonia).<br />
Concluding remarks<br />
Which is the “practical” outcome of the<br />
proposed thermodynamic consideration<br />
(Note: as mentioned before, in this info pack<br />
we are not considering technological aspects<br />
like, for example, limitations in compressor<br />
discharge temperature because of<br />
compatibility with lubricants etc. Please<br />
consider the relevant info packs in ICE-E web<br />
site).<br />
From an “economical” point of view, installing<br />
a two-stage compressor is by far more heavy,<br />
from an investment point of view, than using<br />
two throttling valves or installing an<br />
accumulator (open flash tank).<br />
Given this economical consideration, the<br />
thermodynamic results clearly indicate that<br />
using the system schematic in figure 2 will<br />
offer poor chances of recovering the higher<br />
investment funds in short time (indeed, it is a<br />
relatively rare system schematic, with<br />
ammonia).<br />
The further limited investment costs because<br />
of one more throttling valve and one tank,<br />
looks more promising in terms of shortening<br />
the pay-back period.<br />
As a concluding remark: the rather simplified<br />
thermodynamic approach here proposed can<br />
be considered a starting point if you are<br />
looking for a new refrigeration system.:<br />
If you are not a specialist in thermodynamics,<br />
please remember of Carnot and ask to your<br />
advisor to compare (it is rather simple, for<br />
him), the coefficient of performance of the<br />
system he is proposing to you (even<br />
considering ideal processes of the refrigerant)<br />
with the efficiency of the Carnot’s cycle.<br />
You will have the first, objective, clear,<br />
indubitable number for starting your following<br />
technological and economic evaluation.<br />
Figure 4. Cascade refrigeration system<br />
References<br />
Cavallini A., Zilio C., Brown J.S. (2010).<br />
Sustainability with prospective refrigerants. In:<br />
Proc. of Sustainable Refrigeration and Heat<br />
Pump Technology Conference. Stockholm,<br />
June, 13-16, ISBN: 978-2-913149-81-6<br />
For more information, please contact: Claudio Zilio Claudio.zilio@unipd.it