ice-e info pack 1 refrigerant cycles

8/13/2019 ICE-E Info Pack 1 Refrigerant Cycles

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Refrigerant cycles

In the e-learning section of theICE-E web site, the reader canachieve basic knowledge aboutthe refrigeration cycles. Thechoice of the most suitablecycle, in terms of energyconsumption reduction must besupported by severalconsiderations dealing bothwith thermodynamic andtechnological aspects. In thepresent Info Pack, fundamentalconsiderations aboutthermodynamics arepresented, aiming athighlighting the consequenceson cycle energetic efficiency.

Regarding technological aspects, the reader

may refer to Refrigerants, Operation and

choice of compressors, Heat exchangers,

Expansion device Info Packs.

Back to basicsThe purpose of a refrigeration system is to

transfer thermal energy from a low-

temperature source to a high-temperature

sink. From an energetic point of view, the goal

should be hit utilizing the least amount of

work, i.e. to maximize the Coefficient of

Performance (COP) for a given cooling

capacity and for fixed source and sink

temperatures. More thermodynamically

oriented reader could, alternatively, restate the

goal in terms of entropy: the purpose of a

refrigerating system is to transfer entropy from

a low-temperature source to a high-

temperature sink while generating the least

amount of entropy, or stated in another way

the goal is to generate the least amount of

entropy for a given cooling capacity for fixed

source and sink temperatures.

It is well known that the ideal cycle for

achieving this goal (when both the source and

the sink are isothermal) is the Carnot

Fundamental

considerations

about the

thermodynamics

of inverse cycles

can help ineconomic and

technological

choices for

refrigeration

systems

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Figure 1 - Carnot cycle and ideal vapor compressionrefrigeration cycle


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refrigeration cycle, whose work is depicted by

the area a-b-c-d in figure 1 and whose

cooling capacity is given by the area 1-4-f-g

for figure 1 (taken from Cavallini et al, 2010) .

It is also well known that increasing TLand/or

decreasing T0increases the cycle efficiency.

The Carnot refrigeration cycle, however,

cannot be realized via practical hardware.

Therefore, the widely used reference cycle in

practice is based on the so-called ideal vapor

compression refrigeration cycle. The cycle

shown in figure 1 contains two irreversibilities:

(1) isenthalpic expansion ( exp) and

(2) superheating of the compressor discharge

vapor ( sup) to realize a constant-pressureheat rejection process in the condenser.

In practice, real vapor compression

refrigeration cycles include other

irreversibilities, principally among them are:

(3) non-isentropic adiabatic compression,

(4) non-isobaric heat rejection and

(5) non-isobaric heat addition.

Though not shown in the figure, two other

common modifications to the cycles are

superheating of the refrigerant at theevaporator outlet and subcooling of the

refrigerant at the condenser outlet. Finally,

external to the cycle itself, there are large

irreversibilities associated with the heat

transfers to and from the source and sink due

to the finite temperature differences between

the refrigerant and the external heat transfer

media.

All the design strategies of a refrigeration

system (included two-stage

compression/throttling, as depicted in figures2, 3) are intended for reducing the above

mentioned (1) and (2) irreversibilities.

Accordingly, a detailed analysis of the ideal

vapor compression refrigeration cycle, as

depicted in figure 1, gives cue on how to

reduce energy consumption for any kind of

vapour compression refrigeration cycle.

Evaluating cycle performanceThe most common method for evaluating the

overall thermodynamic performance of thesecycles is based on a First Law of

Thermodynamics approach, namely,

comparing the Coefficient of Performance

Figure 2. Two stage compression withintercooler, single throttling vapourcompression cycle (and related T,sdiagram, below).

Figure 3. Two stage compression withOFT, double throttling vapour compressioncycle (and related T,s diagram, below).

All the design

strategies of a

vapourcompression

refrigeration

system, are

intended for

reducing the

irreversibilities

linked to

throttling,compression and

heat transfer.

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(COP) and the Volumetric Cooling Capacity

(VCC). For a given cooling capacity, VCC

gives an indication about the compressor size

to achieve the specified cooling capacity.

The COP is the ratio of the energy

(refrigeration effect) extracted from the low

temperature source (if h is the specific

enthalpy of the refrigerant, qL=h1h4is the

energy extracted or the so called refrigeration

effect, referring to figure 1) and the input work

(wcomp=h2-h1). The volumetric cooling capacity

VCC is the energy extracted from the low

temperature source per unit of refrigerant

volume processed by the compressor.

One limitation to this approach is that the COP

is a function of the operating conditions (thehigh-side and low-side temperatures). For

example, is a COP of 5 better than a COP of

10? The short answer is: it depends.

To overcome the mentioned limitation one

should compare the COP and the Carnots

cycle COPC(and this can be considered a

Second law of Thermodynamics approach):

= COP/COPC

Another way to consider and quantify the

irreversibilities is to choose an external

reference temperature T0(e.g., as the

temperature of the ambient) which can be

used to calculate the exergy losses. For the

example of figure 1, the external reference

temperature has to be chosen as the

temperature of the external cooling medium

(e.g., air) for the condenser. Once this is done,

the specific exergy losses can be calculated

for the four basic processes for the vapor

compression refrigeration cycle. They are

represented by the hatched areas in figure 1

(ideal reference cycle.

It is worth noting that for the ideal vapor

compression refrigeration cycle in figure 1 the

condenser exergy loss reduces to the

superheating loss ( sup), defined by the area e-

b-2, while compression and evaporation are

no-loss processes.

The magnitudes of the exergy losses for non-

isobaric, non-ideal heat transfer processes

and non-isoentropic compression (i.e.

irreversibilities (3), (4), (5), mentioned above)

described above are determined by

component and system designs, and by the

refrigerant. For example, the refrigerant

circuitry in the heat exchangers, the type of

compressor used and its design, and the

system configuration all will influence several

of the exergy losses.

Performance potential: ammonia

as an example

In the following, we consider a largely used

(old) refrigerant in refrigeration applications:

ammonia. We consider an evaporation

temperature TL= -40C, while the

condensation is T0= 40. With reference to the

set temperatures, the Carnot cycle COPCis

2.91.

We consider no condenser subcooling or

compressor superheat, and a compressorisentropic efficiency of 1. When the single

stage compressionsingle throttling is

considered, = 0.703.

Keeping fixed TLand T0, we want now to

consider the possibility of installing a two-

stage compressor, again ideally with

isoentropic behavior (figure 2). Furthermore,

an ideal intercooler is installed: i.e. it is

possible to lower the temperature of the

ammonia, after the low stage compressor

discharge (point 5, in figure 2) down to the

sink temperature (40 C), that is a limit

situation achievable theoretically only in an

heat exchanger with infinite heat transfer area

and in perfect counter-current configuration.

The intermediate pressure (i.e. the

intercooling pressure) is set equal to the

square root of the product of condenser an

evaporator saturation pressures.

Single throttling is considered (no condensate

subcooling, no vapour superheating).

In this case superheating losses (see hatchedarea indicating supin figure 1) are reduced,

while throttling losses are the same (see

hatched area indicating exp).

According to previous considerations, we

expect an increase in (or in system COP,

since Carnot cycle COPC, is fixed at 2.91).

It is possible to calculate = 0.730. That is an

increase of less than 4 %, in comparison to

the single stage compression arrangement.

Lets now evaluate the possibility to implement

the system configuration in figure 3, again with

fixed TLand T0and with no condensate

subcooling, no vapour superheating.

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Is a COP of 5

better than a

COP of 10? Theshort answer is:

it depends.


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For more information, please contact: Claudio Zilio ([email protected])

Always compare

Carnot cycle

performance andthe performance

of the system

you are going to

evaluate.

You will have

quickly and

easily the first,objective, clear,

indubitable

number for

starting your

following

technological

and economic

evaluation.

In this case, point 5 temperature can be lower

than ambient temperature, since it is not any

more linked to the external heat sink

temperature, as in the case of the intercooler

in figure 2. This should bring about a reductionof ( sup). Furthermore, using two-stage

throttling, reduces the relevant hatched area in

figure 1 ( exp).

Accordingly, increases of about 40 % ( =

0.985). Being lower the refrigerant enthalpy at

the evaporator inlet (h4, in figure 3), also VCC

increases.

Which is the practical outcome of the

proposed thermodynamic consideration?

(Note: as mentioned before, in this info pack

we are not considering technological aspectslike, for example, limitations in compressor

discharge temperature because of

compatibility with lubricants etc. Please

consider the relevant info packs in ICE-E web

site).

From an economicalpoint of view, installing

a two-stage compressor is by far more heavy,

from an investment point of view, than using

two throttling valves or installing an

accumulator (open flash tank).

References

Cavallini A., Zilio C., Brown J.S. (2010).

Sustainability with prospective refrigerants. In:

Proc. of Sustainable Refrigeration and Heat

Pump Technology Conference. Stockholm,

June, 13-16, ISBN: 978-2-913149-81-6

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Given this economical consideration, the

thermodynamic results clearly indicate that

using the system schematic in figure 2 will

offer poor chances of recovering the higher

investment funds in short time (indeed, it is arelatively rare system schematic, with

ammonia).

The further limited investment costs because

of one more throttling valve and one tank,

looks more promising in terms of shortening

the pay-back period.

As a concluding remark: the rather simplified

thermodynamic approach here proposed can

be considered a starting point if you are

looking for a new refrigeration system.:

If you are not a specialist in thermodynamics,

please remember of Carnot and ask to your

advisor to compare (it is rather simple, for

him), the coefficient of performance of the

system he is proposing to you (even

considering ideal processes of the refrigerant)

with the efficiency of the Carnots cycle.

You will have the first, objective, clear,

indubitable number for starting your following

technological and economic evaluation.

ice-e info pack 1 refrigerant cycles

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