Rob's
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Maximum COP of ideal absorption refrigeration system
In case of a single stage compression refrigeration system operating between constant evaporator and condenser temperatures, the maximum possible COP is given by Carnot COP:
If we assume that heat rejection at the absorber and condenser takes place at same external heat sink temperature To, then a vapour absorption refrigeration system operates between three temperature levels, Tg, To and Te. The maximum possible COP of a refrigeration system operating between three temperature levels can be obtained by applying first and second laws of thermodynamics to the system. Figure 1.1 shows the various energy transfers and the corresponding temperatures in an absorption refrigeration system.
Figure 1.1: Various energy transfers in a absorption refrigeration system
From first law of thermodynamics,
where Qe is the heat transferred to the absorption system at evaporator temperature Te, Qg is the heat transferred to the generator of the absorption system at temperature Tg, Qa+c is the heat transferred from the absorber and condenser of the absorption system at temperature To and Wp is the work input to the solution pump.
From second law of thermodynamics,
where  is the total entropy change which is equal to the sum of entropy change of the system  and entropy change of the surroundings  Since the refrigeration system operates in a closed cycle, the entropy change of the working fluid of the system undergoing the cycle is zero, i.e., =0. The entropy change of the surroundings is given by:
Substituting the expression for first law of thermodynamics in the above equation
Neglecting solution pump work, Wp; the COP of VARS is given by:
An ideal vapour absorption refrigeration system is totally reversible (i.e., both internally and externally reversible). For a completely reversible system the total entropy change (system+surroundings) is zero according to second law, hence for an ideal VARS .
 Hence:
Hence combining first and second laws and neglecting pump work, the maximum possible COP of an ideal VARS system is given by:
Thus the ideal COP is only a function of operating temperatures similar to Carnot system. It can be seen from the above expression that the ideal COP of VARS system is equal to the product of efficiency of a Carnot heat engine operating between Tg and T oand COP of a Carnot refrigeration system operating between T oand Te, i.e.,
Thus an ideal vapour absorption refrigeration system can be considered to be a combined system consisting of a Carnot heat engine and a Carnot refrigerator as shown in Fig.1.1. Thus the COP of an ideal VARS increases as generator temperature (Tg) and evaporator temperature (Te) increase and heat rejection temperature (To) decreases. However, the COP of actual VARS will be much less than that of an ideal VARS due to various internal and external irreversibility’s present in actual systems.
Fig. 1.2 Vapour absorption refrigeration system as a combination of a heat engine and a refrigerator.
It’s this author's option that the use of a fan to satisfy 3 distinct heat ranges is marginal at best and as we have seen many posts with poor cooling and the poor results. This should give a better understanding of how to achieve better refrigeration.
(COP) = Coefficient of Performance
(Carnot) = Carnot Heat Engine
* This post was
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edited 08/14/11 03:36pm by Rob's *
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Rob
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trailernovice
Mission TX
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(scratching my head...)...huh????
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Ivylog
Blairsville, Ga. USA
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I'm impressed. So what's the answer?  Pretty sure I even know where you copied this from.
This post is my opinion (free advice). It is not intended to influence anyone's judgment nor do I advocate anyone do what I propose.
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belairbrian
Alabama
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Rob,
Did you just attend a factory training course?
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Jim-Linda
Livingston, TX
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And your point is??
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tvman44
Southwest Louisiana
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Jim-Linda wrote:And your point is??
Jim Exactly my question??
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ScottG
Bothell Wa.
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Probably just about anyone here can spout techno babble about some field they're experts in. So I too must ask, whats the point?
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sh410
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The OPs last sentence sums it up:
Rob's wrote:It’s this author option that the use of a fan to satisfy 3 distinct heat ranges is marginal at best.
All of us that have fans on the back of our refrigerators now know that the use of these fans are "marginal at best" I guess that my refrigerater did'nt know this.
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pianotuna
Regina, SK, Canada
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Hi Rob's,
I agree that no energy may be saved by using a fan to suck air out the chimney. However cool down time to a useable temperature is reduced from 12 hours to 6 hours. Here are the results of a "back yard" experiment.
Fridge Fans do they really work?
I'd be interested in comments.
Regards, Don
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CA Traveler
The Western States
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Thermodynamics wasn't high on my list of classes!
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Bob
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