Refrigerant in Vapor Compression Refrigeration Cycle -- How to decide?

[rheikaus]

Hello All -

I'm a sparky, but am starting a job that will require me to know about cooling cycles. I've re-read my college thermo book to get somewhat up to speed, and looked for answers to this question on the forum -- but I haven't gotten the full story yet.

How do you choose a refrigerant for a cooling cycle (VCRC)? Purely scientifically, not for safety reasons or anything like that. I know that a high Cp is desireable, as shown by Q=mCp(T2-T1). How does thermal conductivity, heat transfer coefficient, or thermal diffusivity affect the performance of a heat pump or refrigerator? How important is the latent heat of vaporization?

I have looked at properties of refrigerants (R-134a and ammonia) in my text book, and these properties seem to be way off between the two. The only thing they have in common is similar boiling points at atmospheric P. It seems to me that ammonia is a superior refrigerant due to it's higher hfg and Cp. Is it strictly because it is toxic that it is not in common use anymore? It does not contain chlorine, so that can't be it...

Thank you all in advance for your insight. 8 years of not cracking a thermo book puts some cobwebs on the brain...

 

[Yorkman]

Ammonia, next to water is the best refrigerant known to man. Let me clarify. It has the highest latent heat value next to water and as you said it doesn't contain any chlorine. The significance of the high latent heat of vaporization value, means that the required pounds per minute circulated, per ton of refrigeration; is smaller than any of the Halogen refrigerants we have. Enviromentaly speaking it is a freindly refrigerant.
It does however have a few draw backs, it is an extreme irritant to the mucus membranes and can be lethan at levels over 5000 ppm. But at high enough exposure levels so is 134a or any other refrigeramt for that matter.It is flammable in an enviroment of 16% ammonia or more. It reacts with copper. This means it can't be used in hermetic/semi hermetic compressors or piping systems containing copper.
In my opinion the toxicity issue coupled with the publics ignorance is playing a huge factor in the limited use of ammonia in commercial air conditioning. That is not to say it's not dangerous it can be. But, I think at some point we the public may have to accept that it is probably the most friendly refrigerant we have when it come to the enviroment. It can be used and has been used in a safe manner, with proper taining .

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Montemayor]

Yorkman is exactly right. There is no better, overall, refrigerant than Ammonia. And I don't think this will change in the immediate future. However, Yorkman is not stating all the specifics about Ammonia. It is not only toxic, it combines with any moisture (especially that found in human beings) to produce Ammonium Hydroxide - a toxic, corrosive and dangerous caustic. It will attack human tissue, especially human mucous membranes - eyes, nose, lungs, etc. and cause severe and deadly pain and damage. My intent is not to cause panic; it is to be as accurate and detailed in describing what you are dealing with.

Having said that, I fully support the use and application of Ammonia as the proper and best refrigerant available. When engineered and designed accordingly, it can impart safe, economic, and efficient cooling to an infrastructure. When the York Corp. bought and took over our facilities at RECO, inc. in San Antonio, TX in 1989, the yankees found our entire office building air conditioned with an indirect Ammonia refrigeration system. It had been running that way for years and I believe they kept it that way since it operated so well.

I have never succeeded in finding any evidence or facts about Ammonia "burning". I know it can be made to burn - but under controlled and engineered conditions. It will not burn out in the open. Study and research has not revealed any evidence to the contrary.

 

[insult2injury]

The only place I've seen ammonia burned is in heat treatment furnaces. It's used to prevent decarburization. The temperatures are usually above at least 800 F I think.

What application is for? Electronics cooling? The proper refrigerant really depends on the application. You should know an approximate evaporating temperature and the types of compressors that will be used at a minimum. In the end, the best choice may come down to what is readily available, but most likely, one refrigerant will provide the highest efficiency. I believe the dupont website has some guidelines or a selection tool.

http://refrigerants.dupont.com

 

[25362]

I adhere to what has been said of ammonia as a refrigerant.
However, it all depends on the intended use. I've seen many lube ketone-based solvent dewaxing plants using propane, instead of anhydrous ammonia, as a refrigerant when going to evaporator temperatures at or below -30^oC.

The latent heats per unit volume of vapor at -30^oC aren't too far apart. The Cp/Cv values for propane are lower than for ammonia, and the higher mol. wt. allows the use of centrifugal compressors for the needed development of the required head pressures without many wheels.

One added benefit of using propane (no doubt a highly inflammable hydrocarbon), vs ammonia, is that the chillers and the compression suction system don't need to be operated under vacuum at temperatures around -40^oC. Inward air leaks to anhydrous ammonia systems have been linked with SCC of carbon steels. Ammonia condensers are normally provided with deaerators.

 

[rheikaus]

That is great info. Thank you all for the thorough explanation. I enjoy this forum because you get such a complete answer to all angles of the posted question, and the resulting questions that arrise. The wealth of knowledge on this site is humbling.

I became an engineer because I wanted to know everything about everything. Turns out that the more you learn, the more you realize you don't know anything. So, in keeping the discussion purely theoritical, it would seem that just about any substance can be used as a "refrigerant" with an unrestricted pressure, temperature, and flow rate (low or high) -- though these cycles would be horribly inefficient. Therefore, it becomes a study in practicallity and efficiency. Just want to make sure I'm understanding the basics.

Thanks again!

 

[Yorkman]

I'll see if I can't find that data on flammability, I know that I saw it some where and I thought at the time, at a 16% level of exposure who the heck is going to be around for that? Little ammonia humor there. I'll see what I can find.
We did not talk much about the refrigerant with the highest latent heat value, R-718 (water). Maybe we can save that for another time and place.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Yorkman]

Members,
I found the data I was looking for. It was listed in two different souces. One source lists flammable limits as a range of 16% to 25% in volume by air at one atmosphere. With an ignition temperature of 651[°]C or 1204[°]F. so an open flame, or electrical arc might produce enough heat to cause ignition. Mind you if the levels are at 16-25% something bad has already happened and if the vapor ignites it's about to get worse.This comes from;
the IIAR: International Institute of Ammonia Refrigeration

Another source was an online site that provided MSDS imformation. they listed the same levels percentage wise but catagorized it as explosive levels which is not the same thing in my mind. I guess it depends on the type of space that the ignition occurs in. They did not list an ignition temperature at that site.
For those that want to know I guess.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[insult2injury]

A mixture of the right fuel and anhydrous ammonia makes a very good explosive and I believe it is a bit volatile as well.

 

[djack77494]

While ammonia is combustible and does have published explosive limits, practically it is difficult to get ammonia to even burn without supplemental fuel. As noted, by the time you get anywhere near the LEL, you've got some really major problems at hand. Also, ammonia's heat of combustion is much lower than for hydrocarbons.
Doug

 

[quark]

The use of ammonia as a refrigerant is picking up again. Unlike freons, you can use mineral oils for compressor lubrication (which are cheaper) and these are generally immissible with ammonia. Oil recovery becomes easier.

At acceptable exposure limit of 50 ppm, the exposure is for 8 hours. You can smell ammonia even at 10 ppm and so you can attend any leakage and this is not possible with freons.

Ammonia systems are tolerant to 300ppm moisture (though you are supposed to keep moisture out of them).

If the latent heat is high, you require lesser mass flowrate of refrigerant. This reduces investment on compressor.

 

[25362]

The following site may be a reasonable tutorial:

www.cibse.org/pdfs/GPG280.pdf​