Low temperature refrigeration
Low temperature refrigeration
(OP)
Hi,
I need to design a heating / cooling system for a small batch process. Its duty will be to supply alternately heating and cooling to any one of four jacketed vessels, either about 5 kW heat load cooling at -100°C (-148°F), or 10 kW heating at +100°C (212°F), as the batch process progresses, one vessel and one duty at a time. Ideally the waste heat from the system when refrigerating would be rejected to atmosphere.
Can anyone suggest the best technology and who can supply it? The plant will be in the UK.
Thanks
Stuart
I need to design a heating / cooling system for a small batch process. Its duty will be to supply alternately heating and cooling to any one of four jacketed vessels, either about 5 kW heat load cooling at -100°C (-148°F), or 10 kW heating at +100°C (212°F), as the batch process progresses, one vessel and one duty at a time. Ideally the waste heat from the system when refrigerating would be rejected to atmosphere.
Can anyone suggest the best technology and who can supply it? The plant will be in the UK.
Thanks
Stuart





RE: Low temperature refrigeration
With your small load, you might want to consider liquid nitrogen.
RE: Low temperature refrigeration
I'd thought of liquid nitrogen and had suggested it to my customer, but he'll need some persuasion that it's the best way. Eliminating alternatives is one part of persuading them, and if it will be a complex and expensive multi-stage machine then no doubt he'll be less enthusiastic about that. They have developed the process that they want to scale up in a lab, where they have a chest freezer that runs at this sort of temperature, so they assume getting a larger version will be easy. I don't know how this freezer works other than it's electro-mechanical.
Does anyone make such cascade machines off the shelf?
Stuart
RE: Low temperature refrigeration
Start with the manufacturer of your customer's existing chest freezer.
Nitrogen has much lower capitol cost and almost zero maintenance.
RE: Low temperature refrigeration
1) Find a readily available, inexpensive refrigerant;
2) Refrigerant should have a boiling point at least 5-7 oF lower than the stated -100 oC;
3) The refrigerant should be "benign" - not toxic, flammable, or explosive;
As MintJulep says, Nitrogen seems to fill the bill. However, for such a small cooling load (5 kW = 17060.7 Btu/hr = 1.42 tons), I seriously doubt you will find interested parties to supply what is normally a complex, cascade refrigeration system. From the data furnished, the simplest and most practical answer seems to be: buy liquid Nitrogen (LIN) as a consumable refrigerant and replace it as needed. You've given a rate of heat transfer, but you haven't mentioned for what length of time this rate continues. It might be for an hour or so; we don't know.
Bear in mind that the LIN is at -195 oC and the excessive cold temperature might cause problems, depending on the process.
RE: Low temperature refrigeration
But, I agree, LIN.
RE: Low temperature refrigeration
RE: Low temperature refrigeration
I’m not familiar with the compound CF4, but I believe it will be practically impossible to condense it with CO2 at a temperature of -70 oC.
The reason this proposal is a practical impossibility is that CO2 has a Triple Point (the point at which the substance exists in equilibrium with its 3 states of gas, liquid, and solid) of -56.558 oC. This means that at -70 oC CO2 exists as a solid (“Dry Ice”) snow in equilibrium with its vapor at a pressure of 75.124 psia. It is exceedingly difficult to use the snow as a cooling fluid.
Although CO2 has been used as refrigerant in the past, it is a very inefficient one and not cost-effective. I have used it as a refrigerant in the manufacture of food grade CO2 and Dry Ice. In that role it proves effective because of its handy availability and cost.
Many people don’t realize that liquid CO2 doesn’t have an atmospheric boiling point. It only has an atmospheric sublimation point – at -78.5 oC. Every technical text book, reference, and data book that I’ve seen has continually made the mistake of labeling CO2 with an "atmospheric boiling point" of -78.5 oC. Even the NGPSA Data Book does this. I make mention of this continuing error because it has, in my opinion, caused engineers to make the erroneous assumption (as in this case) that it can be used as a conventional refrigerant – when it definitely can’t. I’ve used it as a refrigerant successfully, as I’ve mentioned, but at an evaporator temperature of approximately -50 oF at 100 psig. To go any lower is to risk invading the solid phase zone and inheriting a lot of problems.
RE: Low temperature refrigeration
Trick would be to apply a brine (nothing salt based, it would have to be something like benzene or White Gas or delimone) and mix the heated with the cooled.
Obviously, making this thing also heat would be pretty complex as well, though if the highest VP fluid was transcritical at the heat-of-rejection condition, it might all work...
I believe overall that 3-stage conventional Cascade with the lowest stage R508 will get you the temperature. Its still going to involve some pretty exotic machinery and materials but the fluids will do it.
http://www
a presentation by Rudy Stegmann on the alternatives, interesting reading.
Harris Cryostar makes chests for -150Deg. C. Autocascade, backed up by LN2 in most cases.
RE: Low temperature refrigeration
RE: Low temperature refrigeration
RE: Low temperature refrigeration
ht
Here is the refrigerant used in the CP system.
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Montemayor,
Next time down on the coast if you get a chance checkout some of the fish and shrimp freezers. Quite number of them use LCO2 as the refrigerant. They let the Dry Ice formed on the product sublime to continue coolong.
RE: Low temperature refrigeration
I’m glad you brought out the subject of possibly using CO2 as a refrigerant because it allows us all to analyze its merits with other refrigerants on a level playing field. I started to envision using it as a universal refrigerant when I started out in the Compressed Gas industry, right out of college in 1960. When I saw it being used as the refrigerant in producing liquefied CO2 product at 250 psig and -8.5 oF at the Harrison, NJ Liquid Carbonic CO2 plant, I thought I had the universal solution. When I was later was sent abroad to operate and later install other CO2 and dry ice facilities I tried to incorporate it in the Dry Ice production cycle. That’s when I was forced to get into the phase equilibria of CO2 and its thermophysical properties. To my dismay, I found out the bitter truth – there was a tradeoff in applying CO2. The Harrison plant was successful in its application because it seized on a convenient property of CO2: it was readily and cheaply available and it fitted in the type of compressors employed.
Harrison plant used the conventional, 3-stage reciprocating compressors employed at that time to produce what was then called “high pressure liquid CO2” and distributed in high pressure steel cylinders. However, the industry saw the multiple advantages of radically changing the type of product and switched to “low pressure CO2”, which was saturated at 250 psig. This allowed the used of standard, ASME section VII vessels (called “bullets” in the LPG business) fabricated out of ASTM A-212 steel. This allowed the vessels to be designed for -20 oF. To produce the new level of product (called "LiquiFlow"), Liquid Carbonic designed the Harrison plant with the conventional 3-stage compression cycle – but with a major process change: the second stage, which conventionally took the gas from 65 to 250 psig was used as the last stage of compression in the production line. The third stage was converted to serve as a closed cycle, refrigeration stage while using CO2 in a closed cycle. The third stage took the vaporized CO2 from the main CO2 condenser at approximately 200 psig and recompressed it up to the critical pressure of 1,100 psig so that it could be readily condensed at 85 oF with cooling water. The high pressure liquid CO2 was stored in a liquid reservoir and served as the refrigerant in the CO2 condenser, completing the cycle.
When I tried to employ a CO2 refrigeration cycle in the production of Dry Ice I ran smack dab into the Triple Point and I couldn’t go any further. That ended that effort - real fast.
The above shows that indeed CO2 can be used as a refrigerant. However, the caveat is that it is limited by the type of compressor used and the compression suction pressure it has to withstand. A suction pressure of 200 psig on a compressor is a special case – and consequently will be reflected in pricing. Also, as I recall, the Coefficient of Performance (COP) for CO2 is terrible as compared with the conventional refrigerants – especially Ammonia (which I consider the best, practical refrigerant).
Unclesyd:
Nice to hear from you again. Yes, the preservation and transport of seafood has been a traditional application of direct, liquid CO2 refrigeration by expanding the refrigerant directly onto the material to be cooled. My old company, Liquid Carbonic pioneered this type of application many years ago – in the 1930’s, I believe. The method is nothing more than the same effect of using a CO2 fire extinguisher. The free, adiabatic expansion of liquid CO2 produces a 2-phase mixture of dry ice snow and cold CO2 vapor – both at approximately -109 oF. This effect was tried on both interstate trucking applications and rail cars. It was quickly found that direct application of dry ice on the consumable products produced a “burning” of the product and ruined it. The method worked, but not by direct contact; rather, the cold resulting vapors from the expansion and from sublimation were circulated about the consumable product and used to do the resultant cooling through a difficult gaseous film coefficient. The refrigeration and preservation of Ice Cream, however, was a different and more effective application. It is still the principal alternative method to preserving Ice Cream other than mechanical refrigeration.
I’m not familiar with any liquid CO2 or Dry Ice applications on fish or shrimp here in the Texas Gulf Coast. Although I was born, raised and educated in Galveston, Texas (the traditional home of Gulf of Mexico Shrimp and fishing industries) I never saw or heard of it being used there while I grew up in the 40’s and 50’s. However, when I was in Peru in 1963, I furnished a lot of Dry Ice and even some liquid CO2 for refrigerating fish in transport when the Peruvians had the largest fish industry in the world, based on their catches of anchoveta. Again, the system worked there, but it wasn’t cost competitive in most cases because of the cost of the CO2. Dry Ice is used today in such applications as those of hunters and fishermen who have to transport relatively small quantities of preserved meat product. I seriously doubt if one can allow the solid dry ice formed to come in contact with the fish or shrimp in order to cool it. The result, in my experience, would be “cold burns” which would make the product un-salable. I believe if it were to be still used today (which is likely in some situations) the liquid CO2 would have to be expanded in such a way that the solid dry ice were kept away from the product shrimp or fish. The cold, -109 oF vapors would be still used as the effective cooling medium alone. However, I could be wrong since I’ve been away from the CO2 industry for some time.
RE: Low temperature refrigeration
Included with the design using the lower temperature refigerants, you will be faced with oil recovery problems and the lower temp refrigant pressures at ambient conditions. The settling out temperature will put your design pressure too high unless you let all of the refrigerant fade-out to vapor and reduce the pressure to a level that equipment is deigned to operate.
Ken
Ken
KE5DFR
RE: Low temperature refrigeration
The nice thing is no rotating equipment.
Yes, you have ongoing LN2 costs, but for a small system it is an attractive option.
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RE: Low temperature refrigeration
Never mind, he'll be back with a new proposal soon I guess. If he can't find one, LN2 must be the way to go at this scale. Maybe next time he upgrades his plant we might consider a mechanical solution.
RE: Low temperature refrigeration
A lot of interest in using CO2 as Industrial Refrigerant in cascade with NH3. Partially this because most larger food plants have use for Low Temp and Medium Temp and have been running multistage compression systems on NH3 for many years. BUT as production rates got higher and direct flash CO2 to atmosphere got grayer, demand for systems in the minus 60 to 70 neighbourhood led to very large NH3 Boosters (because of the specific volume so low at 20" Hg SSP); some really big pipes; and some very specialized lubricants to maintain transport and distribution at such temperatures. Even screw compressors at these conditions have some pretty specialized starting arrangements, oil cooling arrangements, seal arrangements and so on. And the maintenance issues are quite a burden, to a processor who thinks of refrigeration as utility.
The big pipe topic alone makes over-the-roof installations pretty costly overall: One installation went cascade and reduced multi pipe run load on 34-foot tall flat roof structure by almost 140 lbs per running foot. CO2 also avoids a lot of the occupancy/ leak detection scenarios that lead to some very detailed ventilation/evacuation/general alarm arrangements, required even where NH3 is in such a deep vacuum....
R717 is a great refrigerant, and we don't see transcritical being realistic in a large scale sprawling system, but CO2 installations are becoming very popular, especially for those who package equipment at the 75 to 150 TR capacity levels....