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An Ice Cream Manufacturing unit proposes to install the refrigeration plants with ammonia as a refrigerant. For such application where the temperatures shall be generally between - 40 Deg. C to - 30 Deg. C, what would you advice. Would you reccommend using refrigeration plants with R 404 A.
:) I wouldnt recommend using R404A, but I am biased on this anyway:D
Depending on how much cooling capacity is needed, you might consider using a CO2/NH3 cascade system too.
But then, I love ammonia too!:D
IceMan,Quote:
Originally Posted by US Iceman
Do you know of anyone else besides US Cold Storage using the CO2/NH3 cascade?:confused:
I think Nestle' is using it exclusively for all of their new systems.
I think this is going to catch on. Very low temp. ammonia systems do require a lot of compressor displacement in relation to the capacity. At -40C/F the specific gas volume is very high per pound or kilogram circulated (for ammonia). This in turn means the booster compressors are huge compared to their capacity.
Using CO2 for the low stage makes a lot of sense. The boosters are smaller, i.e., lower cost.
The down side is you need some way to prevent the low stage pressure from increasing if the booster compressors are shut off.
The safest way is to use a fade out drum (expansion tank) to keep the low stage pressure below the relief valve setting.
If not this, then a separate refrigeration system to keep the low stage cold.
Can you describe this in some detail? I have never heard the term before.Quote:
The safest way is to use a fade out drum (expansion tank) to keep the low stage pressure below the relief valve setting.
Hi Dan,
Sure thing.
A fade-out drum is just a large pressure vessel rated at a standard design pressure (maybe 250 or 300 psi or 17.2 to 20.7 bar).
When the CO2 system is off, it can warm up. As the liquid CO2 warms up it begins to vaporize, which causes the pressure in the CO2 system to increase.
The problem with CO2 is; at normal ambient temperatures the vapor pressure of the CO2 is very high (higher than the normal vessel and compressor design pressures).
By adding a large supplemental volume to the CO2 system, this allows the gas to expand without a large increase in pressure.
In essence, the vessel is an expansion tank.
This same thing happens on cascade systems using high pressure refrigerants, i.e., R-23, R-503, ethane, etc.
Does that help?
I understand now
Thanks
I have to agree with US Iceman on this type pplant...CO2/NH3 works great. However there are a number of plants running -30 dF and -40 dF on two stage ammonia systems in a vacuum on the low stage suction. With a tight system and a good purger, these work well.
Ken
That's quite true, and I am not discrediting all of the two stage systems that are well built and maintained. In fact, these will probably be more safe than a CO2/NH3 cascade system. A lot of people have exerience with a two stage ammonia system, while not many will have the exposure to the CO2/NH3 systems.Quote:
Originally Posted by TXiceman
That in itself can present other problems that may negate the benefits, since the technology has to be re-learned. CO2 used to be very common, but since it fell out of use (in industrial refrigeration) many years ago people will have to learn how the systems operate and what to expect.
What worrys me is the high pressures that you are working with on the CO2 side
At operating temperatures, the CO2 pressures are not any worse than anything you already have experience with in ammonia systems.
Now, if the CO2 system is shut off and allowed to warm up, then you have a different story. Easily over 500 psig.
Thanks Iceman. I guess my concern is how one goes about sizing the fade out drum. If it is to contain the equivalent vapor of a large CO2 liquid charge, I see a prohibitively large tank. Small cascade systems with critical charges are one thing, but a large commercial application is quite another. Are these used in large systems with significant liquid charges?
Hi Dan,
Yes these vessels can be quite big or sometimes in multiple quantities. It all depends on the cacade system volume and the amount of refrigerant you have to contain.
In a large system, I think it is more cost-effective to use a separate refrigeration system to maintain the CO2 system at lower temperatures to prevent the high-pressure issue.
I did some preliminary design work on an R-503 system some years ago that about about 6 large vessels for the fade-out drums. Let's just say the refrigerant charge was larger than I would like to have seen it.
The use of the fade-out drums is almost fool-proof, while a separate refrigeration system always has to the oppotunity to not function when it is required.
These have to be reviewed to see what is acceptable from an owners point of view.
In our country cascade with CO2/NH3 is yet to be taken up. What would be your advice if I go for Two Stage Compressors with R 404 A or R 22. R 22 is still in use in our country. I have to provide a system which can be easily attended.
About the largest fade-out drum I have put in was 12' diameter and 36' long. This was on a "critical charge system".
I have used a small refrigeration unit on the cascade condenser to maintain low stage pressures on an ethane unit. The plant produced ethane and could vent the ethane to the flare system if they were in a total power outage and could not maintain refrigeration. The plant had a normal outage about every 9 months.
As a personal preference, I will not use any of the 400 series refrigerants on anything other than a small closed loop unit. You should consider the use of R-507 for your applicaiton over R-404A. The 400 series are all blends and present problems when leaks occur as different portions of the blend will leak out depending on where the leaks occurs in the system. it has to do with the boiling and dew point being differenet in each component. With the 400 series you have to charge liquid into the system and not vapor. R-507 is an azetrope and behaves as a single component. You can easily recharge either liquid or vapor into the system.
The refrigerant and equipment manufacturers have done a good gjob of selling the 400 series, but they need to be restricted to the smaller systems.
Ken
If you were to choose between R 404 A, R 22 and Ammonia for using for -45 Deg. Evap. which refrigerant would you choose. R 22 is still very much in use in our country and we can use two stage reciprocating semi hermetics. The total capacity is about 102 Kw and we have the option to put multiple units. I have used and are using R 22 and R 404 A for various low temperature applications. With ammonia I have worked only down to -5 Deg. C Evap and as such do not have any experience with this.
With your Ref capacity and between the 3: R404A.
But, if i have a vote: I will use R507, glide=0
R-22 will have to go, been trying to convince my refrigeration customer with R22 to convert to R507.
In my case, as an enviromental guy, I have to push my customer to use Natural refrigerant: NH3 for now. Hopefully i would see the day to include Helium as well.
The cost of R 404A is very high. As on today R 22 is very much in use in our country and for the next 10 to 15 years it is going to be there. As it is there are a couple of importers for these alternative refrigerants and they charge very high.