LOW TEMPERATURE REFRIGERATION

[PERFECTAIR]

Looking to cycle an insulated room of size 15ft x 14ft x 8ft between 5deg F and 110deg F at 20 minutes interval. The cycling will be continuous for about 48hrs. Any useful design hints on any refrigeration application or the use Nitrogen to get the 5deg F state will be appreciated.

 

I can help you with this, but need to clarify a few things first to decide whether i'm up for it!!

Do you need help with the system mechanical design and eqiuipment selection only, or do you need help calcualting heating/cooling btu loads as well?

 

[PERFECTAIR]

Just system machanical design and equipment selection on the cooling side if you will apply Nitrogen OR Just system design if using other method.

Thanks

 

Sorry I didn't get back to you last night. I hope this information can be at least a little helpful to you on this project. I can only provide you some insight on possible options or methods you can persue.

First, I am not aware of your position regarding this project, whether this is a job you have, need to bid, or one of your customers has asked you to inquire about. I assume from your post title, Perfect Air is an installation and service company, or mechanical contractor.

Second, what you need to accomplish is not just your simple or conventional - size, install, and startup type installation. There is an enormous amoount of considerations regarding this system in respect to the structural enclosure and insulation materials used for the room, The mechanical cooling system components, The 20 minute time constraints, and controls as well.

I have had the pleasure to be involved in many industrial low temp applications thoughout my career. Unfortunately, none of these has involved the use of nitrogen as the refrigerant. I was involved in a similar project years ago for a facility for testing anti-freeze requiring, a -40 degree booth and wind speeds in excess of 80 mph.

(1) You could attempt, and possibly get the help from equipment manufactures on sizing, to utilize single or multiple Direct expansion, split, R502 systems. The considerations would be the substantial tonnage you'll find that you need to take the room from the 110 degrees to 5 degrees in the 20 minute interval. It would basically be the calculated load/hr divided by three. second, there are refrigeration considerations starting dx systems with the initial 110 degree load and would require the use of refrigeration regulating and control valves to reduce the initial load on the compressors/condensers. Other considerations are concerning any air over evaporator motors and may have to be looked at, and of course the need for any defrost cycle if the room ambient temp in the warm up cycle did not accomplish this for you. Best bet is to let the equipment manufacturers do the home work for you.

(2) Another option would be a direct expansion cascade system. This would involve the use of a single air handler with muliple dx evap coils that are in air flow series. Each evap coil would have its own dedicated remote condensing unit. These you will find to also be of substantial size to accomplish this task. Cascade systems use separate coils and condensing units, and in many circumstances each containing different refrigerants. Different refrigerants are used to obtain multiple evaporating temperatures. There are benefits to a cascade, which are the abilily to get low discharge air temperatures even with a high return air load, and just as important, moisture control. If you would take a moisture laden return air through multiple coils of different evaporating temperatures, this is what would take place: entering air would go through the first coil at 40 degree evaporating temp and moisture would be removed as water (condensate), The next coil lets say with a 10 degree evaporating temp would take the same air volume with less moisture, causing the discharge air temp to be lowered to say 20 degrees and minimize external coil icing because some moisture was removed from the first coil. The cascade cycle then continues to the next coil and so on. The total number of cascade stages and sizes would be designed to accomodate the specific application. Considerations involving the air handler for construction characteristics would have to be looked at, as well as defrost. It would be possible to use
a single air handler for both heating and cooling cycles depending on the heating source. I.E. if it were a hot water coil, glycol at a 20% solution would have to be used, and coil design regarding expansion/contraction requirements needs to be looked at. Further, provisions for preventing any possible thermal shock on the boiler vessel.

(3)The use of a low temp chillers has really excelled in the last 15 years for many low temp applications, especially with all the ice building strategies be applied to produce chilled water from ice during the day, while making ice at night when electric rates are off peak. This is an option to persue but, be aware however this one will probably be the most expensive!! This in your case would involve an air cooled packaged, low temp chiller that would furnish say -5 degree water and glycol solution (in some instances, calcium chloride solution and other solutions are used, but try to stay away from it)This glycol solution would be chilled by the chiller, and circulated by a pump to a chilled water coil in an air handling unit. It is better when the fluid system incorporates a large storage tank and two pumps. One pump would be used to circulate chilled water from the tank, through the chiller and then back to the tank. This gives you storage or reserve water and possibly chiller sizing diversity. The second pump would circulate water from the storage tank through the chilled water coil in the air handler. these systems are expensive due to the equipment and installation cost, especially when heavily insulated storage tanks are used. Piping should be welded steel with suitable pipe insulation. Other factors to consider would be the air handler, coil, defrost cycle method and controls.

I'm sure as you are aware that explaining something in writing is difficult at best in this industry. I'm sure there are more considerations I have overlooked but to be frank, I getting tired.

I'm sorry I couldn't help you with the nitrogen system, maybe its a better way to go, and someone else can give you some direction. The best results i've had when tackling a new and difficult project is by making some phone calls! I'd start with unit manufacturers, Most are willing to help and some even like the challenge. Hell there there to sell equipment, so make them do your homework for you.

Of course theres always the final option of retaining a mechanical engineer to handle the whole project design, but in most circumstances, when you are just bidding a project, or proposing a budget to a customer, you certainly can do that.

Best of luck, hope this was somewhat helpful

Steve

 

Just found this page, all though I may be a bit late. I have done this type of ageing room before using simple Dry Expansion with a single stage Bitzer reciprocating compressor with two ceiling mounted evaporators fitted with electric heater bank.

One tip is to allow for the heat in the fabric and cooler case work when at 110degF.

Let me know if more info required.