Blast Freezer with R22

Hi Everybody:

I'm designing one blast Freezer with the following characteristics:

• Direct expansion system
• R22
• T evap = -37ºC
• T air = -30ºC
• T cond = +35ºC
• Total load = 190 Kw
• 02 open drive screw compressors 119 Kw each one.
• 01 oil separator
• 01 Evaporative condenser of 800KW

Someone told me that for this temperature and load it's better to use a economizer and thermosiphon oil cooling but i'm not sure about it beacuse i always installed NH3 Systems for low temperature.

i will like to know if somebody has some information or coment about this topic.

Luis Sandoval.

Lima - Peru


Best Regards.

Unquestionably, you should use an economizer. Thermosyphon oil cooling is also your best choice although glycol or water cooled oil cooling can work quite well if you have a use for the heat & decent water quality.

you have certainly got the condenser capaity covered at 800 Kw.

Hi again.
as a rule I use the total heat of rejection at a point of nominal -20 C evap at compressor. The THR includes the compressor input Kw the evap., input Kw, the oil cooler Kw input. Ok staged loading on compressor , screw or recip., so add 10%% safety margin, then look at approach temp conditions of condenser- either evap cooled or air cooled. Next select an approach temp for condenser, above highest local DB ambient, say 10 degrees C. max.
Hope this helps. Magoo

If using R-22, you can go down to -42 deg. C, with economiser and definitely air cooled or water cooled oil coolers.

What make of screws are you using?

The evaporative condenser you mentioned, sounds like an overkill, watch out for too low condensing temperatures!

Why DX? This would work more efficiently & the evaporator would be smaller if it was pump or gravity circulated

hello

R22 phase out would stop new design of systems and progress I would of thought.

AWW

1. DX not as efficient as overfeed or gravity recirc but it does result in less R-22 charge.
2. R-22 is already becoming more expensive and will increase as time goes by. Not that R-507 and POE oil are cheap by any means at least here in USA.
3. How are you going to defrost your coils? Just curious because I don't think hot gas is going to work because you have only one blast freezer. You don't say how many coils you have.
4.Yes, be sure to use an economizer. I recommend thermosyphon oil cooler and you can build it into your high pressure receiver if you are using one.
5. I suggest a shell and tube economiser so you have subcooled liquid feed to the TXV's at the coils at high pressure.
6. Your design is just barely above atmospheric suction pressure. Be sure you don't go into vaccum in the suction to compressors and run risk of getting air into the system. Then you will need a purge unit for sure and you will lose R-22 when purge unit runs, purge unit releases roughly 50% R-22 loss and 50% air depending on whose unit.

It would be better to use a different refrigerant suitable for low temperature applications such as R507 or R404a. You would need minimal oil cooling with these refrigerants and could use a simple air cooled or water cooled oil cooler.