Ultra low Temp Freezer Design

[jwasir]

Hi All,

I need to know if someone has experience in designing low temp freezer for -76F (-60C).

Also I am curious to know:

1. What kind of defrost to be used -- Hot Gas or Electric. Will there be a big shock in the system with HG defrosting?

2. Do we need to use any special metal for indoor unit enclosure -- Can alumnium/stainless withstand -76F temp?

3. Will 2 stage or cascade will be better for the application?

4. Accumulators and other components are rated for -40F -- can they be used for -76F?

5. Do we need to use elect. exp. valves?

6. How many Fins/Inch are required for Evap?

7. What are the other parameters needs to be considered for this low temp application.

THANKS IN ADVANCE

[Peter_1]

1. What kind of defrost to be used -- Hot Gas or Electric. Will there be a big shock in the system with HG defrosting?
2. Do we need to use any special metal for indoor unit enclosure -- Can alumnium/stainless withstand -76F temp?
3. Will 2 stage or cascade will be better for the application?
4. Accumulators and other components are rated for -40F -- can they be used for -76F?
5. Do we need to use elect. exp. valves?
6. How many Fins/Inch are required for Evap?
7. What are the other parameters needs to be considered for this low temp application.
THANKS IN ADVANCE
1. Preferable hot gas, you then flush also the coil.
2. Both are commonly used. You must avoid cold bridges at any times, especially in these applications
3. It can't be properly done with a 1 stage (or an autocascade) unless you et run your compressor in vacuum.
4. No problem with this.
5. EEV is indeed possible but also cap tubes are widely used.
6. Same as used in freezer applications, 7 to 10 mm space.
7. Not that easy if you never done it. A thoroughly understanding of log/p is necessary. Don't forget the fan is a serious heat load at -60°C.
Door heaters can give serious problems.
Insulate very well.
Use ozone friendly gas mixtures.

[The MG Pony]

Actualy peter in auto cascade he would be running quite nicely in the possitive pressures.

A 2 stage auto cascade can easily achive -80c with a high suction pressure using only two refrigerants! Not to mention efficient.

I've seen now several people make effective -80 auto cascades out of half horse power rotory compressors with impressive resualts of up to 300W loading!

http://www.xtremesystems.org/forums/showthread.php?t=144007 = here is a thread chalk full of detailed info and schematics, allot to read through but worth it.

[jwasir]

1. Preferable hot gas, you then flush also the coil.
2. Both are commonly used. You must avoid cold bridges at any times, especially in these applications
3. It can't be properly done with a 1 stage (or an autocascade) unless you et run your compressor in vacuum.
4. No problem with this.
5. EEV is indeed possible but also cap tubes are widely used.
6. Same as used in freezer applications, 7 to 10 mm space.
7. Not that easy if you never done it. A thoroughly understanding of log/p is necessary. Don't forget the fan is a serious heat load at -60°C.
Door heaters can give serious problems.
Insulate very well.
Use ozone friendly gas mixtures.

Thanks Peter_1,

Can the HG produce big thermal shock at the evap inlet, this is one evap/comp system?

What are Cold Bridges?

What is log/p?

What will be the problems with comp running in vacumm?

Thanks for the info..

[Peter_1]

Can the HG produce big thermal shock at the evap inlet, this is one evap/comp system?
What are Cold Bridges?
What is log/p?
What will be the problems with comp running in vacuum?
Thanks for the info..
As far as I've seen it , no.
Cold bridges: http://www.cs.arizona.edu/people/jcropper/desguide.html
http://www.e-lindsey.gov.uk/environment/planning/building-control/building-control-guides/upload/023%20Cold%20Bridges.pdf
Perhaps some are using another name for a log/p http://www.kohlenberg.info/_data/Bild42.jpg
If running in vacuum, eventual leaks will cause air entering the system, not sufficient motor cooling, ...
A solution with positive pressure is possible, so go for this.

[jwasir]

Peter_1,

How can we get Positive pressures, when system running at -80C SST?

[The MG Pony]

I would recomend you impliment a cascade system, just what is the evap duty on this thing?

If small ie two door reach in then a nice Autocascade system will be ideal, If larger a 2 stage cascade would be better

[Peter_1]

Jwasir, MG Ponny gave the right answer already.
This is something not so easy if you ask me.

[jwasir]

The MG Pony,

The room size is approx 20 x 20 x 25.

It needs 20,000 BTU's.

I was planning for 2 stage 30 HP Bitzer comp? The operating envelop is down to -95F/99F.

Do U have some documents for casade system?

Thanks..

[Peter_1]

You need for that capacity certainly a cascade system.

How you found you need a 2 stage 30 HP?
Do you mean a two stage compressor or a cascade? This is a complete different set-up.

How you know you need 20.000 BTU? What will be the heat load of the fan of your evaporator?

How will you construct the room? Air tightness is extremely important.
Sliding doors?
What wall thickness have you taken?
Pressure equalization valve?
Door heaters on the outside?

You will need some space for the expansion tank which will have a big size.

You first post was -75°F, now -99°F. Working in this low region temperatures, 1°F can make a huge difference.

Is this for a pharmaceutical application?
Do you really need -99°F?

The only thing which will get really cold is the evaporator.

I think you must be aware that a room of that size is a very difficult and risky job.
But I'm interested in the ongoing results, even in the calculations for this project.

There was some time ago a Belgium poster here where several low temperature rooms were installed and they had serious troubles reaching the desired temperatures for several reasons.
I helped a little the project engineer searching what could be wrong with it. The company who installed it is a very competent Belgium company.
What I learned is that you will have to control yourself every step in the production of every step in the making of this room. If one step isn't done the proper way, you will be finally left with the bad results.
Then try once to say that the door or a simple doorheater is the fault for your problems.

Only trying to avoid as much as possible pitfalls for you.

[jwasir]

Sorry for the confusion.

I still need -76F. -95FSST / 99F Condensing is the opearting limits for the 2 stage compressor I selected.

Bitzer 30 HP will deliver 20,000 BTU @ -95 SST.

The room is built by others and is located inside a freezer room, where temp is maintained at -5F. Wall thickness is 6" all 6 sides.

The fan is 1.5HP and has 2500 Btu's.

Application is Tuna Fish Freezer.

Do U think 2 stage will work -- it would be simpler than cascade?

[Peter_1]

Sorry for the confusion.

I still need -76F. -95FSST / 99F Condensing is the opearting limits for the 2 stage compressor I selected.

Bitzer 30 HP will deliver 20,000 BTU @ -95 SST.

The room is built by others and is located inside a freezer room, where temp is maintained at -5F. Wall thickness is 6" all 6 sides.

The fan is 1.5HP and has 2500 Btu's.

Application is Tuna Fish Freezer.

Do U think 2 stage will work -- it would be simpler than cascade?

I would like to hear the opinion of others. Running a two stage at these conditions will give you a very bad COP.
But it's anyhow simpler to use a prefabricated 2 stage unit.
You're anyway running just within the Bitzer specs as long as you can condens low enough (-95°F te@ 95°F tc or -70°C/35°C)
I should solder as much as possible to prevent air entrance in case of a leak.
Make sure the evaporator gives you the needed warranties
If it was me, I should go for the 2-stage Bitzer.
Make sure you have enough capacity. The job I mentioned before didn't work at all due to the door heaters and air leaks.

Interesting job. Keep us in touch with your progress.

[The MG Pony]

Ok Now that I know what it is a local company here had a blast freezer working @ -80, not as large as yours mind you, they directed the cold air over the racks, the room it self tended to stay at apx (-70)+- but the tuna froze verry well.

[Peter_1]

Why going so deep in temperature?
Is it only for speed reasons?
You're coming close to use a nitrogen tunnel.

[The MG Pony]

Yes peter, it is a blast freeze system to preserve the quality of the fish, then it is stored at -40c usualy, never held at -90 though, only fresh frozen at that temp.

[jwasir]

Peter_1/MG Pony,

Thanks for the motivation.

The only concern I have is that the system will be operating right on the lower limit.

I might have to use LP cut out or HG bypass, incase pr. goes below -90F.

Do U think, if soln valve can be added, which will dump HG below -90 and stop above -85 (just a thought).

Also, I am curious to know, how U guys do comp selection for cascade.

I checked copeland/bitzer sites and the comp's are a/v for min. operating conditions of -40/110F.

When the cascade system is running -90 or -100, do we need special comp's which can actually go down to low SST's?

Are there special comp's for cascade?

[The MG Pony]

mainly you start with evap load then size the compressor to that load and select a gas that gives you the best suction pressure with reasonable discharge temp and pressure, Ethane comes to mind for the low stage.

The second exchanger is sized based on discharge load, and the second compressor is sized to dissipate that load again a gas is selected that will yield good capacity at reasonable temps and pressures, and don't go cheap on condensers or heat exchangers!

[Peter_1]

The only concern I have is that the system will be operating right on the lower limit.

I might have to use LP cut out or HG bypass, incase pr. goes below -90F.

Do U think, if soln valve can be added, which will dump HG below -90 and stop above -85 (just a thought).

Most manufactures have special valves for this, capacity regulating valves which injects proportional hotgas to maintain a very stable LP.

[jwasir]

Peter_1,

What kind of fan motor can we use for -76F room temp?

[Peter_1]

I should not know. In such a cases, my manufactueres must give me the needed warranties.

[jwasir]

Thanks Peter_1,

Its hard to find motors for low temp!

[jwasir]

Peter_1,

You recommend HG defrosting.

This is a 1 evap system and during defrosting, there will be no cooling a/v for intermediate stage.

Can we still use HG defrosting for 2 stage?

Thanks in advance!

[scottishladx]

I would use a 2 stage cascade system!

With temp probes on exchanger rather than a timer

[jwasir]

With temp probes on exchanger rather than a timer

Can U please explain this briefly??

[scottishladx]

Can U please explain this briefly??

Let the first stage get to -25 before the second stage kicks in to whoop it down to -80 rather than a 3 min relay moere efficient.

[ozzy bill]

tx will give you problems,try capillary much easier capillary heaters are needed on off cycle, try r507 high stage and r170 or r508 low stage propane can be added to 508 for oil return but watch pressures on off cycle or at static

[Z4Y]

hai i'm zaenal

can you help me to troubleshoting my cascade system

high stage = R404
low stage = R508 + R290 (9%)
evaporator lenght = 39,8 mtr
i use 1 Hp compressor for 1st and 2nd stage
but why outlet evap temperatur to slow decrease ?
example in evap = -90 c but out evap = 10 c (2 hour running)

[jcook1982]

Z4Y it takes a long time for it to cool down. Maybe only 10 degrees C for each hour of operation.