Modifying cabinet temps

[james]

Okay, here's a new quick question for you all:
all temps in Centigrade, just in case :~)

Some of you may remember the traumas I've had with this cabinet over the last year or so...
I've finally got it operating quite happily at -20, with a reasonable duty cycle and steady state temp, evaporating at about -30C and controlling between -20 and -22 air-on. Inside the cabinet is a pair of 600W evaporators, each controlled by a separate expansion valve (danfoss TS2 fitted with 00 orifice), which common up and return to a 2kW THR condensing unit

We've recently had an enquiry for a cabinet which runs at +4C, instead of -20. Now, ideally I'd like to change as little of the system as possible, and I think I can get away with leaving the same evaporators and, possibly, orifices, inside the cabinet. I'd quite like to keep liquid and suction at 1/4" and 1/2" respectively, and just change the condenser for a HBP version. I think a danfoss SC10 should do the job, but...

What sort of effects am I likely to encounter? Excessive superheat? and is this _really_ a problem, other than not fully using the evaporators. Is there anything I've missed with the spec?

[Dan]

I have had fair luck just turning the thermostat up with several style boxes. No other adjustments. These had CPR valves or MOPD TEV's in them. You could likely disable the electric defrost heaters.

[herefishy]

You'd better check the maximum evaporating temperature rating of the condensing unit. You might find that the minimum expected operating temperature may not be within even the maximum rating of the C.U.

I've run into the mindset by the customer that, well, if it can do -20, why can't it do +40 (if it's supposed to cool). But the space shuttle can't cruise 5mph down the strip to pick up chicks!!!!!

Depending on the situation, you may have some humidity or other critical criteria compromised by changing the application.

DON'T DO IT!!!!!!!!!!!!!!!!!! IF THEY WANT A COOLER, THEY NEED A COOLER. A FREEZER IS NOT A COOLER!!!!!!!!!!!!!!!!

[james]

Yeah, that's basically my thoughts on the situation. Obviously for manufacturing it would be easier to keep the entire system the same, but we're basically already running the condenser with a huge airflow through it during cooldown to keep the head pressure low. If we try to control it at +4, then it's always going to be running loud (2 pole fan running flat out... in a laboratory!).

[dwallan]

What about installing a small heater, possibly proportianal control of the exisiting defrost elements, creating an artificial load and using more of the available capacity.

David Allan

[james]

There isn't actually a heater or defost system on the refrigeration unit. I know this might sound like a bad idea, but it's running in a dry nitrogen environment anyway, and the evaporators icing up are the least of our problems if moisture gets in there!

[zolar1]

Couldn't you change the refrigerant or adjust the charge?

[dwallan]

Why not add a heater and/or a hot gas bypass and pitch it as a constant temperature room capable of maintaining the temperature within 0.5'C of the set point which could range from
-20 to +15.

David Allan

[james]

David,

Can you explain that a little more? We are currently controlling between -20 and -22 air temperature, which gives us a pretty fast duty cycle for the compressor. How would I maintain a +/- 0.5C room temperature with heaters etc?

The comressor we're using would struggle I think if we increased the set temperature, as it would not cope easily with the high back pressures.

[zolar1]

Would a larger reveiver do it? Or maybe a bleed-off line? Or move the temperature sensor to a different location?

I'm just grasping at straws here (I don't know much about this yet)

[Gary]

James, as Dan suggested earlier, just install a CPR to limit the back pressure, and turn up the thermostat.

[Dan]

I am puzzled where the concern regarding high suction and discharge pressures come from if you have a CPR or MOPD powerhead. I think it would be a piece of cake to operate the box at a higher temperature.

[CDF]

A CPR will definately do the trick and turn the stat up. A MOP would also work, however will restrict your flexibility too much. Disconnect the evap heaters and extend your off cycle defrost time. Job done!

[herefishy]

Regarding the TEV, is it an MOP? Have you checked, adjusted superheat for the "new" design temp?

[Simon Butler]

James,

Probably the safest and cheapest modification apart from turning the stat up would be to add a crankcase pressure regulator, thus preventing the increased head pressure from putting to much load on the CU which is probably rated for low temp ie. small motor big piston and small condenser.

Simon.

[CDF]

Simon,

I always thought a CPR was a crankcase pressure regulator!!! Obviously not!!

[Simon Butler]

sorry only read the first page before i posted reply, read the one about putting a heater in the room and thought i should reply quickly!!!

Si