guage manifold readings

[cool_bank]

hi every one
im working in ice cream factory.we have a cold room to hard ice cream .ice cream is in seal plastic canes.we maintain minus 40 degree Celsius temperature in cold room.to achieve and maintain this temperature we have a 30 hp reciprocating unit .it is direct expansion.it have thermostatic expansion valve and water cooled condenser and we use 404a refrigerant in this unit.and evaporators are forced fan type.
my qestion is this that in this unit what will be the cut in & cut out settings of low pressure switch as a safety control? and what will be the lp and hp guage readings at minus 40 degree celcious.
what is the formula to know these two tings ?
can anyone help me?

[taz24]

.

R404a at -40deg c is 0.5 bar gauge so for safety yo need to cut out at 0.1 or 0.2 bar gauge
which would equal a temperature of about -44 to -45deg c. the thing will then need to cut in
again about 1 bar above the cutout. If you have the cut in too high the ice cream will go soft
but if you have it too low then the compressor will cycle.

One way to prevent cycling is to use a delay timer on the compressor start.

All the best

taz

.

[mikeref]

what will be the cut in & cut out settings of low pressure switch as a safety control?


Brrrr, -40C. I would choose the lower pressure setting at just above atmospheric pressure, then the cut in pressure at around 90 Kpa.
Use a pressure /temperature chart to see what your low side pressure corresponds to in Degrees C of F.
As Taz24 says, you don't want the compressor to short cycle on the pressure switch. You need just enough differential, (cut OUT verses cut IN pressure on the LP switch) so that your compressor has several minutes at least, while the pressure slowly rises before compressor restarts.
A short cycle delay timer, ( thanks Taz,) in the control circuit, will ensure there is a minimum "OFF" time before compressor restarts.
Set the timer to 10 minutes.

[Brian_UK]

I would ask why you are storing ice cream at -40°C, it's way too cold for it, a waste of energy surely?

[Teuria]

I would ask why you are storing ice cream at -40°C, it's way too cold for it, a waste of energy surely?

I think that it wasn`t for storage purpose but for making icecream hard. Icecream needs very cold temp to freeze it properly.

[Brian_UK]

^^ Fair comment, I suppose that as I don't eat rock hard ice cream I'm a softee.

Used to work at an Ice cream makers place where he stored it at -21°C in one store and -27°C in another.

[Teuria]

^^ Fair comment, I suppose that as I don't eat rock hard ice cream I'm a softee.

Used to work at an Ice cream makers place where he stored it at -21°C in one store and -27°C in another.

Temp for freezing icecream is lower than storage temp because icecream is full of " ice flakes " if you freeze it in too warm temp (not cold enough) and too slow. ie at home freezer if you freeze your melted icecram it tastes like .... because of that "iceflake" thingie :)