understanding refrigeration pressues, help please

[Haz]

hey all, im a 3rd nearly 4th yr apprentice and am having problems understanding what pressures a fridge/freezer unit should run at. For example what is the correct formula , how would you do it. if say you had a fridge set at 2c on 134a. My boss only tells me what psi they should run at on certain gases and applications but cant really tell me how he got it apart from experience. But surely there is a formula or calculation you would use to have a close idea what the unit should run at.

sorry if I sound dumb but I want to be good at my job not just winging it like most the cowboys in my area.

cheers

[RANGER1]

Has,
Best tool to have is a pressure/ tperature chart.
Convert the pressure to the the pressure for whatever refrigerant you are using.
The temperature after it is converted has to be colder than whatever you are trying to cool or freeze to be able to transfer heat.
Evaporator with fans in say a cool room, temperature difference would we'd to be
approx 6-8 deg C lower than room temperature.

Direct contact with product requires less of a temp difference sY a water chiller.

[Tony]

City and Guilds exams work on a 10°C temperature difference for evaporators. So, if you want a cold room to be say -20°C then the refrigerant should be boiling (evaporating) at -30°C.

If this system was using R404A, at -30°C its pressure would be 1barg (14.7psig)

Hope that helps.

[mattym]

I agree with tony i usually work off the ten degree rule as a rough guide

[Haz]

thanks guys i understand now