View Full Version : Running Pressures

03-04-2011, 02:45 PM
Hi can anyone tell me how to calculate the ideal low and high running pressures for a refrigerant? I have the space temperature and outside ambient temperature.


03-04-2011, 03:47 PM
As low as possible Hp and as a high as possible LP.
Subtract 5 to 7K to room or cooled medium temperature and add 7 to 10 K to outside or condensing medium temperature.

06-04-2011, 08:39 AM
For R410A,
Working pressures @ rated ambient temperature 35DecC as follows:-
High pressure range >> 360 to 420psig
Low pressure range >> 110 to 160psig.
Theoretically working temperatures should be designed by most manufacturers.
For example, condensing temperature target > 40 to 45 Deg C
Evaporation temperature target >> 0 to 5 Deg C
R410A pressure @ 40 DegC = 340 psi and @ 45DegC=380 psi
R410A pressure @ 0 DegC = 100 psi and @ 5DegC =120psi
Hope that will help...

09-04-2011, 01:11 PM
A rule of thumb for the high side is:
Ambient + 15'C = SCT
So, in the UK, design ambient is 32'c (according to BSEN378). 32'C + 15 = SCT of 47'c. So in theory at design ambient, your SCT should be 47'c. However! As it is all about energy efficiency these days, we want to keep the HP as low as possible. For every 1'C you can lower the SCT equals 3% in energy savings. So the push now is to keep the HP as low as possible. Its out with the old condenser fan speed controller and keep the fan running flat out. Either that or lower the FSC setpoint way below 47'C.

Try to raise the SST whenever possible. As with the SCT, you can save 3% energy when you raise the SST. Aim for these evaporating SST temperatures as a ball park figure:

Airconditioning = +5'C
Coolroom = -5'C
Freezeroom = -25'C

Forget about pressures.

Also, the rule of thumb for air cooled heat exchangers is 10K (5K for water cooled). Air off - 10K = SST.

If the air off the evaporator is +14'C (for AC) then you should be evaporating at around +4'c SST

Hope that helps.

09-04-2011, 08:17 PM
Very informative Tradewinds.
Thank you.