View Full Version : Reading High Head Pressure

08-01-2006, 07:15 PM
This is one area Im still unfamiliar with.

When servicing a unit , we take head pressure readings.

How do I ascertain the head pressure is right and within limits taking into account the prevailing ambient temps, etc

I would appreciate if a member could explain


08-01-2006, 11:15 PM
I've always used the theory ambient temp plus 17.5 degC, should give approx head pressure/ condensing temp at gauge, works for refrigeration systems.

09-01-2006, 08:08 AM
Thx Reg,

Is that for all refrigerants??

09-01-2006, 07:18 PM
I should say a difference of 10 to 12 K in cold weather and 15 to 18 K in warm weather.

09-01-2006, 10:08 PM
Also Abe, it can sometimes depend of the compression ratio of the compressor - if the suction is 60psig with a 4:1 ratio then you should start looking at 240psig.

09-01-2006, 10:17 PM
what gas are u running on? r22 about 45 degrees c, when drawing a staight line from psi scale to 22 scale. i also find that 404a runs with a higher hp than other gases. on low temp that is. i also find the units i work on in the uk, there condensors r under sized.

10-01-2006, 04:43 PM
I see no general rule of thumb for this as it tends to differ by application. generally an air cooled unit stood outside in the uk would be 40/45 deg c condensing. the air td over the condensor would depend on other conditions such as fan speed/waterflow control, ie low td in summer when fans are flat out and high td in winter when fans are slow. another example would be a cascade system where the low side would be condensing at roughly the same temp as the high side is evaping. i do remember the old textbook figs for r22 were 70f/90f/110f air on/air off/hp temp respectivley.

10-01-2006, 11:11 PM
Then Brian's taking the piss with his compression ratio antics:)Been waiting to see who was awake :D

15-01-2006, 01:02 PM
I was with Brian, exactly the guide as rule of the thumb it depends on the compression ratio and the ambient temperature. Believe me in the middle east ambient temp 49*C got head press 350psi and the normal suction press 70psi.

15-01-2006, 01:14 PM
If you have 350 psi with an ambient of 49C, then other gasses has to be used or very big condensers or the condensing process must be helped with evaporative cooling during high ambients.
No wonder it remains so warm in the Middle East.:p :D

15-01-2006, 02:48 PM
Not only so warm Peter, extremely so hot varries to 49*C to 52*C. You are right then the unit there designed having a big condensers, yes some helped with evap cooling process. Thats why condenser with cooling tower type are the best to used in there.