View Full Version : Discharge superheat?
02-09-2007, 10:54 AM
How do I calculate discharge superheat? Also can someone explain the differance in absoulte and guaged pressure with an example? Thanks
02-09-2007, 02:26 PM
Answer #1: Discharge Superheat = Temperature of discharge line 6” downstream of the compressor discharge service valve (semi-hermetic) or discharge connection (hermetic) – saturated temperature of the liquid line (obtained from P/T conversion of liquid line pressure).
Answer#2: Gage pressure is the pressure your manifold gages read which DO NOT take into account the atmospheric pressure (14.7 PSI @ sea level). Absolute pressure is a combination of your manifold gage pressure AND the atmospheric pressure.
Example: 0 PSIG = 14.7 PSIA
15 PSIG = 29.7 PSIA
Hope this helps…..
John J. Dalton CM
PS: The highest temperature of a refrigeration system is at the tip of the discharge valve inside the compressor (recip.), usually 50 to 75 degrees F higher than what was measured 6" downstream from the discharge line.
02-09-2007, 08:20 PM
First of all, everything John said is correct.
If by chance you are working on a chiller consider a few things.
Once you calculate the difference between the saturated condensing temp & the discharge line, you now know how much DSH you have, but the more important question is, how much should you have?
Several things cause the answer to vary a little, like wether it is a screw or a centifugal, how much its loaded or unloaded, also lookout for liquid injection as this can trick you on how much DSH you think you have.
The liquid injection (like on a McQuay) would cause the dischrge temp to be low.
At full load amps, I've seen many machines operate between 15* & 20* DSH.
At a very unloade state, such as 40%, I've seen the DSH as high as 40*.
Without any specifications on how much your suppose to have, a good log is your best friend.
If you are at FLA & your cooler approach is at a min, you wouldn't want to see less than 15* DSH & in some cases no less than 20*.
I'm thinking the reason you want to know about PSIA is maybe this is a York machine & your use to PSIG.(or the other way)
Lets say you have a cooler pressure of 16" & a condenser pressure of 5 PSIG.
Step 1, divide 16" by 2 (I know it somthing like 2.015, I never can remember, so I always use 2 because its close enough) -16 divided by 2 = -8 PSIG.
Step 2, -8 PSIG + 14.7 = 6.7 PSIA.
To convert 6.7 PSIA to what you are use to seeing, do everything in reverse.
6.7 PSIA - 14.7 = -8 PSIG. Then -8 x 2 = -16 or 16".
If the condenser pressure is 5 PSIG, add 14.7 which is 19.7 PSIA.
Or, 19.7 PSIA - 14.7 = 5 PSIG.
I'm sure someone can explain it better, but thats what works for me.
Hope it helps.
09-30-2007, 02:20 PM
Ona YORK with a subcooler I don't think you would go on the liquid line you will get subcooled liquid not saturated I beleive YORK says find liquid level by running your hand down condenser until you fell temp change go down about 1 inch this will be saturated temp .Some YK machines have pretty big built in subcoolers tha can do 8-12 degrees. you may assume you need to add gas when in reality you do not .Discharge superheat = discharge temp-saturation
air=actual saturated liquid subtracted from pressure of shell convertedto temp
09-30-2007, 02:37 PM
most chillers will measure the discharge temp and discharge pressure to obtain discharge superheat.
09-30-2007, 04:16 PM
agreed, the only problem i see there it uses panel saturation vs actual saturation if you have air this will be elevated. York used to have an actual sensor on the condenser for actual saturation temp back in the day but they told us to disable it due to the fact of if you were running cold tower water the liquid level could be considerably higher then they thought. so you would be reading subcooled liquid causing misdiagnosis ,not a bad idea to back up your panel readings wwith actual readings using temp gun .
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