Sub cooling from discharge pressure?

[mixsit]

I'm sure this has likely come up at the site befor but a quick search didn't bring it up..
My question is from hearing a tech tell me to get sub cooling he was trained to use the liguid temp from the saturated discharge pressure temp.
I've seen dischagre pressures range 15-35lbs above liquid.
What the heck?

[powell]

I'm sure this has likely come up at the site befor but a quick search didn't bring it up..
My question is from hearing a tech tell me to get sub cooling he was trained to use the liguid temp from the saturated discharge pressure temp.
I've seen dischagre pressures range 15-35lbs above liquid.
What the heck?

He is incorrect. You want to measure the liquid line pressure/temp to get subcooling. The term "discharge pressure (compressor hot gas line)" is sometimes referred to as the liquid line pressure in error.

[coolerinfrederick]

well, measuring discharge superheat could indicate issues and alot of chillers do measure it, but this guy basicly has no clue what he's talking about.

[rundawg]

My question is from hearing a tech tell me to get sub cooling he was trained to use the liguid temp from the saturated discharge pressure temp.

The discharge line is Super Heated Vapor, not saturated liquid.

Stick with measuring subcooling on the liquid line (after the refrigerant leaves the condenser).

We can only hope this is what he means, but he has a hard time explaining it.

[hvacrmedic]

I'm sure this has likely come up at the site befor but a quick search didn't bring it up..
My question is from hearing a tech tell me to get sub cooling he was trained to use the liguid temp from the saturated discharge pressure temp.
I've seen dischagre pressures range 15-35lbs above liquid.
What the heck?

It's easy to get confused when using terms like SCT and SDT. What these are, as far as we're concerned, are the temperature corresponding to the pressure as read directly on our gauges. With that definition it follows that

SC = SCT-LLT, or equivalently SC = SDT-LLT.

It is taken for granted that, according to instructions, you've measured the pressure at the same point that you measured temperature, and that you've measured these either at the condenser outlet, reciever outlet, MD inlet, etc, depending upon where it is you desire to know the SC. HTH.

But, in most cases you can use discharge pressure and LL temp on resi or light commercial units to calculate SC "close enough". This is right in the service manuals and many textbooks. More than a 5psi drop through the condenser would be excessive.

[hvac wiz 79]

That's why it irks me when manufacturers don't install service access on the liquid line in rtus . . .

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[Six]

Educate him on the uses of Discharge Super Heat to trouble shoot a refrigerant issue.

[mixsit]

...It is taken for granted that, according to instructions, you've measured the pressure at the same point that you measured temperature, and that you've measured these either at the condenser outlet, reciever outlet, MD inlet, etc, depending upon where it is you desire to know the SC. HTH.

But, in most cases you can use discharge pressure and LL temp on resi or light commercial units to calculate SC "close enough". This is right in the service manuals and many textbooks. More than a 5psi drop through the condenser would be excessive.

Indeed. This happened to be a 30 ton split and yes, I'll use discharge for 'close enough or go-no-go estimating. Where there's a bunch of units and you know the typical pressure difference or have a track record that would be a decent example in a 'close enough situation.

Thanks all. Guess I just needed a cross check/re-affirm from that 'Everything you know is wrong' feeling!
This is not measure/document sub cooling.

[Dallas Duster]

That's why it irks me when manufacturers don't install service access on the liquid line in rtus . . .

Sent from my HTC Sensation 4G using Tapatalk 2

Must be a Trane.

[hvac wiz 79]

Must be a Trane.

Yep . . . . Then today i open a small precedent & find one on the liquid line. Maybe trane is getting better lol

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[will smith]

Carrier roof top units for example, won't give you a liquid line access port if the unit has a cap tube/piston feed. You'll be charging based on superheat anyway. It's nice to be able to check both, but not necessary in their accountant's mind-it saves them .09cents per unit/circuit by leaving out the access port.