Measure suction line pressure drop with thermometer

[Steve Wright]

I was on a job site a 20 ton split system and an engineer took a temperature reading at the suction outlet of the evaporator and another temperature reading at the suction inlet of the condensing unit.

He then declared that the pressure drop was OK.

How did he do that with no liquid refrigerant present in the suction lone?

[chemi-cool]

Hi Steve,
A good practise would be to measure pressure and temperature and check that it matches.

He must be a magician

Chemi

[US Iceman]

Hi Steve,

I can only guess at what this person was doing. Here is one:

1) He was converting the two temperatures to the saturation pressures at both locations and assuming the difference in pressure was due to the suction line piping.

This would have to be based on more rules of thumb and assumptions to determine the pressure loss than I care to imagine.

If he was looking for liquid refrigerant in a system, would he use a "divining" rod? (Sorry, I could not resist that comment)

I would have rather seen him estimate pressure drop by using the unit capacity and referencing a pipe pressure loss chart for the refrigerant used, pipe size, and refrigerant.

I'm not sure how much this helps you, but this is my best guess.

Regards,
US Iceman

[Argus]

He was converting the two temperatures to the saturation pressures at both locations and assuming the difference in pressure was due to the suction line piping.

This would have to be based on more rules of thumb and assumptions to determine the pressure loss than I care to imagine.

I came to that conclusion. There may have been a confusion in methodologies. It's certainly an unreliable way of estimating pressure loss.

As I recall some of the old text books, suction lines were sized on a pressure loss usually given as the equivalent pressure to less than 2 deg F. (1 deg C) in most instances.

The problem with measuring temperatures along lengths of pipes is that you don't know how much heat is picked up en route.

The only reliable way to measure pressure in my estimation is with a gauge or manometer.

.
________
Jean Behra

[chilly]

I agree with above, He was just converting temp to presure and asuming increase in temp was convertable to presure increase, But not taking into acount temp picked up along the way as Argus mentioned.

Im just wondering why was he looking to measure the presure loss, All suction lines will have some amount of presure loss as will the liquid line and any pipe in the system, Why was he measuring the loss, was there a problem with the system,

Just wondering

[Argus]

Why was he measuring the loss, was there a problem with the system,

Just wondering

If he was carrying out a survey, or acting as a consultant, it's part of the brief: it's called 'Justify the Fees'

.
________
glass pipes

[Steve Wright]

To All

He was checking for undersized lines or crimps. I think line size was 21/8".

This guy is a seasoned professional engineer. There were various groups involved(owner, architect, factory, installing contractor, and me the after fact consultant to installing contractor). This guy represented the factory and is really a good engineer.

What he was saying validates proper installation by my customer so I Didn't want to ask too many questions and throw water on the party.

The problem ended up being mis applied equipment.

Steve

[US Iceman]

Hi Steve,

This almost sounds like the typical architect selection for refrigeration systems. 20 Ton condensing unit requires 20 Ton evaporator capacity. Good for any temperature.

I have seen this happen several times, and in every case problems were the result.

For anyone reading this, please don't take this seriously as a method to select refrigeration equipment and expect it to work. Components must be carefully selected for the exact operating conditions.

It sounds like this was a finger pointing contest. Not too much fun....

Can you share the nature of the problem Steve?

Regards,
US Iceman