Return suction diameter

[sdumper]

What are the size limits for returning refrigerant to the compressor?

What I am trying to determine is if a return suction line diameter of 1/2 inches (12.700mm ) and a length of 5 feet (1524 MM) is to extreme.

Return gas r14 (75%) /r1150 (12.5%)/r170 (12.5%) blend...

[Patrickclouds]

the limit of the suction line is the refrigerant velocity that should be at around 7m/s in vapor rising pipes to ensure that the oil comes back to the compressor. if you have a continously falling pipe and no rising pipe diameter doesn't matter.
you can go under this 7m/s if you are sure that the oil flows back to the compressor. I've seen on an experimental unit a velocity of 2,8m/s that was enough to get the oil back through a rising vapor suction pipe.
You should also care about that above when you are using an oil seperator because they have an efficiency below 100%.

[n00b 0f l337]

Since you have an oil separator you should be fairly good. But over time it can be an issue.
Either way though, many cryogenic process vessels are attached by 50ft of suction that is often 1/4" or so.
Chances are you'll be just fine.

[sdumper]

Good to know guys thank you.

Just had another thought...if return oil is a potential issue (ab 150 freezes at approx -74c) wouldnt I get an early warning long before the compressor became an issue?

Sounds like I could simply shut it down go through a de-ice and start it back up every day or so without any issues.

[wdrzal]

Add a hot gas bypass to the evaporator. Regulate the amount of hot gas so the evap warms to say -50c for several minutes every 30 to 60 minutes or as determined by testing. you can use a solenoid valve and a PID timer. Or you can design a evap with electric heating cartridges (like load testers use) then put the electric circuit on a timer or flip a simple toggle switch to defrost............... Or as you said you can just shut it down.

[teyber]

the limit of the suction line is the refrigerant velocity that should be at around 7m/s in vapor rising pipes to ensure that the oil comes back to the compressor. if you have a continously falling pipe and no rising pipe diameter doesn't matter.
you can go under this 7m/s if you are sure that the oil flows back to the compressor. I've seen on an experimental unit a velocity of 2,8m/s that was enough to get the oil back through a rising vapor suction pipe.
You should also care about that above when you are using an oil seperator because they have an efficiency below 100%.

Do you have a clever way of measuring this? Or have you hooked a mass flowmeter to a system? Ive been searching for a less expensive mass flow to do some system controls. maybe you could shed some light

[Patrickclouds]

yes you need a mass flowmeter to meassure this.

analog massflow meter which is placed in the liquid line before expansion valve.

or a digital sensor somewhere else in the circuit, but that is very expensive.