Liquid Floodback

[ywz]

Hello everyone,

This is the first time I post a question here. I have a small flooded R404a system with a one HP compressor. The accumulator is big enough to hold the entire liquid refrigerant. My question is how to check if there is any liquid refrigerant flood back to the compressor.

When the system starts, I cannot measure any superheat in suction line. As the suction line is well insulated, I wonder if the oil return orifice on the accumulator is too big, some liquid refrigerant flowing back with the oil through the orifice cannot obtain enough heat in suction line to vaporize. Are there any simple ways to check flood back?

Thanks for reading my question.

ywz

[taz24]

Hello everyone,
Are there any simple ways to check flood back?

Thanks for reading my question.

ywz

You can measure superheat at any point along the suction line.
If there is a pressure drop across the suction read the pressure at the drum. If you are confident the suction has no noticeable pressure drop just measure pressure and temp at the comp.
You are just doing a pressure temperature relationship test at any point on the suction.
Expose the suction pipe just befor the comp and measure the temperature.

Cheers taz.

[aldematt57]

Hello everyone,

This is the first time I post a question here. I have a small flooded R404a system with a one HP compressor. The accumulator is big enough to hold the entire liquid refrigerant. My question is how to check if there is any liquid refrigerant flood back to the compressor.

What kind of amp draw are you getting on the compressor? I would want to know what the rated load amp draw is supposed to be and compare it to actual amp draw.

When the system starts, I cannot measure any superheat in suction line. As the suction line is well insulated, I wonder if the oil return orifice on the accumulator is too big, some liquid refrigerant flowing back with the oil through the orifice cannot obtain enough heat in suction line to vaporize. Are there any simple ways to check flood back?

Thanks for reading my question.
ywz

Let the system run for a period of time and then determine actual and required superheat readings.
Inadequate oil return is a possibility, but likely not because of a standard opening at the accumulator.
Are there any restrictions or oil traps?

[Brian_UK]

You say that you cannot measure any superheat. Do you mean that there is no measurable superheat or that you cannot take a reading?

Are there any symptoms that make you think that there is liquid flooding at the compressor?

[ywz]

Thank you for all of your inputs. In my test, the temperature profile shows when the compressor starts, the suction pipe temperature is about 0.8C lower than accumulator bottom temperature for a short period of time. This is the reason I think there is no superheat in suction line and wonder there exist flood back at this time. The amp draw is normal and no other symptoms show there is liquid floodback. I am working on a new product development. I have to convince my boss that there is no flood back in the system in all condition. Now I am thinking of adding a suction line heat exchanger to maintain some superheat.

ywz

[Brian_UK]

You need to compare temperatures to pressures recorded to measure superheat correctly.

[US Iceman]

I have a small flooded R404a system...

Well then, you will not have any superheat. With a flooded evaporator the superheat is zero. The only way you will see superheat on a flooded system is from heat gain in the suction line.

Adding other gadgets will only make your product more expensive.

The other thing to worry about is not the volume of the accumulator holding the entire refrigerant charge, but what happens when the accumulator is full (or partially so) during operation. You can still experience some problems if the accumulator is sized for 150% of the system charge. It just depends on how everything is sized and operated.

[monkey spanners]

If it is product development why not put a sight glass in the suction line (check for temperature aplication of sg) and see whats coming back just before the compressor.

[ywz]

I do put a sight glass in suction line. I will run the test this week. Based on our previous test result, we could found some liquid in the sight glass.

US Iceman, besides the accumulator size, I know that I also need to check the oil return orifice diameter. Orifice size and liquid pressure head in the accumulator will determine oil (and liquid refrigerant) return rate. Could you please advise me any other factors I need to concern?

Thanks
ywz

[US Iceman]

I don't place a lot of faith in those little accumulators. They seem to do OK on small systems, but their ratings are terrible on larger systems. That's been my experience.

You don't want the suction pressure to change quickly as an liquid sitting in the accumulator will violently boil during a pressure reduction. This can lead to liquid carryover with an accumulator which will really make you scratch your head for a while.

If the system is designed properly for flooded operation you should NOT experience ANY liquid flooding.

If you watch you actual discharge temperature at the compressor you can see when liquid or wet vapor is entering the compressor. The discharge temperature will start to decrease quite rapidly and then start to increase after the liquid is gone.

[ywz]

Hello US Iceman,

Thank you for your input. To monitor discharge temperature is a good way to check liquid flooding.

At the time to turn on the compressor, usually there is large amount of liquid refrigerant in the accumulator, the suction pressure drops down fast, and there is some liquid inside the dip tube. Due to these three reasons, we can see liquid in sight glass. Now I am thinking about adding a trap between the accumulator and the compressor suction port. Do you think it works?

Currently, the velocity in suction line is about 2500 fpm. I plan the increase the tube size to reduce the velocity to 1500 fpm. I hope it also helps to eliminate the liquid flooding.

Thanks
ywz

[US Iceman]

Hi,

It's hard to tell you exactly what to do since I do not have a full appreciation for what you have.

In a traditional flooded evaporator arrangement the accumulator will usually be fairly large in volume. It is common to find these with the vessel being partially full of liquid, so that when the compressor starts the liquid does boil, but it stays in the vessel.

It sounds like your problem is related to the actual size of the accumulator.

Another thought would to also use a VFD to start the compressor and then allow the speed to increase slowly. These helps to prevent the sudden decrease in suction pressure and reduces the violent boiling on start up.

Suction line velocity is needed for oil return, however, low velocity in the accumulator is needed to keep the liquid in the accumulator.

If there is any liquid in the accumulator (since you have a diptube) the compressor will suck the liquid right out. Because of this you will continue to see the flooded starts you are experiencing.

It sounds to me as if you have a design problem to solve.