suction line traps

[joep]

Can anyone remind me why have a suction line trap straight out of an evaporator in a room? i.e at the bottom of the riser.

[US Iceman]

Traps in refrigerant piping are used for the purpose of providing an area for oil collection. When the oil accumulates in the trap it will eventually create a seal (blocks off the pipe inside diameter). When the pressure behind the seal increases, the liquid oil is pushed up the riser so that the oil can return to the compressor.

Where the traps are located and their spacing between traps depends on where the equipment is located (above or below the compressor).

[joep]

Traps in refrigerant piping are used for the purpose of providing an area for oil collection. When the oil accumulates in the trap it will eventually create a seal (blocks off the pipe inside diameter). When the pressure behind the seal increases, the liquid oil is pushed up the riser so that the oil can return to the compressor.

Where the traps are located and their spacing between traps depends on where the equipment is located (above or below the compressor).

Thanks mate, I knew that much but more to the fact is there any advantage of having the trap at the riser instead of else where in the pipe run.

[US Iceman]

The traps are at the riser so that you can move the oil. Otherwise, without the traps you are dependent on the gas velocity to return it in much smaller amounts. It may or may not return in sufficient quantities to maintain the oil level in the compressor.

[John Smith]

Traps in refrigerant piping are used for the purpose of providing an area for oil collection. When the oil accumulates in the trap it will eventually create a seal (blocks off the pipe inside diameter). When the pressure behind the seal increases, the liquid oil is pushed up the riser so that the oil can return to the compressor.

Where the traps are located and their spacing between traps depends on where the equipment is located (above or below the compressor).

Our new lecturer at bath college taught us that traps in suction lines are only to prevent large slugs returning to the compressor. He said they cannot help oil return and actually reduce oil return and system efficiency. And the trap on a evaporator is only for compressor startup so that oil and liquid dont effect the txv sensor.

[US Iceman]

Our new lecturer at bath college taught us that traps in suction lines are only to prevent large slugs returning to the compressor. He said they cannot help oil return and actually reduce oil return and system efficiency. And the trap on a evaporator is only for compressor startup so that oil and liquid dont effect the txv sensor.

Sorry to upset the apple cart, but I disagree.

Think about what a trap is: It is a place to collect liquid (could be refrigerant or oil, but it is intended for oil). In a ***** system the oil is moved by velocity. That is why we go through the trouble of trying to size pipes. We are trying to balance the need for velocity to move the oil versus limiting pressure loss in the piping so that performance or capacity is not affected.

As the pipe trap accumulates liquid the velocity through the trap increases. If the mass flow decreases due to lower system load, the velocity decreases through the trap and it begins to seal. Once that happens the pressure in the upstream side of the pipe increases. When a sufficient pressure difference is created across the trap, the seal blows out and the oil slug moves downstream.

The trap is there to allow the oil to collect until the trap seals. All of this is necessary to allow the oil to circulate in the system in a controlled manner so that the oil returns to the compressor.

You want the trap after a TXV bulb so that the oil does not insulate the bulb and affect its temperature sensing which could affect the TXV (could cause hunting on the valve).

[John Smith]

Sorry to upset the apple cart, but I disagree.

No it is very interesting. Our new lecturer Marc O'Brien started a lecture telling us that there were still a lot of people who thought that suction traps help return oil. He is a consultant but lectures one day a week for level 3.

I have typed out the notes from two of his powerpoint slides.

In the design of high or low pressure vapour lines oil and condensed vapour slugs require management.

This applies to suction, discharge, hot gas and latent gas lines.

Generally we want to slope in a downstream direction except in the vicinity of compressors or on the compressor side of surrounding traps where we must always slope away from the compressor for off-cycle protection.

A suction or discharge riser that fails to properly carry oil will not be improved on by the addition of traps in the rising section, as shown top right. Adding such traps to risers will in fact worsen the oil carrying abilities by adding friction thereby reducing vapour velocities.

Traps in riser sections only help to limit the size of oil slugs arriving at the compressor during or after the off-cycle.

Then, seen bottom right, traps are not required if there is an oil separator at the base of the riser. Nor are double risers required.

If there are problems carrying oil up any riser section, suction or discharge, then apart from the installation of a double riser, the best solution is simply a reducing in the diameter of the riser.

In the illustration to the right the reducers and elbows are configured so as to minimise the oil quantities draining back upstream from the base bend or draining back down the riser from the top horizontal section.

Traps along the riser can be added to also provide oil slug size management but their addition will not assist the riser in carrying oil – there will be no improvement in oil lift.

He also says that adding traps to a system that does not return oil does not add a risk of oil slugs hitting the compressor even if the traps are very big.

I can tell him about this forum if you want.

[US Iceman]

I can tell him about this forum if you want.

John,

Don't do anything rash!

Mr. O'Brien used to be a moderator here and left after some discussion. I'm not going to get into specifics in a public venue about this, and would prefer to let sleeping dogs lie.

Double risers are only required if the system/compressor has capacity control. If the system load decreases you need to have a method for unloading the compressors (speed or displacement). When this is done the swept volume through the suction line decreases and reduces the piping velocity. That is why you need double risers (suction or discharge) for oil return back to the compressor. Otherwise, the oil will simply lay there until the gas velocity increases to the point where the oil slug moves.

If you have vapor condensing in a gas pipe you of course need to provide a method for minimizing the liquid slug once it begins to move in the system. This happens a lot in steam system and is called drip draining or something similar to that. This is something different from an oil slug from a P-trap.

You could say an oil separator at the bottom of the riser would work, if you can return the oil automatically to the compressor. There a lot of ways to manage oil in a refrigeration.

The best way is to do it such that no gadgets are used or a method that requires human intervention to make it work.

[Peter_1]

Our new lecturer at bath college taught us that traps in suction lines are only to prevent large slugs returning to the compressor. He said they cannot help oil return and actually reduce oil return and system efficiency. And the trap on a evaporator is only for compressor startup so that oil and liquid dont effect the txv sensor.

If this should be true, then all the refrigeration books from the last 50 years has to be rewritten.
Why an oil trap should prevent for liquid slugging? It's just the opposite: liquid can accumulate as as soon area is reduced enough, all the liquid is pushed through it at once.
And the trap on a evaporator is only for compressor startup so that oil and liquid dont effect the txv sensor
This is not worth arguing on it because both doesn't have a relation with each other.

[John Smith]

If this should be true, then all the refrigeration books from the last 50 years has to be rewritten.
Why an oil trap should prevent for liquid slugging? It's just the opposite: liquid can accumulate as as soon area is reduced enough, all the liquid is pushed through it at once.
And the trap on a evaporator is only for compressor startup so that oil and liquid dont effect the txv sensor
This is not worth arguing on it because both doesn't have a relation with each other.

Hi Peter1

Not liquid slugging. Slugs of oil.

Marc O'Brien showed us image 19.5 on page 455 in my RJ Dossat. I have the 3rd edition. It shows the txv bulb must be installed 30cm up the vertical riser if a trap cannot be used. If a trap can be used then the bulb can be installed before the trap on a horizontal. The trap collects oil during the off cycle so that it does not interfere with the bulb when the compressor starts.

[Peter_1]

Hi Peter1

Not liquid slugging. Slugs of oil.

Quote from your post In the design of high or low pressure vapour lines oil and condensed vapour slugs require management.
Condensed vapor is liquid for me.
And an oil bend in the discharge isn't there for oil reasons like he said. (4th line in the same message)

[joep]

Thanks for your time mate.

[sedgy]

yes the reason for suction line traps is to assist the oil to return to the compressor if the evap is much lower than the con.d unit the traps are normally 3-4 meters apart, and so the oil slugs up the suction line trap by trap

[Tung]

Dear US Iceman:
The velocity is important for oil return to compressor, then how can I uncrease the velocity? thanks!

[US Iceman]

Dear US Iceman:
The velocity is important for oil return to compressor, then how can I uncrease the velocity? thanks!

Make the pipe diameter smaller.

[NoNickName]

Or use double riser

[chemi-cool]

Another useful thing to do is to connect a small diameter pipe [3\8] from the bottom of each oil trap to the top of the next oil trap.

the pressure in the small diameter pipe is slightly higher and the oil goes up faster.

[Tung]

Dear US Iceman:
What I mean is if the suction pipe size is fixed, then is there any other method that I can increase the velocity?
will it work to throttle cooling water to increase the con'd pressure and make the pressure differential between con'd and evap larger?
I am talking about the chiller with a flooded evap.
Thanks!

[US Iceman]

Dear US Iceman:
What I mean is if the suction pipe size is fixed, then is there any other method that I can increase the velocity?
will it work to throttle cooling water to increase the con'd pressure and make the pressure differential between con'd and evap larger?
I am talking about the chiller with a flooded evap.
Thanks!

If you have a flooded chiller that has an accumulator on top of it you do not need traps in the suction line. You need some way to recover the oil directly from the shell of the chiller (assuming the refrigerant is in the shell & on the external surface of the tubes). This is an entirely different issue.