to use or not use p-traps?

[chilltech]

hi guys i am installing a roof mounted condensing unit 4 metres above a coldroom with dual evaporators using r404 refrigerant.unit is supplied with an oil seperator installed.the design engineer at the manufacturing company has asked us to install a p trap on both the liquid and suction lines at the base of the 4 metre rise up to the condensing unit,just before the pipe runs split to feed the 2 evaporators.i am not an engineer just a good old fridgy so i would like to know if this is the best way to install this system as normally we would only use a p trap on the suction line.also if this is the best method could you explain the reasoning behind it as i like to understand why we do the things we do.:)

[750 Valve]

um, ah, you might want to point out to the "engineer" that liquid flows from the condensing unit TO the evaps... thats 4m downhill, gravity is your friend in this case and no traps in liquid line are necessary.... actually traps are not needed in any liquid lines, all they do is provide additional pressure drop, even if the liquid was to rise 4m the only design condiseration is in the pressure difference created by that 4m rise in relation to gravity (pressure = density x gravity x height). The only reason you need traps in the suction lines is to allow the oil to pool and pretty much "hydraulic" its way up the riser with velocity in the line, discharge lines also need riser traps if they are a big rise but they have a greater velocity so you do not always see them as its easier to move that oil along, but as the oil is miscible with the refrigerant it is not required at all with liquid lines

[SteinarN]

I have never understod the reason to use p-traps. I have never installed one myself, and if I'm to do some piping work on an existing system, I always remove any which might happen to be there.

People says the reason to install these traps is for the oil to acumulate or pool there before it flows along with the refrigerant gas up the pipe :confused::confused:

Once more: acumulate.

I have always tried to design the pipes so that the oil will not acumulate anywhere. If the gas speed is sufficient, that is say 4m/s for small diameter (3/8") pipes and up to 12-14m/s for large (2-1/8") pipes, then the oil should return to the compressor without problem. At the same time, if the gas speed is to low to return the oil, how can a p-trap help it? The gas speed doesn't increase and the oil doesn't become thinner or defy gravity.

Actually, I will argue that the oil in fact do not acumulate at any p-traps. It flows through any p-traps as it flows through any other section of pipe as long as the gas speed is sufficient.

The only reason I see to install p-traps is on very long (more than 10-20meter) vertical pipes. When the compressor stops, the thin oil film covering the inside of the pipe will start to slowly creep down the pipe. If the run time of the compressor is very short, then it might be possible that it start to acumulate some oil at the bottom of that section of pipe. But the solution to this problem is not to install a p-trap at the bottom of the vertical section, but instead in the middle of this vertical section, eventually several traps if the vertical rise is very high. If the compressor run time is not very short, I can hardly see any case where it should be necessary with p-traps.

I suppose I will be taken on for this, but it is how I see it.

[Shamel]

Hey, Chilltech. I am also new on this site. I am Also just a fridge mac but i would agree with the response 750 posted. The liquid line poses no difficulties regarding slope and position for oil movement.The suction line however, shoukd have a downward slant to a certain point, then a short u-bend (trap) & then up. the reason for this is because the oil will build a positive flow back to the compressor, (something like priming a pipe to pull water,oil etc.) the oil collects in the u-trap & the suction of the compressor draws this oil back to it.

[Ray123]

We have a case where the design of the plant did not include any p-trap before the vertical rise and the pipe diameter is too large for good oil return. Compressors are dying young and hard for the past few years.

It's a hypermarket which evaporators are located at the basement, and compressors are located at 3 level high. Probably 12 ~ 20 m in height from evaporators.

What do you think is a better way to modify the system for a better oil return? Will it help if a p-trap is installed before the vertical rise, then additional small diameter tube to tap oil from the bottom of p-trap and joint to suction header.

[750 Valve]

We have a case where the design of the plant did not include any p-trap before the vertical rise and the pipe diameter is too large for good oil return. Compressors are dying young and hard for the past few years.

It's a hypermarket which evaporators are located at the basement, and compressors are located at 3 level high. Probably 12 ~ 20 m in height from evaporators.

What do you think is a better way to modify the system for a better oil return? Will it help if a p-trap is installed before the vertical rise, then additional small diameter tube to tap oil from the bottom of p-trap and joint to suction header.


you really need to look at the capacities that each line is carrying at both high and low load situations, it may be as simple as installing a trap, it may require a smaller diameter pipe to increase the velocity, but it may also require double risers to meet the low and high load requirements in terms of achieving the velocity without sacrificing pressure drop. Be aware of the total system pressure drop when selecting risers, I like to keep them around the 1K as per our customers specs, no more than 2K or you begin to have issues with equip selection (need to size them 2K lower in SST as well to achieve capacity required)