Suction Line Pressure Drop

[CamLoch]

What is the result of excessive pressure drop in the suction line? I typically have seen suction line design for 1-2 psig drop.
We have a bus AC system measuring 25-35 psig drop from evap out to compressor suction. The line sizes are obviously too small and that is being changed. However, I received reports the oil site glass in the compressor is "foaming". Can excessive pressure loss and or excessive superheat contribute to this phenomenon?

 

[zdas04]

A compressor provides "compression ratios" like a pump provides "head". Hp requirements, discharge temp, and compressor capacity are all a function of compression ratios. If you lower your suction pressure without changing your discharge pressure then ratios go up. Hp required goes up. Discharge temp goes up. Compressor capacity goes down.

You didn't say what the technology of the compressor is so I can't tell you if the foaming you see in the "oil site glass" is part of the problem. Is that the supply oil to the recip cylinder or a discharge coalesser on a flooded screw?

David

 

[CamLoch]

David,

Thank you for the response....
The compressor in question is a Bitzer F400 reciprocating "transit" compressor. The oil sight glass is located in the compressor sump and is for cylinder lubrication. The Bitzer compressor is used to replace two (2) TM-21 swash plate automotive compressors. The system seems to be running (cooling) well save for the lack of efficiency. I am just trying to find possibilities for the "foaming" oil as the customer says this does not happen with similar systems using a Carrier transit (reciprocating) compressor.

 

[vpl]

One of the causes of "foaming oil" can be water in the oil. Since this a compressor, I don't know if there is any source of water; but maybe a possibility is air mixing with the oil would also cause foaming?

Patricia Lougheed

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[CamLoch]

VPL,

If there was air in the system, our pressure/temperature ralationship would typically show unusual readings and we are seeing "expected" discharge and suction pressures for the ambient conditions.

 

[Compositepro]

It seems reasonable to expect the oil to foam in the crankcase if the pressure is significantly lower than the evaporator. Any refrigerant dissolved in the oil will boil.

 

[CamLoch]

Compositepro,

That was my thought also. Once the cold refrigerant gas mixes with the hot compressor crankcase, any refrigerant absorbed in the oil will boil off.

 

[hacksaw]

how is the suction line routed? You may be getting a trapped section with oil in the line.

 

[dcasto]

mr zadas04, you needed to finish you statement. Since the HP goes up with more rations and the capacity goes down, the NET energy decreases if the original ratios are about 2 or more. If the original ratios are less than 2, then the NET energy use will increase.

See you May 27th, GPA?

 

[zdas04]

Not a member of GPA, never been a processor.

David

 

[chicopee]

Have the oil tested for contaminants to determine cause.

 

[gxmplx]

I have little experience with Bitzer compressors but in order of priority you could have one or more of the following:

1) Liquid refrigerant flood back would be the first thing to check by measuring superheat at compressor suction. Discard this choice if it measures 10F or higher (lower values are possible with accurate instruments and good measurement practice).

Yes this is related to pressure drop if the system uses a fixed orifice as expansion device or fast acting expansion valves like gas charged.

2) If the lines are too small you could have large foaming before the refrigerant enters the compressor and high chance you have liquid refrigerant droplets mixed with the oil and low superheat. You would need a temporary sight glass before the suction valve to check for this. Installing a suction accumulator would help to remove some droplets but not all due to high refrigerant velocity that makes the separation process less efficient.

3) Ask Bitzer and/or the system manufacturer if they use foaming agents in the oil, specially if it uses PolyOlEsther oil.

4) Looking at the compressor blown drawing at Bizer´s website I see you could get lots of foaming in the sight glass should the crankshaft rotate as to hit the oil in the sight glass direction. This would also depend on RPMs and oil level in the crankcase

5) You could have a worn oil pump IF the compressor has one (can't really tell from the drawing). Splash lubrication definitely causes foaming and more with POE.

Discharge temperature and capacity tell you whether suction pressure drop is excessive or not.