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rosetennis
17-05-2006, 07:48 PM
We have two rotary compressors that service blast freezing rooms. The bearings on both compressors both went out at the same time. There were thousands of small metal flakes in the oil separator. We have since rebuilt the compressors (new bearings), had outside services (2 companies) rebuild and have run into the same problem. We are now thinking that liquid ammonia could be in the suction line drawn into the compressor. This would put pressure on the shaft and rotary causing the alignment to change and wearing the bearings prematurely. Is this possible? Has anyone seen this before and what would be the signs to know if liquid was actually being drawn by the compressor? Also, is there any chance that liquid ammonia could make it through the oil separator and into the bearing itself to cause more problems? any help greatly appreciated

Andy
17-05-2006, 08:09 PM
Hi Rosetennis

If your compressors have failed there may still be some steel in the system.

Liquid carryover would be the most likely, but it would be possible that condensed liquid in the oil would cause premature bearing failure.
Are the oil heater working in the oil sep

Is the discharge check valve after the sep working.

I am afraid to say rotary vane compressors are nearly a thing of the past, being quite inefficient and not well supported by service companies.

Beter to buy a new screw or piston compressor to replace the rotary.

Kind Regards. Andy

parry
17-05-2006, 08:11 PM
i would ask the copressor rebuild company they usually can identify the reason 4 ur problem

US Iceman
17-05-2006, 08:29 PM
Hi rosetennis,

Welcome to the RE forum. What type of facility do you have? Cold storage, production freezing, ???


The bearings on both compressors both went out at the same time.

This is usually a sign of a problem in the system. It is highly unlikely that both would compressors would fail at the same time if it was only a mechanical defect or wear.


There were thousands of small metal flakes in the oil separator

Have a sample of the flakes sent to a lab for analysis. They can tell you the composition of the metal. That way you know where the metal came from.

I'm assuming these are rotary vane compressors and not screws. Is this right?


We have since rebuilt the compressors (new bearings), had outside services (2 companies) rebuild and have run into the same problem.

It sounds like they are reacting to the problem and not fixing the root cause. Do you have engine room operator logs? Check and see if the discharge temperatures are very low during certain times. If the discharge temperature is pretty low you can figure liquid is running through the compressors.

My guess is (based on the information provided); you have a periodic flooding problem either caused by insufficient capacity on a low temperature accumulator, or the suction pressure is pulling down too fast causing the accumulator to carryover liquid back to the compressors.

rosetennis
17-05-2006, 10:37 PM
Iceman,

We are a public freezer facility. The compressors are rotary vane. I checked the engine logs and the discharge temp is 150 to 160F for these compressors. This is however the reading after the system has stabilized. Could be that on start up we are seeing much lower discharge temps for a while? We typically run at 12 inch vacuum These compressors would also be turned on and off a few times a week so it could perpetuate itself quickly. Any chance that gas ammonia could be recondensing itself in the oil separator after shut down and then being pulled through the oil pump when the system starts up again causing expansion at the bearing or pump?

Additionally, I checked the blast accumulator and the liquid level is actually not that far away from the suction line (suction line is not at top of accumulator but on the side near the top) and the suction line appears pitched towards the compressors.

Please advise. Appreciate your help very much

rosetennis
17-05-2006, 10:38 PM
Andy,

No oil separator heater on the equipment. I am assured the check valve is functioning correctly

Many thanks

Pete

US Iceman
18-05-2006, 01:26 AM
Hi Pete,


I checked the engine logs and the discharge temp is 150 to 160F for these compressors. This is however the reading after the system has stabilized. Could be that on start up we are seeing much lower discharge temps for a while?


Check the booster discharge temperature at startup. If the discharge temperature does not increase rather quickly you may possibly have liquid running through the compressor. Not a lot, but maybe enough to cause problems.


These compressors would also be turned on and off a few times a week so it could perpetuate itself quickly.

If the liquid level in the accumulator is too high when the rotary compressors start, the sudden reduction of the booster suction pressure can cause the liquid in the accumulator to rise (very violent boiling). This can suck liquid out of the accumulator and dump it right into the compressors.


Any chance that gas ammonia could be re-condensing itself in the oil separator after shut down and then being pulled through the oil pump when the system starts up again causing expansion at the bearing or pump?


According to the old Murphys' Law, anything can go wrong at anytime. Even if the engine room air temperature got down to 40F (4.4C) the refrigerant in the discharge line would probably still be superheated a little bit.

My guess is your saturated intermediate temperature is around 10F (-12.2C). For ammonia to condense in the booster discharge line, the air temperature in the engine room would have to be below the booster discharge line temperature. If the compressors were setting outside on a windy Chicago winter day, then maybe...

It would be more possible for the liquid to be sucked backward from the intercooler back into the compressor if the discharge check valve leaked a little.

Apparently the compressors have failed several times. When did it start? What happened in the engine room before the compressors started to break the first time?

Do you use liquid injection for desuperheating of the booster discharge gas or is the intercooler new?

If the intercooler was recently installed I have seen some new intercoolers where the discharge sparger (the pipe that drops down inside the intercooler below the liquid level) can siphon liquid back to the boosters. At the top of the sparger there should be a small hole for a siphon break. The manufacturers drawing should show this.


Additionally, I checked the blast accumulator and the liquid level is actually not that far away from the suction line (suction line is not at top of accumulator but on the side near the top) and the suction line appears pitched towards the compressors.

The accumulator should have an elbow turned up on the booster suction connection (inside the accumulator) so the liquid would have to travel up higher before it flowed into the suction line to the boosters.

I don't like the suction line pitched to the boosters, this helps to promote drainage of liquid.

If the boosters are shut down on a regular basis you could have condensation occurring in the suction line. Remember, the suction line is very cold at 12 inches of vacuum.

When the compressor shuts off, the suction gas pressure can rise a little bit. When the gas pressure increases, the saturation temperature of the gas is also increasing. If the the suction pipe temperature is lower than saturation temperature of the gas, the gas can begin to condense in the suction line.

As you can see, there are a lot of possibilities and questions here.

What was the condition of the barrel in the compressor?

Were the blades broken or cracked?

Does the suction line to the compressor come off of the top of the main suction header or off of the side?

Have you tried to close the oil drain valve on the oil separator? If liquid is draining through the separator it has to flow backwards through this line to return to the compressor.