Screw compressor oil temperature? 1

[greif1]

I have a small Paletek 10 hp oil flooded screw compressor, and it seems to run hot oil temperature. If the ambiant is 90 degrees F the oil can be 200 or 210 F.

Is this excessive? What is the danger temperature?
Thanks!
gr

 

[zdas04]

See thread1036-183642 (maybe I should write that FAQ). the short answer is that 200-210F is a perfect oil temperature out of the screw. If your 200-210 is oil temp into the screw then with any compression ratios at all will put you into a serious danger region. Mineral oils will break down somewhere around 215F, synthetics and semi-synthetics will be able to go a bit higher, but I wouldn't rely on it.

David

 

[crjones]

Generally at 100 psig discharge the discharge temperature is 100 degF above ambient. The cooling system if air cooled and you have the air exiting against a wall will lessen the flow and raise the temperature or if water cooled then there is insufficent water flow.

Your unit should have a high temperature shutdown switch that is set from 235 to 245 degF.

 

[zdas04]

crjones,
What is 100F above ambient based on? What inlet temp, what suction pressure, what gas, what oil inlet temp, what bullet in the oil-cooler bypass valve, what oil flow rate?

If you are assuming atmospheric air, then at what elevation? At sea level, the machine would be doing 7.8 ratios. At 5,300 ft it is doing 9.3 ratios. If the oil were not present then the gas would have gotten to 1.8 and 1.9 times inlet temp (in absolute units) respectively which is a 65F temp difference if you started at 90F.

For a specific machine and a specific suction and discharge pressure I can calculate the BTU's added by heat of compression for a given gas-inlet temp. I can calculate the rate of heat transfer to the oil based on some pretty gross assumptions about the portion of oil that is going to the bearings vs. the injection oil (if there is a constant-speed oil pump, if the pump speed is based on driver speed or if there is no pump then this is rougher). But knowing what portion of the oil is even going to the cooler is really a crap shoot.

The rule of thumb that I've used is that you get something like 35F oil temp rise for 10 ratios, but I've seen that number as low as 15F and as high as 60F.

I don't mean to pick on you, but that number doesn't make sense to me. Sorry.

David

 

[crjones]

The original design work on oil flooded screw compressors was based on 80degF air at 14.7 psia. THe discharge pressure was 110psig to 150 psig. The system was set up that you have oil cooling, sealing, and lubricating resulting in a 100 degF rise. A thermal bypass valve was put in the system to keep the oil injection at 140 degF or a 60 degF rise. The goal was to keep the oil temperature above the pressure condensation of water.

A 10 hp Palatek is a single stage screw compressor

 

[JRLAKE]

greif1,

Initially that does seem a little high to me. But what has the oil temp been historically? Has the machine been in service long? Is it installed in a dirty environment? Are the coolers clean? Is the thermostat working? Is the inlet filter clean? Are the inlet passages to the canopy blocked? Is the oil level ok?

I combat overheating air compressors on a daily basis. I agree with crjones. I would expect to see an 80 to 100 degree F increase in oil temp over ambient.

 

[greif1]

Initially that does seem a little high to me. But what has the oil temp been historically?
***The hotter the room temp the higher it goes! 190 to 210 if all is well is type, but in higher temps it wants to go higher and sometimes the safety kicks the power off.



Has the machine been in service long?
*** about 2 - 3 years
Is it installed in a dirty environment?
*** No, but is is a small room with a dryer and a backup piston compressor, so heat builds up. There is a fan driven 16" inlet of outside air and fan driven outlet so air flow is pretty good (but not enough!). The only way we keep the Palatek oil cool is to enclose the heat output of it and duct it out, and have a large diam pipe on the inlet and direct that directly at the Paletek.

Are the coolers clean?
*** Yes, cleaned often

Is the thermostat working?
*** Not sure, not sure it has one!

Is the inlet filter clean?
***** Something I haven't checked myself, but I think so,

Are the inlet passages to the canopy blocked?
***Hmmmm.... what's a canopy on a compressor?


Is the oil level ok?
*** Yes

I combat overheating air compressors on a daily basis. I agree with crjones. I would expect to see an 80 to 100 degree F increase in oil temp over ambient.
*** One problem is that the incoming air ducts go through another boiler room and pick up 10 degree extra on the way in.
gr

 

[crjones]

The typical overheating problem on the screw compressor is a result of inadequate cooling air flow. A 10 hp air cooled unit eqruires about 1000 cfm for cooling. The fan on the compressor generally only has about .25"water static head. In small rooms or ducted flow there is a need for more fans.

Also the temperature rise is based on the inlet air and cooling air being the same. If the air going into the cooler is too hot the oil going back to the air end will be too hot and then the discharge temperature will be higher. One way to check the cooler performance, the cooling air temperature rise is usually 25degF.

 

[JRLAKE]

I meant to say I would expect to see an 80-100 degree increase in the compressed AIR temp earlier.

Some compressors have package enclosures, aka canopies. If you restrict air flow through them, by placing a box or something over the inlet or outlet grating, you restrict cooling air flow.

Sounds to me like you have a room ventilation problem. I would also insulate your incoming air in the boiler room, make sure duct work is sized correctly, vent all cooling air exhausts outside and check your filters.

 

[greif1]

I thought I would relate the final solution to my oil temperature problems. Although I originally thought I had plenty of outside air flow (a long duct with fan to draw air from outside and another long duct with fan to push hot air out)the problem was one of ducting. Moving the same compresor so that it sat in a window frame to eject hot air directly outside (with another window 20 ft away providing make up air) made for a very stable and useable oil temp.
gr