Crankcase cooling

[mdossaji]

I am developing an oil-free air compressor and would like to get some feedback from anyone who has any experience in cooling. Inside of my crankcase seems to get quite warm around 250F. Was wondering if anyone has any idea how to cool the housing using natural convection. I have a fan placed beside it to provide forced cooling to my cylinders.

 

[bcs5274]

oil cooler?

 

[iHydrogen]

It would help if you provide a bit more information on the application. It sounds like you are using a piston-type compressor, how many stages does it have, and what is the pressure ratio per stage? Are you sure the heat is originating in the crankcase? you think it is likely being caused by friction in the crankcase and not compression in the cylinders?

 

[CaracasEC]

One of our gas compressor, piston type, 3 stage, vertical type . It has a jacket cooling water. However this are just on the bearing housing, cylinder and cylinder head and not the crankcase where this will be cooled itself by the lubrication system as the crankcase provide the sump for the compressor oil.

 

[mdossaji]

The application is a 2-stage reciprocating oil-free air compressor. The pressure ratio per stage is 3.45. I am sure the heating is due to the blow and friction from the rings but since there is no oil inside the cranckase, the chamber gets quite hot.

I was thinking of bringing in ambient air through an orifice and cooling due to expantion it as it enters the cranckase.

 

[CaracasEC]

An oil-free air compressor means that the sweep of the piston is not lubricated by oil but the piston ring itself, the wear band on which it is the material usually teflon and is self lubricated. However, what is the design of the lube oil system. Is there a separate system such as the API 614 requirements? Inside the crankcase is where the crankshaft and the connecting rod where circulating oil is there?

 

[mdossaji]

The connecting rod bearing is sealed with grease so there is no lube oil system.

 

[CaracasEC]

Is your proposal for the cooling of the cylinders? Or for the crankcase? Or Both?

 

[mdossaji]

its for the crankcase cooling assuming the heat generated inside the crankcase is coming form the bearing friction. Since the rings are polymer based, do you think the heat during compression is not releived through the cylinder wall causing the pisotn to overheat?

 

[CaracasEC]

Though the rings are polymer based, they still generate heat due to friction. Depending on your operating temperature, if the ambient temperature delta T to your operating temperature is not big then cylinder cooling is not required. Usually it is the 2nd stage cylinder that have a cooling water jacket. Also, the heat form the cylinder which will experience by the piston shaft to the crosshead to the connecting rod to the crankcase is very minimal that be considered that this was the cause of your crankcase getting warm. Please review your OEM what is the design limit temperature of the crankcase, if you found that it almost exceed then cooling it with air i think is not preferred. As you will know, air is corrosive especially if the humidity is high especially on an industrial plant setting.

 

[mdossaji]

The ambient temperature delta T to my high pressure discharge is 300F even though I use forced air cooling from the fan that is attached to the crankshaft turning at 1750 RPM. My loew pressure discharge delta is 190F from ambient.

 

[CaracasEC]

At this delta T, you don't have cooling jacket water for the cylinders? If yes, they should have hot insulation? Please review first your specifications if the crankcase temperature is within allowable limit.

 

[mdossaji]

Crankcase temp is within allowable limit of 220F. Although I have seen my connecting rod crank bearing getting up to 300F.

 

[CaracasEC]

If the crankcase temp is within allowable limit, then you don't have to modify the compressor cooling design. Please remember that your bearing such as the connecting rod is an antifriction type as you have stated. Modifying the temperature inside your crankcase will affect the temperature on your outer race that could change the bearing preload. The temperature inside the crankcase was i think consider by the compressor designer in consultation with the bearing manufacturer to compensate any thermal expansion between the bearing outer race to bearing housing. For example you have cooled the crankcase below the minimum temperature requirement, then the outer bearing casing will thermally contract and the outer race preload will increase down to the ball to the inner race and may cause a problem.

 

[gurmeet2003]

This design seems a little unusual. All the compressors that I have seen have some form of oil lubrication in the frame. For low hp compressors the lubrication is splash with a dipstick mounted on the connecting rod. Higher hp machines usually have a force fed lubrication (with sleeve type bearings). However I do not mean to imply that there is anything wrong with the design.

With force fed lubrication the oil is usually cooled outside the frame in an oil cooler (air or water cooled). This helps to cool the frame. Maximum oil temperature in this type of design is around 180 deg F.

There are three sources of heat in the frame. One is the bearings; both main and big end, where the heat will be generated. If the cylinder is single acting there will be some blow by of hot gas into the frame from the cylinder. This will not happen for double acting cylinder. Because the gas gets hot during compression, the discharge side of cylinder gets hot. Some heat is always transmitted by conduction from the cylinder to the frame.

From design point of view there are probably two areas that could be affected by high temperatures:
1. bearings
2. crankshaft seals if any

This temperature is not high enough to affect the allowable stress in cast iron (it would be worth while checking this).

You said you are developing an oil free air compressor. Are you developing it from scratch. Since you are using grease lubricated bearings, I am assuming that you are developing a small compressor. How many hp is it? I have lot of experience in design, analysis and testing of reciprocating compressors of all sizes. If you need any help you can contact me on linkedin.


Gurmeet
Time is an illusion. Now is the only thing there is.

 

[mdossaji]

Gurmeet,

I feed the intake air from the inlet filter into the crankcase and then guide the air out to the low pressure inlet. This allows my crankshaft drive to be little cooler but the heat picked up by the filtered air eventually raises my low pressure intake temperature to 200F whcih does not help with my efficiency.

Please provide me your linkedin contact.

Thanks

 

[gurmeet2003]

Mdossaji,
Here is my public profile on linkedin:

http://www.linkedin.com/pub/gurmeet-cheema/10/b8/847

Gurmeet

Gurmeet
Time is an illusion. Now is the only thing there is.