Annoying Compressor fault....?

[Iron Snail]

Hi all.
I have a older direct drive unit similar to a AIRMASTER 8/250, this has a overload/trip fitted to the top of the motor casing. In cold weather like now, it takes 3 or more attemps to get it to run , reseting the trip each time to get the the oil stirred up , have changed the oil ( 40 SAE grade Clarke Comp oil) Make no differnce with fresh oil or an empty tank.I Have come to the conclusion, its thick oil overloading the motor on a cold start.

Question -- could one use a multigrade mineral oil , or is compressor specific oil only required.

 

[brightspark]

any oil is good oil I use anything I have on my 3 comps

 

[the snooper]

well weve been told its moisture in the air and ive illustrated this theory below
soaking wet air is drawn in, and along with the ship in the cylinder it proves hard to compress, but the final straw comes when the ship hits the cylinder head, in summer when the air is dry, the ship isnt so high in the cylinder and so allows the piston full travel and all operates fine

 

[brightspark]

I flog the guts out of my comps sandblasting. they run for ages dunno what a duty cycle is when they pack in il know as said any oil is good oil

 

[Mee]

If you push a piston into a cylinder that has a vacuum (density = 0) in it you'll get no resistance, then if you force air into the cylinder the higher the pressure means increased density and at the same time the piston would be more difficult to push.

It's the pressure rather than the density that gives rise to the resistance, but you can't have higher pressure without higher density (at a given temperature/volume).

So perhaps sucking higher density air into the cylinder and then compressing it would take more effort.

But in the context of the original topic, I reckon the effect of denser air is insignificant compared to the effect of thick oil. What we need to do is try to start a problematic compressor on a cold day whilst feeding it with hot air.

By jove, someone gets it. It isn't insignificant as it can vary the starting load by up to 20-25% depending on climatic conditions and ambient temperature variation.

 

[Crater Creator]

By jove, someone gets it. It isn't insignificant as it can vary the starting load by up to 20-25% depending on climatic conditions and ambient temperature variation.

20-25% based on cold air? Where on earth do you get that figure from?
With respect that is absolute nonsense. As are many of your posts.

 

[Mee]

Yes, you're right, the density is higher because of the pressure, which is what creates the load.
In my opinion pumping losses due to denser intake air would be negligible, at the low flowrates we're talking about.
I'm ready to be wrong though, if Mee can provide a link to the source of his wisdom.
I think the weight of the oil when very cold is 99% of the problem.

It works at all flow rates and is directly proportional, pumping losses are insignificant as the pumping pressures to density rates are proportional and the more you pump gases up to higher pressures the more air density matters, look up the Bolsche formula which is the accepted formula for compressor designers.

 

[Crater Creator]

Still no link. I'm out.

 

[Mee]

Air Compressors and Cold Weather

Air Compressors and cold weather

Air Compressors are generally designed to operate in ‘normal’ European ambient temperatures between about 5’C and 35’C. Outside these temperatures they start to have problems with either overheating or freezing. It should be remembered that the ambient temperature should be the temperature around the compressor itself, which is not necessarily the outside air temperature. The compressor ambient temperature will vary from the outside air temperature depending on whether it is in a heated compressor room, in an unheated, ventilated compressor room or outside under a rain cover.

It needs to be stressed that air compressors need to be situated in a very well ventilated place, so even if the room is heated there will still need to be plenty of ventilation for the compressor to work properly.

Problems start when the temperature falls below 5’C

As the ambient temperature starts to fall below about 5’C problems can start to occur. The low temperatures cause several issues within the compressed air system. These are primarily related to the fluids within the compressor itself and to the condensate created in the compression process. Note: when the compressor is running and is up to its normal working temperature the ambient air temperature is not too much of a problem, except if the actual compressed air is routed to a low temperature area and has not been adequately dried.

Air Compressors and cold weather – What can go wrong?


Air Compressor Oil

If the compressor is not serviced properly it is possible that the oil will contain a high level of water. This is not a huge problem at normal ambient temperatures but if the temperature falls too far this water can freeze in the oil, causing issues for the compressor at start up or causing damage to the compressor internals.

Air Compressor Condensate

Condensate is probably the cause of the biggest cause of problems for air compressors and cold weather. Condensate is formed when air is compressed and then cooled. Condensate is a mix of water (99%) and oil (1%). Condensate it at its worst in colder temperatures due to the higher moisture content in the incoming air stream and generates a significantly higher rate of condensate over a given period of use.

Condensate tends to collect at low points within the compressed air system. These include, the compressor, separators, receivers, filters and dryers as well as in low points in the compressed air pipework. If temperatures fall low enough this condensate will freeze, blocking pipes or voids within the compressor, dryers, filters and pipework.

If there is frozen condensate in the compressor this can cause damage to the compressor, if it is in the filters or dryers again this can cause damage. If there if frozen condensate in the pipework this can get blown to the downstream equipment, again causing damage to the downstream equipment.


Air Compressor Dryers and Filters

Strangely the best protection devices for Air Compressors and cold weather are the dryers and filters built into your compressed air system. This is because this equipment removes water from the compressed air and this prevents water from freezing in the downstream pipework and equipment. So keeping your dryers and filters working effectively is important to protect the rest of your compressed air system.

Refrigerated dryers only work effectively at ambient temperatures above 5’C. So if your dryer is in an unheated compressor room this could be damaging the rest of your compressed air system. Installation of cabinet heating could help protect your dryer and the rest of your compressed air system.

 

[Crater Creator]

Mee, your post does not mention the density of air causing extra load on the motor.
It does say that:
" The low temperatures cause several issues within the compressed air system. These are primarily related to the fluids within the compressor itself...."

Which is exactly what almost everyone has been saying.

 

[Mee]

Motor problems arise in cooler weather due to overloading and these can be caused by many factors or a combination of factors in a compressor design. We must also remember that many generic designs exist and are bought by many manufacturing company’s around the world and designers base there designs on the Bolsche formulae for European operating temperatures using balanced starting methodologies to improve air output from a specific range of electric motors.

Many people ignore the motor issues and diagnose faults incorrectly by assuming the compressor works in summer but not in colder climatic conditions where temperatures dip below 5’C and attribute this performance drop to the compressor itself and not the motor which is often internally moist through natural condensation. Where a motor is aluminium cased this may exacerbate a problem by a starting torque drop of around 5% which is often significant enough to start a warm compressor but not a cold one or a poorly manufactured compressor or one utilising a marginal design and built to a budget.

 

[Mee]

Mee, your post does not mention the density of air causing extra load on the motor.
It does say that:
" The low temperatures cause several issues within the compressed air system. These are primarily related to the fluids within the compressor itself...."

Which is exactly what almost everyone has been saying

No it isn't, many people have made numerous comments which are factually incorrect, and you are factually incorrect as I will make the assumption that you have misinterpreted the information.

Problems start when the temperature falls below 5’C

As the ambient temperature starts to fall below about 5’C problems can start to occur. The low temperatures cause several issues within the compressed air system. These are primarily related to the fluids within the compressor itself and to the condensate created in the compression process. Note: when the compressor is running and is up to its normal working temperature the ambient air temperature is not too much of a problem, except if the actual compressed air is routed to a low temperature area and has not been adequately dried.

It refers to the fluids in the machine and refers directly to the condensate, it doesn't refer to anything else as the oil is in the crankcase and not in the machine as it is only referring to anything inside the inlet and compression side of the unit.

If you read further it covers it more specifically.

Air Compressor Condensate

Condensate is probably the cause of the biggest cause of problems for air compressors and cold weather. Condensate is formed when air is compressed and then cooled. Condensate is a mix of water (99%) and oil (1%). Condensate it at its worst in colder temperatures due to the higher moisture content in the incoming air stream and generates a significantly higher rate of condensate over a given period of use.

Condensate tends to collect at low points within the compressed air system. These include, the compressor, separators, receivers, filters and dryers as well as in low points in the compressed air pipework. If temperatures fall low enough this condensate will freeze, blocking pipes or voids within the compressor, dryers, filters and pipework.

If there is frozen condensate in the compressor this can cause damage to the compressor, if it is in the filters or dryers again this can cause damage. If there if frozen condensate in the pipework this can get blown to the downstream equipment, again causing damage to the downstream equipment.

Does that explain it?

 

[Crater Creator]

The condensate in the text you've quoted is relating to downstream condensate - which is what happens when you compress air and cool it - condensate is formed, causing problems for downstream equipment, or needing frequent draining of the receiver(s).
I work with a few rather large compressors, recip, screw, and centrifugal, driven by everything from electric motors to industrial gas turbines, and which compress gases of varying composition. So I'm not really new to the subject.
Thanks.

 

[rtbcomp]

...look up the Bolsche formula which is the accepted formula for compressor designers.

Tried that, couldn't find anything - any chance of a link?

 

[WorkshopChris]

and motor on a coupled unit and in cold weather the motor loses some performance when it is cold, particularly if it cold and damp from a little condensation.

and as far as motors losing performance goes it is not a motor gaining performance it is an issue of motors losing slight performance when they are stood in the damp as it is the damp itself which penetrates the motor.

Many people ignore the motor issues and diagnose faults incorrectly by assuming the compressor works in summer but not in colder climatic conditions where temperatures dip below 5’C and attribute this performance drop to the compressor itself and not the motor which is often internally moist through natural condensation. Where a motor is aluminium cased this may exacerbate a problem by a starting torque drop of around 5% which is often significant enough to start a warm compressor but not a cold one or a poorly manufactured compressor or one utilising a marginal design and built to a budget.

Am interested in this as if the motor is the problem due to damp then it should affect all machinery.
I have never found a motor problem attributed to cold or damp with machinery in regular use.
Most of the motor manufactured offer no advise on motors for cold conditions but lots on de rating information for hot environments.
However i have had problems with various machines starting in the cold, all down to increased friction from cold lubricants or electrical supply issues.

 

[daleyd]

Forgive me if I'm wrong, but I also think that motors are actually (A little bit at least, in terms of average UK ambient temperature ranges) MORE efficient when cold as the resistance is less? Isn't this why they are spending millions in research into superconductors and the like, which run at close to absolute zero, or did I just dream that?

I'm also finding it hard to believe that a compressor designer/design team would be reliant on ONE formula (This mythical Bolsche formula??)- there must be several if not dozens of things to take into account (Mechanical forces, pumping losses, motor efficiency, etc) when designing something like this - though granted most basic designs for home workshop type compressors have already been proven over the years so at best most design changes are going to be "tweaking" I would have thought.

I imagine the reason that many are marginal is the fact that they are trying to squeeze the highest CFM from a given motor rating.

 

[daleyd]

Interesting link to motor derating - at 40 deg C ambient a 1.1 kw motor (picked at random) is rated for 1.1 kw, and drops proportionately as temp rises.
At 20 deg C (The lowest the calculator goes) the rating is 1.21 kw.

http://exico.co.uk/deration.html

 

[rtbcomp]

Is Bolsche pronounced Bolshy?

 

[rtbcomp]

Interesting link to motor derating - at 40 deg C ambient a 1.1 kw motor (picked at random) is rated for 1.1 kw, and drops proportionately as temp rises.
At 20 deg C (The lowest the calculator goes) the rating is 1.21 kw.

http://exico.co.uk/deration.html

40 DegC ambient? that's a bit warm

 

[daleyd]

40 DegC ambient? that's a bit warm

Not uncommon though in a lot of industrial environments (or at least not the places I work ha ha) ! I just used the websites own standard which suggested up to 40 deg was "normal" - anything above should be de rated.
Rather than actual figures though the bit I was interested in was at higher temps the rating goes down