Annoying Compressor fault....?

[rtbcomp]

When you talk about de-rating does that mean you have to take less power from the motor to stop it overheating (heat won't dissipate as quickly at high ambient temps, so the motor could overheat) or does it mean the motor will only deliver the lower figure?

 

[daleyd]

Generally it means that if you are specifying a motor for a job, you would go up a size (or several depending on the application) from the one you have calculated to be the correct power at say 20 degrees. I guess as the resistance increases with temperature then at, say a nominal 240 volts, you will get less current hence less power out at higher temps.

Certainly on some inverter drives I've worked on (I'm not really a "designer" as such so this is only what I've picked up from commissioning and hands on work with them, though I have been involved in specifying drive systems before.) they have been overrated due to harsh (i.e. Hot) environments.

 

[eil]

Just to recap then, compressor starting problems in winter is down to dense waterlogged air and damp electric motors. This is particularly true if you are a resident of Atlantis or cloud cuckoo land.

 

[brightspark]

where does the moisture enter the motor when it has sealed bearings .I see them on augers out in all weathers exposed to rain and hail for years on farms there sealed

 

[Mee]

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?

Theoretically you are correct, as highlighted it isn't actually the cold itself which is the problem as it is the cold and damp weather causing moisture in the motor which reduces its efficiency until it spins a few times and generates sufficient heat to dry itself. You actually answer your own question in your final quote.

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

Correct, when a motor is marginal and it loses a little performance its impact is very serious and in the case of a small compressor it is serious enough not to provide enough start torque when we factor in the density of the cold air.

 

[Mee]

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

Most equipment which is electrically powered isn't so marginal as compressors are, de rating is for thermal transfer issues in your quoted example and totally different to cold issues.

In cold weather your electrical supply issues are the indicator, a moist motor draws more current, denser air draws more current.

 

[the snooper]

Most equipment which is electrically powered isn't so marginal as compressors are, de rating is for thermal transfer issues in your quoted example and totally different to cold issues.

In cold weather your electrical supply issues are the indicator, a moist motor draws more current, denser air draws more current.

Brightspark ask a very valid question a couple of posts back, why no answer ?

 

[daleyd]

I've been dealing with drives in one way or another for more than 20 years, admittedly mostly 3 phase and dc industrial stuff but the principles hold the same. "Moist/cold air" within the motor in the range of normal ambient conditions in the uk is so insignificant as to be ignored. The windings would have to be severely waterlogged to cause any appreciable drag, if that's the case you have other problems to worry about as your garage is under water.

I must remember to tell my instructors during my apprenticeship and all the courses I've been on since (hitachi, Lenze, danfoss, ssd and a few others. Oh I also work for one of the worlds largest suppliers of inverters and control equipment and no one here has ever mentioned it either ...) that they are all missing a trick by not mentioning this "moist air" either in passing or in any of their literature.

 

[brightspark]

all this moisture in motors I'm surprised anyone that uses motors and there covered by a 30 milliamp rcd manage to keep them setthere must be thousands of them

 

[WorkshopChris]

In cold weather your electrical supply issues are the indicator, a moist motor draws more current, denser air draws more current.

Yes you are correct but only in a little way.
Yes a moist motor (as in wet) will draw a little more current rising up to a lot more current as the windings flash-over and burn up if the motor is so damp that the insulation brakes down.
I have also never seen a belt driven compressor with a non sealed motor so the moisture in the air will have no affect.

 

[Iron Snail]

As Mr Fuzzy and others suggested.

I just chuck in whatever I have left from doing the car (usually some 5W40 synthetic)

Drained the 40 weight oil and replace with 5/40 semi synthetic, left over night and started from cold this morning ,temp 40F and it ran straight from cold, no triping out. Well pleased

 

[normspanners]

so try sae 30 oil in it when its cold and see if it makes a difference

I was going to give that a try at the next oil change.

thats what I put in mine in winter to help and it normally starts ok, but as time goes on it thickens up.

 

[normspanners]

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.

this quote is very similar to post 53 ???

Think I have posted this once ---twice ---three --- or is it ten times ghttp://www.air-equipment.co.uk/air-compressors-and-cold-weather read 3rd paragraph

 

[normspanners]

hence why people used to change their cars oil twice a year (winter oil and summer oil) now we use multigrade no one AFAIK bothers unless they have seriously old kit. Also why sticking 0W40 or 5W40 in instead of 40W single viscosity makes a compressor go from not starting to starting no problem

All cars still have various oil grades for different operating temps it is just we dont generally have to bother now in britain

 

[normspanners]

Just to recap then, compressor starting problems in winter is down to dense waterlogged air and damp electric motors. This is particularly true if you are a resident of Atlantis or cloud cuckoo land.

I wish I could like this twice!!--- or more, made me LOL and I mean that !

 

[normspanners]

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.

Does that explain it?

No I am afraid it doesnt
it says that the fluids ( read oil ) within the compressor itself, ie IMO means condensate getting into the general lubricants in the motor and compressor that then causes thickening/ice crystals in the oils /grease

it also mentions the condensate in the compressor air SYSTEM ( read pipework etc ) and it clearly says and to the condensate created in the compression process. read further on and it states again except if the actual compressed air is routed to a low temperature area and has not been adequately dried. that surely means that the air has already been compressed and is on route to the air system,that therefore means causing problems with ice forming in pipework further on from the compressor, so actually causing no issues with the compression process.

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.

 

[Omniata]

So thin oil in winter, thicker in summer?

Edit: I just had to go and lie down for a few minutes, I'm struggling with a severe migraine wondering how moisture levels in the realms of Aqua man and Captain Nemo would be present operating a mains powered compressor?!

 

[normspanners]

I have been thinking about this, and if hot air is compressed it gets hotter quicker, the hotter it is the more the pressure rises, therefore hot air will take more power to compress because it creates a higher pressure for each compression process whilst being compressed, because it is 1/ getting compressed but also 2/getting heated by the compression process so therefore raising its pressure further by the heat produced by the compression process AND the latent heat in the cyl ie if you fill a scuba cyl to 3000 psi with the hot air off the compressor, when this cools it drops to 2800 psi. whilst I realise cold air holds more oxygen and is more dense
warm air will attain more pressure quicker than cooler air, that enters the compressor !!

SO cool air keeps the compressor piston bore cooler so it doesnt actually get the accelerated double pressure rise immediately until the whole compressor warms up so in fact the cold air might actually be easier to compress initially , the colder the incoming air is !!
so in fact the opposite is actually true, colder air takes less hp/ amps draw initially. WOW !!

ALSO a cold piston is not as large as a hot piston and therefore its piston rings etc will not seal as well therefore lowering the initial compression ratio until the piston achieves its correct operating tolerances.

HUM found this https://www.physicsforums.com/threa...e-more-work-to-compress-than-cold-air.369289/

Specific Heat[edit]
The specific heat of a substance is the amount of heat required for a unit rise in the temperature in a unit mass of the material. If this quantity is to be of any use, the amount of heat transferred should be a linear function of temperature. This is certainly true for ideal gases. This is also true for many metals and also for real gases under certain conditions. In general, we can only talk about the average specific heat, cav = Q/mΔT. Since it was customary to give the specific heat as a property in describing a material, methods of analysis came to rely on it for routine calculations. However, since it is only constant for some materials, older calculations became very convoluted for newer materials. For instance, for finding the amount of heat transferred, it would have been simple to give a chart of Q(ΔT) for that material. However, following convention, the tables of cav(ΔT) were given, so that a double iterative solution over cav and T was required.

Calculating specific heat requires us to specify what we do with Volume and Pressure when we change temperature. When Volume is fixed, it is called specific heat at constant volume (Cv). When Pressure is fixed, it is called specific heat at constant pressure (Cp).

Latent Heat[edit]
It can be seen that the specific heat as defined above will be infinitely large for a phase change, where heat is transferred without any change in temperature. Thus, it is much more useful to define a quantity called latent heat, which is the amount of energy required to change the phase of a unit mass of a substance at the phase change temperature.

 

[Omniata]

Simple answer yes...

Medium answer: More work has to be done compressing warm air at an equivalent pressure due to the energy already contained in the gas pushing it apart... So cooler air is thermodynamically easier to compress, liquifaction, etc...

Long answer: Phone Manchester University physics department on Monday morning, they'll love you, trust me...
Edit: I would nominate a former student I knew there whilst I was studying Chemistry, but they'd probably hunt me down...

 

[Crater Creator]

I have been thinking about this, and if hot air is compressed it gets hotter quicker, the hotter it is the more the pressure rises, therefore hot air will take more power to compress because it creates a higher pressure for each compression process whilst being compressed, because it is 1/ getting compressed but also 2/getting heated by the compression process so therefore raising its pressure further by the heat produced by the compression process AND the latent heat in the cyl ie if you fill a scuba cyl to 3000 psi with the hot air off the compressor, when this cools it drops to 2800 psi. whilst I realise cold air holds more oxygen and is more dense
warm air will attain more pressure quicker than cooler air, that enters the compressor !!

SO cool air keeps the compressor piston bore cooler so it doesnt actually get the accelerated double pressure rise immediately until the whole compressor warms up so in fact the cold air might actually be easier to compress initially , the colder the incoming air is !!
so in fact the opposite is actually true, colder air takes less hp/ amps draw initially. WOW !!

ALSO a cold piston is not as large as a hot piston and therefore its piston rings etc will not seal as well therefore lowering the initial compression ratio until the piston achieves its correct operating tolerances.

HUM found this https://www.physicsforums.com/threa...e-more-work-to-compress-than-cold-air.369289/

Specific Heat[edit]
The specific heat of a substance is the amount of heat required for a unit rise in the temperature in a unit mass of the material. If this quantity is to be of any use, the amount of heat transferred should be a linear function of temperature. This is certainly true for ideal gases. This is also true for many metals and also for real gases under certain conditions. In general, we can only talk about the average specific heat, cav = Q/mΔT. Since it was customary to give the specific heat as a property in describing a material, methods of analysis came to rely on it for routine calculations. However, since it is only constant for some materials, older calculations became very convoluted for newer materials. For instance, for finding the amount of heat transferred, it would have been simple to give a chart of Q(ΔT) for that material. However, following convention, the tables of cav(ΔT) were given, so that a double iterative solution over cav and T was required.

Calculating specific heat requires us to specify what we do with Volume and Pressure when we change temperature. When Volume is fixed, it is called specific heat at constant volume (Cv). When Pressure is fixed, it is called specific heat at constant pressure (Cp).

Latent Heat[edit]
It can be seen that the specific heat as defined above will be infinitely large for a phase change, where heat is transferred without any change in temperature. Thus, it is much more useful to define a quantity called latent heat, which is the amount of energy required to change the phase of a unit mass of a substance at the phase change temperature.

Norm,
Hot air will not compress more than cold air, or rise more in pressure for a volume change. The pressure rise during the compression stroke is what creates the temperature rise, which is why small compressors with a high overall compression ratio tend to be two-stage with an intercooler between them.
The forces on the piston that the motor "sees" are just 1) ambient at BDC and 2) discharge pressure at TDC.
You're right that hot inlet air will take more energy to compress, as the piston is moving less molecules of air on each stroke and the machine will have to run for longer to fill a cooled receiver on the outlet.