Compressor inlet temperature VS discharge temperature, anyone got the math?

[Josh Kinzey]

I want to figure out how much of a difference the temperature of the ambient air makes in the temperature of the compressed air my compressor produces.

If I move compressor from outside (100 degrees ambient) to inside (70 degrees ambient) what kind of reduction in compressed air temperature should I expect to see?

 

[AndyA]

Leave the compressor outside. Have an aftercooler outside to drop the air temp to 100, then bring the air inside and have another aftercooler to drop the air temp to 70.

Having the compressor and first aftercooler outside will really drop the heat load on your A/C. Reduced noise inside is another good benefit.

I measured the outlet temp on my air compressor. It was winter time and the ambient was 40F, the outlet temperature was around 300F. Having 70F ambient isn't going to help much. The aftercooler(s) are the real solution.

 

[KinzeMech]

Single stage or two stage compressor makes a difference in discharge temp as well.

Two stage pumps build to higher ultimate pressure, but the shed a lot of heat in the inter-stage cooler.

 

[Josh Kinzey]

I appreciate your time, but you didn't really answer my question. I understand your view, and focusing on the details of compressor inside / outside might seem like the real answer I'm looking for, but it's not. I was hoping to be able to calculate discharge air temp.

Sometimes this place jumps ahead so far they miss the point. My wife does the same ****, I"ll say, "Where are your keys?" She will respond, "I already got milk." What. The. ****. I didn't ask about milk, I asked about your keys. I understand that you think I want to take your car to the store to get milk, but I don't. I just want to get my sunglasses out of your car. Now, I have to ask again, "Where are your keys?" Invariably, she will respond, "Why?" ******** it, just tell me where the ******* keys are!

Anyway, like I originally asked, I'm wondering about the affect of ambient air temperature on discharge temperature. If anyone can shed some light on the actual math associated with that, I'd really appreciate it.

 

[Josh Kinzey]

In the interest of full disclosure, what I am thinking of doing is climate controlling the air compressor shed.

I've been through 2 motors on the compressor since it the summer heat started here in NC. I'm not sure that the heat has anything to do with the failures, but thus far the rise in ambient temperature has seemed to be the driving factor in the failures.

I have installed a fan that blows on the motor, which has dropped the case temperature of the electric motor almost 30 degrees. Since installing the fan, I have not had a motor failure, but I'm not comfortable attributing the motor longevity to the reduced temperature - but, the motor running cooler certainly can't hurt.

I have everything necessary to insulate and cool the compressor shed. I am considering this in an effort to prolong the life of the electric motor on the compressor. Also, I was curious as to what (if any) benefit might be had by reducing the inlet temperature on the pump. Lastly, the humidity here is very high a lot of the time. Air conditioning the shed, would dramatically reduce the humidity in the room, which will probably reduce the amount of water in the tank of the compressor. I paint a lot, and getting the water out of the line is always a hassle. If I can reduce the amount of water being drawn into the system, AND reduce outlet temperatures, that will help me keep water out of my paint jobs.

Like I said though, the temperature and humidity reduction is secondary to the longevity of the compressor motor. It's just something I'd like to know before making a decision on whether or not to cool the shed.

 

[zkling]

I appreciate your time, but you didn't really answer my question. I understand your view, and focusing on the details of compressor inside / outside might seem like the real answer I'm looking for, but it's not. I was hoping to be able to calculate discharge air temp.

Sometimes this place jumps ahead so far they miss the point. My wife does the same ****, I"ll say, "Where are your keys?" She will respond, "I already got milk." What. The. ****. I didn't ask about milk, I asked about your keys. I understand that you think I want to take your car to the store to get milk, but I don't. I just want to get my sunglasses out of your car. Now, I have to ask again, "Where are your keys?" Invariably, she will respond, "Why?" ******** it, just tell me where the ******* keys are!

Anyway, like I originally asked, I'm wondering about the affect of ambient air temperature on discharge temperature. If anyone can shed some light on the actual math associated with that, I'd really appreciate it.

OK, dude chill out, he was only trying to help. And to be honest, if you were able to calculate this yourself, you would know how to do it. It is an exercise in thermodynamics, simple charles' law.

The answer is going to be not much. Especially under in everyday use conditions What are you trying to accomplish here?

First off understand that a compressor is technically a mass flow device. Yes they are typically rated in CFM (a volumetric flow rate), but since most of us are compressing atmospheric air we can use that density to determine mass flow rate. Which is going to be dependent on your locations and the density and humidity of the air that day.

Air gets heated by two methods in a compressor. Primarily through basic thermodynamics (Charles law) which state that the temperature increase of a gas under compression is proportional to the pressure increase differential.
http://www.compairusa.com/About_Us/...--02What_happens_when_we_compress_air-q-.aspx

Secondly the minor contributor to heat in the compressed air is through mechanical heat generated through friction of the moving components.

Now, all GOOD compressors are designed to run at a specific temperature that is optimal for the compressor itself. Combination of wear, noise, efficiency and driving off the moisture produced when the compressing happens. Quincy is big on this specific temperature. So basically no matter what the inlet (ambient) temperature is, the output is going to be roughly the same under normal operating conditions. You could add additional cooling to the pump, that is in addition to the cooling fins already designed on the flywheel. Then again you are getting into the issue of working around the pumps designed operating temperature.

What are you trying to accomplish here?

 

[zkling]

In the interest of full disclosure, what I am thinking of doing is climate controlling the air compressor shed.

OK, this is the post you should have started with.

I've been through 2 motors on the compressor since it the summer heat started here in NC. I'm not sure that the heat has anything to do with the failures, but thus far the rise in ambient temperature has seemed to be the driving factor in the failures.

I have installed a fan that blows on the motor, which has dropped the case temperature of the electric motor almost 30 degrees. Since installing the fan, I have not had a motor failure, but I'm not comfortable attributing the motor longevity to the reduced temperature - but, the motor running cooler certainly can't hurt.

I guarantee it does. Heat is the #1 killer of electrical motors, besides obvious mechanical failure. Heat increases, resistance increases, things go bad fast.

I have everything necessary to insulate and cool the compressor shed. I am considering this in an effort to prolong the life of the electric motor on the compressor.

Good idea, go do it, what are you waiting for? Have you monitored the temperature inside the shed while the compressor is running?

Also, I was curious as to what (if any) benefit might be had by reducing the inlet temperature on the pump.

You will get a SLIGHTLY higher mass flow rate.

Lastly, the humidity here is very high a lot of the time. Air conditioning the shed, would dramatically reduce the humidity in the room, which will probably reduce the amount of water in the tank of the compressor. I paint a lot, and getting the water out of the line is always a hassle. If I can reduce the amount of water being drawn into the system, AND reduce outlet temperatures, that will help me keep water out of my paint jobs.

Like I said though, the temperature and humidity reduction is secondary to the longevity of the compressor motor. It's just something I'd like to know before making a decision on whether or not to cool the shed.

Best bet would be to draw the air from a cool dry source into the compressor. Maybe from inside the shop? Then make sure your compressor is running at the minimum recommended operating temperature. Finally a good aftercooler between the pump output and the storage tank will take care of any residual moisture. That way you will be storing semi cool, dry air.

What compressor do you have?

 

[shieldcracker]

The air discharge temperature is a function of the heat compression (itself a function of the compression ratio, CR), the inlet air temperature, and any stray heat transfer (negligible). I agree with AndyA its not gonna help much to move the compressor inside.

Tdischarge= Tinlet*(Pdischarge/Pinlet)(K-1)/K
K=air polytropic constant 1.4

 

[KinzeMech]

Like I said though, the temperature and humidity reduction is secondary to the longevity of the compressor motor. It's just something I'd like to know before making a decision on whether or not to cool the shed.

If the longevity of your motor is the primary concern, then analyzing compressor discharge temperature is the equivalent of unlocking the car with a gallon of milk so you can get your sunglasses out of it. I would suggest instead just using the keys.


What kind of failure did the motor experience? If you've been through 2, then your two dead motors are a pretty good body of evidence for finding the cause, if you look at them.

How many amps are they drawing under load? If the motor is overloaded or overheated, this will show as excessive amp draw.

What is the supply voltage? How much does the voltage drop when the compressor is running? Voltage sags will cause amp draw to increase.

zkling is dead-on as far as I can tell. The load on the motor, from the pump, is a function of the amount of air (mass) being compressed multiplied by the pressure differential. 30 degrees cooler could actually increase the load on the motor, in that a pump inhaling 30 degrees cooler air is inhaling denser air, thereby increasing the mass flow rate of the pump across the same pressure differential.

 

[shieldcracker]

I appreciate your time, but you didn't really answer my question. I understand your view, and focusing on the details of compressor inside / outside might seem like the real answer I'm looking for, but it's not

I dont want a combo, I dont want fries, I dont want to super size,
All I want is a burger...
This **** happens all the time

 

[koditten]

I think you are going to have only a short term solution by air conditioning the shed. The compressor "consumes" the refrigerated air at a constant rate. Once the compressor pulles in all the cooled air and compresses it,you are now back to pulling moist air from the air. I don't see how an air conditioner is going to be able to keep up with the demand of air the compressor is pulling in.

Just to be clear, you have 2 problems that need adressed. 1) excess moisture, 2) premature motor failures. correct?

In refrence to compressors running at a specific temperature, I don't know if this applies. Quincy "rotory screw" compressors are specked out to run at 180+/-. This is to ensure any moisture in the oil is "cooked off". Recirical compressors are dependent entirely by the ambient temperature.

The moiusture can only be battled by dropping the exit air below the dew point, thus causing the moisture to turn from a vapor to a liquid. You need to have aftercoolers, desicant coolers and blowdown taps with a place to drain the liquid out the taps. Be aware there are no free lunches with compressed air. You will lose CFM's to intentional leaks (blowdowns). You may need a bigger compressor if you are a large air consumer.

The motor failure is the result of excess heat. You proved this by fan cooling the motor and noticed a lack of failures. Now you can cool the compressor shed with an a/c unit if the intention is to cool the motor only. This will by you some motor life.

If it was me, I would have the air compressor intake plumbed so it pulls outside air in to be compressed. Then I would have an a/c unit cooling the shed along with an external aftercooler located in the now cooled shed. I would also invest in a timed blowdown valve for the bottom of the tank.

A final dessicant dryer placed inside the shop near the point of use would be the final bit of insurance. Make sure the dryer is within 25' of the spraygun. You don't want any more hose than you need running on the cool concrete floor. This can condese water as well.

 

[Fixnair]

I can't quote exact numbers but when you compress air to 100 PSIA you increase it's temperature 600*. A lot is shed through the compressors cooling devices but discharge temps in the 300* range are not uncommon. That discharge temp will vary directly with ambient temp but with discharge temps near 300* what's another 20 or 30*.

Interesting sidebar here, this compressional heating is exactly what makes a Diesel engine run. Imagine a compressor compressing air up to 450 PSIG. That would increase its temperature to around 2500*. Now inject fuel into that inferno.

On the other end of the scale consider our weather. You have all heard the weatherman talk about high pressure zones. The weight of the air pressing down on us varies and when we are under a high barometric pressure our weather is considerably warmer.