How to Calulate Heat Generated by a Reciprocating Gas Compressor

How about some data, or is it "20 questions" time again?

Make a good usage of the thermodynamics knowledge you were given in school and compute the energies after compression. make a balance and consider the total efficiency.

If this is a new compressor that you have purchased, contact the vendor of the equipment and ask him.

He has answered this question many times.

If, more likely, this is equipment relocated from somewhere else and moved to Pakistan, contat a vendor of similar sized equipment and make an estimate.

May we assume from this statement: "I need to calculate the total heat generated" that you don't know how to do this?

You haven't actually asked us to do the calculations for you, yet. Is that what you want? If not, I grant you permission to proceed with the calculations.

I'd just find out how much gas it burns in an hour, then find the energy content of same and use that number.

It's obviously not rocket science, maybe homework?

What for the compressor should burn gas?

What do you do if it has an electric motor?

I am a bad boy to make such remarks isn't it?

You know, for you it is not rocket science but for some others it can be as the relativity theory (not restricted).

I assumed that the compressor ran, was powered, by gas.

I did not consider that it might be a machine that compresses gas and that it might be powered by electricity.

If it's powered by electricity, it makes his calculations simpler, I'd think.

At any rate, I don't believe that the OP knows how to proceed.

Very simple to get ball park figure if electric powered:

Kilowatts of motor X 3200 = BTU's (this is just for the motor)

Total BTU's out cannot exceed the BTU's input.

Add the power of the controls (Motor starter,solenoids, etc. ,which will be a small amount).

Now you have a SWAG of the total heat generated by the compressor and motor.

You still must consider ambient temperature, which will add to this number, and do not allow the motor to exceed it's maximum temperature rating.In an electric motor, temperature damage is accumulative, so the cooler it runs, the longer it will last.

Better to calculate for the highest anticipated ambient temperature to be safe.

Your best data will be in the form of real world measurements. Measure it!

I totally agree.

Ron

Please explain how you would measure the generated heat I am very interested which method you will use (since you fully agree with this suggestion). I would appreciate a clear and full explanation.

Well i would enclose the unit and measure the temperature in the container at ambient, then measure the temperature( thermocouple or thermistor) when the unit was running. The simple volume displacement will establish the CFM required.

Ron

Sorry , i lost my train of thought. The above is the method i am using for cooling a TOF laser in a steel mill. I found out that mounting the laser away from the 1600 degree F plates really did not work as i was reflecting the beam off of a 45 degree mirror and the heat was distorting the fixture which was causing inaccurate readings. So i am cooling the laser and aiming directly at the plates. By liquid cooling at an air to water radiator mounted away from the heat i was able to achieve accurate readings.

Ron

I don't see how that helps the OP much in this case. If there is already a fan installed, he could estimate the flow from ΔP measurement and the fan curves, and from that with temperature rise get approx heat generated. But he wants to calculate the heat generated in order to select his fan, from that and acceptable temperature rise.

Codey

May i know when you get your answer how will you calculate the air flow?

hi!

is this compressor air cooled? check with manual.

After that try to take real readings of temp. at most heated parts. Consider max. temp.

Now decide size of the canopy. Treat compressor as heat generating body inside canopy.

Calculate heat generation and reqd. removal. Considering volume of air in the canopy and its removal rate to get the desired temp. select Axial Fan.

I have not given actual calculations as it is your project

Hi Fawad

Apply the 2nd Law of Thermodynamic.

Energy input = Compression energy + heat loss

Compression energy can be estimated from gas flowrate, pressure and temperature at inlet and outlet.

From inlet and outlet pressure & pressure you can determine the polytropic efficiency.

Energy input from the driver data e.g voltage, current (if electrical) or steam consumption if steam turbine.

I have briefly explained, the rest is in your hand.

Hi Fawad,

I think this is the best practical approach, You have the power input, you know the work done by compressor in form of pressurised flow, rest energy is wasted in form oh heat and efficiency. You can consider theoritical efficiency of the particular type of compressor to estimate the heat produse or energy lost.

The trouble with that is it ignores the fact that the compressed gas contains power which does not end up in the canopy. This power is ~ q*P₁*ln(P₂/P₁). This is the power for isothermal compression, actual figure is a slightly higher due to raised temperature, but isothermal power gives a little safety.

So if you take compressor input power (from supplier or calculated), divide by motor efficiency to get total power into the canopy, and subtract the above you won't be far out.

Codey

A fair bit of the heat is waste heat from the motor. For motor efficiency η, waste = P*(1/η -1) where P = motor power. Using rated power will give some safety, if you want to be more precise use the actual motor power (= compressor input power).

For the rest, need to estimate the area of hot surfaces (including pipework) and study convection heat loss, but it's likely to be somewhere round 1000watt/m².

If you just want a covering figure, you could use motor power/η. This is the electrical power input, and heat produced cannot exceed that.

Codey

When is this assignment due?

I'd just suggest a canopy, with no sides. That would negate the need for any cooling fans. You can safely say that all the heat generated by the undefined system will find its way into the atmosphere and maybe get an A for originality.

So many factors, so little information.

What is the R Value of the insulation, if any, of the shelter?

What is the max ambient temperature outside, inside?

I presume you want to have a little overkill to allow for unforeseen variances.

That is why I stated the simple method of converting motor KW to BTU, to err on the safe side.

To reduce heat inside of the shelter, insulate the discharge piping.This alone will move a lot of heat outside.

If the compressor is oil cooled, locate the oil cooler outside in a shady area, and fan cool it.

Whatever you choose, pull filtered air in at the bottom, and force hot air out at the top.This puts nature on your side.You won't believe the installations I have seen where this simple factor was ignored.

Good luck with your project.

I am working as an senior engineer specilized in air screw compressor design at a sino-america JV manufacturer.

In my opinion you could use the energy conservation equotion to caculate the cooling heat load of compressor. the power input=gas entropy increase+cooler load, actually if the discharge temperature is almost the same as inlet, the gas entropy could be taken as same and no increase, the power input is the cooler load.

if the compressor is driven by a CNG generator, you should add these heat load up.

just mind you if you have heared about the ATLAS COPCO's Zero discharge compressor, the conception is all the cooling heat could be recycled, they conceal this recycled energy in low temperature, only for heating water, pre-heating for vapor and so on.

hope help you.

No, even if the gas discharge temperature is the same as inlet there is still energy in the compressed gas which does not have to be shed as heat from the compressor. See my post #16.

Dude I know the guy who asked this "question". I'm sure he understands my reply. No need to worry. :) Nonetheless thank you for your response.

I just hope that you contacted the vendor and he never sent you a reply. You might want to look through the "specifications" in the "manual" provided by the vendor unless of course ASME made this particular model.

P.S. Try looking for the term "sparger" on Google. Really helpful.

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