Air Compressor Burning Up!!

[MEnut]

I have an application where an air compressor frequently needs replaced. It seems that it runs constantly, never catching up with itself. My first opinion was that the motor was too small. However, our HVAC service guy says that the air tank is too small.

I do not have too much experience in air flow systems, has anyone else encountered this problem? If so, which way should I go (bigger motor or bigger tank?) I'll post the specifics is anyone is interested.

Thanks, guys!

 

[quark]

Perhaps the bigger compressor. Give us the details.

Regards,

 

[MEnut]

Okay, Here's the Spec's!

1.5 hp oil-less 4 clynder motor - rated at ~7 CFM for 60 psi
30 gallon tank

Line Feed (consumption) = 1 CFM

It does seem like I actually have some over-kill on the motor size, doesn't it?

 

[pennpoint]

What is the compressed air used for?
Can you verify that 1 cfm is the actual consumption?

 

[mdonner]

Your tank is about 4 cubic feet volume.
With 1 cfm diverted for consumption, 6 cfm is pumped into tank, and it will work about 40 minutes to pump 4 cu ft.
Than it will rest no more than 4 minutes (obviously less than 4 minutes, to keep pressure).
Increasing compressor size will never give you more than 4 minutes off time (this is the time of 1 cfm consumption to empty the tank).
Increasing your tank will increase off time almost linear. Knowing the compressor'sdesign duty cycle you can calculate the optimal tank volume.
For example, by adding a 60 gallons tank the compressor will run 2 minutes to pump 90 gallons and will rest about 10 minute. With 120 gallons tank it will be <3 min on and ~15 min off.

 

[quark]

Generally you should not have any problems with your system provided your 1 cfm consumption is SCFM.

Motor is properly sized. The calculation goes like this.

for adiabatic compression

BHP = (4.36/1000)[k/(k-1)](cfm)(P1)[(P2/P1)[sup](k-1/k)[/sup]-1]

Where k = ratio of specific heats and for air it is 1.4
P1 = initial pressure in psi (atmospheric 14.7)
P2 = final pressure abs = 74.7 psi
cfm = 7 (scfm)
therefore, BHP = (4.36/1000)(1.4/0.4)(7)(14.7)((74.7/14.7)[sup](0.4/1.4)[/sup]-1) = 0.928

Considering 80% motor efficiency and 80% compressor efficiency IP = 1.45 HP, so motor is well sized.

A thumb rule for receiver sizing

Receiver capacity in gallons = compressor capacity in cfm. So you generally don't have problems until an unless you have some equipment which require more air for short span of time. You should tell us your load pattern and equipment.

1 cfm is SCFM (14.7 psi and 60 deg.f) or ACFM (at 60 psi g)

Do the following checks.

1. Shut off receiver outlet valve and start compressor. Check whether compressor unloads (or stops)in about 35 to 40 seconds.
2. After filling receiver, stop the compressor, open air outlet valve, operate the machine(s) and check rate of pressure drop. This will give you an idea about your consumption.
3. Check for proper operation of pressure switch. (sometimes it may malfunction)
4. Clean the suction filter.
5. If the compressor is loading and unloading type check for proper operation.
6. Clean the suction valves of compressor. If dust accumulates on seat and control plate, discharged air will be bypassed to suction.

Regards,

 

[MEnut]

Thanks for the help, guys! I will put a new tank online. Quark, I wish I could give you the load pattern, but the compressor on it right now is burned up, and I did not know much about this application until it broke (typical, right?). All I know is that the compressed air is used in conjunction with an ozone system. As for the 1 cfm air consumption, I am not completely confident that's the true useage of the application . . . that is what the machine manual specified. As soon as I get the new compressor online, I will run the checks about and monitor the pressure drop.

Thanks again!

 

[ChevyBuilder]

Air Compressor sizing is dependant on the # of permissable starts/hour and the duty cycle of the compressor. Duty cycle or run time is directly linked to consumption or load (pressure,Displacement,RPM). The lower the RPM the better. Air cooled, oil lubed compressors should not exced a 33% duty cycle. Any greater duty cycle cuases overheating and promotes oil carry-over. Oiless compressors can have have varing Duty cycles based on ring and bearing quality, anywhere from 10-75%. # of permissable Starts/hour is mostly motor dependant, single phase and smaller compressors (1/3 thru 2hp) 4/hr, three phase and medium sized (2.5-4hp) 6/hr, 5 and up call the rep, are general rules of thumb. To minize starts/hour and the wear that is produced, get a bigger tank, the differance in a 120 vs a 200 gallon tank is around $100. Adding a tank has the same effect and may be less of a installation issue.