Air compressor run time

[70-K5]

Long story short, I bought a very used Big Red Air ( SnapOn brand) for $200, got it all back together. I'm wondering if it meets performance specs after I honed cylinders and put new rings in.

It is Ingersol Rand 2340 pump and 80 gallon tank. It took 2 min 45 seconds to cycle, 140psi to cut off at 175psi.

This is more than enough air for my shop, just wondering if it's operating close to original spec.

 

[vanapplebomb]

Sounds right in the ball park for a two stage 5 hp compressor. I would say you are good.

 

[ToolFanGeoff]

70-K5,
I have found this formula to be reasonable (easy, and close enough to get a feel for performance): Calculating compressor actual cfm = TANK GALLONS x .536* x PSIG divided by SECONDS
Using the formula above, I get just about 9.1 cfm.
Geoff

 

[Citation]

My calculation is you are getting about 9.5 CFM average over the 140 to 175 psi range. That looks like a nicer 5 or 7.5 hp unit so I would expect something better than that. 15 cfm would suggest about 1:45 for that same fill while 20 cfm would do it in about 1:15. You might have a valve that is leaking. Or maybe that is all the pump is really good for. Also, I assumed you were around sea level. If you are in the mountains flow will be lower.

 

[GeoBruin]

Using another approximation, with Boyle's law, we can calculate that your compressor holds 101.9 CF of air at 140 PSI and 127.38 CF of air at 175 PSI. That's a difference of 25.48 CF. So if your compressor compressed 25.48 CF of air in 2.75 minutes, that's 9.26 CF in one minute, or 9.26 CFM.

To double check my math, I plugged your tank size, initial, and final pressures along with the estimated 9.26 CFM number into this air compressor pump up time calculator and I got 2.74997 minutes which is exactly what you reported.

 

[scooby074]

Sounds about right to me.

 

[70-K5]

Thanks for the math everyone. sounds like it isn't operating at full capacity... The cylinder walls were pretty wiped when I started, I ran the hone through it and tried to get a good cross hatching, I didn't mess with the valves at all, maybe I should have replaced those while I had it apart? Or, am I under driving the pump? It is an IR 2340 and my electric motor is turning 1,725rpm. I haven't put a tape on the pulley's but the pump would be considerably under that

It is really more air than I think I need and a nice quiet upgrade compared to my harbor freight compressor that was a noisy beast and I'd say put out half the air.

 

[GeoBruin]

Thanks for the math everyone. sounds like it isn't operating at full capacity... The cylinder walls were pretty wiped when I started, I ran the hone through it and tried to get a good cross hatching, I didn't mess with the valves at all, maybe I should have replaced those while I had it apart? Or, am I under driving the pump? It is an IR 2340 and my electric motor is turning 1,725rpm. I haven't put a tape on the pulley's but the pump would be considerably under that

It is really more air than I think I need and a nice quiet upgrade compared to my harbor freight compressor that was a noisy beast and I'd say put out half the air.

If it's making enough air and it's quiet, that sounds like a win. That said, valves seem to be a common place to start seeing performance degradation.

 

[Citation]

Thanks for the math everyone. sounds like it isn't operating at full capacity... The cylinder walls were pretty wiped when I started, I ran the hone through it and tried to get a good cross hatching, I didn't mess with the valves at all, maybe I should have replaced those while I had it apart? Or, am I under driving the pump? It is an IR 2340 and my electric motor is turning 1,725rpm. I haven't put a tape on the pulley's but the pump would be considerably under that

It is really more air than I think I need and a nice quiet upgrade compared to my harbor freight compressor that was a noisy beast and I'd say put out half the air.

Take a look at the specs here

https://www.grainger.com/product/INGERSOLL-RAND-Air-Compressor-Pump-Splash-4KR37

Per these specs you should see 14cfm at 175 psi at around 1500rpm (pump speed). If your pump speed is say 60% of that rated mac speed your air delivery rate will be 14cfm*.60 . If your pulleys are sized for a 3450 rpm motor then your 1725 rpm motor is the issue. You would need to correct the pulley sizes. The motor pulley should only be slightly smaller than the pump pulley.

 

[70-K5]

Compressor is doing great, apnost done cleaning up shop so I can start using some tools.

I looked more into this sheave sizing.

Motor is 8" while pump is 14". According to a calculator the motor at 1725 would be driving the pump at 986 rpm.

The pump is rated at 1725, obviously I could put a 10, 12 or even 14" sheave on the motor.

But would I gain anything besides decreased runtime? Is running a pump for 90 seconds at 1700 rpm gping to put less wear than running for 3 min at 1,000 rpm, or is it a push?

 

[The Cobbler]

higher rpm puts more stress on everything, including the motor. it may not have the HP to spin the pump at that pulley ration. it will certainly load it down more than the lower rpm.
I would leave it alone

 

[GeoBruin]

Compressor is doing great, apnost done cleaning up shop so I can start using some tools.

I looked more into this sheave sizing.

Motor is 8" while pump is 14". According to a calculator the motor at 1725 would be driving the pump at 986 rpm.

The pump is rated at 1725, obviously I could put a 10, 12 or even 14" sheave on the motor.

But would I gain anything besides decreased runtime? Is running a pump for 90 seconds at 1700 rpm gping to put less wear than running for 3 min at 1,000 rpm, or is it a push?

You will get more flow. Yes, you will fill the tank faster if you're just using a little air and the tank pressure drops below the cut in pressure, but if you're using a lot of air constantly, the amount of flow will dictate how long you can work before dropping below the regulated pressure.

Spinning the pump faster is how a lot of compressors meet the advertised CFM, but often it is at the cost of additional heat and noise. The best compressors are able to make high flow numbers at low RPM, minimizing heat and noise.

If you can measure the current you're drawing as the compressor reaches nearly full tank, you can guess at how hard the motor is working with the current pulley setup. If, for example, the motor never draws more than 15 amps, (depending on the motor rating) you've probably got some headroom to run a little larger drive pulley. But if you're already near the max nameplate current, you might be taxing the motor too much to increase the load.

It will also get noisier if you run it faster.

 

[Citation]

Compressor is doing great, apnost done cleaning up shop so I can start using some tools.

I looked more into this sheave sizing.

Motor is 8" while pump is 14". According to a calculator the motor at 1725 would be driving the pump at 986 rpm.

The pump is rated at 1725, obviously I could put a 10, 12 or even 14" sheave on the motor.

But would I gain anything besides decreased runtime? Is running a pump for 90 seconds at 1700 rpm gping to put less wear than running for 3 min at 1,000 rpm, or is it a push?

The pump should have a minimum rated speed. So long as you are over that speed you will be fine. Under that speed and lubrication can be an issue and the pump will wear out faster because of the slow speed. Slower, but over the minimum speed, does reduce heat and noise. Probably doesn't affect pump life much as you still need about the same number of revolutions of the pump to fill the tank. It's easier on the motor but probably not enough to matter. It's easier on the pressure switch which might matter.

My guess is you are still within the ok speed range for the pump but I would feel better if I could verify that.

 

[timgunn1962]

As a general rule, and subject to keeping within the design range, it's best to run the compressor at the lowest speed that gets the job done. It's quieter, cooler and puts less stress on the mechanical components that way. If it isn't getting the job done, it might be worth looking at speeding it up, but that doesn't seem to be the case for the OP.

 

[PoorUB]

The specs I come up with give a max RPM of 1575, so about a 12-3/4" motor pulley.

I wonder if it didn't have a 3450 RPM motor on it originally, although that would run the pump a too fast.
Who knows what the previous owner did! At least it is a popular compressor and you could find some local store selling a 5 HP with the same pump and see what motor and pulley they are running.

On the other hand, if it supplies enough air for you, don't worry about it. I would run it as is, unless you buy some tools that need the extra CFM. You can also drop the max PSI on the pressure switch too if you do not need 175 PSI. It may run a tiny bit more often, but shorter cycles. Lower pressure equal less heat and wear on the pump.

I run my Curtis at 150 PSI max pressure and regulate my shop air to 120 which is higher than recommended for most air tools, but I am not to concerned on wearing them out.

 

[Citation]

The specs I come up with give a max RPM of 1575, so about a 12-3/4" motor pulley.

I wonder if it didn't have a 3450 RPM motor on it originally, although that would run the pump a too fast.
Who knows what the previous owner did! At least it is a popular compressor and you could find some local store selling a 5 HP with the same pump and see what motor and pulley they are running.

On the other hand, if it supplies enough air for you, don't worry about it. I would run it as is, unless you buy some tools that need the extra CFM. You can also drop the max PSI on the pressure switch too if you do not need 175 PSI. It may run a tiny bit more often, but shorter cycles. Lower pressure equal less heat and wear on the pump.

I run my Curtis at 150 PSI max pressure and regulate my shop air to 120 which is higher than recommended for most air tools, but I am not to concerned on wearing them out.

I was also wondering about a motor replacement. But looking at that compressor and seeing things like the magnetic starter I could see it being a nicer unit that would have a 4 pole motor. Looking at pictures on the web I don't see those units coming with IR pumps. I generally don't see IR pumps on anything but IR compressors. My guess is the pump is not original and the pulley sizes are just what was on the pump and motor.

 

[Citation]

Per this listing the minimum rpm for that pump is 700 so you are well inside of the safe speed range

Ingersoll-Rand 2340 Air Compressor Pump, 1.5-5-hp, 2 Stage

2340 Ingersoll Rand type 30 two-stage bare pump. Includes two-stage, 100% Cast Iron Pump and air filter canister.

www.portlandcompressor.com

You have some overhead if you ever need it.