Compressor rpm vs heat

[Flybyjohn]

Just got my compressor build almost finished up. I am using the HF 5 hp two stage compressor pump with a marathon 26.2 amp 1750rpm motor. The compressor’s rated rpm is 1180. I ordered a pulley for the motor to get it as close to the 1180rpm, wanting as much CFM as I could get out of it and I overshot it by about 6%. I used a tach and the motor is actually pulling 1785 rpm, a bit higher than expected. That put my measured compressor rpm at 1260 rpm. That is about 6.77% higher than the recommended speed. The pump psi rating is 175psi but I have the pressure switch set to 150 psi.

I ran the pump for about 40 minutes without load and changed oil, then I pumped up the tank and blew out the sprinklers. It took me about 3 hours. Compressor ran 90% on 10% off during that time.

The 5/8 tubing from the head to the compressor tank turned yellow for the first bend out of the head. I tried to measure the temp but the thermometer would not read accurately because of the shiny metallic surface.

So two questions, 1. Will this higher rpm blow up my compressor. And 2. Is the pump head putting out too much excessive heat from the higher rpm’s.

I was thinking that the 6-7% within recommended rpm would be within the safety factor (not sure what safety factor is built into a Chinese pump) but with the tubing discoloring, it makes me wonder if I’m getting too hot. Attached pictures of the tubing discolored. The second picture shows some unused tubing next to the discolored one.

 

[txvwnut]

Compressed air is hot no matter what the RPM of the compressor is. Three hours of basically non-stop run time is a lot of air getting moved and a lot of heat getting created. I bet when you run it under normal conditions you'll see a difference in how hot the tube gets. I would see if you can route that charge tube around the other side of the pump so it will have a little more air flow across it.

 

[Milton Shaw]

My compressor has another copper line for cooling that runs behind the flywheel with a couple of coils and then from stage one output to stage two intake as an intercooler. That helps with the output temp from stage two. Can't see if yours does the same. A longer coil between the stage two output to the tank check valve would help with the life of the check valve as that can cook with such a short loop and continuous use. A safety item would to go on and get another check valve and keep it taped somewhere on the compressor so you can find it when that one goes. That way it will never go bad, much less on a Saturday with a car paint job coming up.

 

[stonesfan68]

I would recommend using a different pulley to stay below the maximum compressor RPM. Your setup will work for awhile, but the compressor will fail prematurely.

 

[finn]

Drop the speed with a new pulley.

It’s not uncommon for consumers to exceed manufacturers recommendations, figuring that the designers throw in extra safety factors. If that was true, warranty claims would be nil, but they aren’t.

You’re not smarter than the engineers that designed and validated the product. The real issue is probably piston and ring temperature at the higher speeds, although reducing cutoff pressure somewhat mitigates that, at least directionally.

Same goes for amateur “tuners” that pedal electronic modifications to increase your truck’s power output. You can get away with it for a while, but you’ll never be as smart as the engineers who have to stand behind the product.

 

[rlitman]

Is that tach the no-load speed of the motor? Is it really running that fast when the compressor is pumping against some pressure in the tank? I'd guess it slows down a bit as the tank pressure goes up.

If you want to get a good infrared thermal reading off the pipe, hit it with some white paint. Or Kapton tape if you've got it.

I would also think that a shroud that keeps the fan air at full force over the intercooler tube and cylinder heads could help a little.

 

[Flybyjohn]

That was the load speed of the motor cranking the pump but not sure exactly the psi in the tank when it was measured. I’m using a propeller tach from my rc airplanes. I could hook up my magnetic tach from my lathe and confirm my numbers.

I’ll order another pulley if it’s still reading too high. I didn’t over size it on purpose, just calculated it for the motor running a little slower than it actually does.

I do have a Derale 15300 I was planing to hook up between the pump outlet and tank inlet, but am still on the fence as I can feel quite a bit of resistance loss over the 5/8” tubing I’m using now just trying to blow through it.

 

[The Metric System]

I used to work as a compressor engineer.

Piston compressor heat with respect to crank speed is highly non-linear due to the limitations of reed valves as piston speed increases and the increasing impact of the reexpansion volume. Basically, the faster you spin the shorter the duration that the valves are open, so the lower the percentage of the compressed gas that makes it out of the cylinder on any given stroke. This retained hot gas not only increases the total heat energy remaining in the cylinder, it (along with shorter intake valve opening duration) decreases the percentage of new cool air entering.

The result is that small increases in shaft speed drive small increases in heat right up until you hit a threshold where another small bump in speed causes a very large temperature increase.

I can't say for sure if this is what happened with your compressor or if you are overheating it, but that sort of color change in copper generally indicates temps of ~200C or higher and this would be well above the working limit of any consumer-grade air compressor I'm familiar with.