My IR 3hp, 60 gal. compressor is on it's last legs again. I rebuild it about every 2 years. Will a 5hp pump work on this without changing the motor? I use the crap out of it and really can't afford to buy a larger unit right now, and I'm tired of rebuilding it. A new 3hp pump is only $200, but if the larger pump will work, I would rather do that. Figured it might not have to work as hard.

 

A 5hp rated pump will work if you use a larger diameter pulley or smaller motor pulley so it turns slower. They tend to run the smaller units faster than they should so you get a decent CFM rating, at the expense of life. Most of the 5-10hp ones only turn 500-600rpm, where the 3-5hp ones turn upwards of 1000-1200rpm. This higher speed along with smaller cooling fins and the smaller fan pulley makes them run at a higher temp that just cooks everything.

 

How can I figure pulley size's needed?

 

This gets real sticky. Keep this constant in mind as you read the following: It requires one horsepower to deliver about 4.3 SCFM of compressed air @ 90 psig at sea level and
78°f. Thats an industry standard for all recirocating or rotary screw compressors. Some will deliver a little more air, some less depending on their efficiency.

When you say 3 HP, do you really mean 3 HP or is it 3 Peak Hp? A real 3 HP (17 amps @ 230v) makes about 13 SCFM @ 90 PSIG of air. A real 5 HP (28 amps @ 230v) makes about 22 SCFM @ 90 PSIG. The larger pump makes more air because it is physically larger. (larger pistons)

Now if you are talking a compressor from a big box store then a 3 peak HP compressor will only make about 7 SCFM @ 90 PSIG. and a 5 peak HP only about 13 SCFM @ 90 PSIG.

In either case to drive a 5 HP compressor with a 3 HP motor you would severely overload the motor unless you changed the pully/belt system to slow the 5 HP pump down to stay within the amperage load of the motor but then you would still only get 3 HP worth of air from the 5 HP pump.

Hope this is clear to you. I have tried to explain it as best as I can.

Also why does your I/R need to be rebuilt so soon? Most industrial rated machines are good for 100% duty cycle. It should run all day at full load W/O harm. Is the area around the machine clear? The real killer of compressors is heat. You should do everything possable to keep it cool.

Here is another thing to consider. Compressing air up to 100 PSI increases its temperature 600°. Cooling devices help to dissipate this and usually we only see discharge temps in the 250°-300° range.
Now take your diesel truck, it compresses air up to 350-400 PSIG without cooling devices. Its temp is now nearly 2000° and you know what happens when you inject fuel into that don't you.

 

Very clearly explained. It's a true 3hp, something like 5hp peak, 11.3cfm@90psi. It does say continuse duty, and I do use it as such. With the pumps warrantied for 2 years, it's about right on my rebuilds, but I have only been doing the top end. With the price of a full rebuild kit, I may as well just get a new pump. It'll last me till I can afford a bigger compressor. You have verified it would be a waste of time to just upsize the pump. Appreciate the help.