Compressor conversion question

[dogfix]

I have acquired a scuba compressor. Price: "take it or I'm turning it into a boar anchor."

It is an American Bristol, FreshAir, Model 3S4
PSI: 4200
RPM: 2400
CFM: 4
5 HP gas engine.

I'm interested in converting it from gas to electric, but I'm not sure how to determine what size motor to use.

Any ideas?

 

[loosebits]

Call Porter at August Industries: http://www.augustindustries.com/

He's probably one of the most knowledgable compressor people out there and he has no problem spending lots of time with the hobbiest.

 

[pescador775]

You need a 4 hp motor. Recommend 220 volt/single ph.

 

[loosebits]

And you will notice that they come in two different price ranges. The cheap ones are usually in too small a frame and are overrated. Everyone has told me that they are junk. Our 5HP motor was $450 new (there were other 5HP motors that were around $250).

 

[captain]

Gas and electric HP needs are slightly different. Generally more HP is needed when using a gas engine because torque which is what makes things turn is the same on an electric motor no matter what the RPM where as with a gas engine torque varies with rpm. My compressor was originally 3 HP electric but requires a 5 HP gas engine.

 

[Ed Palma]

to be on the safe side, is it safer to go with more torque then?

 

[cool_hardware52]

to be on the safe side, is it safer to go with more torque then?

Yes. Too much motor is better than too little. I'd first check to see you have sufficient electrical service, first into the building, and second near where the compressor will live.


Tobin

 

[captain]

Say it takes 10 foot/pounds of torque to start the compressor turning. Say a 3 HP electric motor produces 10 foot/pounds of torque. It will produce that 10 F/P of torque the instant the switch is turned on. Gas engines produce very little torque at low speed and it increases as speed increases. Now say a 3 HP gas engine will produce 10 F/P of torque but it needs to be turning 3000 RPM to do so. It will not start the compressor turning but it would keep it running if it was already running. In order to start the compressor turning you will need a gas engine that will produce the 10 F/P at the start and it would take maybe a 5 or more HP engine to do that.

 

[Thalassamania]

don't know about your Bristol but many compressors have an unloader that lowers the torque needed to get it going. Then can often be retro-fit (but you likely know this).

 

[pescador775]

The military uses 3-4 hp motors to drive their 4 cfm compressors. My Capitano, rated 5 cfm, has a honking big 5hp, and it is plenty. Today, it is fashionable to poo-poo the old rule, 1 hp per cubic foot. However, that is something which only people like myself, who build and overhaul compressors and pumps for a hobby, would likely know. The guy who owned American Bristol retired about ten years ago. However, back in the early 70's when Bauer offered the limp wristed Purus at 1.7 cfm and used a laughable condensator design, Bristol came out with the 4 cfm and one of the first integrated filter/condensators for portable compressors. Among our circle, this filter was admired; so, one fellow decided to reproduce the design in his machine shop. To this day, these are the filters which I use on my home made portable Kidde compressors. Bristol's Mr L didn't object, he was part of the group.