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.

[Doc Ed]

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.

[rcontrera]

That compressor is a Coltri Sub MCH6 that American Briston adapts to their own use and needs at least a 4 HP electric motor. The rated operating speed is 2400 to 2800 RPM.

[pescador775]

The American Bristol compressor was based on a block made by Bristol in England. It was not the Coltri Sub. I doubt that the small "fresh air" is still available in the US. Parts for it may be hard to find. As I said in previous posts, the owner of American Bristol retired and the company went in another direction.

[dogfix]

Here is some additional information on my compressor. It was made in 1979. American Bristol is now owned by Scott Health & Safety, out of Monroe, North Carolina. I have talked with Scott H&S and they assure me that parts are still available, at least for a while.

I don't know what "an unloader" is, at least by that name.

I received no manual or documentation with the compressor. Scott H&S no longer has any documentation for that old of a product.

When I originally got the compressor I took it to Breathing Air Systems in Reynoldsburg, OH. They gave it the twice over, and pronounced it to be in sound working order.

[pescador775]

Believe it or not, here is a Bristol, electric for sale:
http://cgi.ebay.com/Fresh-air-scuba-paintball-dive-breathing-air-compressor_W0QQitemZ290024364728QQihZ019QQcategory Z106436QQrdZ1QQcmdZViewItem

It was the smallest model offered at that time.

[twosheets]

Have boat, need anchor! Will pick up for F R E E!

(just had to say this, I am sure you will find someone knowleable but if you don't...)

[dogfix]

What is the process for calculating pulley ratios if I need to make an adjust to the speed?

[rjack321]

Do you really want to change the speed and know why? Some compressors won't get enough lubrication if you slow them down and speeding them up past design speed is a bad idea too.

Regardless its a simple ratio, diameter drive pulley to diameter compressor pulley. Like 3":12"

If you made that 6":12" - the compressor would turn 1/2 as fast.

[dogfix]

The reason I was asking about pulley ratios is: I'm assuming which ever motor I get will run at a different RPM than the gas engine. So I would need to adjust the pulleys.

Is there a control that I could put on the motor to adjust the speed?

How can I measure the speed the compressor is running at?

[rjack321]

RPM motor / RPM compressor = diameter motor pulley / diameter compressor pulley

You need to know any 3 of those 4 numbers, calculate the 4th by cross-multiplication.

You have the RPM of the compressor (~2500) and can measure the compressor pulley's diameter. The plate on the electric motor gives you its RPM. So the remaining value is the motor pulley diameter.

Or if you want to reuse the motor pulley calculate the other way and figure out what RPM motor to buy.

[pescador775]

If you install a 3450 rpm electric motor you can use the same size pulley wheel. The faster the motor spins, the faster the motor cooling fan turns. Stop worrying about non existent problems like speed controllers and ratios.

[rjack321]

Are you saying over or underreving the compressor isn't a concern?

[pescador775]

The compressor was originally driven by a 3600 rpm engine. What possible harm would be done by using a 3450 rpm motor? It is normal practice to run an electric powered pump a bit slower anyway.

[rjack321]

Gotchya
I didn't have a clue about the gas engine RPM. I agree a slightly lower speed on this relatively high rpm unit would probably be a good thing.

[Hoosier]

How about adding a gas motor to the electric one? I would like to add a gas option, but I am not sure what I need for Alkin portable compressor.

[pescador775]

The Alkin will work with 4-5.5 hp. The Airetex motor is the Honda GX, 5.5hp. Virtually all the small engines have a common bolt pattern for mounting purposes so that is rarely an issue. Some of you guys don't get it. The manufacturers aren't about to supply pumps with different sized pulleys, and the vendors sure don't want that. The motors and engines have to match whatever the stock pump is fitted with. That means motors and engines will have similar rev rates.

[Thalassamania]

An unloader takes the "load" off the compressor at startup by opening all the vavles so that no air is acutally compressed untill the compressor has roatated for a little while.

[pescador775]

The Davey MC-1, etc has an unloader. My homemade 4 cfm portables are equipped with a clutch for the belt drive. The clutch consists of a steel boss which has been drilled, tapped and fitted with large knob. A bearing is pressed onto the boss and when the clutch is installed into its track in the compressor frame the belt presses against the bearing. When starting the engine, the clutch is in the lowered position. When the engine is firing good, the knob is loosened and the clutch is raised to tension the belt and is fixed in that position while operating the compressor.

[pescador775]

Compressors require a substantial torque to get things rotating, and some smaller engines can get bogged down trying to start up a compressor even with all drains open. The 4 cfm Kidde needs about a 6 hp engine to make for a reliable start "in gear". However, with the clutch, the Kidde portable can use the low profile, light weight Briggs 5 hp. Those engines are not EPA friendly and no longer made. Lately, guys have been using the 5.5 Honda for most of the small machines and this is understandable. Right now, I'm working on a direct drive application which will use the 6 hp Kohler. With a six quart tank, this engine should run all day without topping up. Technical issues are several but one interesting problem is reversing the rotation of the Kidde block and this requires modifications to the oil pump, piping and fitting a CCW rotation fan.

[116fire]

Hey I just got a sw 4 cfm, and took it apart. My question is I want to use an electric motor-- can I do a direct drive? The shaft that the gas continetal engine has is strange--tapered and female thread. Do I have to be concerned about introducing axial loads if I use a pully system. Thanks