RPM control of a pneumatic motor

[ports394]

I'm not sure if this is the right section to ask in, but I'm a MechE, so I'll ask my brethren.

I've been looking for an XP/TEFC (class 1 div 2 electrical area) peristaltic pump. A small pump for flows from 1 GPH to 10 GPH. Not necessarily those exact limits, but in that range.

I can find Masterflex, Watson Marlowes that are "relatively cheap" for regular DC/120VAC motors.

If I get lucky and see one that has a TEFC/XP motor on it, yikes the price. So I started looking at air motors. And I'm not seeing very many out there.

Peristaltic really depend on a set RPM to give you a given flowrate. Is it really difficult to get a consistent RPM on a pneumatic motor?

ports

 

[ports394]

http://www.envcoglobal.com/catalog/product/peristaltic/masterflex-bt-air-powered-pump.html

This is the type of stuff I'm finding with an air motor. It's not controllable.

 

[micalbrch]

Why don't you try an air driven diaphragm pump (AOD pump)? There are plenty of manufacturers and the pumps are not expensive.

 

[Compositepro]

Air motors can be fairly accurately speed controlled. Here is an air motor handbook:

http://www.gastmfg.com/pdf/publications/airmotorhbk.pdf

Check out page 28 for control valving.

An AOD would work and is probably more durable. Use the AOD as a constant pressure source of fluid then control the liquid flow.

 

[ports394]

the AODD is what we're using right now. It's a creative setup.

We need to keep the feed drum circulated and sometimes use an air motor/agitator with it.

A feed tube goes into the drum from the AODD inlet.

AODD outlet = tee. Leg 1 = recirc tube back into the drum WITH pressure gauge and ball valve. Leg 2 = feed into a volumetric (gravity float) flow meter (without needle valve), outlet of the flow meter goes to a needle valve, and then into the dryer.

The dryer is a vacuum drier.

The ball valve on the recirc tube is used to create back pressure in the line and force material into the flowmeter.

The problem we're having is that over a minute or so the flowmeter will drop from 10 GPH to 0. The original flowmeter had a needle valve at the bottom, and the needle valve was plugging. That was replaced with a different downstream needle valve.

The upstream needle valve [past the flowmeter] is necessary, otherwise the material bubbles and evaporates in the flow meter.

The AODD is controlled with a needle valve on the air inlet. I've got a spare pressure regulator that I'm going to try installing on there.


This isn't my system, another engineer built it. But I'm trying to fix it up because it's giving us a ton of trouble. Usually I would just use the AODD, and set a flow rate by pushing it forward into the dryer with no recirc.

 

[Compositepro]

If you don't get the details right changing to a different style pump will not fix any problem. It will just change which details you need to get right.

The simplest flow control scheme is a restrictive orifice with a controlled differential pressure across it. The restriction can be a needle valve, a hole in a plate, or a length of tubing. A ball valve on a recirc. line is not really a pressure control. A needle valve on the air to an AODD is a very crude control on the pressure or flow out of the pump. Plant air is not very constant pressure.

 

[ports394]

Compositepro,

You're right.

My plan is to put a peristaltic pump (electric 120V) for now on the system. It'll take feed from the drum, push into the flowmeter, through the needle valve and into the dryer.

If that works well, then I'll order an air operated peristaltic.

I'm trying out the 120V one for now because I found a complete one on ebay for $150. It's feasibility check.