Any HVAC guys here?

SMSW

Joined
Sep 4, 2013
Messages
8,130
Location
Cleveland
Bike
2010 ST1300
I have a Robinair vacuum pump that I use for everything but it's designed purpose (evacuating an air conditioning system prior to charging it). Most recently, I used it to apply veneer to a substrate, and it was pulling 20 - 23" of mercury vacuum. I get a fine oil mist out of the vent tube. I've noticed this virtually every time I use the machine. Is this normal? Is there any way to prevent it? Because of this oil mist, I don't use the machine indoors.

Were I in the business and using this regularly, the pump would be quite a mess unless I wiped it down after every use.
 
Not an HVAC guy but I work in an industry that uses lots of large vacuum pumps on industrial plants. We mainly use liquid ring vacuum pumps but in my early days sliding vane pumps were common but no longer used due to noise and oily emmisions. If the Robinair uses a sliding vane design then it needs lubricating oil as the vanes slip in their guides and ride upon the casing with every rotation, and there is bound to be some oil that is lost to the air stream and not captured by any filter.
 
TerryS said:
Not an HVAC guy but I work in an industry that uses lots of large vacuum pumps on industrial plants. We mainly use liquid ring vacuum pumps but in my early days sliding vane pumps were common but no longer used due to noise and oily emmisions. If the Robinair uses a sliding vane design then it needs lubricating oil as the vanes slip in their guides and ride upon the casing with every rotation, and there is bound to be some oil that is lost to the air stream and not captured by any filter.
Click to expand...
I think my pump has a 2 stage rotary vane design, and yes, I do fill it w/ oil and am supposed to change it after a certain number of hours of operation. There is no visible external filter on what I call the 'exhaust'. If there is one, it must be a mesh type and internal.
 
Your pump is designed to pull down fairly close to 29.92 inches and achieve a vacuum of some odd 20 or better microns [760,000 microns or so in an an atmosphere 14.7 psi] this pressure will not only evac the air but boil out any moisture prior to charging with refrigerant [provided the oil is kept clean and changed as needed]. My guess might be that your's was no good for that anymore since it couldn't pull down. Even a bad one though can be better than nothing and used for a tripple evac as long as a system hasn't been exposed to moisture. There will probably be a black plastic cap sort of plug looking thing on top of the fill reservoir that you can loosen up. If you run the pump in this way, it will operate in single stage mode and move more volume.
I've had and used dozens and all of these pumps spit oil mist and drops during operation.
Note that they are not designed for any back pressure and historically manufacturers have made warnings fairly clear so be carefull of any coalescing or other type of filter you might want to install. I just cover the discharge with some loose rags to catch the oil if I'm using mine in a clean enviroment.
The only back pressure mine sees is in instances where I'm not using a micron gauge when the pump starts to make it's signature sound that it's close to a complete vacuum [or the best it's gonna get] I'll hold my thumb over the discharge briefly and see if how long it takes to build up some pressure.
[added] and it's also not designed to begin a pull down at anything above atmospheric pressure, don't expose it to anything more than a slight puff above atmospheric pressure at start.
 
Last edited:
SMSW said:
I get a fine oil mist out of the vent tube. I've noticed this virtually every time I use the machine. Is this normal?
Click to expand...
Kinda normal; they do have an oil trap, but fine mist will pass/get drawn by the gas flow, hence more visible upon starting the pump at atmospheric pressure (max volume)...
It is kinda worse when the pump is overfilled, like by foreign oil drawn in when vacuuming refrigeration lines on units already been in operation...
It also changes with ambient temperature, like cold, damp air vs hot dry days...
Also possible is water vapor, i.e. some condensate trapped in the oil or getting drawn out the hose/pipes/device attached; maybe other substances (like when used on an epoxy/resin cast pot)...

Quarterly oil change kinda mandatory to get rid of any contamination, ensuring max performance and avoiding premature wear of the sliding vanes...
However, do not overfill, see manual for detailed oil specification/brand and recommended max level in the view-glass...
 
You must log in or register to reply here.
Back
Top Bottom