Understanding CFM?

[unc23win]

I found another used vacuum pump out of a dairy. I was just wondering if my thinking is correct about the CFM capabilities and the hg levels.

From what I can tell using used pump information on the Lapierre site in most cases with the bigger dairy pumps the maximum CFM would be @ 15hg and as the CFM required goes down the hg level will rise possibly to 27 hg.

So when sizing one of these pumps if I want 20 hg I just need to make sure my CFM requirement is 25-50% less than the CFM required to achieve 15% hg. Is this correct thinking?

[maple flats]

While that relationship is valid, most dairy pumps will not achieve 27"HG. It depends on the type. Rotary vane dairy pumps are rated at 15" but with good lubrication you can (and I do) get 20". For this you must open up the oilers or replace them with adjustable industrial oilers. A piston pump can get higher, but again, the oilers must be opened. Higher vacuum generates more heat, and with higher vac levels you have less air (less dense) cooling on the inside. You must address the oilers and you may need supplimental cooling. Maybe a fan blowing on the pump. On an old dairy pump, designed for 15" you could totally ruin it in less than 1 season if you go too high vac and don't address both lubrication and cooling.
As for pump size, I think you are correct. Most if not all dairy pumps in good condition should be OK at 20" vac. but open the oilers. To do this with an Alamo pump, you can unscrew the oiler tube and shorten the wire inside. To do this, break it, don't try to cut it or a burr will be formed that will slow the oil flow. McMaster Carr and Graingers also sell oilers that are made to be adjusted for flow. You want about 5-6 drops/minute on each bearing. Vacuum oil is best but some use other oils with good results. I've heard ATF or any light wt hydraulic oil for some, I use Vacuum oil.

[unc23win]

This is the repsonse I got from Carl Lapierre. Might help some people out trying to achieve higher vacuum.


Happy new year…

Usually pumps are rated at 15 in/hg but some company rates their pump at acfm… (w/o any vacuum)

The formula to get approx cfm is quite simple, you take the rated ACFM of the pump lets go with 100 at no vacuum.

The you divide that 100 by 30 (should be 29.9) then you multiply the result by the difference of vacuum, if you want to know how much cfm is left at 20 in/hg, you substract 20 from 30, your left with 10, so (100 / 30) * (30 – 20) = 33.3 cfm

If the pump is really rated 100 cfm @ 15 in/Hg, it means its 200 acfm so again (200 /30) * (30 – 20) = 66.6 cfm

From there you just need to find out if it is rated at no vacuum or at 15 in/Hg

[michiganfarmer2]

my sp22, 2 piston dairy vacuum pump holds 22" all day. ive been using it that way for 3 years now. Not a lot of cfm though. ....about 18 I think.