Heat exchanger

[nymapleguy607]

So here is kind of an off the wall idea I had about my vacuum setup. Right now I am running an old delaval pump and I keep a vacuum regulator on it to help keep the pump from over heating. This works well for the most part but the pump will still get warm to the point you don't want to hold your hand on it. (it still passes the spit and sizzle test) So my thought was what if I were to build a heat exchanger and place it in a tank of water to chill the incoming air into the pump? I would run it ahead of the moisture trap just in case any water got into the system. I thought it might be something to help keep the pump cooler besides running a fan over it.
Thanks Jeff

[DrTimPerkins]

....(it still passes the spit and sizzle test)

Not sure about the answer to your question...probably a bit more complicated than that, but it might work.

If however you're sitting around the evaporator drinking a few boiling sodas with your friends, you can impress them by calling the above "spit and sizzle test", the Leidenfrost Effect Test (the technical term for the way water hops and skittles around in a frying pan when it is hot). http://en.wikipedia.org/wiki/Leidenfrost_effect

[Dennis H.]

Not sure that it will work, let me try to explain.

Once the tubing system is under vac and has reached the desired vac level there will not be very much air movement in the system unless there is a vac leak, not a good thing.
This is where the vac regulator comes into play as been mentioned. It lets in a little air that not only regulates the vac level but allows the cold air,"leak", to help cool the vac pump.

Or you can feed oil thru it and then cool the oil with a radiator of some kind.

[Haynes Forest Products]

NYM607 Im with Dennis You would get better results with a coil into cold water that carried hot oil from the pump and back into the intake. The air that does make it into the pump is cool because it has been mixing with the cool sap all the way down the lines.

[Teuchtar]

Jeff
You are exactly right. Chilling the inlet air is a very effective way to limit exhaust temperature. Vane pumps are essentially little air compressors which behave according to the thermodynamic relationship T2 = T1 * Pr **(.286). Use degrees Rankine, not F in this equation. DegR = DegF + 460.
T2 is the exhaust temperature, and the one that causes all the hurt to the machine.
At 20" vacuum, your pressure ratio is 3:1. So the temperature relationship is T2= 1.37(T2). In other words, 1 degree of inlet temperature change is worth roughly 1.5 degree at the exhaust.
On a warm spring day, with sun on your pipeline, you'll easily see 100 degF air into the pump. That gives 308 degF exhaust.
If you can reduce that temperature back to 40 degF by use of a heat exchanger with coldest water you can find, your exhaust will be down to a comfy 225 degF.
With that cooler inlet temp, you could go ahead and crank down the vacuum to 23 inches Hg before the exhaust rises back up to the same 308 degF.
So, inlet air cooling just bought you ability to make more vacuum before you toast your pump. Actually the toast thing takes a while. As temperature rises, your pump makes more varnish (brown coating on hot parts) and carbon (hard wear particles). The varnish coating causes more tip leakage (vacuum loss), and the hard carbon wears everything. Below 300 degF it takes forever to hurt, but as temp goes above 350 degF, it really takes off. Oil will smoke at some point, and youve gone too far. Local hot spots (air temp, aggravated by rubbing tip seals) give you hotter temp yet, with a snowballing effect the older this old horse is. Your mileage may vary.
So, cool your inlet, keep the oil fresh and cool, and cool the casing also.
Better yet, Buy a liquid ring pump. Instead of Adiabatic polytropic compression, they use isothermal compression because the entire pumping cycle is flooded with cold water. Much less sensitive to air temp, and much less pumping work. You can crank the vacuum all the way down to 29 "Hg and as long as you have cool service water, the pump parts will not increase (much) in temperature.

[Dennis H.]

I like the info you gave, when it is explained that way it makes sense.

Question for you, won't the cooling effect of the incoming air to the vac pump be minimal when the entire system has stablized at the desired vac level?

I can see when you 1st fire up the vac and it is moving large volumes of air but when it running at the desired vac level I thinking there will not be a large volume of air moving. Well I hope not anyway, otherwise there are leaks.
I know that there are gases that get pulled in from the trees but is it that much?

If adding a heat exchanger to the inlet is worth it I am more than willing to give it a go.

Again thanks for the info.