I bought a Varian VHS-4 oil diffusion pump off of ebay about a year ago. The pump was stored outside by the people I bought it from so it suffered mostly cosmetic issues. Recently I have acquired all the necessary components for my UHV (Ultra-High Vacuum) system and began to test the pump. After about 30 minutes of warmup the pump is only able to acheive a chamber pressure of about 1.6 mTorr (this seems to be the max where the water temperature exceeds the rated value). I suspect the issues lies in the fact that the cooling coil (meant to chill the chamber wall to about 60F) has lots of dents which heavily restricts the water flow rate. The water isn't able to remove the heat quick enough which results in decreased performance. Here are some photos of a couple of the worst indents in the coil.



What I want to know is if you guys have any ideas as to how I can repair these dents to increase flow rate. The copper pipe is very small (maybe 1/8" OD) which makes it difficult to replace. The piping has some compound between the piping and pump body but I dont know what it is made of. I thought about TIG brazing or soldering copper pipe to bypass the kinks but Im not sure I would be able to get a water tight seal easily. The pipes are made of copper and the pump body is made of stainless steel so welding is not an option either. There is a powder coating on the pipe presumably for corrosion resistance but I'm not apposed to removing some as it would only be for a few small sections of tubing.

Here is some information from the pump manual regarding cooling coil requirements

"Connect the inlet water fitting (near the inlet flange at the top of the pump) to a continuously running water supply at 0.15 gpm (VHS-4) and at a temperature of 60 to 80 °F".

  • $\begingroup$ Well I might consider annealing those sections then pressurising the pipe using water or oil - but you may need to go quite high and it may split either in those regions or elsewhere. $\endgroup$ – Solar Mike Jun 16 at 16:54

Do this: set up the water flow plumbing with the pump OFF, turn on the water flow, and measure the flow rate by running the discharge into a bucket of known volume for a known period of time, and measure the source pressure. If the dents are restricting the flow, then 40PSI (typical for residential water supply pressure) will not result in a 0.15 gallon a minute flow through the system.

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The cooling load requirement on the water flow is

$$\dot{q} = \dot{m} \tilde{C}_p \Delta T$$

where $\dot{m}$ is the mass flow of water, $\tilde{C}_p$ is the specific heat capacity, and $\Delta T$ is the temperature difference of hot oil to cold water.

Measure the flow rate and prescribed by Niels Nielsen. When it is lower than required, the easiest approach will be to chill the inlet water to account for a lower flow rate. Take 15 gpm at 60 $^o$F as the base. The required water temperature $T_w$ for the lower flow $\dot{V}_w$ with an oil temperature $T_o$ will be found from

$$\dot{V}_w (T_o - T_w) = 15 (T_o - 60)$$

An alternative extreme approach would be to braze entirely new copper cooling cools onto the outer pump wall. Or, as an intermediate answer, cut away those portions that are dented and braze in new sections or caps over them.

Finally, in case you have not done this, you should check the operation of your roughing pump on the backside of the diffusion pump. It takes only a modest decrease in the roughing pump inlet pressure to entirely defeat any attempt to use a diffusion pump to get to vacuum.

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