Wizard, the point I was trying to make is that I don't think sub-zero cooling is worth the hassle. You need to add antifreeze and the combined effect of adding anitfreeze and the low water temperature increases the viscosity of the water so much that you need several times more work pumping: this heats the water, so you need much more cooling. All this to get from 5 mbar Hg to 3 mbar Hg.
If you just float loads of ice in circulating water, you can cool it to almost freezing point and enjoy 5 mbar vapour pressure.
If you look at commercial recirculating aspirator stations, they are often like this:
10 L water capacity
150 W water pump
2 aspirators
Optional refrigerated cooling
This goes with what I have been saying: you want as low a power water pump as possible to do the job - anything more wastefully heats the water. If you calculate from flow and pressure requirements, a couple of aspirators need less than 150 W. High pressure, low flowrate pumps are hard to find, so everyone uses massive 0.75 H.P. jobs for a single aspirator and consequently melts loads of ice. Insulating the system helps too.
Refrigerated cooling is a complicated: difficult to break up an old fridge/freezer and suspend the cooling element in the water. I was thinking: use a separate cooling circuit. First, get your recirculating aspirator station working well, then if you want to avoid ice, add a cooling circuit.
Get a fridge/freezer and place a bucket of antifreeze solution inside and use an aquarium pump to circulate the antifreeze through a cooling worm in the aspirator water. Haven't tried this, but a domestic freezer would have have enough power to cool the water, maybe a fridge would too (with fridges, constant workload is the issue - overheating the compressor).