Low Cost Low Cost Molecular Sieve Trap

Update 5-28-23 – We tried this on two different pump set ups and it seemed to work well at first. But on one of the set ups the fine mesh O-ring became clogged with small Zeolite pieces and plugged up the line. We removed the Zeolite from that set up, the other one is working fine so far. It could be a function of the mesh size, we will continue to monitor set up #2.

Molecular sieve traps use zeolite pellets to adsorb water vapor, oil vapor and other gas molecules. They are particularly useful in preventing the back-streaming of oil vapor from the rotary vane pump into the turbo pump and vacuum chamber.

Molecular sieve oil mist trap

Molecular sieve traps are placed between the rotary vane backing pump and the turbo pump. Most molecular sieve traps also have built in heaters that are used to regenerate the zeolite once it becomes saturated with contaminants.

If you have an oil filled rotary vane pump, a molecular sieve trap is a must.  However, molecular sieve traps can be relatively expensive running from $500.00 to $1,000.00 or more depending on the size.

This blog post shows a way to use existing flexible metal hose to make a molecular sieve trap for a lot less.

The general idea is to fill an existing flexible metal hose which is connected between the roughing pump and the turbo pump with zeolite pellets.

Step one is to remove the flexible metal hose.

Next, insert a mesh screen centering ring on the end of the flexible metal hose that is closest to the roughing pump. Connect the flexible metal hose to the roughing pump.

I used this one from LDS – https://www.ldsvacuumshopper.com/nwstceri.html

The next step is to fill the flexible metal hose with Zeolite pellets.  I used these Zeolite pellets (also from LDS) – https://www.ldsvacuumshopper.com/mositrb1zepe.html

Once the flexible metal hose is filled completely (leave a little bit of room at the top) connect the flexible metal hose back to the turbo pump roughing port using another mesh screen centering ring.

Finally, label the flexible metal hose to indicate that the flexible metal hose is filled with Zeolite pellets.  

That way when the flexible metal hose is removed at some point in the future who ever removes it will know that they need to remove the top of the flexible metal hose first and to empty the Zeolite pellets before removing the flexible metal hose from the roughing pump.   You can also hold the bottom centering ring as the flexible metal hose is removed from the roughing pump. (That is one advantage that a regular sieve tap has is that the Zeolite pellets are contained and will not spill out when the sieve trap is removed from the backing pump).

In the experiment where I tried this idea the turbo pump and backing pump worked normally, there was no noticeable increase in the pumping time.   At some point in the future when the Zeolite needs to be baked out heating tape could be used.  Or the Zeolite pellets could just be replaced with new ones. Note that the volume of Zeolite pellets in the flexible metal hose is much less than what a regular sieve trap has, so the amount of adsorption would be reduced proportionally.  Even so I think that this is worth considering, especially in situations where there is no existing molecular sieve trap installed.

Total cost for the two mesh screen centering rings and the Zeolite pellets was under $90.00 Finally, if you are not familiar with LDS you should check them out.  They have a wide selection of vacuum related hardware at good prices and fun categories like REALLY Cheap Stuff and Surplus items.  https://www.ldsvacuumshopper.com/drypumps1.html