My Vacuum Sucks...or not

[JStevens]

Greetings,

I have recently been given a problem to solve and I am not quite sure how to tackle it.

The production group where I work uses vacuum chambers to dry 96 well plates. For sometime they were using 4 chambers all connected to the same pump. Recently they purchased a large chamber, equal in volume to the 4 small chambers, they are using the same size pump and the vacuum gauge reads -28.9 inches of Hg. However after the pump has been running for 16 hours the relative humidity is 80% in the large chamber and is practically 0% in the smaller chambers.

This is my first day on this project and wanted to get some ideas before beginning. My thoughts are as follows and hopefully are right, or pointed out as wrong before I make myself look foolish.

1) The tubing going to the smaller chambers is much smaller(ID) than the tubing being used on the larger chamber. Is it possible that this increases the suction in the chamber?

2) The smaller chambers have baffles over the exit port where the pressure is coming out of the chamber, does this in some way affect the suction?

3) Would it be reasonable to bring the chamber to vacuum and then back fill with nitrogen, and then repeat the process until the plates are dry?

4) The large chamber has one large exit port, each of the small chambers has one small exit port. Would making more exit ports in the larger chamber improve air flow and therefor reduce the humidity faster?


Any help will be greatly appreciated.

[mgb_phys]

Do you mean you are running the large chamber connected with the same pump as the smaller ones or it has it's own pump the same size?

You want the largest possible diameter shortest connection between the chamber and the pump. Ideally in high vac stuff you mount the pump directly on the chamber.

Whats the temperature in the chamber, if it's cold it's very difficult to pump out water vapour.

Is the pressure gauge on the pump or on the chamber? Is it possible something is blocking the line and you aren't pulling much of a vac in the chamber?

[JStevens]

The Large chamber is connected to a pump of the same size as the smaller chambers. The pressure gauge is connected in the vacuum line, maybe 4 inches away from the chamber. I will try reducing the distance between the pump and the chamber. I didn't mention before but there is a freeze trap between the pump and the chamber to collect the water vapor. Other than that the chamber is at room temp. Thanks for the suggestions though.

[mgb_phys]

Most of my experience is on relatively small high vac cryogenic chambers rather than industrial freeze dryers.
New chambers take a while to settle, there is water from the atmosphere absorbed into the surface and machining oil and cleaning materials left in the surfaces.
It's normal to need to bake in the chamber by heating it (the hotter the better) while pumping.

[andy taylor]

hi j stevens...the reason you have a high humidity on the larger chamber , is basically down to the sheer size of the chamber...with vacuum systems the larger the chamber the more condensation catalyst surface are you have....depend also what you are doing the process...as mgb says you could bake the system also..i would doa back fill of nitrogen will keep the chamber surface less likely to sukc moisture from the atmosphere in...another consideraion is the matreial finish of the chambers..