Partial pressure and dehydrators

[bkelly]

I am a systems engineer responsible for some RF hardware. This system has a dehydrator installed that provides dry air to the feed of a satellite tracking antenna. The dehydrator is connected to the feed via about 30 feet of plastic hose about 1/4 inch ID. The feed is about 3 or four cubic feet is size.
The problem is that contrary to the manual from the dehydrator manfacturer, the feed has no air leakage. There is no air flow and the compressor in the dehydrator does not run or runs very little. This means that while there is about 0.3 psi in the feed, there is no air flow.
The vendor claims that since there is positive pressure that keeps out moisture. I claim that this does not remove any moisture that may have been trapped when the feed was last opened and that flow is required.
It seems to be that the partial pressure of water vapor can over come the slight pressure in the feed and moisture will infiltrate the feed. If there is a low air flow of dry air, that flow will pick up the moisture and remove it. If there is no flow, the moisture will accumulate.
The feed has a plastic transparent window at its front and uses a rubber O-ring to provide the pressure seal where it is dissassembled. The O-ring has an electrical resistance of about 1/2 meg ohm per inch in order to keep the feed and its housing at the same electrical potential. That suggests there is carbon in the O-ring. I say this as it might make a difference in its permeability to moisture.
The climate here and in places where this equipment will be used can be quite humid. In some places we expect significant temperature and humidity changes.
As I have not been able to convince the vendor, I am looking for additional advice. Does anyone here have knowledge along this line? If not, can you give me a reference?
As I may well be wrong, I am sending this from home so as to avoid the possiblity of casting unwarrented doubt on a vendor. I will check here for repllies, but feel free to email me at home.
Thanks for your time,
Bryan Kelly
b1@bkelly.ws

 

[Bystander]

You'll want to hunt up the permeability of the plastic used in the line; it might be low enough that the system stays "dry" after sealing.

When you say, "three or four cubic feet," are you describing a refrigerated ballast volume? Such that a failure of the compressor means loss of the refrigeration, and release of whatever water has been trapped at whatever dew point temperature the system has been designed and manufactured to handle?

 

[bkelly]

The volume I refer to is the simple volume of the container holding the electronics that must be kept dry. The compressor forces the air through a dessicant and into the feed (the container). The manfacturer says there should be about 1 to 5 hundreth's of a cubic foot per hour of flow. This feed has no flow.

I don't need any exact figure, just the concept. Is it valid for me to say that simple pressure of about 0.3 psi will not keep out moisture? The problem for the vendor is to establish a leak of the right volume. Too little and the feed will not stay dry. Too much and the dehydrator will need early maintenance.

 

[Bystander]

Compressor to dessicant cartridge through line to sealed volume containing electronics, running at atm. plus 0.3 psi (presumably on-off from a pressure switch) --- okay, got the picture.
Switching to metric to do a little estimating: 30' x 1/4" is ca. 0.2 m²; membrane permeabilities are in the neighborhood of 10 m³/m²/MPa/day for 10-30 μm membrane thicknesses; water vapor pressure is 3 kPa for soggy environments.
So, 0.2 x 10 x 3 x 10^-3 /100_{(membrane thicknesses)} = 6 x 10^-5 m³ water per day. Takes about 2 m³ of dry air per day to flush water that'll come through a plastic tube manufactured from the types of plastic used for semi-permeable membranes.
Permeabilities of plastics used for low pressure fluid handling ought to be 2-3, maybe 4 orders of magnitude lower (no guarantee, that's why you need to check w' mfr.), which gets things down to liters/day or less, which is in the neighborhood of "about 1 to 5 hundreth's of a cubic foot per hour."
Engineered to the "razor's edge." You do want to pin the guy down on the dessicant cartridge capacity --- doesn't sound like you want to be standing around waiting for replacements.

 

[bkelly]

Well, that is an impressive analysis, and I believe it is just what I need.

FYI: The dehydrator is configured with two decissant chambers and a switching mechanism to swap between the two. Air is pumped through one of the chambers into the feed, and the feed should have some small amount of planned leakage to establish an air flow. The off line chamber is heated to dry it out and when the on line chamber reaches some threshold, the status of the two chambers is swapped. Status values are provided to indicate when the chambers and compressor need servicing.

Thanks again for your response, it is just what I needed.
Bryan