[evlatests] EVLA Antenna Feed Moisture Condensation Conflaguration

[Bob Dickman]

It will be interesting to see whether this holds up over time.  The problem
with GoreTex is that its micropore structure is easily plugged up by dust
and dirt.   

Bob


On 7/28/08 8:49 AM, "Peter Napier" <pnapier at nrao.edu> wrote:

> Just to throw another idea into the pot I mention the following. Mark
> and I were recently on a review panel for the Large Millimeter Telescope
> (LMT) and saw an interesting idea for moisture control. The panels for
> LMT are aluminum honeycomb with surfaces of electro-deposited nickel.
> The panels are not very strong and cannot withstand changes in
> barometric pressure without bulging and losing their surface accuracy,
> so the panels have to be vented to ambient air to equalize inside and
> outside pressure. Since the LMT is at 4600 m temperatures are generally
> low and any moisture ingress into the panel during the barometric
> breathing would be devastating with the panels soon being destroyed by
> freeze-thaw cycling. LMT's solution to this problem is to have a vent
> hole which has a thin membrane of Goretex over it. The Goretex lets the
> air through but keeps the water vapor out. After an initial purge with
> dry air the Gortex essentially does the same job as a desiccant with the
> advantage that it does not need to be maintained. Worth considering.
> 
> Peter
> 
> Bob Broilo wrote:
>> Chuck says:
>>> moisture in the very bottom of the feeds. No moisture was observed at or
>>> near the top of the feeds, which rules out feed leaks as the culprit.
>>> The problem is due to condensation.
>> 
>> I don't think you can rule it out.  Gravity, time, and
>> evaporation/recondensation could move water around in the feed.  I'll
>> bet if you checked them on a cold night the drops would be located at
>> the top, on the cold metal.  I don't think you can rule out leaks from
>> purely the location of the condensation.
>> 
>> My concern is that if the feeds are leaking water, the dry air system
>> could easily be overloaded and become useless.  This happened at the
>> VLBA and we had to ensure that the feeds were tight.  The pressure
>> that can be applied to the feed is limited by the strength of the feed
>> window.  1" WC (approx 1/27 PSI) on the L-Band window is 290 pounds of
>> force.  This severely limits the flow rate of dry air that can be
>> supplied through the purge port.  Each unit of dry air can only absorb
>> so much moisture at a given temperature before saturation.
>> 
>> A positive pressure is not a impermiable barrier against water
>> ingress.  The partial pressure of water in the air wants to pump
>> moisture into the dry space (just like those mystery houses, it is not
>> really going uphill, it just looks like it :-).  So there has to be
>> enough flow to keep the air exchanged and dry.
>> 
>> It sounds like the EVLA feeds have been verified water tight by your
>> inspection, so hopefully that would help the dry air system to work
>> correctly.
>> 
>> The dry air system incurs maintenance costs as well.  There is a
>> compressor, a membrane or reciprocating dessicant system, filters,
>> regulators, etc. that all require maintenance.  The system must be
>> monitored for proper operation.
>> 
>> The VLA is a very dry site compared to the some of the VLBA antennas.
>> The dry VLBA sites (including PT and LA) rarely had moisture problems
>> as the dessicants were PMed twice a year.  The dry air system was
>> primarily for NL and HN, where the purge procedure was hopelessly
>> inadequate for the amount of moisture in the air, SC and BR, where
>> water was ingressing the FRM motors, and then every other site because
>> all the VLBA antennas are the same :-).
>> 
>> The EVLA vertex room is a slightly moister environment during parts of
>> the year that the VLA was.  The cooling coils are at 13C with the
>> glycol instead of 6C that the R12 operated at.  This means that the
>> dewpoint in the EVLA vertex room is limited to 55F instead of 42F for
>> the VLA.  This may not sound like much, but when the diurnal cycle
>> brings the night temperature below 55 degrees then BAM, you have
>> condensation.  If the dessicant is saturated, then that moisture is in
>> the feed, even after the ambient air becomes dry again.
>> 
>> So if the feeds are truly sealed tightly against rainwater, and the
>> dessicants really are becoming saturated relatively quickly, then we
>> should install a dry air system on the EVLA.  Or, if the maintenance
>> on the dry air system is less than changing dessicant packs regularly.
>> 
>> I believe that the dry air system will require LESS maintenance than
>> dessicants.  A diaphram/membrane setup will probably go three years
>> without trouble, and is fairly easy to rebuild.  The feeds MUST be
>> sealed well and we may need to plumb the feeds serially which means
>> drilling a second purge port.  Another benefit is that if water does
>> ingress into the feed, a dry air system will slowly remove it without
>> human help.  The existing dessicant packs also do this in a less
>> active manner, and cease to work when saturated.
>> 
>> I'm just making sure that the prerequisites are met and the
>> performance, installation and maintenance issues of the dry air system
>> are understood.  Also I want to instigate discussion so I get free
>> engineering support from ya'll, at least those of you who survived the
>> above droning.
>> 
>>> Conflaguration
>> 
>> What a great word!
>> 
>> Bob.
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