[evlatests] EVLA Antenna Feed Moisture Condensation Conflaguration

[Peter Napier]

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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-- 
Peter Napier
NRAO, PO Box 0, Socorro NM 87801
Ph 505 8357218, Fax 505 8357027