[evlatests] Temperature control in vertex room

[Bob Broilo]

Vivek and all,

> out of spec.  A list is below, and a plot has been
> circulated to the powers that be in charge of HVAC.

Yes, I have the plots.  The problem appears to be during long periods
of low cooling demand (when the outdoor ambient is cool but not cold).
There is a motor-controlled Hot Gas Bypass Valve (HGBP) which allows
the cooling to be modulated.  We have confirmed that this valve opens
and closes completely.  However, when the valve is full open (full
bypass, minimum cooling), some refrigerant still goes through the
evaporator, chilling the water loop slightly.

The reason for this is that the system was designed for the estimated
larger heat load than the EVLA vertex room electronics actually ended
up emitting.  The recent lowering of the supply air temperature helped
to mitigate (or mask) the problem for the warmer months of the year.

There is a device in the loop called the Thermal Expansion Valve (TXV)
which is allowing this refrigerant to flow to maintain the proper
superheat value te keep the compressor happy.  Fortunately, the TXV is
adjustable, and we have a few degrees of leeway in the superheat.

By nudging the superheat up we may be able to allow full modulation to
zero cooling with the compressor running.  In fact we have tried this
on one antenna with success.  We will be adjusting the superheat on
all antennas, prioritizing 17 and 19.

If this plan doesn't work on all the antennas, then a short-term
backup strategy is to inject a tiny amount of heat to balance the
"bleed-through" cooling.  This is a simple program change that takes a
few minutes per antenna.  This is not a good long-term fix because it
wastes a few hundred watts of power for part of the day.

Bob.