hot gas / desuperheat adjustment

[envirotech]

we have environmental chambers which have a discharge bypass valve and desuperheat TEV. The same system can be
seen on sporlans doc. http://www.sporlan.com/90-40.pdf figure 3.
These systems have water cooled condensers and normally run with 30psi suction and 200 psi discharge while pulling
down, R404A.
How are these systems adjusted ? With all this gas at the evaporator outlet how can I find true superheat of the liquid TEV ? Should the regulator keep the suction pressure constant during bypass ?
all help appreciated.

[Andy Schoen]

The TEV at the evaporator coil should be adjusted for proper superheat control at or near the design conditions of the environmental chamber. Now that might be a bit of an oxymoron for some of these types of systems, but it is safe to say that adjusting the evaporator TEV at low loads with the hot gas bypass valve regulating suction pressure is problematic.

The hot gas bypass valve should be adjusted at minimum load conditions to maintain desired minimum suction pressure. Return gas temperature should be check at these conditions to make sure the desuperheating TEV is cooling the gas sufficiently.

[txguy]

First you need to know what your minimum suction pressure should be at design.

disable the solenoids to the hot gas and desuperheating cicuit so they stay out of the picture.

set evap superheat at pull down and monitor at low load to make sure it does not flood back.

reconnect hg and ds solenoids

as suction pressure drops below design adjust hg reg to maintain minimum suction pressure.

check compressor superheat and adjust ds injection valve to maintain compressor superheat

load the system to make the suction pressure raise and note closing of hg and ds valves

decrease load to note proper suction pressure maintained

should be done