Originally Posted by
icemeister
The superheat could still stand to be bit lower to around 4-6°F. You'd be amazed at how much difference it can make. But be sure you're measuring the suction pressure at the coil and not at the compressor when you figure the SH because that couple of pounds of pressure drop in the suction line will throw you way off in a hurry at these temperatures.
Your subcooling usually won't get much higher than what you've got simply because it's got a receiver in the system. There's no adjusting that with charge...or anything else. And yes, by definition at 3°F SC you'll have a clear glass.
Get it out of your head about altering the suction pressure by messing around with the expansion valve. It's not a pressure regulator...it's a superheat regulator.
As for the nozzle question, we've already made a fix of sorts by upsizing the TXV capacity. This did one of two things. Either the nozzle was actually OK sizewise and the TXV was undersized...or the other way 'round. So at this point, unless you find you can't get the SH down to where you need it, I wouldn't be concerned about the nozzle sizing. The superheat is a major factor in telling you whether the coil is performing or not.
The one thing which stands out here is the relatively high TD on the evaporator. You say the box is at 0°F and the coil is at -20°F SST, which gives a 20°F TD. At design conditions it should be a 10°F...but what are the design conditions for this job?
By the numbers we have, the coil has a specific capacity of 9400 Btuh at a 10°F TD and the condensing unit has a capacity of about 9400 Btuh at -20°F SST and 90°F ambient, so I'm assuming the system was selected for a -10°F box temperature...correct?
I think a lower superheat will help lower the TD and subsequently raise the suction pressure & lower the box temperature...but another 10°F lower may be a push. Of course there may be other factors in play here.