Low subcooling ,high superheating

Hi, I am a R&D engineer at a technical refrigeration firm. The firm decided to use hot-gas defrost system in commercial cabinets. In our system we have a suction line heat exchanger (now only heat heat exchanger). In normal refrigeration loop, the gas which comes from evaporator, enters the heat exchanger and it warms. After that, it enters accumulator, then it reaches the compressor. On the other hand, the liquid(maybe liquid-gas mixture) which comes from condenser, enters the heat exchanger that return gas enters the evaporator. Now we expect, when the liquid cools, the return gas warms. Our goal,we should obtain both subcooling and superheating. However, as the superheating is really high, subcooling is almost never. our subcooling is only 0.5 degree celcius, our superheating is 30 degree celcius. What is the problem. Our fluid is R404a, and 1 kg refrigerant is in our system. Is the refrigerant sufficient?

Sadly by the symptoms you described here your system is undercharged.

Normally I would expect the system to be charged until the sub-cooled liquid coming out of the condenser is 4-6K (ºC), then the metering device would be adjusted to achieve the required super-heat and the sub-cooling remeasured to ensure correct charge.
The additional heat exchanger/sub cooler would be in addition to this an a bonus.

But, to correctly diagnose your system all the pressures and temperatures need to be taken in to consideration.


:cool:

What type of expansion device are you using, and is there a liquid receiver in the system ?.
Have you a pipe circuit schematic of the system?

Attachment 14941 Termostatic Expansion Valve

We looked at sight glass in refrigeration loop, we've seen liquid-gas mixture. However, Technicians said 1kg R404A is really high for this system. Probably , We have leakage in this system.