Query re. Expansion Nozzle

[AndrewT]

I'm busy doing an analysis on our R22 fridge plant and am
having difficulty understanding which parameters control which.

Firstly: With a expansion nozzle, if the pressure drops, the temperature remains relativley constant, and its isenthalpic, where does that energy go??

Secondly: We seem to have a constant discharge pressure on the compressors, with a slide valve to control compressor output. Is the suction pressure dependant on pressure in the evaporator, which is in turn dependant on our (R22/glycol evaporator) glycol inlet temperature, or is it primarily more dependant on condenser pressure, which is turn is controlled by cooling water.

Thirdly: How does the expansion nozzle control refrigerant flow, and what variable does it look at??

 

[tombmech]

Firstly: You defined that part of the cycle in your question. The pressure drops, but energy is conserved. The conserved energy remains to be used for a cooling heat-exchange in the evaporator. In a purely theoretical sense, a refrigerant gas is chosen because it exhibits significant energy transfer when changing state from a liquid to a gas (and vice-versa) at temperatures that are useful. The compressor and expansion valve are the two book ends of the refrigerant cycle, both adjust pressure up and down, only so that the change of phase of the refrigerant occurs at the condensing and evaporating temperatures available to your system. There is very little change in energy to the refrigerant through either the expansion valve or the compressor - compared to the heat exchange that occurs through the evaporator and the condenser. That is what makes a refrigeration cycle work. Of course, in a real system, there are frictional losses and the compressor adds heat, so superheat and subcooling are often used to compensate.

Secondly: Suction pressure is directly dependent on the saturation temperature of the refrigerant in the evaporator. As you state, that temperature is most likely affected by the heat exchange process in your scenario, and the resulting temperatures. Head pressure is directly affected by condenser pressure/temperature.

Thirdly: The most common refrigerant expansion valve is a thermostatic expansion valve. A thermostatic expansion valve measures temperature at the evaporator outlet to maintain a pressure that guarantees some amount of superheat (setpoint TBD). This in turn guarantees that the evaporator coil is completely filled with refrigerant with no danger of liquid carryover into the compressor.

 

[rmw]

I have a question that I think is generally on topic to this thread. What effect does too much refrigerant have on the cycle? In a similar system, I know I have too much refrigerant in the receiver downstream of the condenser (and before the dryer). Liquid level is supposed to be between two sight glasses, and it is well above the top of the two sight glasses.

rmw

 

[quark]

The main problem with more refrigerant is that you can't provide enough subcooling to the liquid refrigerant (keeping the cooling water temperature and flow rate constant) and you may loose something at energy front. This problem is severe with DX systems. This may not be a big problem with flooded type chillers with liquid receivers but an analysis is always good to do.

 

[sterl]

You might want to offer some insight into this "expansion nozzle" topic. If its a purchased item the manufacturer and anything else you can read off the label would be interesting.

There are refrig systems that depend entirely on gas expansion to do cooling, in which the pressure change is effected at a ture Delaval type critical flow nozzle.

There are arrangements that use geometry much like a venturi to effect refrigerant recirculation, some called Inductors and others Eductors.

And certain systems use a fixed, formed plate type of a nozzle, as per the ANSI test orifices.

Besides TXV's (thermostatic expansion vlaves) there are AXV's Subcooling COntrol valves, Float Valves and so on...Some of which are active in establishing the machine's Suction Pressure.

And most slide valve machine's are controlled off suction pressure; some are controlled from a process variable (for instance, a brine temperature).

Maybe we can offer more if you describe this thing in a little greater detail?