PDA

View Full Version : Liquid receiver volume .vs coils/heat exchangers volume







ascorone
21-07-2016, 07:05 AM
Hello guys,

I with my colleagues are working in air conditioning sector and step by step we are developing refrigeration systems. Refrigeration is quite new thing for us, so we don't know all the information (factors) that could affect performance of the system. We have made refrigeration system that we are testing in our climate chamber. So shortly about application:

we are using finned air cooled coils (evaporator and condenser), variable speed compressor, eletronic expansion valve and other stuff (like suction accumulator, liquid receiver, filter drier and so on). Our application is reversible, so that means it's able to heat and cool.

coils specification:
- R410A refrigerant
- in summer evaporator capacity 15,3 kw (at 8 C evaporating temperature)
- evaporator inside volume - 8 litres
- in summer condenser capacity 18,9 kw (at 47 condensing temperature)
- condenser inside volume - 15 litres
in winter system is reversed so evaporator works as condenser and condenser contrary (as evaporator).
- in winter evaporator capacity 5,9 kw (at -14 C evaporating temperature)
- evaporator inside volume - 15 litres
- in winter condenser capacity 7,4 kw (at 27 C condensing temperature)
- condenser inside volume - 8 litres

system specification:
- short piping system
- liquid receiver volume - 2 litres

I've made some refrigerant charge calculation according to defra calculator and it says that:
- in summer maximum refrigerant charge approx 10,7 kg and minimum approx 7,1 kg
- in winter maximum refrigerant charge approx 5,9 kg and minimum approx 4,4 kg

Now our system is charged about 8 kg of refrigerant.

The problem is that my colleagues say that system is overdesigned (coils inside volume differs almost 2 times). Also, when we're doing defrost in winter conditions (system is cooling, because evaporator is iced) we're almost working on low pressure limit (with the intense of low system charge).

My questions:
- What do you guys think, if we will add larger receiver (approx 7 litres, because coils inside volume differs of such amount), will our system work more properly?
- If changing liquid receiver won't help, maybe there's only opportunity to reselect new coil that volume would be closer to 8 litres (instead of 15 litres)? What do you think?


If you need any additional information, please write and I'll try to provide it.

Magoo
22-07-2016, 07:20 AM
Hi, Short answer remove liquid receiver in increase size of out door coil volume and surface area for pump down situations, a larger out door coil in cooling mode will increase sub cooling and efficiency, in heating mode will add surface area for heat recovery and reduce de ice cycles requirements.

ascorone
22-07-2016, 11:27 AM
As I understand if I will remove receiver my system will suffer, because on different conditions (heating/cooling) I need different refrigerant charge (so receiver has to accumulate it). Removing receiver, will cause refrigerant conglomeration in condenser.

monkey spanners
22-07-2016, 04:12 PM
Sounds like you need a charge compensator such as this if i have understood your problem correctly,

http://www.emersonclimate.com/Documents/FlowControls/pdf/systemprotectors/catalog/ACC-Charge-Compensator.pdf

ascorone
25-07-2016, 06:56 AM
I've read about that charge compensator and for other guys discussion I've realised that it's actually the same as liquid receiver.

What do you guys think, if we will add larger receiver (approx 7 litres, because coils inside volume differs of such amount), will our system work more properly?

monkey spanners
25-07-2016, 10:06 AM
A charge compensator is not the same a a receiver. Basically depending on reversing valve mode either suction gas or discharge gas is piped through a tube in the center of it so either heating up or cooling down the body of the compensator. The compensator body is piped to the liquid line so when its cooled by suction gas the liquid refrigerant will condense in the body of it, and when its heated by discharge gas the liquid will be driven out. So totally unlike a receiver in operation. There is only one liquid connection so no flow through the device of liquid.

sterl
29-07-2016, 10:26 PM
Charge compensator accelerates the transition from AC to Heat Pump or vice versa and also shifts the charge from the low pressure duty (htg...) to the high pressure (cooling...). You have already identified the extent of charge shift, and your 2-litre HPR tolerates a swing charge certainly less than 2.2-Kg because as stored (heating) the HP liquid temp is about 27 C and its specific gravity about 1.08. That would appear to be substantially less than the charge swing so if you stick with an HPR it needs to be larger.