charge and condensing pressure

HI there,

hope you are fine.
Please, I would like to discuss with you about following topic related to condensing temperature and charge of refrigerant.

Following is the topic.

Let's suppose to have a system with an electronic expansion valve.

I would expect that this valve would regulate the flux so that we would have always the same superheating.

The base case is having the unit charged so that I can have a subcooling of 9F.

Let's suppose that I will now keep away some refrigerant - I will therefore reduce the refrigerant charge.
I would expect
- condensation temperature to reduce
- subcooling to reduce
- superheating to stay constant.

What would I expect from evaporation temperature ?
According to my point of view, since I have an electronic valve, it should be constant.

And about capacity of the unit ?
Basically, if condensation will reduce, considering the compressor curves, I would expect the capacity to increase.
Nonetheless, if this would be true, the lower the charge - up to when we get an acceptable subcooling, the higher the capacity.

Please, can anybody help me to understand better this scenario ?

Many thanks

There is one major flaw in your logic

Your expansion device, whatever type it might be, requires a solid liquid feed to operate. It is to make sure the expansion device get pure liquid that we ensure there is subcooling.

If an expansion valve, be it electronic or mechanical, not receive liquid it will open fully in it's attempt to maintain the superheat. If liquid then is reinstated the valve will only recognise this as it leaves the evoporator and then start to shut down again. This will lead to unstable running conditions with rapid changes of Superheat and it might even damage the compressor.


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Hi Mr Viking,

many thanks for your answer.

I think that indeed the case you are describing is the extreme case that occurs when reducing charge.

I agree with you.

But what I was interested in was what occurs in intermediate cases that the starting one - when we have a specified subcooling - ex 5K, and your case, when we have so much poor charge that from the condensers we have not only liquid but a gas-liquid mixture.

Let's suppose that with a quantity x of refrigerant I have a 5 K of subcooling. I will call this as scenario 1.
Let's suppose I will decrease this quantity so that I get 3K of subcooling. I will call this as scenario 2.

In this case I would consider this subcooling still acceptable for R410A, as example.

I will expect to have a decreasing condensing pressure.
What will happen to
- discharge temperature ? I will expect it to be higher in scenario 2
- evaporating temperature ?
- superheating ?
- overall capacity ?

Thanks in advance.

PaulL,

In my point of view it's very complicated to give a correct answer because with depends of many things.

For example if you have a liquid receiver the liquid can be freely discharged and so the condensing pressure would not be affected by the subcooling at condenser outlet. Also because the condensing pressure is lower you will get lower discharge temperature, lower power consumption, more life time for the compressor and better volumetric efficiency (so more mass flow sucked » more capacity).

(The liquid at the receiver outlet is saturated » no subcooling because there is a equilibrium of two phases, saturated vapor an saturated liquid).

Then you can mount a good coaxial heat-exchanger on the suction line and the liquid before entering on the expansion valve. In this way you can superheat more refrigerant mass flow or opening more a little bit the expansion device and because you sucooling more liquid. The result is you will get more capacity with the same compressor because the difference of enthalpy h3-h4 will be bigger.
On R404A, R507, R410A having much useful superheated is positive to get more compressor capacity. At contrary for R717 having useful superheated it's not positive because the mass density decrease to much and what we can get on vapor sensible heat refrigerated effect is not significant.

Hope this can help you a little bit.

Regards

Sandro Baptista

There should be now rule on liquid sub cooling. You should not compromise condensing pressure to to achieve a high level of liquid sub-cooling.
The primary purpose of sub-cooling "as viking notes" is to ensure a liquid seal at the expansion valve (device).
The system will reach equilibrium, lowing your discharge pressure (and liquid temp) will increase capacity, but then you have to consider how the evap will absorb more energy, is simple terms the suction pressure will drop, to reach the new energy equilibrium. So not quite as simple as first impression indicate

I'd like to thank you all about the interesting and constructive answers.


Mr. Batipsta
You are right concerning your observations.
Indeed I was not clear enough.
What I meant was a basic system, without liquid receiver and heat exchanger to improve superheating.
Therefore the system would have a condenser, a compressor, an evaporator and an expansion valve.

Mr Mad Fridge.
I agree with what you are writing concerning the purpose of subcooling.

Concerning the second part of you answer, may I summarize that what you are saying is:
even if the charge is reduced so that the subcooling is decreased (ex, from 5K to 3K), and therefore condensing pressure is reduced, it is not sure the refrigerating effect will increase, since there is not a rule to predict how the evaporator will react to this change ?
This is a question mark for me: I would expect that the expansion valve would keep the evaporating pressure constant.

sorry to be so stickler, I have many things to understand.

Many thanks,

There many different types of expansion devices, most common one used to day or thermostatic or electronic, which basically do the same thing which is to control superheat, not evap pressure.
If you drop your head pressure by "x" amount and for example have a 20% potential increase in net cooling at the same SST. If this was the original evap, how would this evap absorb this extra energy. Something has to change, and most commonly this the temperature between the refrigerant and the load (the SST will drop) equilibrium is reached. Of course you could just redseign the evap to suit the new design conditions, and get the 20%.

Quote:

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Originally Posted by PaulL

This is a question mark for me: I would expect that the expansion valve would keep the evaporating pressure constant.

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Your valve , TEV or EEV will try to keep a constant SH, not a constant evaporating pressure

Reduction of refrigerant will mean reduction of performance. Removing refrigerant will cause superheat to increase and cause part of the evaporator to run alot colder than specifications, therefore there will be extended running times.Because of less cooling capacity, the system will not allow any off cycles to defrost ice buildup. ( feed me more rum). Less refrigerant than the designed systems amount, will not allow the expansion valve to function correctly, as mentioned above. therefore it, (the valve) will open and will not have any control over superheat, as there will be under standard running conditions. Hope this helps answer your questions. Mike.

The liquid line temperature will rise, which will result in a higher evaporator pressure and lower capacity. This is assuming that the charge is initially correct. Capacity typically peaks at very slightly above a full charge, and will fall with a change in charge level in either direction from that point.