effect of high subcooling adiabatic process (flash gas effects.)

Does a subcooled liquid (affect the adiabatic process"flash" when exposed to a low pressure drop and sst say (freezer) ie. The wider the td the bigg r r the flash and reduction in potential latent cooling or is it all irrelevant?

Hi Rensei,
In a dx system, too much subcooling in the liquid may affect the efficiency of the evaporator because the first rows of the evaporator will have much less turbulent flow, so lesser heat transmission. But with big td (freezer ie.) you can acomodate a big subcooling without many problems (maybe limited to half of the td).
With flooded systems this may be irrelevant.

Rensei,
You are a rare beast as have experienced apprentices that ask nothing at all for very long periods of time, let alone read an extra book they did not have too.
For a long time there is more questions than answers, so good luck to you.

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Originally Posted by cduque

Hi Rensei,
In a dx system, too much subcooling in the liquid may affect the efficiency of the evaporator because the first rows of the evaporator will have much less turbulent flow, so lesser heat transmission. But with big td (freezer ie.) you can acomodate a big subcooling without many problems (maybe limited to half of the td).
With flooded systems this may be irrelevant.

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May you explain to me why the subcooling would affect flow's turbulence? I mean, isn't it related to discharge pressure and pressure drops along piping? I understand higher subcooling could usually come together with lower discharge pressure, but does the subcooling alone reduce the turbulence?

Thanks

Well, it's not the subcooling by itself that affects turbulence, it's the fact that the flash expansion with a big subcooling produces a low percentage of gas. After the distributer the area is calculated to a certain distribution liquid/gas, with less gas, the velocity will also be lower, with a lower velocity you will have a lower heat transmission coefficient.

Very well explained, thanks ;)

Question: How can you know if a high subcooling is due to a refrigerant overcharge or just to another cause (e.g. oversized condenser)?

I think that a possible effect would be a to get a higher condensing pressure, but if you have a big condenser maybe the difference in condensing pressure with respect to a correct refrigerant charge should not be too much.

Hi mrr,
Excessive Liquid subcooling at the condenser outlet, can be caused by system overcharge.
(I removed 20kg of refrigerant from a chiller onetime before the subcooling dropped to a satisfactory level!)

Air in the system!
(Resulting in high power absorbed by the Compressor and or High delivery Superheat)

Choked Liquid strainer ( Measure the temperature diff across it!) ( Also causing High suction superheat!)
(LP Cut Out (System being starved.)
Grizzly

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Originally Posted by mrr

Question: How can you know if a high subcooling is due to a refrigerant overcharge or just to another cause (e.g. oversized condenser)?

I think that a possible effect would be a to get a higher condensing pressure, but if you have a big condenser maybe the difference in condensing pressure with respect to a correct refrigerant charge should not be too much.

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Hi mrr,
A refrigerant overcharge implys always, as a consequence, an increase in the difference between the condensing temperature and the ambient temperature. You always have to compare with a "normal" situation to be able to classify as overcharge or undercharge

Hi cduque,

I think I know what you mean, thank you. However, in my case im getting about 13 ºC difference between condensing pressure and ambient temperature. I do not know a "normal" situation since this is my first experience with a real system. In literature I have read that the higher subcooling, the better, but I also read that subcooling should not be higher than 5-7 ºC.

I always get about 14 K of subccoling. I have a liquid receiver after the condenser. This element help to avoid any problem with overcharge?

The problem is that i do not know if my condenser is oversized and I should be getting less than 13 ºC of difference between condensing pressure and ambient temperature.

Thank you,

Hi mrr,
If you have 13ºC between condensing temperature and ambient temperature it is impossible to have 14K of sub cooling, as you can never cool to a temperature below that of the cooling fluid.
Anyway if, in nominal conditions, the fluid leaving temperature of the condenser is near the ambient temperature then probably the condenser is over sized or overcharged.
With the usual dimensioning of equipments, the temperature difference between condensing and ambient temperature is 10 to 15ºC. The temperature of the fluid leaving the condenser normally is not far from the middle of those temperatures.

Hi cduque,

Thank you. I guess that degree (14 K instead of 13 as you said) is due to probe error and special characteristics of the condenser. The condenser is inside a room and ambient temperature that arrives is emulated. However the surrounding temperature can be some degrees smaller and thus the refrigerant output can be cooled 1-2 degrees more.

Let ask you one last thing:

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Anyway if, in nominal conditions, the fluid leaving temperature of the condenser is near the ambient temperature then probably the condenser is over sized or overcharged.

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If the condenser was over sized, shouldnt the difference between condensing and ambient temperature be even less than 10 ºC?

If I have some overcharge but a liquid receiver after the condenser? Will this affect to the system performance?

Thank you.

I believe that probably, those conditions you describe, indicate simultaneously some overcharge and some oversize.

With a condenser only oversized it's correct that the subcooling increases and the DT (difference between condensing and ambient temperature) reduces.

With only overcharge that DT increases and subcooling increases also.

Thank you cduque,

There is just a remaining question I have. It is known that, in general terms, , the higher subcooling, the better COP, since the heat transfer rate in the evaporator is increased. However, at the same time, high subcooling may denote refrigerant overcharge. In this last case my guess is that the heat transfer in the evaporator (and thus COP) should decrease.

How can we see this in the mollier diagram?

My guess is that (for overcharge) the condensing pressure rises and thus the quotient_condensing pressure/evaporating_pressure. This increase the energy consumption of the compressor for mantaining the same refrigerant flowrate. I'd like to know if that is the main reason or if there is something else to be added to this explanation.

If I am right, given an ad-hoc condenser, should I add refrigerant until condensing pressure begins to rise? I am trying to understand if 15 K of subcooling can be a good value or always indicates overcharge.

Regards,

If your condenser is at all circuited and sized correctly, it should have .4 to .6 bar of pressure difference inlet to outlet. If your outlet pressure is no more than .08 bar higher than saturation corresponding to your measured condensate temperature, there is very little subcooling. But you can't measure inlet side pressure and compare it to outlet side temperature because the pressure difference of the condenser makes that comparison look like "big" subcooling.

Without knowing what the valve and pipe arrangement between the condenser and the receiver might be: you might want to look at liquid temperature in and out of the HP receiver. If there is a regulating device maintaining a set pressure on the receiver, (an outlet pressure regulator) you can easily have the liquid leaving the receiver warmer than the condenser drain. The regulator is passing a little discharge gas to the top of the receiver and keeping the liquid line pressure High Enough; but that will result in an apparent high subcooling from the condenser if you measure only its outlet pressure and temperature because at the same time, the bottom portion of the condenser will be flooded with refrigerant.