In practice, airside flow and refrigerant side maldistribution is very important issues. Although refrigerant flow maldistribution is able to be improved by using good produced distributor, air side maldistribution (based on limitations in the coil design or the distance between fan) could affect the refrigerant side superheats, but does it affect total capacity? I have calculated that air side heat transfer capacity have been changing significantly depending on airside maldistribution degree but I don't know this condition whether to valid on refrigerant side heat transfer coefficient or only its superheat temperatures. (you know that while heat transfer coefficient of air is about 35 W/m2K, refrigerant side is about 100000 W/m2K)
I want to fix something about my question. Namely, refrigerant side heat trasnsfer coefficient is about 10000 and I have calculated air side heat transfer coefficient not to air side capacity. Besides, I think that decrease of air side heat transfer coefficient depending on airside maldistribution must be proportional with total capacity if ref. side capacity isn't affected by air side, is it true? Thanks.
In practice, if you have thermostatic expansion valve, the Temp of superheat is rather constant. The pressure in evaporator will change and pressure in condenser decreases. Also subcooling is smaller.
Below you can see some diagrams I made few years ago during classes. (you have to remember that this was made with high measurement errors, just to see the effect, not for exact numbers)
1 is a normal work of evaporator, 2 is with blocked air inlet to the evaporator.
Thanks a lot your answer but I have said wrong, so that it was misunderstood. Not refrigerant side superheat. That is refrigerant outlet temperatures from each circuits. Sorry again.
Air side maldistribution may seriously deteriorate the heat transfer of air side. Considering the fact that the controlling or largest thermal resistance of the evaporator heat trasfer process is the one of the air side, so the overall heat transfer coeficient is reduced and thus the maldistribution of air will certainly results in the change of the operation state of the evaporator. And the cooling capacity is decreased, so the outlet temperature of the refrigerant may change as well, its superheat may be reduced due to the reduced heat trasnfer from the air side.
Agree with Zhongliang Liu. The overall effect is reducing the cooling capacity, like in ordinary evaporator. I understood you have distinct circuits, that are subjected to different heat transfer rates because of the unequal air distribution. But the distinct refrigerant fluxes are mixed together at evaporator outlet or elsewhere, and the TEV is probably measuring superheat at this point, so the TEV will behave as mentioned above, reducing the evaporating pressure. In any case, the system will adjust accordingly, but the overal effect will be reduced capacity and COP.
Dear Liu and Dear Neto, Thanks a lot for your valuable opinions and advices, I would like to add something. Actually, I have been using electronic expansion valve in my lab, so that my evaporation pressure is always constant, but you know that if my circuits outlet isn't same due to airside and ref.side flow maldistribution, expansion valve throttles,thus cooling capacity decrase as mentioned by Dear Liu. I think that I have solved refrigerant side by using good manufactured distributor but it doesn't valid for airside and I have been investigating this issues. Thanks again.
So, you have an EEV controlled by discharge pressure, so it behaves like an ordinary pressostatic expansion valve, is it? In this case, unequal heat transfer rates at each circuit wiil result in unequal mass flow rates (because pressure loss will have to be equal, since all circuits are linked at the outside). In any way, overall efficiency is degraded (if compared to an ideal situation). Best thing to do is trying to equalize air distribution.
Dear Mr. Neto, evaporation pressure is ensured by second EEV located on auxiliary line owing to PID control, so that we should'nt think the effect of maldistribution on system dynamics. As you mention above, the best thing is to ensure uniform air distribution in order to determine real effect of air. Thank you very much for your advices and opinions.
I think the short answer is that airflow maldistribution at the evaporator will cause the refrigerant-side of the system to behave as though the evaporator were undersized. In the steady-state case this will cause the heat rate at the evaporator to fall resulting from reductions in both vapour pressure and compressor suction gas density. Because compressors tend to be more or less constant volume machines, a fall in suction gas density will result in a fall off in refrigerant mass flow and compressor power input. This will lead to a fall in the heat needed to be rejected at the condenser and so the saturation temperature too will fall there too. If airflow maldistribution is not too great then a well sized TEV should cope. However, if the flow path is badly blocked then it is likely that the TEV will hunt - non-steady case. I think what I've written is pretty much what others have said. There are some publications in Science Direct that may be of interest. To determine more precisely the effect you need experimental data of a good computer model.
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