Mil75HK

I joined mostly because I had a problem with my thesis and wanted to see if anyone had any ideas how I might solve it. No luck so far, but getting there... I'm a Swedish guy doing my MSc in Energy Engineering and living in Hong Kong. Will start PhD studies in January at Hong Kong PolyU. =) I like Math, Energy Optimization problems and Acoustics...which will be the area for my PhD studies.

Ahhhh...I found it =) Newbie here you know..hehe..

Oooopps...sorry.. One tend to get a bit blind when working on something in such detail for an extended period of time. =) AHU - Air-Handling Unit CAV - Constant Air Volume I'm writing the Discussion & Conclusion of my thesis so I just need some of these facts straightened out before I hand it in for the last time =) Thanks for all the pointers, it's really great...makes me look at the problem from different angles.

I agree...and I feel so stupid..hehe.. I feel there should be a straightforward way to solve my dilemma, but I just cant see it. I've tried to contact some professors at the uni, but so far no replies, they seem awfully busy. =) Another problem that complicates some of the calculations is that the manufacturer of this AHU wont give me the heat exchanger area or the U-value. The AHU has 3 settings and works as a CAV-unit. It's equipped with two single inlet centrifugal fans with forward-curved impellers and maintenance-free EC motors. I've tried asking a few times for the U-value at these setting or at least the area, but alas...it's classified. However, I calculated the UA-value to see what the relationship between the air flow, UA-value, RH and efficiency was. Oh...and the temperature difference between the hot and cold air streams. So larger temperature difference, higher efficiency...etc..

I love calculations =) I've used the e-NTU method for my calculations for dry air, where you express the effectiveness exactly as you say: e = Q/Qmax Where Q is for the air stream that undergoes the largest change. m(hot)*Cp(hot) = Chot or m(cold)*Cp(cold) = Ccold and then determine which one is Cmin e = (Ch*(Th,in - Th,out))/(Cmin*(Th,in - Tc,in)) = (Cc*(Tc,out - Tc,in))/(Cmin*(Th,in - Tc,in)) based on hot or cold stream, whichever has the largest change The thing is that I'm doing my MSc thesis in energy engineering and all the measurements are finished. There is unfortunately no way that I can conduct any more tests with the equipment either. The equipment I've evaluated is a residential AHU with a counter-flow heat exchanger, that is used for energy recovery. So I'm calculating temperature efficiencies and the effectiveness based on the data I acquired. But, I just cant figure out what formulas or equations to use to calculate the effectiveness when condensation occur, since e-NTU is only valid for dry air. I've been trying to find formulas for this online and in books...but no luck.. I know what happens...and I can explain why the efficiency and effectiveness is poor. But I cannot show it by calculations... I think I'll just have to write the why instead of showing with calculations.. =) Thank you for the help CaptainPanic =)

Thank you for your input CaptainPanic. The idea is to simulate sub-tropical conditions, with hot and humid outdoor air. The indoor air is controlled to be about 25 °C and 50-60% RH. Of course a cooling device is needed indoors. I'm just evaluating the heat exchangers performance and I'm expecting the efficiency to be quite low. But my question is if there is any way to calculate the effectiveness with consideration to the condensation that occurs? Since the formulas or equations I'm using now are only valid for dry air.. Yes, that is one of the points I'm going to make as well.. That in order to cope with the high latent heat a larger heat exchanger area is needed. This equipment is designed for northern Europe, north America etc.. The air-handling unit has two EC fans driving the airflow, but the flow is very low, about 150 m³/h at nominal speed. Going to check if the flow is turbulent or not.. thank you =) I just found the equation here http://en.wikipedia.org/wiki/Dew_point for calculating the dew point temperature. It's only an approximation though..

I've been working on a project to evaluate the performance of an air-handling unit with a counter-flow heat exchanger. When I simulate outdoor air that is hot and humid, in order to investigate the efficiency my results are generally very low. A typical simulation might be: Tout = 33 deg C, Tin = 25 deg C & RHout = 85%, RHin = 55% I measure all the temperatures and humidities in the ducts and calculated the efficiency and get a low result, usually less then 50%. I calculated the dew point temperature (Td) to be about 30 deg C in the conditions described. Td = Tout - ((100 - RHout)/5) approximately Is the reason for the low efficiency due to condensation? Do I need to calculate the efficiency differently when dealing with condensation? Trying to determine the reason for the low efficiency, is it due to the small temperature difference between cold and hot air streams, compared to winter conditions, or/and that condensation occurs? Cheers kep