How to calculate heat exchanger effectiveness when condensation occurs

[Iprenis]

I've been testing an air-handling unit with a recuperative counter-flow heat exchanger. Everything is smooth when I calculated effectiveness and temperature efficiency when there is no condensation.
However, as to my knowledge you cannot calculate the effectiveness when condensation occur, at least not with the same equations. I'm using the e-NTU, which is only valid for dry air.
Can anyone please give some advice on how to calculate the effectiveness when condensation occurs?

Cheers

Kep

 

[Dan1978]

Assuming you are talking about cooling air by passing it over a heat exchanger, the condensation represents a change of state of water vapor to liquid water. So in addition to any sensible temperature change you could quantify the amount of condensation and work it that way. Not sure of any formulas off-hand. There was a post on here about a month ago where on of the senior guys on the site layed out how to figure out your coil leaving temperature - what your asking is very similar so could try that.

Hope that helps.

 

[Iprenis]

Yes that's right.. Hot, humid air is passed over the heat exchanger and is cooled, and condensation occur. I tried to search the forum for something similar but could not find any information that really helped.
I thought it was a simple matter to calculate the effectiveness when considering latent heat.. but maybe it's complicated? I can't find information anywhere online.. Maybe I should just write in the report that the effectiveness is lower probably because of condensation and since it's not readily calculated bla bla etc etc.. something =)
Thank you for the help Dan =)

 

[Dan1978]

When you say your measuring effectiveness, how are you determing that, and what measurments are you taking?

Either way the condensation shouldn't be ignored, it takes quite a bit of energy to lower the temperature of water vapour enough so it condenses out as a liquid as compared to the same amount of air - it is signifigcant. Even if you had some crude way of measuring the weight of a drip pan before and after testing or the entire coil assemble, if possible? you could work out the energy it took to transform the water vapour into liquid. If you know the entering air temp. and relative humidity, and then the exiting air temp, and relative humidity you could do it that way also, probally easier than measuring and weighing condensed water :]

It kind of sounds like you are doing doing performance testing for a product.

 

[imok2]

Unless I'm missing something, isn't that what a moillier chart does?

 

[mauricestoker]

If you are talking about efficiency in terms of energy, I'd consider buying an off-the-shelf differential enthalpy economizer controller and measure effectiveness directly from DC voltage, assuminjg that 24 VDC would be the cheapest electonic analog output. You could probably pick one out of the RE Michels catalog. If you really want to calculate it, I'd recommend looking at the ASHRAE Fundamentals Handbook psychometrics chapter first.

 

[marcoh]

Problem is similar to theoretically calculating latent capacity of cooling coils, and need to know velocity, turbulence, fin spacing, surface temperatures etc etc. Realistically you need tested data as the theory is dependant on too many variables to give an accurate answer.

 

[mauricestoker]

You are very right, Marcoh, I made assumptions. From that standpoint, it is a moot point to begin with and the proper response would be to send the apparatus to a certified testing laboratory.