Re: [TRNSYS-users] Heat Exchanger & Units

["Diego A. Arias" <daarias@wisc.edu>]

Dear Sasa,

>  I am modelling the simple project similar to the "begin" example 
> included in the TRNSYS.  The difference is that instead  of the 
> Auxiliary Heater, I am using the Type 91 Heat Exchanger (air to 
> water) with constant effectiveness of 0.8 .
>  The inlet conditions are as follows:
>  cold side: water, inlet temperature 20degC, 0.09 kg/s
>  hot side: air, inlet=outlet of the solar collector (Temperature 
> is varying over the time), 0.162 kg/s 
>  As a result, I am getting that the outlet temperature on the cold 
> side is almost  constant(20degC) over the August 1st for Madison 
> (USA). Around noon there is a slight increase (cca 1degC)  and then 
> decrease. The curve shape is similar that of the the hot side outlet.
>      
>  1) Regarding the heat exchanger performance with the constant 
> effectiveness...    a)Why does  the outlet temperature on the cold 
> side  decrease or stay flat?  What seems to be the problem?
>  b) If the inlet temperature condition on the cold side is 
> constant, the outlet temperature on the cold side will always 
> decrease. Therefore, how can  change that condition to have 
> temperature  inlet on the cold side as a function of time.

You can compare your results with the calculations using the
effectivenees method.
According to your values, the capacitance of the water is twice the
capacitance of the air.  You should expect a greater change on the air
side (hot in your case) than on the cold side (water).

I am afraid that I did not understand the second part of your question.
Remember that this model is very basic, and is based on first law only:
i.e., it only takes into account energy balance and, under some
combinations of effectiveness and capacitances, you can get that the
cold fluid leaves at a hotter temperature than the hot fluid, and
viceversa.  Please feel free to send me your project if you continue
getting strange results. 

>  2) Regarding the units ...
>   How can I change the units to be e.g. W/m2 when I plot the 
> results on the on-line ploter?

You have to add another component that converts the units: Type57 (unit
conversion routine).

Best regards,

Diego



>   
>  Thank you for your time and consideration.
>  Regards Sasa Pantic
>   
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