Re: [TRNSYS-users] overall heat transfer,pressure drop, controlling

[Michaël Kummert <kummert@engr.wisc.edu>]

Florian,

> 1) I have a problem  to verify the "Overall heat transfer coefficient of 
> exchanger" of type 5b .
> The heat exchanger i use,  has the temperature-spreading  primary 
> 85/38°C ,secondary 80/32°C and the power of 15kW.
> Can anyone tell me ,how i get the "Overall heat transfer coefficient of 
> exchanger"?

If q is the heat ransfer rate accross the heat exchanger and UA the 
overall heat transfer coefficient:

q = UA (dTlm)

dTlm is the log-mean temperature difference, which has to be calculated 
taking the heat exchanger configuration into account.

dTlm = (dT2 - dT1) / ln(dT2/dT1)

Counter-flow:
  dT1 = Th,i - Tc,o
  dT2 = Th,o - Tc,i
Parallel flow
  dT1 = Th,i - Tc,i
  dT2 = Th,o - Tc,o

h and c refer to "hot side" and "cold side"
i and o refer to "in" and "out"

[ref: Introduction to heat transfer, Incropera & Dewitt, John Wiley and 
sons, New-York - ISBN 0-471-38649-9]

In your case, I would say:
Counter-flow heat exchanger, dT1 = 5 and dT2 = 6
UA = 15kW / 5.48K = 2.735 kW/K = 9845 kJ/h-K (for TRNSYS)

> 2) Can anyone tell me, if trnsys implicate the pressure drop caused by 
> pipe and does this have an effect on the flow rate in trnsys ( i don´t 
> think so) ?

Pressure drops are not taken into account. The flowrate imposed by the 
pump only depends on its control signal.

> 3) To control the solar circle i use the Type2b old control strategy. 
> (Upper dead band dT 10/lower dead band 2 ) Upper input temperature Th is 
> the output temperature of the  collector, and  the  lower input 
> temperature  Ti  should be the lowest temperatur of the tank . I´m using 
> the tank type4e with 5 nodes . If i had understand it correctly, than 
> node 1 is the top node ( hot input) and node 5 the bottom node (cold 
> input). The type 4e gives as outputs "Temperature to heat source"  and 
> "Temperature of node 1+-1" ,"...-2.",."....-3". So for the" lower input 
> temperatur Ti"  of the controller  i can use " Temperature to heat 
> source" as lowest temperature in the tank.

You are correct, the "temperature to heat source" is the temperature at 
the bottom of the tank (node 5 for you) and the "temperature to load" is 
the temperature at the top of the tank.

Kind regards,

Michaël Kummert

--
_________________________________________________________

Michaël Kummert

Solar Energy Laboratory - University of Wisconsin-Madison
1303 Engr Res Bldg, 1500 Engineering Drive
Madison, WI 53706

Tel: +1 (608) 263-1589
Fax: +1 (608) 262-8464
E-mail: kummert@engr.wisc.edu

SEL Web Site: http://sel.me.wisc.edu
TRNSYS Web Site: http://sel.me.wisc.edu/trnsys