[TRNSYS-users] TYPE 33 Pipe heat transfer coefficient

[David Ferrari]

Hello all,
 
I am using TRNSYS to model a solar water heater based on a template deck
provided by a well-respected source. I am concerned about the
calculation of heat transfer coefficient U for input into the TYPE 33
(pipe) components. The TYPE 33 component spec requires U based on inside
surface area, so the following calculation seems correct to me:
UP1 = 1/(1/hin+dpipe1/2/kinsul*ln(dpipeo/dpipe1)+dpipe1/dpipeo/hout)
(where hin and hout are the internal/external surface heat transfer
coeffs; dpipe1 is the internal diameter; dpipeo the external diameter;
and kinsul the thermal conductivity of the pipe material).
The result is used in "UNIT 11" as follows:
UNIT 11 TYPE 31 collector INLET PIPE
PARAMETERS 6
dpipe1 Lpipe1 UP1 1000 4.18 25
 
However, a calculation for another pipe elsewhere in the deck is:
UP3 = 1/(Dpipe3/2/k_insul3*ln(Dpipe03/Dpipe3)+Dpipe03/Dpipe3/hout)
(Dpipe3 is the internal diameter; Dpipe03 the external diameter;
k_insul3 the pipe material thermal conductivity)
 
The result's application (in UNIT 21, below) assumes that UP3 is based
on the INTERNAL area:
 
UNIT 23 TYPE 31 pipe between solar preheater and gas booster
PARAMETERS 6
Dpipe3 Lpipe3 UP3 1000 4.18 25
 
The general form of the equation looks reasonable, assuming hin is very
large in this instance. However, it appears to me that the second term
in the equation for UP3 has the ratio of diameters the wrong way around.
I would like to propose a correction of the calculation of UP3 as:
UP3 = 1/(Dpipe3/2/k_insul3*ln(Dpipe03/Dpipe3)+Dpipe3/Dpipe03/hout)
 
Is anyone able to confirm the correctness of this, or am I missing
something obvious?
 
Regards,
David Ferrari
Engineering Consultant
Solar and Thermal Systems Group

 


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