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