Hello All + Happy New Year!
I am modeling a liquid-source heating coil
with known liquid inlet temp that conditions outside air
entering with a constant flow rate to a constant desired
outlet air temperature. My intention is to use the bypass
fraction scheme to select a liquid-side pump by recording the
coil liquid flow rate (fraction of max liquid flowrate that is
NOT bypassed and passed thru coil to condition airstream).
This is performed using a Type753e heating coil, a Type2
aquastat (heating), a Type 15 weather data, and an equation
block:
1.
The EWT for the liquid loop
(60C) and desired outlet air temp are constant (20C) and
directly specified as fixed inputs to Type753e.
2.
Type2 observes the outside air
temp from Type15 and generates “1” or “0” depending on whether
the outside air is above or below a specified fixed setpoint
including deadband (20 +/- 3 C).
3.
This control signal is passed to
an equation block where it is multiplied by a maximum liquid
flow-rate that is passed to Type753e as the liquid inlet flow
rate in order to shut off the liquid flow entirely when
heating is not required.
4.
The not-bypassed coil liquid
outlet rate required to provide the necessary heating is
recorded to get an idea of what sort of pump capacity is
necessary.
The sim runs fine, but the liquid side temp
drop is unreasonably low – something like 40C drop for a
corresponding air temp rise of 3-4 C. Correspondingly the
not-bypassed coil liquid outlet rate is very low as the temp
drop of the liquid is so high.
I have executed the exact same scheme to
size a pump for a cooling coil using the equivalent cooling
coil component Type 508c… and this works fine (reasonable rise
in liquid temp and so reasonable not-bypassed liquid flow
whenever the coil is in operation).
All inputs/params are in appropriate units
and the max liquid inlet rate is sufficiently high but still
on a reasonable order of magnitude.
I am aware that Type753e guesses an initial
air outlet temperature (equal to the initial guessed average
coil temp) – is it possible that an excessively low guess
value is the cause of the problem here?
I appreciate any and all input!
Best of luck in 2014.
Ajit
Ajit
Naik
Energy
Engineer
dbHMS
303
W Erie Street, Suite 510
Chicago,
IL 60654
p
(312) 915-0557 x308
f
(312) 915-0558
www.dbhms.com