[EnergyPlus_Support] AHU & FCU

["beyangj" <beyangj@xxxxxxxxxxx>]

Dear all,

I¡¯m modeling a system of FCU with fresh air handling unit. I defined
an AIR PRIMARY LOOP which treats the fresh air and use DIRECT AIR to
feed the zone, and the fan coil unit serve the net remaining load. In
the ZONE EQUIPMENT LIST, I set the heating and cooling priorities to 1
for the DIRECT AIR and 2 for the fan coils. 

There are some problems for this model,

1. There are two inlet nodes for each zone, one inlet node is for
outside air processed from the Primary Air Unit, and the other inlet
node is for FCU. But I found that INLET NODE 1:System Node
MassFlowRate and INLET NODE 2:System Node MassFlowRate are the same
for the same zone. I set INLET NODE 1 for Direct Air and INLET NODE 2
for FCU. The mass flow rate of these two nodes shouldn¡¯t be the same. 

2. For my model, the AHU processes all the outside air, and no outside
air for FCU. But I have 2 times the required fresh air for some
working hours, half is from the AHU, half comes from the fan coil unit
even when I set the maximum outside airflow rate of the fan coil unit
as 0. For some hours, the flow rate of the outside air are smaller
than the sum of the zones needed fresh air, even sometime it is less
than that of one zone. Why they are not the same all the time? I
wonder how I can set the flow rate of fresh air for FCU as 0, and just
let the AHU have the desired amount of fresh air.   

3. I found that the air flow rate of the AHU AIR LOOP INLET:System
Node MassFlowRate is constant, much greater than the flow rate of the
AHU outside air. I thought there must be return air recirculating. I
also found the System Node MassFlowRate are constant for different
nodes in a year, respectively. How can I set the flow rate of the
return air as 0 for the AHU? The System Node MassFlowRate should be
variable for specific node at different hour?
 
4. I also found something strange. The AHU COOLING COIL CHW
OUTLET:System Node Temp[C] is about 21[C] in the summer, but CHILLER
CHW INLET:System Node Temp[C] is about 7.6[C], and the CHILLER CHW
INLET:System Node Setpoint Temp[C](Hourly) is 6.67[C]. Why the AHU
COOLING COIL CHW OUTLET temperature can be such a high temperature?

5. There are 5 zones in this model, and there is a FCU for each zone.
I set the 5 FCUs as the same pattern, but the air loop in the SVG file
are different, 2 FCU air loop have return air loop, but 3 FCU loops
seem not have return air loop. What may make the difference?

Thanks a lot in advance!

Young
 



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