Hi All,
I was wondering if anyone would be able to clarify how you model your proposed fan and pump powers. Do you typically use the nameplate motor HP or do you use the total pressure along with the mechanical and motor efficiencies. Using the nameplate motor HP to get a W/cfm or W/gpm seems to overestimate the fan/pump energy because the next available motor size is chosen (or the motor is oversized). The baseline on the other hand only uses the BHP with the motor efficiency for fan power and a straight W/gpm number for pumps.
On another note related to fan power, are laboratory exhaust fans (for general and fume hood exhaust) considered part of the "System Fan Power" or are the exhaust fan powers modeled so that the baseline and proposed are kept the same. The exhausted air is made-up via the air handling unit which also serves other non-laboratory spaces and has a return fan. I've read some discussions on this forum leaning towards including it as part of total system fan power but I've also seen LEED review comments stating that independent exhaust fans should be modeled identically between baseline and proposed models. It seems like keeping the exhaust fan powers the same in both cases helps boost the energy cost savings as the fan power allowance when excluding exhaust fan is distributed to only the supply/return fans.
Last question is do you break up your baseline total system fan power into supply/return/exhaust using the power or flow fraction of each applicable fan in the proposed design? I typically use the flow fraction but I was checking out the "Energy Model Input Translator" put out by RMI and it seems like they calculate baseline fan power by distributing the total system fan power by power fractions for each fan.
Example:
10,000 cfm supply: 0.0013*10,000 = 13 BHP
HP = 13/.91 = 14.3 HP
cfm
% flow
HP
Supply Flow
10000
0.5
7.1
Return Flow
8000
0.4
5.7
Exhaust Flow
2000
0.1
1.4
Total
20000
1
14.3
cfm
% flow
HP
Supply Flow
10000
0.56
7.9
Return Flow
8000
0.44
6.3
No exhaust fan
Total
18000
1
14.3
Lab system with 100% OA
cfm
% flow
HP
Supply Flow
10000
0.50
7.1
no return fan
Exhaust Flow
10000
0.5
7.1
Total
20000
1
14.3
A lab system with 100% OA could also just be 14.3 HP supply and the exhaust would be the same as the design exhaust power depending on how you treat the exhaust fan.
Any light shed on this would be very helpful. Thanks in advance.
Mirza
