[Bldg-sim] Fan power calculation for proposed model

[Nicholas Blanton]

Cory/Jim,

Because a fan selection only gives a single operating point for a specific system curve, I was interested in seeing if a proposed alternative, i.e. basing the system curve on a point produced by the simulation, could be a more accurate method of approximating fan energy consumption. But of course, the only known operating point for the fan is based on the fan selection.

Given that this is the case, would you agree with me that there is no industry standard for establishing a fan power curve, for the purpose of energy modeling? Also, would you agree that the modeler could claim false savings based on a fan curve that contains low part load input kW? I am not aware of any requirements on the curve itself, and that is really the root of the issue I was trying to bring up.

Thanks,

Nick Blanton

From: Jim Dirkes [mailto:jim at buildingperformanceteam.com]
Sent: Wednesday, September 02, 2015 6:04 AM
To: Duggin, Cory
Cc: Nicholas Blanton; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Fan power calculation for proposed model

I agree with Cory.
The ASHRAE 90.1 committee would be hard pressed to integrate the different manners that all modeling software could be used to come up with specific simulated flow rates using unspecified (for the model) operating schedules and internal loads.  The designer is already tasked with selecting the fan using best available information, so I think that's a more reliable basis.

On Tue, Sep 1, 2015 at 5:08 PM, Duggin, Cory <Cory.Duggin at tlc-eng.com<mailto:Cory.Duggin at tlc-eng.com>> wrote:
It is my understanding that the proposed model should always be the design values used in selecting the fan not what is calculated by the energy simulation.  I agree the fan most likely will not operate at that flow rate, but that is what we have fan curves for.

Cory Duggin, PE, LEED AP BD+C
Associate/Energy Modeling Wizard, j.g.
[GreenWeek2015_FINAL-Email-Sig]
TLC Engineering for Architecture
direct:

615-346-1939<tel:615-346-1939>



From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org>] On Behalf Of Nicholas Blanton
Sent: Tuesday, September 01, 2015 4:03 PM
To: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: [Bldg-sim] Fan power calculation for proposed model

Please comment on the following:

Based on Table G3.1.10.a, for an HVAC system in the proposed design, the system is determined by the following:

“Where a complete HVAC system exists, the model shall reflect the actual system type using actual component capacities and efficiencies.”

In design of AHU systems, the scheduled or nameplate flow rate of the equipment is typically higher than the design flow rate of the system, as calculated by the simulation software. The system fan power calculation is based on the efficiency of the fan and ultimately the design flow rate of the fan. Also, in operation the fan typically does not operate at 60 Hz at peak flow rate, suggesting that the operating flow rate in this condition is more in line with the design flow rate calculated by the simulation software, rather than the nameplate flow rate of the equipment.

For these reasons, it is likely the case that the fan power calculation should be based on the design flow rate calculated by the software, rather than the nameplate flow rate.

Thanks,

Nick Blanton

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