[BLDG-SIM] Ventilation Fans

Graham Carter & Megan Lyall hamnmegs at ozemail.com.au
Sun Nov 11 16:23:18 PST 2007


Gavin,
If you input 250,000 CFM at the equipment level and let zones auto  
size a few things could happen.  If the fan is larger than the sum of  
the zone design airflows (diversified or undiversified) then the fan  
will always operate at part load.  Depending on the part load curve  
you have assumed and the actual method of fan speed control you could  
have dramatically different part load performance (% power as a  
function of % flow) - check both of these things.

Also DOE-2 allows you to input a kW/CFM or an efficiency combined  
with the pressure and flow.  Even the best backward curved airfoil  
fans may only achieve a wire to air efficiency of 67% - Fan total  
efficiency of 80%, belt drive / sheave efficiency 95%, VSD drive  
efficiency of 96%, motor efficiency of 92%.  And keep in mind this is  
applied to total pressure not static - static wire-air efficiency  
will be lower by the ratio of static to total pressure.  More  
standard Air Handlers probably have a wire to air efficiency of 55%  
or less.

Also don't underestimate duct leakage or damper leakage if the system  
is an existing high pressure system.

Regards,
Graham
On 10/11/2007, at 3:03 AM, Gavin Gui wrote:

> Jim,
>
>
>
> The fan static pressure has been set to the maximum of 8.0 WG (any  
> higher and the model will not run) despite the equipment schedule  
> is stated as 10 WG.
>
>
>
> As for the CFM, it is input at 250,000 cfm (according to the  
> equipment data).
>
>
>
> As for logger inaccuracy, we logged both supply fans and they  
> checked out against one another.
>
>
>
> I have tried looking at all the parameters in the simulation that  
> might affect the vent fans energy and demand, including operating  
> schedules, but were unable to find any that might justify the  
> differences.
>
>
>
>
>
> With regards
>
> Gavin Gui
>
>
>
> From: Zarske, Jim [mailto:jzarske at nexant.com]
> Sent: Friday, November 09, 2007 10:52 AM
> To: Gavin Gui
> Subject: RE: [BLDG-SIM] Ventilation Fans
>
>
>
> It sounds like you need increase the fan static pressure or kW/CFM  
> in eQUEST (you stated these are high pressure fans, so I assume the  
> power use is higher than a typical fan), confirm you have the  
> correct CFM inputted into the model, or double check the accuracy  
> of the logger data.
>
>
>
> Jim
>
>
>
> From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of  
> Gavin Gui
> Sent: Friday, November 09, 2007 8:03 AM
> To: BLDG-SIM at gard.com
> Subject: [BLDG-SIM] Ventilation Fans
>
> Hi, I am using eQuest to simulate a mid-rise commercial building.  
> The HVAC system is a simple setup whereby huge axial vane supply  
> fans provide 55F cold air, under high pressure, to the VAV boxes  
> located at the respective floors. The air is returned via ducts  
> using similar fans located near the system. However the modeled  
> results obtained for the vent fans electric demand and consumption  
> differ greatly from the logged data obtained from the site. The  
> actual data shows about 75% higher than the model results, both in  
> demand and consumption.
>
>
>
> I would greatly appreciate if anyone can advise me on where the  
> problem is or if there is any way I can fine tune the model to be  
> closer to the actual data. Thank you.
>
>
>
> With regards,
>
> Gavin Gui
>
>
>
>
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