[Bldg-sim] Altitude impact on airflow and fan power in eQUEST

[Steven Gates]

Bill is correct.  Fan power is a function of static pressure and mass flow,
it is not directly a function of CFM.  However, if the the ductwork was
sized the same at altitude as at sea level, then the increased CFM at
altitude would result in an increased velocity and static pressure loss. In
this case, the supply static pressure should be increased.  (The increase in
static pressure would be approximately linear with CFM, rather than the
square, as the density is less, and static is proportional to density*V*V.)

 

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From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Bill
Sent: Wednesday, February 27, 2008 3:37 PM
To: 'Zhen Tian'; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Altitude impact on airflow and fan power in eQUEST

 

I didn't see an answer to this yet, so my apologies if it has already been
addressed.

 

If I understand your situation correctly, the fan power should NOT be
multiplied.  The CFM goes up because of the reduced density at altitude.
However, the mass flow does not change because of altitude--it is whatever
is necessary to satisfy the load.   Power is a function of mass flow, not
CFM, so the power doesn't change.

 

Similarly, I think the outdoor airflow rate should increase, otherwise the
ventilation mass flow would be decreased at altitude.   Also, for the
packaged unit, the proportions of ventilation and return air will be
constant for constant damper positions, so if the total CFM goes up the
ventilation CFM would increase proportionally.

 

Therefore, my opinion is that the eQuest calculation is OK.

 

--Bill

 

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From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Zhen Tian
Sent: Monday, February 25, 2008 3:58 PM
To: bldg-sim at lists.onebuilding.org
Cc: james.love at shaw.ca
Subject: [Bldg-sim] Altitude impact on airflow and fan power in eQUEST

Hi guys,

I faced an interesting situation when I used eQUEST to model a simple
roof-top unit conditioned small one-zone building in Calgary, Canada. The
Altitude of Calgary is 1050m (3444 ft). When I put the altitude value in the
"site data", in the ASHRAE basecase, the autosized fan airflow rate was
multified by 1.14, which is correct. However, I found the the fan power was
not multiplied while it should be multiplied. Meanwhile, the outdoor airflow
rate was also multiplied by 1.14 which should not be multiplied.

Some data for you reference,
1. Altitude factor included: system air flow rate by autosizing is 8830 cfm,
according to Appendex G3.1.2.8 and G3.1.2.9, the fan powe should be 6.4 kW,
the calculated fanpower by eQUEST is 5.61 kW (6.4/1.14) (SV-A report). I
input the outdoor air flow rate as 1320 cfm, then the SV-A reports it as
1505 (1320*1.14).

2. If I did not input the altitude factor, the autosized system air flow
rate is 7746 cfm, eQUEST calculated fan power is 5.62 kW, which matches with
ASHRAE G3.1.2.9 value and outdoor air flow goes back to 1320 cfm.

Does any one knows what is the problem in the eQUEST calculation? I try to
solve this problem by put the system sizing ratio as 1.14 and altitude as 0
to represent the altitude factor. Is this method OK for the  LEED
requirement?

Thanks a lot.

Regards,

Zhen

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