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

[Zhen Tian]

Thanks Steve and Bill for your kind response.

I think this is really interesting issue to me and maybe to other people doing the energy simulation for LEED according to ASHRAE 90.1-2004.

In my proposed case, the engineer designed the system air flow rate as 10000 cfm and outdoor air flow rate as 1320 cfm. When I input the altitude of 3444 ft in the site data, and the air flow rate and outdoor air flow rate (10000 cfm  and 1320 cfm) in the air-side HVAC, I will get 11400 cfm and 1505 cfm in the SV-A report. Should I input values divided by 1.14 (e.g. 8772 cfm, and 1158 cfm) in eQUEST inputs?

In the corresponding ASHRAE basecase, when I input the altitude of 3444 ft in the site data, the sized system air flow rate will also multiplied by 1.14 (e.g from 7746 cfm without altitude factor to 8830 cfm). Bill is right. The fan power is  related to  mass flow rate. But the problem is in ASHRAE 90.1-2004 G3.1.2.8 and G3.1.2.9, the basecase building fan power has nothing to do with the air density but air flow rate only. In this case, the basecase building fan power will also roughly mutliplied by 1.14 (e.g. from 5.61 kW without altitude factor to 6.4 kW). Steve, if I undertand you comments correctly, so we also need to increase the fan pressure drop to get the same fan power of 6.4 kW which corresponding to the 8830 cfm air flow rate. Is this correct?

I appreciate your help.

Regards,

Zhen


Steven Gates <steve.gates at doe2.com> wrote:        v\:* {behavior:url(#default#VML);} o\:* {behavior:url(#default#VML);} w\:* {behavior:url(#default#VML);} .shape {behavior:url(#default#VML);}           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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