[Equest-users] Default Performance Curves

[Bill]

The physics that says energy use of a fan or pump is proportional to the
cube of the speed is for a fixed system.  Most building systems are not
fixed, since there are dampers or valves that regulate flow to satisfy a
setpoint, and the speed is varied to maintain a static pressure (air
systems) or differential pressure (fluid systems).  A system with perfect
pressure setpoint reset--which is all but impossible for a system serving
multiple zones--can approach the cube "law."  I'm not up-to-speed on current
eQuest defaults.  But for most systems, earlier default DOE-2 curves were
not conservative, but were optimistic at lower flows, partly because of the
fixed pressure the system is serving, and also because of the VFD/motor
efficiencies start to drop at low flows.  There has been a number of ASHRAE
papers on system correlations.  One study you may wish to review is 
http://www.energy.ca.gov/2003publications/CEC-500-2003-082/CEC-500-2003-082-
A-21.PDF
 
Start with Figure 9 on page 14. 
 
William E. Koran, P.E. 
Senior Engineer 
Q u E S T 
Quantum Energy Services and Technologies 
Voice: 503-557-7828 
Web:  <http://www.quest-world.com/> www.quest-world.com 

  _____  

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Erik
Kolderup
Sent: Wednesday, October 13, 2010 2:37 PM
To: 'James Hansen'; 'Clifford McDonald'; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Default Performance Curves



The DOE2 EIR-FPLR curves include PLR in the value. In other words, you might
expect the   value of the curve to be (EIR at current load)/(EIR at full
load), but it's actually PLR*(EIR at current load)/(EIR at full load). So if
you have a piece of equipment that has constant efficiency across the full
load range, then the curve will be a straight line between (0,0) and (1,1).
You have to look pretty closely at the background docs, such as the DOE2.1A
Engineer's Manual, to see that.

 

-Erik 

 

Erik Kolderup, PE, LEED AP

Kolderup Consulting | 415.531.5198 | erik at kolderupconsulting.com

 

 

 

From: James Hansen [mailto:JHANSEN at ghtltd.com] 
Sent: Wednesday, October 13, 2010 1:50 PM
To: Clifford McDonald; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Default Performance Curves

 

Someone will be able to explain this better than me, but eQuest curves are
not intuitive.  I wish they were, but they aren't.

 

As an example:  when you enter a chiller EIR curve as a function of PLR, you
would think this would be really easy, right?  0.1, 0.2, 0.3 etc as the PLR,
and then the corresponding EIR (KW consumed vs current tonnage).  But that's
not the case..you end up basically having to square the EIR to get proper
outputs.  Perhaps the same thing needs to be done here? 

 

Again, someone will chime in with the real reasoning for this, as I've seen
it discussed before.

 

GHT Limited
James Hansen, PE, LEED AP

Senior Associate

1010 N. Glebe Rd, Suite 200

Arlington, VA  22201-4749

703-338-5754 (Cell)

703-243-1200 (Office)

703-276-1376 (Fax)

 <http://www.ghtltd.com/> www.ghtltd.com

 

 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Clifford
McDonald
Sent: Wednesday, October 13, 2010 4:29 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Default Performance Curves

 

Hello,

 

I'm fairly new to eQuest, but I was looking at the default performance
curves for VSDs on fans and pumps (EIR as a function of PLR), and with the
exception of the cooling tower fan (htrej-Fan-Pwr-fSpeed) which is a cubic
equation, they are all quadratic equations.  Why would this be when, in
theory, energy use should vary with the cube of motor speed with a VSD?  Why
would it be quadratic for supply fans and chilled water pumps but cubic for
cooling tower fans?  Is there anywhere I can go to get more information on
these default values?

 

Thanks for the help...

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