[Equest-users] Default Performance Curves

Thu Oct 14 10:13:51 PDT 2010

Clifford,

When DOE2 calculates fan power, the equation is (kW current hour) = (kW at
full load)*(EIR-FPLR curve at current part load ratio). 

So the output of the EIR-FPLR curve is defined as PLR*(EIR at current
PLR)/(EIR at full load). And PLR is (current hour airflow)/(peak design
airflow). The form of the EIR-FPLR curve can be linear, quadratic or cubic
and the choice is up to you (the defaults are typically quadratic or cubic).
If it's quadratic then the form is EIR-FPLR = a + b*PLR + c*PLR^2. If, for
example, you have constant EIR across the load range, then coefficients a
and c would be zero and b would be 1. Then you have a linear curve EIR-FPLR
= 1*PLR. 

Since that's probably clear as mud, another place to look for more info is
the engineers manual

http://doe2.com/download/DOE-21E/DOE-2EngineersManualVersion2.1A.pdf (note
it's a 16 MB file), see page IV.208. 

-Erik

From: Clifford McDonald [mailto:mcdonald.cliff at gmail.com] 
Sent: Thursday, October 14, 2010 6:09 AM
To: Erik Kolderup
Cc: James Hansen; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Default Performance Curves

Thanks for the help on this, but I'm still confused. Are you saying the form
of the equation is: 

PLR = (PLR * (EIRcurrent / EIRfull) ) + (PLR * (EIRcurrent / EIRfull) )^2 

?  Then, if the equipment has constant efficiency across the full load
range, the EIR/EIR term is one, and you get:

PLR = PLR + PLR^2, which doesn't make sense.

If the form is just:

PLR = PLR [ (EIR / EIR) + (EIR / EIR)^2, then this would mean that the
efficiency has to be constant (divide by PLR on both sides).

What am I missing here?

2010/10/13 Erik Kolderup <erik at kolderupconsulting.com>

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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