[BLDG-SIM] eQuest Default f(PLR) Skepticism

[Michael Tillou]

Actually Kevin didn't get the boiler hourly energy equation quite right.
The actual equation is:
 
Hourly Boiler Energy = DesignCapacity * HIR * HIRf(plr)

The part of this that Kevin didn't explain is that the boiler HIRf(PLR)
curve includes the PLR which explains why the curve is nearly linear.  The
value that the performance curve returns is actually  
 
(HIRadj) * PLR
 
HIRadj  =  the multiplier that indicates how the full load HIR changes with
respect to part load.   If the boiler efficiency at a given part load goes
down, HIRadj > 1.  If the boiler efficiency goes up at a given part load
HIRadj<1.  HIRadj is really the ratio HIR-partload over HIR-fullload. 
 
PLR = hourly load on the boiler / total capacity of the boiler
 
To create a curve that describes boiler HIR at various part loads you will
need to divide the performance curve output at each part load point by the
part load value and then multiply by the full load HIR.
 
Taylor - You should double check your "custom" chiller curves I'm pretty
sure from what you describe they are not correct.  The Vol6 -New Features
user manual does a good job describing how the chiller curves work.  I
suggest you review this.  You can use the Excel function "LINEST" to create
the necessary coefficients for a bi-quadratic curve from manufacturers
chiller data.  Typically you will need to request data for a specific
chiller from the chiller rep.  The hardest data to get is the chiller
capacity data at various CHW/CW temperatures.  Remember total chiller
capacity is different than the rated 100% part load point, most chillers can
provide 10-20% extra capacity.  
 
I have had good experience creating custom chiller curves for DOE2.2 and I
think the default curves in eQuest are representative of the various chiller
types.  Obviously if you are evaluating a specific chiller you should try to
create custom curves.
 
 Mike
 
 
  _____  

From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Kevin
Madison
Sent: Thursday, October 04, 2007 9:46 PM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] eQuest Default f(PLR) Skepticism


Perhaps it would help to clarify how DOE-2.2 (the simulation engine behind
eQUEST) calculates hourly energy input for boilers and chillers.

For boilers, the hourly energy input is:
  Hourly Energy = Cap(hour) * HIR * HIRf(plr)

So while the HIRf(plr) may increase as part load decreases, which is not
uncommon for standard atmospheric boilers, the energy use will certainly
decrease with plr because the required output of the boiler for the hour
decreases.

For chillers, DOE-2 uses the following relationship to calculate the
electricity input to the chiller each hour:

  Caphour  = Capacity * CAPf(t1,t2)
  PLR      = Load / Caphour
  dT       = t2 – t1
  Elechour = Caphour * EIR * EIRf(t1,t2) * EIRf(PLR,dT) / 3413 Btu/kW

where

  Caphour      hourly capacity, Btuh (this is dependent on condenser and
evaporator conditions for that hour)
  Capacity     rated capacity, Btuh
  CAPf(t1,t2)  correction to capacity for temperatures, curve CAP-FT
  t1           leaving chilled-water temperature, °F
  t2           condenser temperature, °F
  PLR          Part load ratio
  Load         Hourly load, Btuh
  dT           Temperature differential across chiller, °F
  Elechour     electric input to the chiller, kW
  EIR          rated electric input ratio
  EIRf(t1,t2)  correction to EIR for temperatures, curve EIR-FT
  EIRf(PLR,dT) correction to EIR for part-load ratio and dT, curve EIR-FPLR

Again, the primary factor affecting chiller energy use is the cooling
capacity needed for that hour. Just because you don't have access to the
dual function information doesn't mean you shouldn't be accounting for it in
the simulation. Chiller performance is dependent on all operating conditions
including load, condenser conditions and evaporator conditions.

For a more complete discussion on these simulation concepts, refer to the
DOE-2 documentation included with the eQUEST installation. Look in
Dictionary:HVAC Components:Boiler:Boiler Energy Consumption and
Dictionary:HVAC Components:Chiller:Chiller Energy Consumption.

Kevin Madison
Madison Engineering PS
Seattle WA
USA

Taylor Keep wrote: 

eQuest models boiler and chiller plants with default part load curves that I
think may be incorrect. As I understand it, the f(PLR) curves are used as a
direct multiplier on the HIR for boilers and EIR for chillers, with full
load (1.0 PLR) corresponding to a 1.0 multiplier. If this is true, the
f(PLR) curve should increase at part load for atmospheric boilers
(atmospheric boilers become somewhat less efficient at part load). The
default atmospheric boiler curve decreases almost linearly down to zero! I
am having a tough time wrapping my head around this.

On the chiller side, the default f(PLR) is a bi-quadratic function using
both dT and PLR as variables, so it is f(PLR,dT). Since I never have this
dual function information in my general chiller selections, I have been
using a standard f(PLR) function quoted at a fixed dT from the manufacturer.
The curve I get from a McQuay 400-ton chiller selection is quadratic, with
decreasing EIR down to 50% load and increasing EIR below 50% load. I
seriously doubt that the eQuest default corresponds with this entry because
changing the function produces a huge change in performance.

Do any of you have any thoughts or suggestions about the accuracy of default
f(PLR) curves? Should I scrap my "improved," real curves - they are
drastically changing the model performance?!?!

Taylor 


________________________________________________________ 
Taylor Keep 
Mechanical, LEED® AP 
                          
Arup 
901 Market Street Suite 260 
San Francisco, CA 94103 
tel: 415 946 0279 
fax: 415 957 9096 
taylor.keep at arup.com 
 <file://www.arup.com> www.arup.com 


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