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@xxxxxxxx [mailto:BLDG-SIM@xxxxxxxx] *On Behalf Of
*Kevin Madison
*Sent:* Thursday, October 04, 2007 9:46 PM
*To:* BLDG-SIM@xxxxxxxx
*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@xxxxxxxx
_www.arup.com_ <file://www.arup.com>
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