|
Steven--- Regarding the last question you posed in your e-mail, this is a
response I received from our local Trane representative on their chiller design
with multiple impellers. Andrew, Unloading: Trane multistage design allows us to
unload lower (without surge happening). This is especially true if you do not have
low temp condenser water available at low loads. Trane still has an unloading advantage
with the multistage, but, it has gone down a bit with the addition of VFD
options to other's chillers. Add VFDs to our multi-stage design, &
we have even better unloading characteristics. Part Load Efficiency: Same is true of part load
efficiency. Multistage/economizer has the effect of having multiple sized
compressors better optimized for the part load than say a single
compressor sized for full load. Trane has a part load efficiency
advantage due to the multistage/economizer design, but, it has gone down a bit
with the addition of VFD options to other's chillers. Add VFDs to our multi-stage design, &
we have even better part load efficiency. Often, Trane's multistage design will have
a better part load efficiency with a starter than it's competition with a
VFD. Full Load & VFDs: Only thing to remember about VFDs is that
you do take a electric resistance/heat loss. So, same chillers, one with starter &
one with VFD, will result in the starter chiller having better efficiency close
& up to full load, but, the VFD chiller having better efficiency at
part load. So, if you have a bunch of chillers, you
may want to have one as the ramping chiller with the VFD and the others with
simply starters as they will basically run close to full load. Please
contact me with any questions or concerns. Thank You, Steve Welch Andrew Craig, EIT, LEED® AP | Mechanical Designer INTERFACE ENGINEERING direct: 503.382.2696 office: 503.382.2266 fax: 503.382.2262 email: Andrew_C@xxxxxxxxx web: www.ieice.com Consultants of Choice to the Built Environment for over 35 years -----Original Message----- A few more comments regarding boilers: We based the default curve for atmospheric boilers on information
received from a atmospheric boiler consumes 45% fuel at 40% part load, so efficiency
does deteriorate. At full load, the efficiency is 80%; calculated as
(PLR=1.) / (HIR=1.25). At 40% load, the default curve yields an efficiency of
71%; calculated as (PLR=0.40) / (HIR=1.25 * HIRfPLR=0.45). We believe that a 71% efficiency at 40% load is reasonable for a modern atmospheric boiler. The default was based on a packaged water-tube
boiler with internal convective recirculation, but performance for other types
is expected to be similar. The default atmospheric boiler unloads to 40% part-load ratio, and then starts to cycle on/off; the HIRfPLR curve is valid only for part-load
ratios above 40%. The loss during the off-cycle is characterized by the
“standby time”, which is the equivalent full-load time in hours required
to keep the boiler hot, if the boiler has no load at all. The default is 0.027
hours, meaning that, at no load, the boiler would have to run about 2
minutes/hour at full load to offset the jacket and flue losses. The documentation describes the use of the standby-time vs. the part-load curve in
further detail. At very low loads, efficiency is seriously degraded; by
definition the efficiency at 0% load is 0%. You can observe this if you set up an hourly report for the boiler and observe it at very low loads. Our experience to date is that manufacturer’s part-load data for
boiler performance is quite difficult to obtain. If any of you have
well-documented data that deviates significantly from the defaults, we would appreciate hearing from you. As eQUEST/DOE-2 supports a library of equipment, it
would be possible to add performance data for various types of boilers to the library. For chillers, let me further clarify Kevin's comment regarding the EIRf(PLR,dT) curve. As Kevin explained, the dT term need be developed
only for variable-speed centrifugal chillers; it is of quite limited value
for constant-speed centrifugals, and of extremely limited value for positive-displacement machines. The discharge/suction temperature differential is closely associated
with the discharge/suction pressure differential. If the cooling tower is controlled to a fixed, high setpoint such as 85F, then the chiller
impeller may not be able to slow down much at all, even at low loads; the
majority of capacity modulation will then be via the inlet vanes rather than speed.
This is because the maximum possible pressure rise across the impeller drops
off as the square of the impeller speed. Chiller impellers are typically
closely matched to the design pressure rise specified by the engineer. If the required pressure rise does not drop off substantially as the load
drops, then the impeller must maintain speed to avoid surge. Data commonly published by manufacturers (and implicit in the IPLV)
ASSUMES the condensing temperature drops with load; thereby allowing the
impeller speed to drop off. That may not actually be the case in real life! Nor
is it true in eQUEST unless you specify a low tower setpoint, or utilize a
tower reset scheme. The default EIRf(PLR,dT) curve for variable-speed centrifugals was
developed using a manufacturer's proprietary software package that was loaned to
us; it password-expired after a short time. To our knowledge, these data
cannot be developed from software available to the general engineering
community. (You need to be able to vary the chilled-water and condensing
temperatures independently of the part-load ratio. This is also why IPLV data is worthless for an hourly simulation program; the "condenser
relief" is built into the part-load performance.) Let me know if you are aware of any centrifugal chiller manufacturers that make this data generally
available. This brings to mind another interesting point that perhaps others can respond to. Over the years I have heard unsubstantiated rumors that a multi-impeller chiller, such as a Trane, may have significantly
different part-load performance compared to a single-impeller chiller, such as a Carrier. Does anybody have information regarding multi-impeller vs. single-impeller part-load performance? Regards, Steven Gates eQUEST Development Team -----Original Message----- From: BLDG-SIM@xxxxxxxx [mailto:BLDG-SIM@xxxxxxxx] On Behalf Of Kevin Sent: Friday, October 05, 2007 8:45 AM To: BLDG-SIM@xxxxxxxx Cc: BLDG-SIM@xxxxxxxx Subject: [BLDG-SIM] eQuest Default f(PLR) Skepticism You are too generous Mike. I should heed my own advice on the documentation. Thanks for clarifying this. I also thought and consulted a colleague (thanks Steve) on chiller question: For chillers, the second term in EIRf(PLR,dT) is important only for variable-speed centrifugal chillers. It is of negligible importance in other types of chillers. It is important in variable-speed, because the
temperature differential is strongly correlated with the required speed
of the impeller. If the dT is high enough, then the impeller may have
to run at full speed even at low loads. The DOE-2 default curves are just that, defaults. Some folks find it necessary to create new curves to reflect their specific equipment. Cautions offered * verify the conditions (condenser/evaprator) for efficiencies at lower
part loads are the same as for above 50% * make sure the points are normalized around the rating condition Sorry for my hasty response. Kevin Madison Michael Tillou wrote: > 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. > > 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 > > > > 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 Keep >> Mechanical, LEED® AP >> _ _ >> Arup >> >> >> tel: 415 946 0279 >> fax: 415 957 9096 >> taylor.keep@xxxxxxxx >> _www.arup.com_ <file://www.arup.com> >> >> >> ____________________________________________________________ >> Electronic mail messages entering and leaving Arup business >> systems are scanned for acceptability of content and viruses >> >> ====================================================== >> You received this e-mail because you are subscribed >> to the BLDG-SIM@xxxxxxxx mailing list. To unsubscribe >> from this mailing list send a blank message to >> BLDG-SIM-UNSUBSCRIBE@xxxxxxxx >> > ====================================================== > You received this e-mail because you are subscribed > to the BLDG-SIM@xxxxxxxx mailing list. To unsubscribe > from this mailing list send a blank message to > BLDG-SIM-UNSUBSCRIBE@xxxxxxxx > > ====================================================== > You received this e-mail because you are subscribed > to the BLDG-SIM@xxxxxxxx mailing list. To unsubscribe > from this mailing list send a blank message to > BLDG-SIM-UNSUBSCRIBE@xxxxxxxx > ====================================================== You received this e-mail because you are subscribed to the BLDG-SIM@xxxxxxxx mailing list. To unsubscribe from this mailing list send a blank message to BLDG-SIM-UNSUBSCRIBE@xxxxxxxx ================== You received this e-mail because you are subscribed to the BLDG-SIM@xxxxxxxx mailing list. To unsubscribe from this mailing list send a blank message to BLDG-SIM-UNSUBSCRIBE@xxxxxxxx =====================================================You received this e-mail because you are subscribed to the BLDG-SIM@xxxxxxxx mailing list. To unsubscribe from this mailing list send a blank message to BLDG-SIM-UNSUBSCRIBE@xxxxxxxx |