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Thu May 8 09:17:00 PDT 2014


HTH...

=20
Brandon Nichols, PE, LEED=AE AP
Mechanical
HARGIS ENGINEERS
600 Stewart Street
Suite 1000
Seattle, WA 98101
www.hargis.biz

d | 206.436.0400  c | 206.228.8707
o | 206.448.3376  f  | 206.448.4450

=20

-----Original Message-----
From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Michael =
Tillou
Sent: Friday, October 05, 2007 12:56 PM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] eQuest Default f(PLR) Skepticism

Steve,

A great DOE2.2 feature would be the ability to create boilers and =
chillers that could be put into the library.  That way you can recall =
specific combinations of keywords without having to recreate from =
scratch.

Mike



-----Original Message-----
From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Steven =
Gates
Sent: Friday, October 05, 2007 1:48 PM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] eQuest Default f(PLR) Skepticism

A few more comments regarding boilers:

We based the default curve for atmospheric boilers on information =
received from a Northern California manufacturer's representative. The =
default atmospheric boiler consumes 45% fuel at 40% part load, so =
efficiency does deteriorate. At full load, the efficiency is 80%; =
calculated as (PLR=3D1.) / (HIR=3D1.25).  At 40% load, the default curve =
yields an efficiency of 71%; calculated as (PLR=3D0.40) / (HIR=3D1.25 * =
HIRfPLR=3D0.45).=20

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.=20

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.=20

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.=20

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.=20

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 at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Kevin =
Madison
Sent: Friday, October 05, 2007 8:45 AM
To: BLDG-SIM at gard.com
Cc: BLDG-SIM at gard.com
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 Taylor's =
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=20
> right. The actual equation is:
> Hourly Boiler Energy =3D DesignCapacity * HIR * HIRf(plr) The part of=20
> this that Kevin didn't explain is that the boiler
> HIRf(PLR) curve includes the PLR which explains why the curve is=20
> nearly linear. The value that the performance curve returns is=20
> actually
> (HIRadj) * PLR
> HIRadj =3D the multiplier that indicates how the full load HIR changes =

> with respect to part load. If the boiler efficiency at a given part=20
> load goes down, HIRadj > 1. If the boiler efficiency goes up at a=20
> given part load HIRadj<1. HIRadj is really the ratio HIR-partload over =

> HIR-fullload.
> PLR =3D hourly load on the boiler / total capacity of the boiler To=20
> create a curve that describes boiler HIR at various part loads you=20
> will need to divide the performance curve output at each part load=20
> 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=20
> 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=20
> "LINEST" to create the necessary coefficients for a bi-quadratic curve =

> from manufacturers chiller data. Typically you will need to request=20
> data for a specific chiller from the chiller rep. The hardest data to=20
> get is the chiller capacity data at various CHW/CW temperatures.
> Remember total chiller capacity is different than the rated 100% part=20
> load point, most chillers can provide 10-20% extra capacity.
> I have had good experience creating custom chiller curves for DOE2.2=20
> and I think the default curves in eQuest are representative of the=20
> various chiller types. Obviously if you are evaluating a specific=20
> chiller you should try to create custom curves.
> Mike
> ----------------------------------------------------------------------
> --
> *From:* BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] *On Behalf Of=20
> *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=20
> behind eQUEST) calculates hourly energy input for boilers and =
chillers.
>
> For boilers, the hourly energy input is:
> Hourly Energy =3D Cap(hour) * HIR * HIRf(plr)
>
> So while the HIRf(plr) may increase as part load decreases, which is=20
> not uncommon for standard atmospheric boilers, the energy use will=20
> certainly decrease with plr because the required output of the boiler=20
> for the hour decreases.
>
> For chillers, DOE-2 uses the following relationship to calculate the=20
> electricity input to the chiller each hour:
>
> Caphour =3D Capacity * CAPf(t1,t2)
> PLR =3D Load / Caphour
> dT =3D t2 - t1
> Elechour =3D Caphour * EIR * EIRf(t1,t2) * EIRf(PLR,dT) / 3413 Btu/kW
>
> where
>
> Caphour hourly capacity, Btuh (this is dependent on condenser and=20
> 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, =B0F
> t2 condenser temperature, =B0F
> PLR Part load ratio
> Load Hourly load, Btuh
> dT Temperature differential across chiller, =B0F Elechour electric =
input=20
> 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=20
> EIR-FPLR
>
> Again, the primary factor affecting chiller energy use is the cooling=20
> capacity needed for that hour. Just because you don't have access to=20
> the dual function information doesn't mean you shouldn't be accounting =

> for it in the simulation. Chiller performance is dependent on all=20
> operating conditions including load, condenser conditions and=20
> evaporator conditions.
>
> For a more complete discussion on these simulation concepts, refer to=20
> the DOE-2 documentation included with the eQUEST installation. Look in =

> Dictionary:HVAC Components:Boiler:Boiler Energy Consumption and=20
> 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=20
>> are used as a direct multiplier on the HIR for boilers and EIR for=20
>> 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=20
>> atmospheric boilers (atmospheric boilers become somewhat less=20
>> efficient at part load). The default atmospheric boiler curve=20
>> decreases almost linearly down to zero! I am having a tough time=20
>> wrapping my head around this.
>>
>> On the chiller side, the default f(PLR) is a bi-quadratic function=20
>> 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=20
>> from the manufacturer. The curve I get from a McQuay 400-ton chiller=20
>> selection is quadratic, with decreasing EIR down to 50% load and=20
>> increasing EIR below 50% load. I seriously doubt that the eQuest=20
>> default corresponds with this entry because changing the function=20
>> produces a huge change in performance.
>>
>> Do any of you have any thoughts or suggestions about the accuracy of=20
>> default f(PLR) curves? Should I scrap my "improved," real curves -=20
>> they are drastically changing the model performance?!?!
>>
>> Taylor
>>
>>
>> ________________________________________________________
>> Taylor Keep
>> Mechanical, LEED=AE AP
>> _ _
>> Arup
>> 901 Market Street Suite 260
>> San Francisco, CA 94103
>> tel: 415 946 0279
>> fax: 415 957 9096
>> taylor.keep at arup.com
>> _www.arup.com_ <file://www.arup.com>
>>
>>
>> ____________________________________________________________
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