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

[Andrew Craig]

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
TRANEOREGON 
503-431-2562 direct
503-620-8031 or 800-208-7263 receptionist (can forward to mobile)
503-639-1454 fax
sewelch at trane.com
www.trane.com <http://www.trane.com/> 
www.TraneOregon.com <http://www.traneoregon.com/> 

 

 

Andrew Craig, EIT, LEED® AP | Mechanical Designer

INTERFACE ENGINEERING 

708 SW Third Avenue | Suite 400 | Portland, OR 97204 

direct: 503.382.2696

office: 503.382.2266

fax: 503.382.2262

email: Andrew_C at ieice.com

web: www.ieice.com 

Consultants of Choice to the Built Environment for over 35 years

Kirkland, WA | Portland, OR | Sacramento, CA | Salem, OR | SanFrancisco, CA

 

-----Original Message-----
From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Steven Gates
Sent: Friday, October 05, 2007 10:48 AM
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=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 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 

> 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

>> _www.arup.com_ <file://www.arup.com>

>> 

>> 

>> ____________________________________________________________

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