[BLDG-SIM] Condenser Water Reset in eQuest

[Alec Stevens]

Steve,

We’ve recently worked on a project with VFD-controlled 450-ton centrifugal
chillers that can handle ECWT of 55ºF or less.  When we have simulated
chillers in eQuest, we generally make our own curves that cover the range of
expected operations.  I would support the idea of a warning instead of an
error.  Perhaps the warning should suggest that the appropriate curve fits
be confirmed?

Sincerely,

Alec Stevens, PE, LEED AP

DMI

450 Lexington St., Newton, MA 02466

p: 617-527-1525 x105 f: 617-527-6606

e: astevens at dmiinc.com

 

 

  _____  

From: BLDG-SIM at GARD.COM [mailto:BLDG-SIM at GARD.COM] On Behalf Of Steven Gates
Sent: Thursday, August 17, 2006 4:41 PM
To: BLDG-SIM at GARD.COM
Subject: [BLDG-SIM] Condenser Water Reset in eQuest

 

We are checking eQUEST to confirm the issue of MIN-COND-T, and will fix it
if it is missing.

 

Mike is correct that the capacity and power performance curves were not fit
for condenser temperatures below 65-70F, and the default MIN-COND-T reflects
this fact.  The reason is because the majority of chillers cannot operate
satisfactorily much below this temperature.   For many reciprocating
chilllers, the expansion valve requires a minimum pressure differential in
order to operate and deliver the rated capacity.  Centrifugal chillers may
also be limited.  Recently, we were investigating a ‘frictionless’
centrifugal compressor, and learned that motor cooling would be inadequate
if the condenser temperature (and pressure) drops much below about this
value; insufficient liquid refrigerant would be injected into the motor.

 

If you want to investigate lower condenser temperatures, we recommend that
you inspect the capacity and eir curves in the regime of interest to confirm
whether they yield reasonable results.

 

The best way to use the DIAGNOSTIC NO-LIMITS override is to first debug your
input at a higher diagnostic level so that the BDL can issue errors,
warnings, cautions.  Once you are ready to run, then set NO-LIMITS.

 

Depending on feedback we get from people, we can reduce the lower limit of
MIN-COND-T, or change the error checking so that it issues a warning rather
than an error.  As the curves are NOT fit for temperatures lower than about
65F, the lower limit should probably remain the same, but the checking
reduced from an error to a warning.  Comments?

 

Steve Gates

 

 

  _____  

From: BLDG-SIM at GARD.COM [mailto:BLDG-SIM at GARD.COM] On Behalf Of Mike Tillou
Sent: Thursday, August 17, 2006 10:58 AM
To: BLDG-SIM at GARD.COM
Subject: [BLDG-SIM] Condenser Water Reset in eQuest

 

 

Hi all..I thought I would share a couple of tips on using Condenser Water
Reset Strategies in eQuest . Hopefully some of you will find this useful.

We just finished modeling a chiller plant where we looked at using low
condenser water temperatures ( below 60F ) as an energy efficiency measure.
The project is in a climate with very low wetbulb temperatures .  There are
a couple of quirks I thought I would share about trying to model a strategy
like this.  

The Condenser Loop Control tab has several options for resetting the loop
temperature.  If you notice MIN-RESET-T always defaults to 70F. 

Issue  #1:  If you reset the Condenser loop MIN-RESET-T lower than 70F (say
65F) and then run an hourly report to check the loop temperature everything
looks like it's working fine.  BUT..if you also check the chiller entering
condenser temperature you will find it never goes below 70F even though the
condenser loop says it should be lower.  This is because the chiller has a
keyword MIN-COND-T that defualts to 70F and cannot be changed from within
eQuest.  You have to change the keyword manually in the input file by adding
the line, MIN-COND-T = 65 underneath the definition of your chiller.

  
The most likely reason this defaults to 70F is because the default chiller
performance curves were not designed for condenser temperatures below 70F  (
someone should correct me if this is wrong)  

If you want to try and model condenser water temperatures below 70F I highly
recommend you create custom performance curves that include data points that
describe the operation of your chiller at the appropriate condenser water
conditions.

That brings me to Issue #2. 

The MIN-COND-T keyword has a lower limit of 60F.  In other words even if you
create the appropriate performance curves  and then try to set MIN-COND-T
lower than 60F you will get a simulation error.  

You can work around this by using the DIAGNOSTIC keyword "NO-LIMITS".  In
effect this turns off both the Max and Min keyword limits.  You can then
simulate chiller operation with very low condesner water temperatures.  

If you use the "NO-LIMITS" keyword do so very carefully.  This is a very
important error checking function of DOE2.2 eQuest.

Mike 

 

Michael Tillou, PE
etc Group, Inc.
PO Box 7, Williamstown, MA 01267
ph. (413) 458-9870 fx. (413) 458-9875

 

  

 

 
 
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