[BLDG-SIM] Static Pressure Reset modeling in DOE 2

[Jeff Haberl]

Hello:
 
Here's my $0.05 worth.
 
First, I doubt seriously, that DOE-2 is  +- 2 % accurate, compared to what?
This claim would need justifying. 
One way of testing DOE-2 in the near future is with ASHRAE new accuracy test
for air-side systems, 865-RP.
Tests are available for 7 systems, under varying conditions. The "accuracy"
of 865-RP has been developed
to be +- 1%, which caused the two contractors much pain, and many years of
effort.
 
This should help resolve some of the issues regarding accuracy.
 
Jeff


8=!   8=)  :=)   8=)   ;=)   8=)   8=(   8=)   8=()   8=)   8=|   8=)   :=')
8=)   8=)   8=? 

Jeff S. Haberl, Ph.D., P.E.............................jhaberl at esl.tamu.edu 
Associate Professor....................................Office Ph:
979-845-6507 
Department of Architecture...........................Lab Ph: 979-845-6065 
Energy Systems Laboratory...........................FAX: 979-862-2457 
Texas A&M University..................................77843-3581 
College Station, Texas, USA...........................URL: www-esl.tamu.edu 

8=/   8=)  :=)   8=)  ;=)   8=)   8=()   8=)   :=)   8=)   8=!   8=)   8=?
8=)   8=)   8=0   

-----Original Message-----
From: postman at gard.com [mailto:postman at gard.com]On Behalf Of Steve Gates
Sent: Thursday, October 10, 2002 12:00 PM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] Static Pressure Reset modeling in DOE 2


Mark, 
 
In many cases I fully agree with you about the value of keeping the problem
simple.  Indeed, we have been criticized numerous times in the past for
DOE-2's 'overly simplistic' equipment algorithms.  Interestingly, when a
couple of recent independent studies compared DOE-2's chiller and cooling
tower algorithms to more sophisticated algorithms published in the
literature, they found that DOE-2's 'simple' algorithms gave the most
accurate results. (One of these studies found that the ASHRAE Toolkit
chiller algorithm is so complex that it is next to impossible to calibrate
to manufacturer's data, and generates errors on the order of +/- 10%; while
DOE-2's accuracy was on the order of +/- 2%).
 
But I have to disagree with you in the case of variable-speed fans and
pumps. How many of you have heard the claim of VSD salemen that "power goes
as the cube of the flow"?  That claim is patently false when applied to
airhandlers and pumps.  It is only true in the case where the pressure falls
off as approximately the square of the flow, such as in a cooling tower, or
a piping system with no valves.  In the case of systems where a significant
portion of the pressure loss is associated with non-turbulent components,
pressure losses do not follow the flow-squared law.  And a fixed pressure
setpoint further reduces the potential savings.  In fact, if the pressure
setpoint is set high enough, the VSD will run at full speed regardless of
flow (I have observed this situation).
 
So it can make sense for DDC control systems to reset the setpoint as a
function of the pressure requirement of the worst case component (but watch
out for the rogue VAV box or coil!)  And, since one of our goals is to allow
engineers to utilize DOE-2 to study the cost-effectiveness of more
sophisticated control sequences, the program needs to do a better job of
simulating this situation.  Granted, we will keep the algorithms as simple
as is consistent with this goal!  And the default control sequence will
utilize a fixed setpoint so everyone can keep asking "how come the power
consumption doesn't drop off as the cube of the flow, like the salesman told
me?"
 
 

----- Original Message ----- 
From: Mark A.  <mailto:MAW at abacus-engr.com> Webb 
To: Steve Gates <mailto:steve.gates at doe2.com>  ; BLDG-SIM at GARD.COM
<mailto:BLDG-SIM at GARD.COM>  
Sent: Thursday, October 10, 2002 4:11 AM
Subject: RE: [BLDG-SIM] Static Pressure Reset modeling in DOE 2


Has anyone ever told you guys that you were nuts!  By the time you get done
with all of your guessing, manipulation, trickery, and manual curve creation
- you could have just made an intelligent guess and been closer.  With the
added bonus of saving a bunch of time.  Unless it is some kind of PhD kind
of thing.
 
Mark

-----Original Message-----
From: Steve Gates [mailto:stvgates at pacbell.net] 
Sent: Wednesday, October 09, 2002 7:44 PM
To: BLDG-SIM at GARD.COM
Subject: [BLDG-SIM] Static Pressure Reset modeling in DOE 2


Currently there is no "optimized static pressure control" for fans/ducts in
DOE-2.  However, J. Hirsch & Associates has funding to implement such a
mechanism in DOE-2.2 within the next few months.  The funding is provided by
Southern California Edison as part of their "Energy Design Resources"
program ( http://www.energydesignresources.com/
<http://www.energydesignresources.com/> ).  These enhancement will be
incorporated into both DOE-2.2 and eQUEST (not PowerDOE), and will be made
available to the user community at no charge.
 
This control mechanism will be similar to the head reset controls already
implemented for variable-flow pumping systems in DOE-2.2, where the pump
head setpoint can be reset based on the hourly head demand of the worst-case
valve. The actual pump head is then a function of the worst-case valve head,
plus the additional variable-flow head losses of the loop piping and the
central plant equipment.  This control sequence maximizes the potential
savings of variable-speed pumps, as the required speed of a pump is
dependent on both the flow as well as the head requirement; with the head
requirement usually being the dominant factor. (The maximum head of a pump
varies roughly as the square of the impeller speed; if the head requirement
does not decrease as the flow drops off, then the speed cannot decrease
significantly).
 
For fans/ducts, the simple curve-fit that varies fan power as a function of
flow will become obsolete. Instead, DOE-2.2 will keep track of the static
requirement of the worst-case VAV box, as well as ducting, coils, filters,
etc.  Each component must be accounted for separately, as the head of all of
these devices varies differently for a given flow.  (Some components, such
as ducts and sound traps, experience turbulent flow, and their head varies
as the 1.85 power of flow.  Other components, such as coils and filters, are
more laminar, and their head typically varies in the range of the 1.1-1.6
power of flow.  Still others, such as a VAV box without an intelligent DDC
system to monitor damper position, will have a constant head requirement;
i.e. a constant head setpoint out toward the end of the ducting.)  These
changes will require DOE-2.2 to know about the flow through each component
in the system, the component's head characteristic, as well as the fan map
for a given type of fan (forward-curved, airfoil, etc.)  Based on flow and
static requirement, DOE-2.2 will then be able to determine the fan speed and
power consumption.  
 
In the meantime, you can approximate "optimized static pressure control" by
generating a new curve that causes power to fall off more rapidly with flow
than the default curves.  However, this is not a straightforward task, as
you would have to calculate by hand all of the algorithms we have designed
for DOE-2, and repeat the calculations for a series of flows.  Perhaps
someone has already done this exercise, and can share their curves with you.
 
Steve Gates

----- Original Message ----- 
From: babetsk at jbb.com <mailto:babetsk at jbb.com>  
To: BLDG-SIM at GARD.COM <mailto:BLDG-SIM at GARD.COM>  
Sent: Wednesday, October 09, 2002 2:41 PM
Subject: [BLDG-SIM] Static Pressure Reset modeling in DOE 2


To Doe 2 Users:
 
Hello,
 
Does anybody know if there is a way to model (in DOE-2) an "optimized static
pressure control", that is a control method where the controller dynamically
adjusts the static pressure set point based on the position of the
modulating devices in the VAV terminal units? 
 
Thank you
 
 
Konstantin E. Babets, Ph.D.
CFD/Energy Modeling Engineer
Jaros Baum and Bolles
Consulting Engineers
80 Pine Street
New York NY 1005
Tel:   212.530.9455
Fax: 212.269.5894

 
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