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

[Steve Gates]

MessageMark, 

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. Webb 
  To: Steve Gates ; 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/).  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 
      To: 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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