Is anyone able to explain Supply air reset - Page 2
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  1. #14
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    Quote Originally Posted by chesehd View Post
    Not trying to be rude, just exploring this train of thought further.

    With the exception of engineering the system with some diversity built in, if you had the AHU & VAV'S TAB'ed properly wouldn't you be able to maintain a constant static setpoint regardless of damper position? Even if all the VAV's were at a minimum position the VFD on the AHU should ramp down to maintain the setpoint. Conversly, as the boxes open the VFD ramps up to meet the demand. Why complicate things by changing multiple factors?

    Carnak: Thank you. I already understood the DAT reset, I've just not heard about resetting the static setpoint prior to this and am trying to play this out.
    It’s not a matter of complicating things as a way to save more energy.

    I am no expert and haven’t had a chance to fully implement static pressure reset, but have experimented with it and can understand the savings by using it.

    A typical VAV can operate properly with .5” at the flow ring. Let’s say you have a setpoint of 1.5”. If most of the VAV’s are at minimum, why run the VFD harder to maintain 1.5”. You can slow the VFD down with reset to 1.0”. Then reduce energy costs and achieve the same result at the VAV.

    I believe that static pressure reset will be seen a lot more in the future. Just like vari-prime systems that we are starting to see spec’d. It is another way to conserve.
    Quote Originally Posted by Qui-Gon Jinn (Star Wars Episode 1)
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  2. #15
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    Quote Originally Posted by chesehd View Post
    Carnak: Thank you. I already understood the DAT reset, I've just not heard about resetting the static setpoint prior to this and am trying to play this out.
    Thanks but I was just responding to the OP.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

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  3. #16
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    I saw Tom Hartman at an ASHRAE dinner one evening where he was the technical speaker. Very nice and very interesting. In a nutshell and in simplistic terms, what he was trying to get across was the fact that rather than varying only certain pieces of equipment (ie:just the fan for static pressure) he varies everything including the central plant (as many pieces as possible) - at least that was my understanding.

    I believe it would be rather complex in terms of set up, but once it's done it seemed like it would work. I think one of the most difficult things would be to get a control guy to understand what he wants done and then doing it.

    BTW...this whole thread has taken an interesting twist. I'm very curious about the Asian/ Japanese method.

    It seems to make sense, because in the end all we care about is getting the CFM to the space as it is required.

  4. #17
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    Hartman's stuff is very interesting, but it tends to ignore cause and effect. By that I mean it seems to work well when you are turning the entire system down as requirements for cooling/heating/air movement etc. diminish. But when that demand reverses and you apply Tom's logic, it means you turn everything back up again to remedy an unbalance. This sequence will probably "fix the problem", but it does so my throwing a handful of pebbles at it instead of a well aimed single rock. It's more efficient to simply react to an event by altering the one restorative variable that's going to effect the biggest change for the smallest effort.

    But - that aside - I've used Supply Air Reset extensively in VAV systems and consider it a mandatory component of good VAV system design. The setpoint itself floats between 1.0" and 2.5" as measured 2/3 the way down the main trunk on the floor furthest away from the AHU. The reset scheme is run every 10 minutes and adjusts the set point up or down by 0.05". This decision is a function of monitoring the damper positions of all the (occupied) VAV boxes associated to the AHU and discarding any that are open more than 98% and closed more than 50%. Boxes that fall beyond these parameters are usually involved in trying to heat/cool a space that's not being used to the original design load and need to be physically investigated to find out why they can't provide the heating/cooling required.

    The theory behind this is simple - you want your main supply fan(s) to ride their curve. Fan energy is typically larger component of total system energy use than the energy required to cool a building and this reset scheme allows you to maintain your facility but to do so efficiently.

    Nikko

  5. #18
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    Likewise Nikko.

    We use that strategy for many VAV applications and have the same results as you.

  6. #19
    Mr. Hartman's theory comes from a very simple question. What is the most efficient way to deliver certain amount of cooling/heating capacity? And he argued that when there is a over cooling/overheating, equally uploading all energy consuming equipments in the system will save the most energy.

    I googled the term "The Equal Marginal Performance Principle", trying to understand the basics behind it. But it seems that no one has mentioned that before in the engineering field. And Hartman is the first one. I can only find some similar term in economics.

  7. #20
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    Tom's theory is quite interesting. Basically he says that instead of a bunch of independent control loops running a system, all systems should be treated as one big loop. In otherwords - think "system loop". If you don't need that loop to produce effort (which costs $$) to combat an unwanted situation, turn it down. And if you need to produce more restorative effort, turn it up.

    But I'm not convinced that his method is any more efficient than a bunch of smaller independent tasks running together to control a piece of equipment. He uses chillers as the premise for his example, but let's look at a basic VAV AHU. In this unit you have several things under the direct control of the unit:

    1) Discharge air temperature control (Heating/cooling)
    2) Mixed Air temperature control (economizer)
    3) Discharge pressure control
    4) Building pressure control.
    5) Minimum outdoor air (volume)

    Dischage air temp control is simple in a VAV system. The system is designed to function to a static temperature setpoint and the unit controls to this value. I'll often tuck a small "if, if if, if, if" loop in there that says if everything else is completely backed off and the system's satisfied, start to reset the discharge air temp and sometimes I'll incorporate a small reset schedule based on OAT (if the system design is at all marginal), but generally, as a rule - I'll leave it alone.

    Mixed air temp is also simple. You control it to a setpoint that's equal to the discharge air temp setpoint minus the delta T accumulated due to the supply fan heat (I start at 2 deg. F). If it's cool out but not cold enough to damage the unit, and if it's not too warm out, throttle the OA dampers to achieve setpoint.

    For Min OA, never close the OA dampers beyond a point at which your minimum OA volume is being satisfied unless the unit's in danger of hurting itself. In which case, both of these previous loops quit, the OA damper closes and the unit recovers.

    Supply air static pressure was explained in my last

    Building air static pressure is controlled by the exhaust fan speed or damper position from a measurement point usually located i the main lobby but away form the elevators and front doors.

    If you examine these various control strategies, none of them are really connected except maybe the Static pressure reset and Discharge air temp reset. But that's it. So there's no real advantage to putting all the various control-measurement point values into one big pot, mixing them up and magically deriving a bunch of control output points. It just doesn't work that way. In fact and as I said, It's easier and the "tunability" of the unit is far better if each of these control strategies is treated as an independent.

    Now, chiller control may be different, but I'm still not convinced it's all that much different and that there's value in incorporating measured values into an equation where the result of the equation has no cause or effect on those measured values. Seems like a lot of extra work (and risk) for no reward.

    Unless I'm missing something.

    Nikko
    Last edited by nikko; 08-29-2007 at 11:45 PM.

  8. #21
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    Quote Originally Posted by Carnak View Post
    Thanks but I was just responding to the OP.
    Oh...yeah...I kinda hijacked this one. 1 Duh to me!

    To all, thank you for the lesson. Next VAV job I deal with I'm going to play with this idea and see where it goes.

  9. #22
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    Quote Originally Posted by chesehd View Post
    Next VAV job I deal with I'm going to play with this idea and see where it goes.
    Just be aware that the more complicated the control sequence is, the more confused the end-user may be

    I have programmed lots of sites using different DAT reset schemes dreamed up by various engineers. The more complicated ones require assistance from me a lot of times when a new service man sets foot on-site or if the end-users have personnel changes, because a lot of them can't understand anything other than "Outdoor Temp Reset of DAT".

    Part of the confusion is when there are several AHUs that all have different discharge temps when it is rather warm outside. The zones may all be OK, but the end-user starts to freak out because some of the DATs appear a little high, and they are getting worried about loosing control of their building. I've trained and created documentation until I'm blue in the face, and some end-users just can't get it. It seems the guys that work for Hospitals and high-end industrial are the best with the complicated systems.

    Also, dehumidification can be very important and if you don't keep a handle on it, then your higher AHU DAT is going to make things go downhill fast.

    So I guess the moral of the story is, you need to find out what the end-user really wants, needs and can understand and then go from there. Otherwise you are going to be getting lots of confused phone calls and situations where the end-user starts overriding points to "Fix" his system.
    Last edited by tuncos; 08-30-2007 at 10:21 AM. Reason: sp

  10. #23
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    Quote Originally Posted by tuncos View Post
    ...because a lot of them can't understand anything other than "Outdoor Temp Reset of DAT".

    ...some end-users just can't get it.

    Otherwise you are going to be getting lots of confused phone calls and situations where the end-user starts overriding points to "Fix" his system.
    I am disturbingly famaliar with all 3 of these situations and no, not because I came up with a new/creative way of doing things. I'll get calls from boneheads that have been there for 8-10 years and just don't bother remembering which building on the campus has which type of equipment or how it's supposed to work.

    After all, why should they remember? That's what we get paid the big $$$, right?

    Thanks again.

  11. #24
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    Quote Originally Posted by chesehd View Post
    I'll get calls from boneheads that have been there for 8-10 years and just don't bother remembering...

    After all, why should they remember? That's what we get paid the big $$$, right?
    You know, I think you hit the nail on the head. Some of the guys I deal with the most are long-timers in their buildings and a lot of times they call with the most stupid questions.

    I've heard the phrase "Wow, how do you remember all this stuff" so many times. It's really kind of sad

  12. #25
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    Quote Originally Posted by nikko View Post
    Tom's theory is quite interesting. Basically he says that instead of a bunch of independent control loops running a system, all systems should be treated as one big loop. In otherwords - think "system loop". If you don't need that loop to produce effort (which costs $$) to combat an unwanted situation, turn it down. And if you need to produce more restorative effort, turn it up.

    But I'm not convinced ......

    Unless I'm missing something.

    Nikko
    I think Tom's theories are more suited to an Iterative control structure and not the industry-standard PID structure.

    I try to limit my SA Temp reset as you get into humidity problems with increased IAQ concerns. I'd rather vary the fan static a bit on terminal loading and tend to minimize any SA reset unless it's low outside RH levels. (FALL/WINTER). Trane had an interesting set of articles on low temp delivery systems. Currently, I'm investing alot of time into minimizing air moving systems. I personally think that's a bad way to provide space comfort.

  13. #26
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    I pull in a stockmarket RSS feed from the internet and reset my discharge setpoints based on the value of Siemens Stock.

    When the stock goes up, I reset the setpoint down to make everyone cool and happy. If the stock price goes down, I raise the setpoint so that people get hot and stuffy and then they call for service.
    Dingman from Iowa~
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