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  1. #1
    Join Date
    Jul 2010
    Location
    Knoxville, TN
    Posts
    14

    reheat coil control

    I have the opportunity to revise the sequence of operation for several air handling units. The units have cooling coils and reheat coils.

    Cooling coil control valve will simply modulate to maintain 55F.

    I am thinking of having the heating coil control valve modulate to maintain a discharge air temperature which varies between 55F and 100F depending on space temperature deviation from setpoint. For example, if space temp is 5F from setpoint, DAT = 100F. If ST = SP, DAT = 55F. And linear between.

    Does this sound OK? Is a 5F deviation a good number?

    Supply air temperature reset will also be incorporated. If return humidity is <55% and heating coil is energized, CC DAT increases by 1F every 5 minutes until HC closes or H>55%.

    Comments?

    Thanks,
    BK

  2. #2
    Join Date
    Feb 2010
    Location
    Cal
    Posts
    1,596
    Single zone , constant volume? Any remarkable load diversity?

  3. #3
    Join Date
    May 2008
    Location
    Central Fla.
    Posts
    311
    Next time a cheaper but better way on control humidity is a RAWAL device. No reheat coils no B.S. you install it and walk away.

  4. #4
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
    Posts
    1,781
    That sort of proportional only control does work well for keeping a steady temperature in the room, but what it won't do is maintain the temperature steady at set-point. It will stabilize out anywhere within that 5F band, at whatever offset puts the discharge air at the temp it needs to be to be stable. Whether this is okay or not simply depends on the room usage, but what it more than likely does is that the temp in the room will fluctuate over the course of the day, such that in the morning, when it needs more heating, it will actually run colder, and in the afternoon, when less heating is required, it will run warmer. This is what integral control is designed to overcome (where it will, over time, ramp the output based on the offset - to bring the space to setpoint. this does introduce some tendency for hunting though.

  5. #5
    Join Date
    Dec 2007
    Location
    Hopkins, South Carolina
    Posts
    160
    What is this unit used for? Is it a single zone unit? Is it constant or variable volume? Are you doing dehumidification? The more details you can give me the better. I deal with sequences all the time.

  6. #6
    Join Date
    Jul 2010
    Location
    Knoxville, TN
    Posts
    14
    Single zone constant volume.

    Dehumidification will be done at the cooling coil if return air humidity exceeds 55%.

    Sorry Mr. Fuzzy, I am a little confused by your post.

    Currently the heating coil control valve will modulate to maintain space temp. The further from setpoint, the more the valve is open. THis results in supply temps > 100F which stratify and never reach the space. I thought by controlling the valve to supply a max of 100F it would give better space conditioning.

    Anybody do this? Pros? Cons?

    Thanks for all the input so far.

  7. #7
    Join Date
    Dec 2007
    Location
    Hopkins, South Carolina
    Posts
    160
    Single Zone Unit (typical of 1)
    Run Conditions - Scheduled:
    The unit shall run according to a user definable time schedule in the following modes:

    • Occupied Mode: The unit shall maintain
      • A 74F (adj.) cooling setpoint
      • A 70F (adj.) heating setpoint.

    • Unoccupied Mode (night setback): The unit shall maintain
      • A 85F (adj.) cooling setpoint.
      • A 55F (adj.) heating setpoint.


    Alarms shall be provided as follows:

    • High Zone Temp: If the zone temperature is greater than the cooling setpoint by a user definable amount (adj.).
    • Low Zone Temp: If the zone temperature is less than the heating setpoint by a user definable amount (adj.).

    Zone Setpoint Adjust:
    The occupant shall be able to adjust the zone temperature heating and cooling setpoints at the zone sensor.
    Zone Optimal Start:
    The unit shall use an optimal start algorithm for morning start-up. This algorithm shall minimize the unoccupied warm-up or cool-down period while still achieving comfort conditions by the start of scheduled occupied period.
    Zone Unoccupied Override:
    A timed local override control shall allow an occupant to override the schedule and place the unit into an occupied mode for an adjustable period of time. At the expiration of this time, control of the unit shall automatically return to the schedule.
    Freeze Protection:
    The unit shall shut down and generate an alarm upon receiving a freezestat status.
    Supply Air Smoke Detection:
    The unit shall shut down and generate an alarm upon receiving a supply air smoke detector status.
    Supply Fan:
    The supply fan shall run anytime the unit is commanded to run, unless shutdown on safeties. To prevent short cycling, the supply fan shall have a user definable (adj.) minimum runtime.

    Alarms shall be provided as follows:

    • Supply Fan Failure: Commanded on, but the status is off.
    • Supply Fan in Hand: Commanded off, but the status is on.
    • Supply Fan Runtime Exceeded: Status runtime exceeds a user definable limit (adj.).

    Cooling Coil Valve:
    The controller shall measure the zone temperature and modulate the cooling coil valve to maintain its cooling setpoint.

    The cooling shall be enabled whenever:

    • Outside air temperature is greater than 60F (adj.).
    • AND the economizer (if present) is disabled or fully open.
    • AND the zone temperature is above cooling setpoint.
    • AND the supply fan status is on.
    • AND the heating is not active.

    The cooling coil valve shall open to 50% (adj.) whenever the freezestat (if present) is on.
    Heating Coil Valve:
    The controller shall measure the zone temperature and modulate the heating coil valve to maintain its heating setpoint.

    The heating shall be enabled whenever:

    • Outside air temperature is less than 65F (adj.).
    • AND the zone temperature is below heating setpoint.
    • AND the supply fan status is on.
    • AND the cooling is not active.

    The heating coil valve shall open whenever the freezestat (if present) is on.
    Dehumidification:
    The controller shall measure the return air humidity and override the cooling sequence to maintain return air humidity at or below 55% rh (adj.). Dehumidification shall be enabled whenever the supply fan status is on.
    Return Air Humidity:
    The controller shall monitor the return air humidity and use as required for economizer control (if present) or humidity control (if present).
    Alarms shall be provided as follows:

    • High Return Air Humidity: If the return air humidity is greater than 70% (adj.).
    • Low Return Air Humidity: If the return air humidity is less than 35% (adj.).


  8. #8
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
    Posts
    1,781
    Quote Originally Posted by bk911 View Post
    Single zone constant volume.

    Dehumidification will be done at the cooling coil if return air humidity exceeds 55%.

    Sorry Mr. Fuzzy, I am a little confused by your post.

    Currently the heating coil control valve will modulate to maintain space temp. The further from setpoint, the more the valve is open. THis results in supply temps > 100F which stratify and never reach the space. I thought by controlling the valve to supply a max of 100F it would give better space conditioning.

    Anybody do this? Pros? Cons?

    Thanks for all the input so far.
    If you're having stratification problems, no amount of controls changes are going to overcome them. You need to increase mixing in the space. Limiting the heating valve is just going to limit the amount of heat that can be introduced into the space. Pumping less heat into the space certainly isn't going to increase the temperature at the floor.

  9. #9
    Join Date
    Jul 2010
    Location
    Knoxville, TN
    Posts
    14
    Quote Originally Posted by CraziFuzzy View Post
    Pumping less heat into the space certainly isn't going to increase the temperature at the floor.
    Actually it does. Discharge air temperature should be no more than 30F above room temperature, or it becomes too buoyant and stays at the ceiling. This is easy to see if your return temperature is higher than the space temperature. Most old school VAV box programming has this issue. Newer cascading control which limits discharge temperature to 100F eliminates this problem, if there is enough airflow.

    Thanks for the sequence USC. Unfortunately it is not exactly what I am thinking. It simply controls HCV and CCV to maintain space temp.

    I have a temp sensor downstream of the HC and one downstream of the CC. I want to control each control valve from its respective temp sensor.

    CC to 55F for dehumidification and space comfort. Reset up if reheat is needed and humidity is ok.

    HC varies between 55 - 100F depending on deviation from space setpoint. I am just confused about the deviation amount. I will start with 5F and see how it goes.


    Edit: THinking about this a little more, maybe HC varies between 100F and room setpoint. So when at setpoint we are introducing neutral air. yea, I will try that and see if it works.

  10. #10
    Join Date
    Dec 2007
    Location
    Hopkins, South Carolina
    Posts
    160
    Do you have that great of a humidity problem?

    I am not saying you are wrong with the limiting of supply air temp but we have never done that and to my knowledge we do not have any problems.

    As far as humidity control it seems doing it the way you are saying you are creating an energy hog.

    The way we would do it is control space temperature using chilled water and hot water valves. Now when it comes to dehumidification we would open chilled water valve 100%. When in dehumidification to prevent subcooling we would modulate heating valve to maintain a space temperature of 2 to 3 degrees below cooling setpoint.

    That typically works for us and we are in a humid part of country.

  11. #11
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
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    1,781
    Quote Originally Posted by bk911 View Post
    Actually it does. Discharge air temperature should be no more than 30F above room temperature, or it becomes too buoyant and stays at the ceiling. This is easy to see if your return temperature is higher than the space temperature. Most old school VAV box programming has this issue. Newer cascading control which limits discharge temperature to 100F eliminates this problem, if there is enough airflow.
    I understand the process of stratification, however, there is no 'cutoff' point where if you are <30F delta it will somehow mix more. Even a 5F delta between space and supply temp will result in stratification. As a guideline though, the 30F delta IS something to avoid, though it should be avoided by the sheer fact that if you have proper air change, your controls should never NEED 100F supply air to maintain a space. If you are in a situation where you genuinely need, steady state, >100F supply air to get enough BTU's into a space to maintain temp at setpoint, then this should be fixed by increasing air change/supply cfm, not artificially limiting the hot water valve.

  12. #12
    Join Date
    Jan 2011
    Posts
    72
    Quote Originally Posted by bk911 View Post
    I am thinking of having the heating coil control valve modulate to maintain a discharge air temperature which varies between 55F and 100F depending on space temperature deviation from setpoint. For example, if space temp is 5F from setpoint, DAT = 100F. If ST = SP, DAT = 55F. And linear between.

    Does this sound OK? Is a 5F deviation a good number?

    Supply air temperature reset will also be incorporated. If return humidity is <55% and heating coil is energized, CC DAT increases by 1F every 5 minutes until HC closes or H>55%.

    Comments?

    Thanks,
    BK
    Here is what I think,

    When dehumidification is not required, coiling valve is modulated to maintain a SAT. When Return Humidity is above a setpoint which means dehumidification is required, Return humidity PI control loop modulates the cooling valve and overides the original SAT PI loop. then SAT is maintained by modulating the reheating valve.

    SAT = Final Supply Air Temperature to the space

  13. #13
    Join Date
    Jul 2010
    Location
    Knoxville, TN
    Posts
    14
    Quote Originally Posted by The_USC_2001 View Post
    As far as humidity control it seems doing it the way you are saying you are creating an energy hog.

    The way we would do it is control space temperature using chilled water and hot water valves. Now when it comes to dehumidification we would open chilled water valve 100%. When in dehumidification to prevent subcooling we would modulate heating valve to maintain a space temperature of 2 to 3 degrees below cooling setpoint.

    That typically works for us and we are in a humid part of country.

    Hmmm, your method is pretty typical if you do not have a temp sensor leaving each coil. Looking at a psych chart, 55F leaving a chilled water coil can maintain a typical office space at 75F / 50%. By opening the chilled water coil 100% you are potentially cooling the air too much, say down to 50F or below. That is at least 5F of unnecessary cooling AND unnecessary reheat. That is a huge energy hog.

    With temp sensors after each coil and return humidity sensors we can dictate exactly what discharge air temperature needs to be to maintain space temp and humidity. The air is cooled exactly where it needs to be, no more no less. This reduces chiller AND boiler load, pumping energy, cooling tower fans, etc...


    @Mr. Fuzzy - You should google. Lots of good research already done on this subject. Most experts agree 30F is the limit for good air mixing. I have personally seen this happen in several buildings I have commissioned. I realize it is counter intuitive, but it works. By using a laser thermometer, we confirmed the stratification reduced and space temp at the occupied level increased.

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