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  1. #1
    Join Date
    Mar 2008
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    103

    Cooling Tower Fan Control

    What is the best way to stage/control cooling tower fans with vfd's?
    Have a project to install vfd's on a cooling tower with 3 fans and a bypass valve. Plan on enabling fans only after bypass valve is 100% over the top of the tower. Then would you stage the fans with a deadband between start/stop of the fans based on water temperature, or speed of the operating fan/fans? Or, would it be best to start all fans and just let them ramp up together? Don't have any sequence of operation to work off of. Currently staging the 3 fans based on increase/decrease in temp.
    The fans are gear box driven, so need to find out minimum speed fans can run.
    Any thoughts or best practices would be appreciated.
    Thanks
    b-man

  2. #2
    Join Date
    Aug 2000
    Location
    USA
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    4,995
    Ive had good success ramping all the fans at the same speed. Keeps with .3F leaving tower water.
    Your poor planning does not constitute an emergency on my part!!!!

  3. #3
    It sounds like how to stage may depend on the Minimum they can take.

  4. #4
    Join Date
    Nov 2009
    Location
    karachi
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    174
    I have a site in which there are two fans for cooling towers controlled by the vfds.
    I hve used a loop block and if the output of the loop block goes from 0-50 then the first vfd will operate from 0-100 and if the o/p crosses over 50 then my second vfd will come in operation...If one fan can maintain the temp. then there is no use of starting a second one..The consultant told me d/f seq. but I gave him a idea and its working fine.
    so you hve three fan you can use 0-33 for 1st fan,34-66 for 2nd and 67-100 for the third. I hope this might help.
    and for bypass I think it must be there for pressure controlling

  5. #5
    Join Date
    Mar 2005
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    Pacific Time Zone
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    Controlling all 3 fans at the same speed and on at the same time would be the easiest, but I would do something like this.
    1 - Bypass opens to tower
    2- Fan 1 comes on at min speed
    3 - Fan 1 ramps as needed
    4 - Fan 1 gets above 80% for x time frame and/or loop temp continues to rise, then Fan 1 ramps down to min speed and fan 2 comes on matching that speed.
    5 -Fan 1 & 2 ramp up together as needed.
    6 - repeat 4 with fan 1&2, bringing on Fan 3.
    7 - all 3 fans ramp up together.
    Then as the fans ramp back down, once fan speed gets below 30% for x time and/or temp continues to drop, then fan 3 turns off and fans 1 & 2 ramp up to 100%, then continue stage down sequence.

    With that I would think you are going to have to set up some kind of breaking on your 'off' fan(s), so they don't start spinning backwards, and causing a lack of overall heat transfer on the running fans, from the air being sucked back in through the non-running fan. Do the fans have individual isolation dampers? Then last guy I asked this looked at me cross-eyed, but in colder climates stg 1 enable flow to tower, 2 open isolation damper(s), 3 bring on fan...
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
    BIG Government = More Dependents
    "Any 'standard' would be great if it didn't get bastardised by corporate self interest." MatrixTransform
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  6. #6
    Join Date
    Nov 2009
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    karachi
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    the seq is good ..but why we need to ramp down the speed of the first fan and then match the speed of first n second? cant we just start ramping up the second fan if the temp. continues to increase and the first fan continue to run at maximum speed

  7. #7
    Join Date
    Feb 2008
    Location
    Purcellville Va.
    Posts
    745
    I'd think the type and condition of the tower would factor into this.
    We put drives on an older BAC VXT470 ton that would stage the fans as needed but when we added the drives we now ramp them together as it almost completely eliminated any blow by out of the fan housings. I only have two fans so and they aren't gear driven so I don't have an issue with a minimum. I'd think you could modulate the bypass to maintain temps at minimum speeds if needed.
    Does the on-site engineer have any opinions?

  8. #8
    Join Date
    Mar 2005
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    Pacific Time Zone
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    Quote Originally Posted by mujtaba12 View Post
    the seq is good ..but why we need to ramp down the speed of the first fan and then match the speed of first n second? cant we just start ramping up the second fan if the temp. continues to increase and the first fan continue to run at maximum speed
    You could do it that way too - I was thinking more along the lines of the 4 towers I just did, but they each had individual fans. In that situation I had added surface area for evaporation to account for, therefore staging down the fan speed when another tower was brought online.

    BTW - anyone heard of city inspectors wanting to know the cooling tower fan run time? I had a request by a customer to start logging that info so they can provide it. I am thinking I was told they want something to calculate how much chemical is being evaporated into air from the towers. I don't quite get how they can even do that calculation, especially with a fan on a VFD and considering the tower will run and evaporate w/o the fan. Even that combined with water meter usage data, I just think they would need a lot more data to get a proper calculation - Outdoor Temp, Humidity and fan speed plus...
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
    BIG Government = More Dependents
    "Any 'standard' would be great if it didn't get bastardised by corporate self interest." MatrixTransform
    http://threedevilskennel.com/ - not my website.
    Versatile Hunting Dog Federation - www.vhdf.org/


  9. #9
    another example:

    We have three gear driven tower cells sized for one chiller each with a common supply, return, and sump equalizer piping.

    One VFD signal to all three fans based on common supply temp to chillers.

    Individual tower cell isolation valves and fan enable based on number of chillers on-line since any chiller can run with any cond pump and any tower.

    Our by-pass valve is primarily used to quickly warm the water switching from free cooling back to mech cooling.

    I had to do some minor program changes from start up since the towers cycled on cond water temp only at the beginning. The towers did not know how many chillers were running. Found 2 chillers pumping into one tower in mild weather and the tower over flowing due to the extra volume.

  10. #10
    Join Date
    Jan 2002
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    Fort Worth\Dallas, Texas
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    Quote Originally Posted by mujtaba12 View Post
    the seq is good ..but why we need to ramp down the speed of the first fan and then match the speed of first n second? cant we just start ramping up the second fan if the temp. continues to increase and the first fan continue to run at maximum speed
    It is cheaper to run two fans at a lower speed than one at max.
    Go Rangers!

  11. #11
    Join Date
    Oct 2005
    Location
    Albuquerque
    Posts
    4
    Tower airflow varies almost linearly with fan speed, however fan power consumption varies as a cube of the speed. I would stage them by staging water sequentially over every tower in sequence then start each towers fan on minimum in sequence. Once they are all at minimum i would ramps all the fans together. I would also add provisions to swap the lead tower so as the same tower is not always the lead tower. You may have to play with the sequencing.

  12. #12
    Join Date
    Jan 2002
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    Fort Worth\Dallas, Texas
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    Quote Originally Posted by captain-insano View Post
    Tower airflow varies almost linearly with fan speed, however fan power consumption varies as a cube of the speed. I would stage them by staging water sequentially over every tower in sequence then start each towers fan on minimum in sequence. Once they are all at minimum i would ramps all the fans together. I would also add provisions to swap the lead tower so as the same tower is not always the lead tower. You may have to play with the sequencing.
    I agree.
    Go Rangers!

  13. #13
    Join Date
    Nov 2009
    Location
    karachi
    Posts
    174
    crab do u hve the program for this.I use invensys controls and used workplace tech tool...

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