Results 1 to 9 of 9
  1. #1
    Join Date
    Aug 2018
    Posts
    21
    Post Likes

    Blowers and Air Density

    Kind of an equipment design question.

    Given that air density is higher at a lower temperature, why are most large equipment blowers located downstream of cooling coils? Every piece of commercial equipment I've worked on or around has been set up this way.

    With a correction factor of 1.03 when comparing 50 degree air to standard 70 air, which would be close to normal RA temperature, wouldn't it be more effiecient to push air through the coil rather than pull it.

    Ive read here that True has reduced deforst heater wattage and number of defrosts per 24hrs to meet efficiency standards on newer, stringent equipment.

    The difference in power consumed by fans across the commercial/industrial world could really add up as well, correct?

    The one thing I can think of would be that the pressure drop from the coil would immediately drop the total pressure on the supply of a large ventilation system. But on a 3-5 ton RTU the ventilation is not any different from residential where almost all the cooling coils are downstream of the fan.

    If there is some design consideration im missing I would apreciate any advice, knowledge or resources. Thanks in advance.


    Sent from my SAMSUNG-SM-G891A using Tapatalk

  2. #2
    Join Date
    Sep 2008
    Location
    VA
    Posts
    3,081
    Post Likes
    Well, since volume is the inverse of density, does it really matter? You're still moving the same amount of air molecules or load. With warmer air you're moving a higher volume with less density, cooler air you're moving a lower volume with greater density.

    Seems to me there's more laminar flow with the draw through design, but it's probably such a small difference it doesn't much matter.

    I always seem to get more accurate results with a flow hood on the return air rather than the supply air.
    "The only true wisdom is in knowing you know nothing" Socrates

    Visit us at Dr. Degree°s HVAC
    or like us on Facebook

  3. #3
    Join Date
    Nov 2001
    Posts
    37
    Post Likes
    I am an engineer for a large company that makes air handlers. This is a very common question.

    In North America, it is much more common to place the fans after the cooling coil. This is called draw-through operation. The advantages are, roughly in the order of importance cited by designers:

    1. The fan provides a little bit of reheat which reduces condensation formation in the ductwork. Often, the air coming off a cooling coil is completely saturated with moisture. In that condition, any pressure drop will create condensation. The 1.5 degrees F or so added by the fan generally prevents this from happening. When I discuss this issue with building design engineers, this is by far the most often cited reason.

    2. As long as the fan inlet is far enough away, a wheel diameter or so, the airflow over the coil will be more even, which will provide better cooling coil performance.

    3. If you have a housed fan and the discharge of the fan connected directly to the duct you will have more efficient fan operation than if the air has to leave through an opening.

    4. If the fan is last-in-airstream, the entire unit is under negative pressure (i.e. air leaking into the unit rather than out of it) In general, AHU's will leak less at a given negative pressure than they will at the same positive pressure. Given the increase in the use of final filters over the last twenty years, along with the improvements in AHU construction, this is not a big factor anymore.

    In Europe, on the other hand, it is typical to have the fan before the coil (blow-through). The advantages are:

    1. You do get colder air coming off the coil, since the fan heat gets absorbed by the coil, so slightly lower airflow for the same amount of space cooling.

    2. The coil can be closer to the end of the unit, so the area of the unit cabinet that is subject to cold air, and therefore thermal losses and/or sweating is reduced.

    The vast majority of AHU's that we sell are draw-through configuration.

  4. #4
    Join Date
    Aug 2018
    Posts
    21
    Post Likes
    Thread Starter
    Thank you for those considerations.

    Please correct me if I'm oversimplifying.
    Wouldn't power consumption rise while CFM would remain constant because fan inlet air desity would be higher in a draw-through design?

    Coolcoil, is it possible that europeans design to correct for this and reduce power consumption?

    Sent from my SAMSUNG-SM-G891A using Tapatalk

  5. #5
    Join Date
    Nov 2001
    Posts
    37
    Post Likes
    It is true that if you have two identical fans running at the same speed, and the one with the higher density entering air will use more power because it is moving a larger mass of air. In the cooling scenario we are discussing, the downstream fan would use a bit more power. If the system switches to heating, then the downstream fan is moving less mass and uses less power. However, there is a lot more to consider both for energy consumption and practical considerations.

    First, I doubt that manufactures' decisions about the fan location has anything to do with energy consumption. It is all about keeping the motor out of a hot air stream. Keep in mind, Europe is all about heating. It's much colder there and many building do not even have air conditioning, especially back in the days when manufacturers were first designing equipment for large-scale use. And, back then there was a lot of steam heat or very hot water going to the coils, so the air temperatures were much higher than they are today. Also, motors were a lot less resistant to heat. Even today there are people in our industry who believe that putting a motor downstream of a heating source is malpractice. I actually had a disagreement with one of my colleagues on that topic within the past month or so. Of course, being downstream of a cooling coil is thought to be good for a motor.

    For what it's worth, a fan will always be upstream of an indirect gas heat exchanger, because you want positive pressure at the heater in case it develops a leak. You don't want to actively pull dangerous gases into the air stream.

    So, back to the energy discussion. First, I will say that we are talking about small differences here. And, they reverse between heating and cooling mode. The practical considerations like having the motor in a hot air stream, getting some reheat to avoid condensation in the duct and even airflow over the coil are higher on designer's minds. Further, if you are talking about a housed fan (vs. a plenum fan) the housed fan at the end of an air handler that is hooked up directly to the duct (or with flex) will operate more efficiently than that same fan placed upstream in the AHU.

    Housed fans need some length of straight duct at their discharge to reach their highest efficiency. Very few fan manufacturers tell you the efficiency difference between ducted and unducted operation. Even as a large manufacturer, we cannot get that data from our suppliers, as they claim they have not tested in those conditions. We do lots of our own testing in our AMCA-accredited lab, and I can tell you with confidence that the efficiency losses are at least 5%, and usually somewhat larger, depending on the fan design and what is placed downstream of the outlet. That's why you see fewer housed fans in AHU's and rooftops today, though that is also driven by the fact that it is a lot easier to do direct drive with a plenum fan than a housed fan.

    If you would like to read more, here is a link to an article in ASHRAE Journal from an engineer, who's opinions I respect, arguing in favor of putting fans upstream of the coil for energy savings. I still don't think it outweighs the practical considerations, but given the wider use of plenum fans and the fact that motors are much more temperature-resilient today, I think it is worth considering.

    http://www.nxtbook.com/nxtbooks/ashr.../index.php#/52

  6. #6
    Join Date
    Oct 2016
    Posts
    590
    Post Likes
    One more advantage to the fan after the coil. The air will get a little colder as it passes through the coil, thus removing more moisture.

  7. #7
    Join Date
    Oct 2016
    Posts
    590
    Post Likes
    If you have final filters after the coil in a blow through unit, they will become damp, moldy, and restrictive.

  8. #8
    Join Date
    Nov 2001
    Posts
    37
    Post Likes
    Yep - both of the above are true, though the coil temp difference will be very small. The filters are a bigger problem, and we have definitely seen that happen. In the article that I linked, the author suggests doing a little free reheat, but that negates the benefits of putting the fan upstream. That’s one of the things that made the suggestion not-so-compelling in my eyes.

  9. #9
    Join Date
    Aug 2018
    Posts
    21
    Post Likes
    Thread Starter
    Thank you both for the information and the link

    Sent from my SAMSUNG-SM-G891A using Tapatalk

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Related Forums

Plumbing Talks | Contractor MagazineThe place where Electrical professionals meet.