Blowers and Air Density

[Guyinfield]

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.


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[mgenius33]

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.

[coolcoil]

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.

[Guyinfield]

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?

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[coolcoil]

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

[Scoobie]

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.

[Scoobie]

If you have final filters after the coil in a blow through unit, they will become damp, moldy, and restrictive.

[coolcoil]

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.

[Guyinfield]

Thank you both for the information and the link

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[motoguy128]

For what it's worth, cooler air being more dense, will allow the same fan to spin at a lower RPM to move the same CFM. May improve fan economy slightly. Not a big deal with a 30F delta T however.

On concern there, is that with severely clogged filters, some fans can get noisy as you suction pressure drops. Depending on fan type, it can even go into a rotating surge condition where the surge of the previous fan blade pocket stalls the next, and the surge rotates opposite the fan rotation.

[Guyinfield]

Thanks Moto. I guess my initial thought was considering scale and totality of power consumption. On a 2 hp blower obviously very little saved, but if you add up all the 2 hp blowers running across the US well how about that. On the other hand a 200 hp blower would really add up over the course of a year or so. We could unload our one of our aging nuclear plants or something like that.

I have done some reading on laminar flow benefits on the draw through thanks to cool coil

I do wonder why then residential equipment is largely set up with fans upstream of heat exchangers. Id assume that on a typical nat. gas, split system the gas heat exchanger would help to laminarize before the air hits the cooling coil. Also having the coil runs perpendicular to the blades of the blower would prevent overloading/unloading certain refrigerant circuits.

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[coolcoil]

I do wonder why then residential equipment is largely set up with fans upstream of heat exchangers. Id assume that on a typical nat. gas, split system the gas heat exchanger would help to laminarize before the air hits the cooling coil. Also having the coil runs perpendicular to the blades of the blower would prevent overloading/unloading certain refrigerant circuits.

Fans are always upstream of gas heat exchangers for safety reasons. Any combustion HX needs to be in the positive pressure section of the air handler so that if it leaks, the products of combustion are not drawn into the air stream.

[Guyinfield]

Fans are always upstream of gas heat exchangers for safety reasons. Any combustion HX needs to be in the positive pressure section of the air handler so that if it leaks, the products of combustion are not drawn into the air stream.

Understood. My focus would mostly be on the cooling coil.

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[coolcoil]

I suspect that in residential systems the coils are typically after the fans because the coil section is an add-on to the furnace. Ideally, you would stick the coils below the furnace instead of on top, but then you would have to construct the coil section to hold the weight of the entire furnace assembly. You also might have problems getting rid of the condensate if the coils were on the floor.

You could put the coil section to the side of the furnace, and I have seen that. However, you would then have to move the filter to the inlet of the coil section and still need some elevation or a pump to handle the condensate. More floor space is needed too.

From a manufacturer's point of view, either of these options adds cost, and this is an industry where they fight for pennies in savings. Also, draw-through could have a small negative effect on the EER/SEER based on the way those are measured.
d

[ch4man]

I suspect that in residential systems the coils are typically after the fans because the coil section is an add-on to the furnace. Ideally, you would stick the coils below the furnace instead of on top, but then you would have to construct the coil section to hold the weight of the entire furnace assembly. You also might have problems getting rid of the condensate if the coils were on the floor.

You could put the coil section to the side of the furnace, and I have seen that. However, you would then have to move the filter to the inlet of the coil section and still need some elevation or a pump to handle the condensate. More floor space is needed too.

From a manufacturer's point of view, either of these options adds cost, and this is an industry where they fight for pennies in savings. Also, draw-through could have a small negative effect on the EER/SEER based on the way those are measured.
d

i believe the reason lies in the fact the innards of a heat-x in the summer are exposed to outside air(read humid). a cooling coil before the heat-x can and will drop the heat-x below dewpoint and rot out the heat-x. the mfg would not want to warranty that

do you really believe the manufacture cares how hard it is for you to install it?

[coolcoil]

i believe the reason lies in the fact the innards of a heat-x in the summer are exposed to outside air(read humid). a cooling coil before the heat-x can and will drop the heat-x below dewpoint and rot out the heat-x. the mfg would not want to warranty that

do you really believe the manufacture cares how hard it is for you to install it?

I don't disagree with your assessment. However, I was talking about the location of the cooling coil upstream or downstream of the fan, not the location of the HX. As noted above, the HX must always be downstream of the fan. Having the cooling coil downstream of that makes sense for manufacturing and the reason that you cited.

Though, in rooftops the order of components in direction of airflow is almost always cooling coil -> fan -> HX.