Fan Power

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This is a DOE-2 question, I think, but I am applying it to EnergyPro. I
am asking here in the hopes that some will have a more in-depth
knowledge of DOE-2 and how it relates to fan power. I am also hoping
that some here will be able to reflect on the theory and application of
fan power and energy as it relates to actual systems and modeling.

When inputting indoor fan power, I have used many sources for the fan
power. I sometimes use the motor nameplate, which I believe is too high
in every case.

I also sometimes use FLA or RLA, which I also do not believe are
accurate, but sometimes give more realistic numbers.

Some manufacturers give fan BHP at design conditions, which I find to be
very helpful, but not everybody is that helpful.

It has been suggested that I calculate the fan BHP using the theoretical
formula *** Fan BHP = (cfm x static press "w.c.) / (6356 x fan
efficiency) *** and then use a conservative fan efficiency (maybe 50%)
for a good number. This gives a much lower value than the other
methods, and I am trying to see if there is anything wrong with this
approach.

As I understand the usage, indoor fan power is divided by motor and
drive efficiency to come up with fan energy. So the fan BHP is the
starting point. Having this too high drives up fan energy, sometimes
dramatically, and can influence the proposed case quite a bit if there
are lots of fans, even small fans.

Any thoughts on this approach would be very much appreciated.

--
Robert Wichert P.Eng. LEED AP BD&C

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Robert,

I've also run into problems obtaining combined (fan and motor) efficiency
and input power, mostly with terminal units such as heat pumps and fan coil
units but also with individual fans. What I've often done is used the watts
listed in their electrical tables. For example, below if you take their
size 20 at high fan speed, they classify the motor HP at 1/30HP or 25W
(output power). I've taken the subsequent columns to mean input power so
the watts at 115V is then 57W which would make the motor is 44% efficient
and that seems reasonable to me. The listed amps don't necessarily equate
to the same wattages, they are usually higher so my only theory is that this
is the starting amps.

I'd be interested in finding out what other have to say. I have tried
talking to some local equipment reps without any luck.

Aaron Smith, P.Eng

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A 50% efficient MOTOR cannot be correct. 50% efficient fan, yes.

I actually think a lot of the amps given are something akin to MCA.

Let's see what others say...

Robert P. Wichert P.Eng. LEED AP BD&C

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Based on Aaron?s calculations, it sounds like 44% = motor efficiency x fan efficiency, That?s the net which affects energy use ? how it is composed doesn?t matter too much.
Small motors often have terrible efficiency. The same is true for small fans! Put them together and it?s pretty dismal.

James V Dirkes II, PE, BEMP, LEED AP

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Small FHP HVAC motors can have efficiencies below 50% which is why ECM motors are becomming popular in that size range - they are typically 65-85% efficient. See in the presentation below where w/cfm is 1/3 with ECM vs PSC.

http://www.energy.ca.gov/title24/2013standards/prerulemaking/documents/2011-08-17_workshop/presentations/08%2520EC%2520Motors.pdf

Aaron Smith, P.Eng

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The data is very hard to come by, but 44% for the motor only I believe is about right in those smaller motors. 35% wouldn't surprise me with maybe a 60-65% direct drive blower so 20% wire-air wouldn't be out of the ballpark in my opinion. The motors are historically single phase split capacitor historically and a poorer quality than a small 3 phase induction motor.

On a tangent, 3 phase induction motor efficiencies really drop off below 1.5 HP or so. Unless of a very high quality / HE then the part load efficiency can also be very poor despite the full load efficiency looking OK. So a small 3 phase motor could have 85% full load / full speed efficiency but at 25% load (say 50% speed and 50% torque) the efficiency can be dramatically poorer. We have managed to get part load motor efficiency data from TECO and others in the past that go down to 50% torque but at full speed (thus 50% load) so not that helpful as variable speed / variable pressure systems spend time at much lower loads. The data does typically show a peak efficiency at 75% load however where I2R losses have dropped but eddy current and other losses haven't started to dominate the losses and thus drive efficiency down which they do at lower loads.

We have a large project where we have asked contractors and thus suppliers to test select equipment on their test rigs to generate better part load data. We will see how we go.

Fortunately with a lot of smaller equipment digital motors or electrically commutated motors have lifted efficiencies dramatically as they are direct drive and being digital the variable speed controls are intrinsic to the motor.

Getting part load efficiencies out of VSD drive suppliers we have found to be like drawing blood out of a stone. I assume that there is a large fixed component of losses so at part load in % terms the VSD losses could grow from 3% say to 7-10%. Don't forget your belts as well if you are taking a fan manufacturer's bhp figures as they typically do not include the belts - maybe 3-5% in losses if well aligned and tensioned.

If you have variable flow, variable pressure systems that spend a lot of time at very low loads, the VSD can affect the motor efficiency at lower loads. There was an ASHRAE Transactions paper or RP from the late 90's I recall that showed motor part load efficiency without a VSD drive and then with and there was a difference. If I remember correctly they did the test with a SE motor but may have also done with a HE motor and the HE motors are more resilient to the impacts of a VSD.

I guess all I am trying to say is that getting a full load design point is one thing, but make sure it includes all the losses and more importantly make sure you take a view on what happens at part load as that is where larger modern systems operate. Smaller systems may be constant flow / constant speed but increasingly we are seeing variable speed FCUs etc available on the market using ECMs.

Regards,
Graham

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Well, OK, I will bow to your expertise, but a 44% motor sounds pretty
dismal to me, as you suggest.

Would a conservative number be 20%, air to wire? Is so, I am looking
for FAN efficiency. NOT motor or drive efficiency, which are taken care
of by the modeling software separately.

Would 50% for FAN efficiency by reasonable?

If so, this allows me to use the theoretical air BHP equation to
calculate fan BHP at design conditions using 50% fan efficiency. This
would be a big breakthrough for me, actually.

I do like the idea of actual testing, which is rare as hen's teeth.
Even amp numbers are way too high. I have had RLA that show more HP
than the nominal motor HP. That's gotta be wrong.

I have not seen a single ECM fan coil unit with an improved fan BHP.
Not one. If you got one, please share your knowledge!

Cheers from sunny California.

Robert Wichert P.Eng. LEED AP BD&C

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Why don't you see if you can get fan curves for some small fans - they'll show fan efficiency.
p.s., I think 50% for the fan is darn good for most rooftop equipment; anything smaller will almost certainly be worse. I did some modeling for a manufacturer a while back who used ~2,000 cfm backward inclined fans - 40% static efficiency

James V Dirkes II, PE, BEMP, LEED AP

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Yes, good ideas, Jim, but I am disappointed in your 2,000 CFM fan with
40% static efficiency. That's scary.

We use Aspen fan coils, and I don't know if they have fan curves. I'll
ask. They are most likely smaller though, so even more scary.

Robert Wichert P.Eng. LEED AP BD&C

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Robert,
It's a major fan manufacturer! I'm not too surprised (just a little). These happen to use ECM and the variable speed feature covers a lot of "sins", but you are right - a bit scary.

James V Dirkes II, PE, BEMP, LEED AP

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