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Old 10-11-2011, 09:15 AM   #16
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Ductwork handle smaller unit?


Great. beenthere, I really appreciate your help and expertise. I guess the last thing I've been struggling with is trying to accurately determine the friction rate to use for sizing the branches.

I've been planning on using a Carrier fe4 fan coil, which it looks like has a maximum static pressure of 1.0" w.c. This is referenced on page 8, "Maximum Static Table". It also makes reference to 0.8" and 0.6" w.c. but it looks like these are probably higher CFM than I would be using. Although I confess, with a variable speed blower I do not fully understand what numbers I should be working with.

http://www.xpedio.carrier.com/idc/gr...t/fe4a-3pd.pdf

Page 11 also reference static pressure drops and pressure correction, although I do not completely understand how to use the data. I think a 10kw auxiliary unit would suffice, meaning no correction is needed. If I am not using a factory installed filter, then I would not use the corrections listed on the second table. I think I would need to use a "Air Delivery Performance Correction Component Pressure Drop" value (which I think would be subtracted from the 1.0" w.c.) but I am not sure which value to use. Maybe the 0.028 for the 2-ton air handler?

Using the data that I think is accurate, I end up with the following.

1.0 (Max Static Pressure) - 0.028 (Component Pressure Drop Correction) - 0.1 (filter) - 0.03 (supply register) - 0.03 (return grille) - 0.03 (balancing damper) = 0.782 ASP

With a TEL of 469', that gives a friction rate of 0.17" w.c. Does that look correct, or is that high?

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Old 10-11-2011, 03:40 PM   #17
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Ductwork handle smaller unit?


Using the 1" of ASP, would mean that you have little load up ability on the air filter. Meaning you would need to change it very often.

Best to use .8" ASP or less. That gives your air filter the ability to catch dirt, and not have to be changed every 2 to 3 weeks. A .17" FR will tend to make a noisy duct system.

Remember, the higher the static pressure, the more electric an ECM blower uses. They use less electric at .5" TESP and lower.
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Old 10-11-2011, 03:51 PM   #18
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Does that mean that since it has a maximum of 1.0" wc, I can choose to use a lower number? I just have to look up the value to know that I can not use a higher number, is that correct?

So with the ECM blower and using 0.5", a better calculation would be.

0.5" (Static Pressure) - 0.028 (Component Pressure Drop Correction) - 0.1 (filter) - 0.03 (supply register) - 0.03 (return grille) - 0.03 (balancing damper) = 0.282 ASP

With a TEL of 469', that would give a friction rate of 0.06". That is much lower, and more in line with what I have seen. Does that make more sense?

That is also with a basic filter, a better filter would give more of a pressure drop I think. Would it make more sense to start with a higher static pressure (maybe 0.65") and use a filter that has a pressure drop ~2.5?
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Old 10-11-2011, 04:27 PM   #19
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looks better in that calc.

Works best to use the PD for a better air filter, even though you only intend to currently use a standard air filter now. And see if you can still use .5" as your ESP. Go to .6 if needed, but try to avoid if possible.
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Old 10-11-2011, 11:36 PM   #20
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Wow, very informative, thank you. That sounds like quite a task though. Just by doing the math, I would have to decrease my TEL to ~235' for using a filter with a PD of 0.24. That is basically eliminating my return side, which isn't realistic. There are not a whole lot of other variables to change.

Another option would be to eliminate the 10kw auxiliary heat strips. But even that with a 0.24 PD from a filter would only give a static pressure of 0.04" w.c.

What seems to me to be the most realistic option is the Honeywell (or similar) pleated filter. The Honeywell looks like it lists a 0.23 PD at 492 fpm It would appear that (if I follow the curve down) the 20x25 filter has a pressure drop of only ~0.09" w.c.-0.1" w.c. at 900 CFM. If I need 2 CFM/sq in. for the return, this would slightly exceed that and give me a lower velocity (meaning less pressure drop) and this filter would make the situation work. Does that sound realistic or am I missing a variable somewhere.

Last edited by kmachn; 10-11-2011 at 11:41 PM.
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Old 10-12-2011, 04:31 AM   #21
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Ductwork handle smaller unit?


Turning vanes in the return drop can reduce its TEL, along with a scoop in the supply plenum at each trunk line attachment.

Often, you do have to use .6 or .7", in order to meet space confinements.
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Old 10-12-2011, 06:08 PM   #22
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Ductwork handle smaller unit?


Turning vanes and scoops on the supply side sounds like it should be easy enough to have done during the installation of the new unit. Right now the plenum to the supply trunks is just a 90-degree Tee (TEL of 40') and the return basically has 90-degree bend. How much would these changes reduce the TEL?

Also, what about my idea on the 20x25 Honeywell air filter at 900 CFM?

http://customer.honeywell.com/techli.../68-0239EF.pdf Page 11, "Capacity and Pressure Drop" table

Does this sound reasonable? Realistic?
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Old 10-12-2011, 06:14 PM   #23
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The 20X25 air filter would work great.

vanes would drop the supply tee to 15, and the return ell would also drop to 15.
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Old 10-12-2011, 08:54 PM   #24
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Great, I'll factor all that in. I feel like I have a pretty good grip on where to go from here (finally). Thanks a lot for your help, I would have been pretty lost.
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Old 10-13-2011, 03:54 AM   #25
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Your welcome.

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