The Sheet Metal Shop .Com

Duct Construction and Uses - Air Velocity

pricer - Aug 06, 2004 - 02:00 AM
Post subject: Air Velocity

Would you think that by slowing the air's velocity through a DX coil, that it would help more to remove the mosture out of the air or not?

Bud - Aug 06, 2004 - 02:30 AM
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I would think to have the correct temperature drop across the coil is what's important. The air flow across the coil is one adjustment that will help with maintaining the correct temperature drop. If you have to little air flow, the coil will freeze. Often techs put on the gauges and never access the entire system..lack of airflow across the coil will look like the system is undercharged...
I would think (thats think) if you were to increase the size of the coil, you wouldn't get as cold of a temperature at the register, but you'd remove a greater amount of humidity...Thats from the non refer guy, me:)

Am I close:)

pricer - Aug 06, 2004 - 02:46 AM
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Thats exactly what I would think too, if you were to slow down the air speed across the coil it would make it colder. I am being told now that this does not help in removing moisture, that its just the oposite. I was told that if the correct amount of air is passing thru the coil for the size of the unit, that it removes more moisture. The idea is that more air sees the coil more often. This information came from a short class,that I was unable to attend, from an engineer that designed a dessicant unit that we recently installed. I wish I could have had the time to attend the class then I could elaborate more.

BobLuland - Aug 10, 2004 - 11:31 PM
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This is a misunderstood subject matter. It’s not the speed of the air but coil surface area. You can take a ten-seer condenser and match it with an over sized evap coil and get 12-seer efficiency level. The same thing applies to humidity. The larger the coil surface the more humidly you’re going to remove. Bob

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pricer - Aug 11, 2004 - 04:15 AM
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BobLuland wrote:

This is a misunderstood subject matter. It’s not the speed of the air but coil surface area. You can take a ten-seer condenser and match it with an over sized evap coil and get 12-seer efficiency level. The same thing applies to humidity. The larger the coil surface the more humidly you’re going to remove. Bob

The Second amendment, the original home land security.
1-800-NRA-JOIN

Sounds like a winner to me. The way it was explained to me, if you move the correct amount of air across the specified coil, you would remove more humidity than if you had slowed the air speed down. A colder coil lowers the sensable temp to quickly causing the unit to cycle off reducing the time it takes to remove the latent heat. As you have said about the larger coil surface, I can see where this would help greatly in removing humidity. If you can remove the latent heat, the humidity, a higher sensable temp feels good. I have seen buildings and houses that the sensable temp was droped so fast that the humidity levels remaining in the air created lots of mold problems, but they had a cool house! In may peoples minds that is the most important. Never mind the mold!

BobLuland - Aug 12, 2004 - 02:33 AM
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Pricer, The whole idea of doing a proper load calculation on a space is to achieve a balance of heat removal at a rate that is significant to the amount of humidity removed. If you don’t remove the humidity you have fail at what you set out to do in the beginning. You are better under sized than over if anything other than proper. Bob

pricer - Aug 12, 2004 - 02:42 AM
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BobLuland wrote:

Pricer, The whole idea of doing a proper load calculation on a space is to achieve a balance of heat removal at a rate that is significant to the amount of humidity removed. If you don’t remove the humidity you have fail at what you set out to do in the beginning. You are better under sized than over if anything other than proper. Bob

Thanks for sharing this, I am sure a lot of people will benefit from it. If you care to change the topic on the subject of air flow, I would like to hear more of your thoughts. Sounds like you have much wisdom to share.

Gate - Nov 25, 2005 - 09:40 PM
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First, due to my newbness in HVAC, how can using a larger evap increase the SEER (system energy efficiency ratio) doesn't SEER require 1 evap model# capacity, etc directly linked to a matching condenser?

With the DC Variable speed motor in higher-end residential HVAC systems (ex: Carrier Infinity Series) the "old" 400CFM per ton has been lowered (slowed) to 350CFM per ton for better dehumidification, comfort cooling, and easier energy consumption. Is this incorrect?

Thanks for your inputs.
- Gate

danski0224 - Nov 25, 2005 - 10:27 PM
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The variable speed equipment I am familiar with all use a specific time interval where the blower spins at 80% capacity at the start of a cooling call. The desired effect is to remove extra humidity. If the call for cooling is not satisfied, then the blower ramps up to 100% capacity until the call for cooling is satisfied.

It stands to reason that a colder coil (less airflow) should attract more moisture from the air. It should also lower the register temp because of that slower airflow- just like turning down the blower on your car AC.

Those that check the AC systems on a VS drive best make sure the blower is operating at 100% when checking charge. Some boards have an LED that indicates 100% operation. I suspect that proper charge will be more important than ever with a VS drive.

A bigger coil gives you more BTU's of capacity (to a point- the condenser can condense only so much), hence the higher SEER number. SEER is just a number for a quantity of BTU's in exchange for a quantity of kWh, plus a percentage guess factor (the S part). EER is a true measure of BTU's produced per kilowatt of power used (no fudge factor). The S part also factors in duct tightness, so a leaky system will never deliver the SEER rating. In essence, your "13 SEER" matched system could have an EER of 11.5- or less if the ductwork is full of extra holes (why California is requiring that ducts be tested and sealed for equipment replacement).

My not so technical understanding of how it all works. Smile

cliffwilson - Dec 01, 2005 - 02:46 AM
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OF COURSE LOWERING THE AIRFLOW WILL AFFECT MOISTURE REMOVAL. IT WILL ALSO CHANGE
TOTAL HEAT REMOVAL. BY REDUCING AIRFLOW YOU LOWER THE COIL SURFACE TEMP,WHICH
LOWERS THE COIL DEW POINT. THIS WILL INCREASE LATENT HEAT REMOAL,BUT ONLY SLIGHTLY.
IT'S BEST TO DESIGN THE SYSTEM TO REMOVE THE ANTICIPATED LATENT LOAD .DON'T TRY TO
RE-DESIGN A POORLY MATCHED SYSTEM

Subzerori - Nov 06, 2007 - 12:51 AM
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If you slow the fan down the less air that is moved over the coil, the less air the less humidity, and as what was said earlier the less heat removal. means frozen coil. to slugged compressor, maybe hurt valves at this point, meaning new compressor.
Just my opinion.

BobLuland - Nov 08, 2007 - 12:37 AM
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I will adress this weekend in between building a house and saving the world! YE HA! Bob

bordontn2 - Nov 08, 2007 - 06:28 PM
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SEER (seasonal energy efficiency ratio) based on cooling load days;
not sloppy duct installation. That's factored in when you run your load.
bordontn2

Hittman - Feb 10, 2008 - 02:55 PM
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[quote="BobLuland"]Pricer, The whole idea of doing a proper load calculation on a space is to achieve a balance of heat removal at a rate that is significant to the amount of humidity removed. If you don?t remove the humidity you have fail at what you set out to do in the beginning. You are better under sized than over if anything other than proper. Bob

Bob nailed it here, This is about the only way to remove the proper amount of humidity efficiently. This being said in general a colder coil will remove more humidity in a given amount of time. There are two ways to do this on a system that is not properly sized
#1 You can install a TXV (thermostatic expansion valve), this will adjust ref. flow to coil load. #2 Slow the air flow across the coil, This will give you a colder coil and more humidity removal but less sensible heat removal in a given amount of time (but not so efficient)