increse CFM ??

[jmiles]

If I had a 3 ton ac with an ahu that derlivered 1200cfm- would installing a fan that produces 1400-1600cfm improve the amount of cooling being delivered???

[hvacbear]

What symptoms do you have that you are trying to correct? The scenerio you suggest could just make it worse.

[dan sw fl]

... Would installing a fan that produces 1400-1600cfm improve the amount of cooling being delivered ?

NO !

[smokin68]

If I had a 3 ton ac with an ahu that derlivered 1200cfm- would installing a fan that produces 1400-1600cfm improve the amount of cooling being delivered???

It would remove less humidity.

In short, NO.

400 CFM/ton is about perfect here in Florida.

[Mstrav]

no!!! more cfm = air temp would be warmer, less air, air would be colder. The out-put is determined by the comp btu, moving more air will not change that, it will change moisture removal or run the risk of freezing the coil (if you slowed it down)

matt

[Jabarco]

Yes, more CFM would increase the suction pressure and in turn increase the capacity of the condensing unit. However, that increase in capacity will be an increase in sensible heat and less latent heat which will probably result in less comfort; depending on your geographical area.

Jabs

[beenthere]

Might, or it might not.

Depends on the duct system, you might not get more then another 1000 CFM.

You will lose humidity control if it does move more air.

[freeeze43]

400 cfm per ton of cooling, more airflow across the coil will cause the coil to freeze up, you must try to keep a load up on the coil for an efficient system..

[k-fridge]

more airflow across the coil will cause the coil to freeze up

Huh?

You'd better think about that one.....

[t527ed]

400 cfm per ton of cooling, more airflow across the coil will cause the coil to freeze up, you must try to keep a load up on the coil for an efficient system..



:confused: :confused: hows it do that??:confused: :confused:

[Airmechanical]

It would remove less humidity.


delete, i need my reading glasses!



.

[Airmechanical]

more airflow across the coil will cause the coil to freeze up


can't agree with this either!



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[RoBoTeq]

Less humidity control. Increased air pressure problems. Increased load problems. All around bad idea.

[RoBoTeq]

Quote:
Originally Posted by smokin68
It would remove less humidity.


can't agree with this!

.

It is true. Air requires "hang time" on the coil in order to have moisture removed. The more air across the coil, the less air that actually touches the coil, the more moisture that gets past.

[Airmechanical]

Quote:
Originally Posted by smokin68
It would remove less humidity.



It is true. Air requires "hang time" on the coil in order to have moisture removed. The more air across the coil, the less air that actually touches the coil, the more moisture that gets past.

yap i read it wrong sorry, it would remove less humidity!:o :o :(



.

[Jabarco]

It is true. Air requires "hang time" on the coil in order to have moisture removed. The more air across the coil, the less air that actually touches the coil, the more moisture that gets past.

Hang time? You say it requires "hang time"? The fact that the evaporator temperature would increase if more air was moved across it..... wouldn't have anything to do with the "hang time" theory would it?



http://i199.photobucket.com/albums/aa214/jabarco/psc_01.gif

Jabs

[contactor]

Roboteq.....The more air across the coil, the less air that actually touches the coil,.
Now that statement I disagree with.

[freeeze43]

ooops, my bad, i meant not enough airflow.

[yp5]

Quote:
Originally Posted by smokin68
It would remove less humidity.



It is true. Air requires "hang time" on the coil in order to have moisture removed. The more air across the coil, the less air that actually touches the coil, the more moisture that gets past. I strongly agree, very well said.

[mark beiser]

Quote:
Roboteq.....The more air across the coil, the less air that actually touches the coil,
.
Now that statement I disagree with.

Increasing the CFM across the coil would increase the bypass factor, so a lower percentage of the air moving through the coil would actually come in contact with the coil.
A larger quantity of air would contact the coil simply because more air is being moved through the coil though.

[Jabarco]

Increasing the CFM across the coil would increase the bypass factor, so a lower percentage of the air moving through the coil would actually come in contact with the coil.
A larger quantity of air would contact the coil simply because more air is being moved through the coil though.

Mark, you almost got it right.

CFM = Volume
FPM = Velocity

Volume ≠ Velocity

The faster that air moves over an evaporator coil, the higher the coil’s bypass factor.

Jabs

[Oldman1]

Yes, more CFM would increase the suction pressure and in turn increase the capacity of the condensing unit. However, that increase in capacity will be an increase in sensible heat and less latent heat which will probably result in less comfort; depending on your geographical area.

Jabs

Most correct answer!! Above

Most likely answer to the bottom question!

More airflow will not cause coil to freeze? Most cases. Most likely attempting to increase airflow requires ductwork modifications, larger size ducts, increased motor HP, or a higher duct static pressure is created. The problem with installing a larger fan into a smaller ducted system will cause the blower RPM to increase not allowing the FC fan to picking up the airflow required by the new fan. If the larger fan isn't picking up the required airflow due to the tip speed of the wheel you won't be moving the 400 cfm/ton requirements. And possibly could freeze the coil.

[Old CFM/New CFM]2 x Old SP = New SP
[Old CFM/New CFM]3 x Old Motor HP = New Motor HP

Most systems are designed orginally to handle only what is necessary to controlling cost.

[billb7581]

One of those register booster fans will help.
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:p

[Shophound]

Hang time? You say it requires "hang time"? The fact that the evaporator temperature would increase if more air was moved across it..... wouldn't have anything to do with the "hang time" theory would it?



http://i199.photobucket.com/albums/aa214/jabarco/psc_01.gif

Jabs

More air moving across the evap coil increases the heat load imposed on the coil. That is what elevates suction pressure. This in turn raises the apparatus dew point of the evap coil. Higher ADP means less water condensed out of the air passing over the coil.

So, yes, increasing air flow increases the sensible capacity of the system, but not the latent.

As for "hang time", I see it as Mark does...increased flow increases coil bypass factor...the amount of air moving over the coil surfaces that is unaffected by the refrigeration process. I see your point re: CFM and FPM not necessarily being equal, for if the CFM is increased and the ductwork is properly resized for the anticipated volume increase, the FPM could remain unchanged. Therefore the discussion becomes one of air volume/quantity vs. air speed. The "hang time" might reduce if the FPM is increased, but if CFM is increased yet FPM is adjusted for, the "hang time" might remain the same but the load on the coil has changed due to the volume of air moving over the coil.

[beenthere]

Ya need a Mollier chart yet.

[Jabarco]

So, yes, increasing air flow increases the sensible capacity of the system, but not the latent.

Yes, we agree, increasing air flow increases the sensible capacity of the system However, air could "hang" all day long but if the evaporator temperature increases (which it will do, due to increased air flow) the latent heat that is collected to remove moisture is reduced due to the higher evaporator temperature....not "hang time".

Jabs

[Jabarco]

400 cfm per ton of cooling, more airflow across the coil will cause the coil to freeze up,......
http://i199.photobucket.com/albums/aa214/jabarco/youdo.jpg

Jabs

[Shophound]

Yes, we agree, increasing air flow increases the sensible capacity of the system However, air could "hang" all day long but if the evaporator temperature increases (which it will do, due to increased air flow) the latent heat that is collected to remove moisture is reduced due to the higher evaporator temperature....not "hang time".

Jabs

If the air was indeed "hanging all day long", would that not imply a reduced airflow...essentially NO airflow, hence the coil temp would drop and the air "hanging" around the coil would experience a hell of a lot of moisture extraction?

One of the points of my earlier post was that increased airflow imposes a higher heat load on the coil, due to increase of volume. This is what raises saturated suction pressure (SSP) of the refrigerant in the evap and therefore raises sensible capacity. Since the temperature of the return air is the same in our example, and the only thing that has changed is the volume of return air, the increased SSP also elevates SST, which in turn elevates the apparatus dew point (ADP) of the evap coil surfaces. The dew point of the return air has not changed, but the temperature of the coil has, it being an increase, so less latent heat removal occurs.

When RoBo used the term "hang time", the term made sense to me and does not seem out of place to describe what is occurring over an evap coil when air volume is varied. You seemed to bolster this point, perhaps inadvertently, when you said that the coil could experience "hang time" all day long. Doing so would refer back to my point regarding air volume affecting heat load imposed on coil, affecting suction pressure, affecting the sensible heat ratio.

[mark beiser]

Mark, you almost got it right.

CFM = Volume
FPM = Velocity

Volume ≠ Velocity

The faster that air moves over an evaporator coil, the higher the coil’s bypass factor.

Jabs

I'm well aware of that fact, nothing wrong with my answer though...

By default, raising the blower speed, or installing a bigger fan (see the first post in the thread), to increase the CFM through a coil will increases the velocity.
More air through the same size space = higher velocity.

[mark beiser]

Yes, we agree, increasing air flow increases the sensible capacity of the system However, air could "hang" all day long but if the evaporator temperature increases (which it will do, due to increased air flow) the latent heat that is collected to remove moisture is reduced due to the higher evaporator temperature....not "hang time".

Jabs

Actually latent cooling is reduced for both reasons.
Increased evaporator temperature and increased bypass factor both contribute to the reduction in latent cooling.
Most coils have design features to reduce bypass, like fins that change directions, multiple rows of tubing, enhanced fins, and tight fin spacing, so bypass is usually only a small part of it.

In a perfect coil, bypass would not be an issue, but there are no perfect coils.

[Ed Janowiak]

^ I think there is a guy on this site that will say that Coleman coils are perfect ;).

[mark beiser]

^ I think there is a guy on this site that will say that Coleman coils are perfect ;).

Coleman/York make evaporator coils? :confused:

:D

If all you had to go by was the systems I see in my area, you would think that York no longer manufactures evaporator coils, other than the ones that come in their air handlers.
All I see matched up with York/Coleman condensers and furnaces on newer systems in my area are ultra cheep 3rd party coils.
Kinda makes me wonder why...
Are they that bad, to expensive, or something else? lol