How does an oversized evaporator coil affect humidity removal in an A/C system?

[Saturatedpsi]

Hello everyone...

Since this is my first post, I'll intro myself as a Physics major some 40 years ago, who never got around to actually practicing physics and ended up as a Heating/Cooling serviceman...well, I'm a one-truck-service-company-owner and I drive the truck.

To the question: I was stumbling around on one of the HVAC forums, reading comments from the "pros" relative to an indoor humidity question; one of which said "an oversized evaporator coil will reduce moisture removal from the conditioned space".

Now, the gentleman may well have omitted some qualifying statements...but way back, an HVAC authority of sorts, told me oversized evaporator coils would improve moisture removal, due to "increased surface area". Which made some practical sense to me. He did however, qualify his statement by saying the airflow and refrigerant metering would have to be "adjusted" for optimal coil performance...which also makes sense.

So, assuming the oversize factor is reasonable, the coil is "full" of liquid refrigerant and 400 CFM per ton nominal is maintained, will humidity removal be increased/improved?

Thanks for allowing me to member up and thanks in advance for any responses.

 

[sophiecentaur]

This problem must involve many different variables so I think there can't be a definitive answer with such little information.
Shifting heat and shifting water are two different issues, I think. A massive heat exchange coil can shift a lot of heat without a large temperature difference and that can involve no water extraction at all, if the dew point is above the coil surface temperature. otoh, a really small coil, operating at a really low temperature, may not remove much water because not much air is in contact with it - condensation may occur as the 'very cold' air leaving the coil comes into contact with the surrounding warm air but the resulting 'cloud' will re-evaporate without touching the coil and being removed.

The optimum coil dimension and temperature will depend on the humidity and actual air temperature, I guess. There must be tables and calculations for which those damned qualified Engineers expect to be paid! (The cheek of it!)

 

[russ_watters]

Welcome to PF!

If you oversize the evaporator coil a little, the air will spend more time in the vicinity of the coil, giving it more opportunity to transfer heat to the coil. You'll get both additional sensible and latent heat transfer (more capacity overall). And the ratio of latent heat to sensible will increase a little.

There is a limit, though. If you oversize your coil by double or triple you may end up with flow so slow over the coil that it becomes laminar. Laminar flow means the air isn't turbulent-ly mixing as it flows across the coil and heat transfer drops dramatically. I don't know that this is relevant here, but I just wanted to cover all the bases.

Shifting heat and shifting water are two different issues, I think. A massive heat exchange coil can shift a lot of heat without a large temperature difference and that can involve no water extraction at all, if the dew point is above the coil surface temperature.

That doesn't fit the scenario defined by the OP: in order to have a low delta-T you need a lot of air for the amount of heat you are transferring and the OP defined the airflow/heat ratio to be a pretty standard 400CFM/Ton. In reality, if you take a matched system operating at 400 CFM/Ton and swap out the evaporator for a slightly bigger one, your airflow will stay about the same, your capacity will go up a little (so slightly lower CFM/Ton and your latent/sensible ratio will go up.

otoh, a really small coil, operating at a really low temperature, may not remove much water because not much air is in contact with it

No. As you said above, the first few degrees of delta-T involve no condensation (latent heat), so the colder you make the air, more latent vs sensible heat you get. And as above, airflow isn't varying much.

...condensation may occur as the 'very cold' air leaving the coil comes into contact with the surrounding warm air but the resulting 'cloud' will re-evaporate without touching the coil and being removed.

Entrained room air can condense briefly in the airstream or on a diffuser, but that doesn't really apply here. The OP is talking about what happens on the coil.

The optimum coil dimension and temperature will depend on the humidity and actual air temperature, I guess. There must be tables and calculations for which those damned qualified Engineers expect to be paid! (The cheek of it!)

Ehh - for residential applications, there isn't really anything for we damned qualified engineers to do. The manufacturers do all the work matching the condensers and evaporators so all you do is select the 3 Ton (for example) evaporator to match your 3 Ton condenser for your 3 Ton load. The manufacturer usually even has tables that show performance characteristics of a few selected mismatchings at different inlet conditions and airflows.

 

[sophiecentaur]

As one damned qualified Engineer to another, I hope you will forgive the non-insult! I am always fascinated by other people's areas of expertise (Electronic Engineering's my thing). They know so much more of the relative importances of all the factors involved.

You have obviously considered all this stuff much more than I and I suppose we are comparing one properly designed system with another rather than my notional systems. But, it surely must be true that very chilled air will cause condensation in the room. I guess what you are saying is that this doesn't happen with most systems because the rate of air flow prevents that happening. I was thinking of the effect when you open a freezer, for instance - which is essentially low speed air flow.

 

[russ_watters]

As one damned qualified Engineer to another, I hope you will forgive the non-insult!

No insult taken - just needling.

But, it surely must be true that very chilled air will cause condensation in the room. I guess what you are saying is that this doesn't happen with most systems because the rate of air flow prevents that happening. I was thinking of the effect when you open a freezer, for instance - which is essentially low speed air flow.

Yes, you were correct on that point. It isn't very common and requires a specific set of conditions, but I have seen it happen.

In your home, it would require you to come home from vacation on a warm (not hot) and extrordinarily humid day and turn on your AC. You may notice a "fog" coming out of the vent and condensation on the vent itself. Both of which are actually entrained room air condensing. After a while, the room's humidity will drop and the condensation will stop/go away.

 

[Saturatedpsi]

I thank each of you for taking the time to reply.

 

[Saturatedpsi]

The manufacturers do all the work matching the condensers and evaporators so all you do is select the 3 Ton (for example) evaporator to match your 3 Ton condenser for your 3 Ton load. The manufacturer usually even has tables that show performance characteristics of a few selected mismatchings at different inlet conditions and airflows.

Were you ever (or are you) involved with some phase of high temp equipment design? You have a lot of relative knowledge...or does that come with the ME degree?

 

[russ_watters]

Neither - an ME degree doesn't give you such specific knowledge. You get that through the practice of being an HVAC engineer.

 

[Saturatedpsi]

Neither - an ME degree doesn't give you such specific knowledge. You get that through the practice of being an HVAC engineer.

In my HVAC related capacity, I've managed to learn most of what I need to know, to do what I do... But it involves a lot of, shall we say "empirical" conclusions, not all of which I'm intellectually comfortable with...:tongue2: So, I hope you'll allow me to impose on your expertise occasionally with some dumb questions...