Membership Chair/ARP Committee / Professional Member*
Not as much evaporator is devoted to superheating vapor as is apparently being pictured. This is because vapor has almost no density (as compared to liquid) and so it is very easy to increase it's temperature. Superheating vapor does not require very much heat - so very little evaporator length/surface area is required to do it.
And if you think I'm crazy you can prove it to yourself with a heat sensing gun - just check the return bends and note how many are the same temperature.
PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
Professional Member*
Suppose return air dewpoint is 50°. The coil is running with a SST of 50.5°, so no condensation takes place. The charge is reduced until SST is 32°. SH is high. Latent capacity is still zero? I'm surprised by your response.Originally Posted by teddy bear
Last edited by hvacrmedic; 09-03-2014 at 08:36 AM.
Sponsor & Professional Member*
I did not say a lot of coil, just less coil is dehumidifying. 10^F is a minimal. many are much high and than consider hunting of the valve. All reduce the latent removal.
Regarding the comment on SST of 32^F, I did not mean to imply zero latent only that cooling 70^F, 50%RH air down to a 40^F dew point with a 32^F coil is getting about a 40% latent removal. Not to say anything about all of the sweating ducts and grills.
Thanks for the come back. A 45^F coil plenty of trouble. No matter what you do, no latent removal without significant sensible cooling loads. Setup for 50%RH with typical cooling loads use a small whole house dehumidifier for the times of low/no cooling loads. When the home is unoccupied, let the dehu maintain 50%RH with a/c off. This will take less energy, medium investment, minimal maintenance, and ideal temp/%RH.
Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
God Speed, Jim. We'll miss you - 2-26-23
His example wasn't of a house at 50%RH at 70 to 75°F. It was of a house with a very high RH.
Sponsor & Professional Member*
Unfortunately, providing low humidity is more than turning an a/c in high humidity home, It's high humidity and then it is 75^F, 50%RH. The real job is maintaining 50%RH 24/7. This means we must setup the a/c to maintain 50%RH.
Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
God Speed, Jim. We'll miss you - 2-26-23
My post wasn't anything to do with maintaining low humidity. You should read it again. It was simply about the state of the refrigerant in the evap coil at a certain condition.
Professional Member*
I was addressing this comment of yours in particular. "...totally flooded evaps provide the highest latent removal ratio."
What did you mean by that? I think we're on different pages. Maybe you can restate that in terms of "sensible heat ratio" since I don't know what "latent removal ratio" is.
Membership Chair/ARP Committee / Professional Member*
As I presently understand it - I would have to agree with that statement: a flooded evaporator coil (0º SSH) would provide superior latent removal - all other things being equal - as compared to a evaporator coil which is also superheating vapor.
What I think it means is: the more coil surface area which is the furtherest below the dew point of the air - the more moisture is condensed out of the air stream.
Re-evaporation volumes aside for the moment; it seems to agree with my basic position that a 50K BTU coil operating at 35º F. removes more moisture from the air than does a 36K BTU coil operating at 35º F. <g>
OH!!!!!!! That reminds me of a question -
PHM
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PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
Professional Member*
Latent capacity and SHR are different things. A low SHR is what you want for better dehumidification, the latent capacity is immaterial since the sensible capacity may be higher too. The shorter runtime will not improve the ability of the system to dehumidify the space.
A starved evap, if it is bottom fed, will have a lower SHR in general.
Membership Chair/ARP Committee / Professional Member*
I'm a little slow today - what is SHR in full words?
PHM
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PHM
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When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.
God Speed, Jim. We'll miss you - 2-26-23
Sensible Heat Ratio.
Professional Member*
When I do an install, I perform a room by room Manual J load calc, select equipment by the Manual S procedure and design the duct system with the Manual D. Confirm charge, airflow and balance the system.
I end up with what I predicted with the design (or very close)... equipment selected and setup to match the unique needs of the building.
If a certain manufacturer doesn't have equipment that matches well enough, I look at other manufactures equipment.
I handle fresh air with an energy recovery unit. I've not done hundreds of installs but I've yet to need a whole house dehumidifier to satisfy the latent load at design conditions for central Virginia.
just my
Professional Member*
The Sensible Heat Ratio is the sensible capacity divided by the total capacity. At an SHR of 0.50 the sensible and latent capacities are equal. At higher SHRs the total capacity is mostly sensible, and at lower SHRs the total capacity is mostly latent. For maximum dehumidification you want the SHR as low as possible and runtime as long as possible.
Regular Guest
A starved evaporator will also have a minimal to non existent dehumidification ability. Bottom line is you need a flooded coil for optimal dehumidification with some sort of reheat.
As far as expansion valves vs pistons/ cap tubes go, in my opinion, expansion valves are far superior. The only reason it may appear like they are not is because most are preset at 15 degrees and non adjustable. Get an adjustable one and set it at 5 degrees for example and they will be far more efficient than a piston. The only time they wouldn't be would be the time when conditions result a piston to produce a SH below 5.
Respectfully Theo.
Professional Member*
The behavior of the system as far as moisture removal and a low charge depends upon whether the coil is TXV or piston metered and upon how the evap coil circuit is configured. Be careful when applying blanket rules of thumb.
Regular Guest
As far as moisture removal alone goes it will vary based on the above info you mentioned and the constantly changing environment. Not much to dispute there. What I was trying to understand was your insinuation of a starved coil providing better dehumidification. I'm assuming because the heat exchanger would theoretically have a lower pressure, therefore a lower saturation temperature. The problem with that would be high SH so whatever your HX temperature started off at would increase significantly resulting in poor dehumidification, if any, half way through. A flooded coil, even a few psi higher/ a few degrees warmer would dehumidify more would it not? There would be an extended cooler surface at pretty much a constant temperature throughout (below dew point) with additional capacity (Btu's) available for additional phase change of condensate.
Or did I misinterpret you and your "insinuation" :-)
Respectfully Theo.
Professional Member*
I don't believe that I insinuated anything, I stated outright that the starved coil, if bottom fed, will dehumidify better.
Again, like PHM you're looking at size of the stream of water leaving the condensate pipe, and not at the SHR. Let's suppose that we're running at 50% part load. Now we reduce the charge until the sensible capacity drops to the thermal balance point. The system is now running nonstop at reduced capacity. With the longer runtime and lower SHR the system will now bring the indoor RH down much lower. I would define that as better dehumidification.
As for the vapor filled portion of the coil, well it isn't condensing any/much moisture. But it isn't doing much sensible cooling either. We can actually pretend for all practical purposes that that part of the coil is just an extra length of uninsinuated suction line tubing. The portion of the air moving over that liquid free portion of the coil is more or less just bypassing the busy part of the coil.
FWIW bypassing some of the air around the coil is one of the many methods of achieving enhanced dehumidification. The fact that we don't have an actual bypass duct or damper in this case doesn't change the physics here, the effect is going to be pretty much the same. The coil ADP is going to drop, and this means the SHR will drop, that part is pure physics and cannot be argued against. You really need to look at this on a psychrometric chart in order to see the relationships between ADP, BF, and SHR. When you see that on the chart this will all become instantly clear.
Professional Member*
Theo, here's another way to say what I did above. Suppose the system is initially dumping 1 gallon of water per cycle. It's running twice each hour, for 15 minutes each time. 50% load. The condensate rate is 4 gallons/hour. It isn't dumping 4 gallons every hour, that's just the flow rate when the stream is running . Since the stream only runs 30 minutes on each hour it's dumping only 2 gallons total each hour.
After reducing the charge to starve the evap the system is running constantly, 100% load (at 1/2 sensible capacity). The condensate rate is 3 gallons per hour, and since it's running nonstop the system is actually dumping 3 gallons each hour. Removing 50% more moisture per hour than the fully charged coil did.
Now as you look at the size of the stream of water, the stream will be larger in the first case (system with full charge), a 4 gallon/hr stream vs a 3 gallon/hr stream, and going by this alone you might conclude that the fully charged system is doing a better job at dehumidifying. If not for the fact that it's only running that stream for a half hour each hour, well there's the catch. Dehumdification can be very poor even with extremely high latent capacities. The change in sensible capacity has to be figured in too.
Regular Guest
That makes perfect sense and thank you. I was thinking in those terms as well with the only exception of, instead of reducing capacity to achieve constant dehumidification at a lower rate, we operate at full capacity with some type of reheat. So in essence we don't undershoot our sensible temperature which then allows system to operate flooded at full capacity (4gl/ hr) without staging and cycling off.
Respectfully, Theo
Sponsor & Professional Member*
Thanks for taking the time to verbalize all of the facts. I have seen this done several times. Shorten the charge for a orfices/cap tubes. Another is to move the bulb of the expansion valve upstream in the coil. Or slow the air flow to make the coil colder.
These ways to max the amount of moisture removes and slow the sensible cooling. This needs to be done before adding supplemental dehumidification and should provide good humidity control during significant cooling loads. But during low/no cooling loads and outdoor dew points (+60^F), supplemental dehumidification is need to maintain 50%RH. Adequate fresh air change and moisture from the occupants is typically a 3-5 Lbs. per dehumidification load. Without dehumidification expect outdoor dew points plus the moisture from the occupants for a final indoor results.
Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
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