High humidity comfort cooling

[Thermatech]

For engineers who live in hot high humidity locations arround the world.

1/ Please can you outline your ideal solution for providing comfort cooling with DX system for high humidity ambient conditions.
2/ Please advise your prefered manufacturer & model of any DX packaged, split or multi split systems which works to your satisfaction in high humidity applications.
3/ Please describe the technical inovation & control stratagy deployed by the manufacturer to cope with the high humidity application to provide comfort cooling to your satisfaction.
4/ Please indicate if your prefered manufacturer & product is
a/ low volume production specialist manufacturer or b/ a mass produced product sold globally.

[Abby Normal]

I find that the way we build homes and buildings has a greater impact on indoor comfort, temperature and humidity levels than any air conditioning system installed.

I believe that if you can control how outdoor air moves through your home, you will have no humidity problems and vitually any air conditioning system on the market these days will work fine.

If you can build a tight structure like they can in Sweden and Canada ( except realize that vapour barriers are opposite in the tropics) you have almost eliminated infiltration which is the PRIMARY source of moisture.

You then need your air conditioning system to draw in fresh air whenever it runs to pressurize your structure with dry air.

In residential applications the ventilation can be intermittent at a higher rate than you would use compared to continuous ventilation.

In commercial applications you would most likely have a cooling coil that was always cold and you can get away with steady fresh air.

From a pyschrometric standpoint there are certain situations that would force you to use a reheat or dehumidification strategy and these would usually involve a dense occupancy level, an active occupancy.

All the enhanced dehumdification tricks that new equipment does in reality compensates for the lack of control over infiltration.

[Gary]

De-humidification is about coil temperature and run time.

If the coil isn't cold enough to remove the moisture, then running it 24/7 won't get the job done.

If the coil is cold enough, then the system must run long enough to get the job done.

On a psychrometric chart, locate the spot where the desired room temperature intersects with the desired humidity. Draw a horizontal line to the left where it intersects with the 100% humidity line, then vertically down to reveal the coil temperature needed. The coil temperature needs to be at or below this temperature or the battle is lost.

[Thermatech]

Chaps

You are not providing any answers to my questions.
Or is it that you dont know of any manufactures who can provide a suitable standard comfort cooling DX system that provides good humidity control ?

1/ Please can you outline your ideal solution for providing comfort cooling with DX system for high humidity ambient conditions.
2/ Please advise your prefered manufacturer & model of any DX packaged, split or multi split systems which works to your satisfaction in high humidity applications.
3/ Please describe the technical inovation & control stratagy deployed by the manufacturer to cope with the high humidity application to provide comfort cooling to your satisfaction.
4/ Please indicate if your prefered manufacturer & product is
a/ low volume production specialist manufacturer or b/ a mass produced product sold globally.

http://www.refrigeration-engineer.com/forums/images/VA_RE/misc/progress.gif

[Gary]

It's not about the manufacturer. They are ALL capable of doing the job.

It's about equipment sizing (run time) and blower/compressor speed (coil temp).

[Thermatech]

But which one would you recommed so that I can look at the technical specification.

[Abby Normal]

De-humidification is about coil temperature and run time.

If the coil isn't cold enough to remove the moisture, then running it 24/7 won't get the job done.

If the coil is cold enough, then the system must run long enough to get the job done.

On a psychrometric chart, locate the spot where the desired room temperature intersects with the desired humidity. Draw a horizontal line to the left where it intersects with the 100% humidity line, then vertically down to reveal the coil temperature needed. The coil temperature needs to be at or below this temperature or the battle is lost.

I have an intersting reaearch project going on right now. At 520 CFM per ton,with a single stage 13 seer PSC blower luxaire system I can consistenly hold indoor rh down around 40% in the tropics.

A worst case humidity design conditon for Miami, is like an average condition here in the rainy season.

I have taken the approach of keeping outdooor humidity out of the home, and I basically purge humidty generated inside the home to the outdoors.

It is where I get the building itself has more of an impact then any cooling system you install.

When my dreaded pleated filters begin to load up they slow down my airflow- RH holds below 40%, so I sort of change my filters when it starts getting too dry

[Abby Normal]

I would be so bold to say that it is not so much that an "oversized air conditioner drives up RH", it is more like "an oversized air conditioner cannot handle excessive infiltration of humid air"

[Abby Normal]

i gues my approach is like practising safe sex or birth control rather than getting treated after the fact for a disease or getting an abortion. Control outdoor air and have no problem with humidity.

[Gary]

I have an intersting reaearch project going on right now. At 520 CFM per ton,with a single stage 13 seer PSC blower luxaire system I can consistenly hold indoor rh down around 40% in the tropics.

I'm guessing that your coil leaving air temp is about 21F/11.5C lower than your room temp. Am I close?


I have taken the approach of keeping outdooor humidity out of the home, and I basically purge humidty generated inside the home to the outdoors.

It is where I get the building itself has more of an impact then any cooling system you install.

Reducing the load is always a good approach.


When my dreaded pleated filters begin to load up they slow down my airflow- RH holds below 40%, so I sort of change my filters when it starts getting too dry

That makes sense. Reducing the airflow drops the coil temp, removing more moisture.

[Thermatech]

So what is the control stratagy for your system ?

Do you use a humidty monitor on the return air & then slow down the fan if the return air rises above a set humidity target ?

What control stratagy is used to maintain the coil temp at optimum temperature for dehumidification ?

Is this a standard mass produced system or a one off custom engineered job ?

In UK conditions runing any standard split system at room RH of 40% & return on coil at normal 21 deg C DB will result in indoor unit coil frost prevention which makes the unit underperform as it stops very 5 or 6 mins for 3 min compressor restart & let indoor coil defrost.
Most mass produced split systems have minium operating temp of 15 deg C WB & any lower than that unit starts stopping on coil frost prevention.
So a common problem for us is too much dehumidification.
Lots of aplications would ideally have higher SH ratio , larger indoor coil & more air volume.
But manufactures want to keep indoor size as small as possible.

[Gary]

In UK conditions runing any standard split system at room RH of 40% & return on coil at normal 21 deg C DB will result in indoor unit coil frost prevention which makes the unit underperform as it stops very 5 or 6 mins for 3 min compressor restart & let indoor coil defrost.
Most mass produced split systems have minium operating temp of 15 deg C WB & any lower than that unit starts stopping on coil frost prevention.

I assume this refers to air on temp of 18C WB as air off temp of 18C WB would mean little or no dehumidification, resulting in very high humidity for a 21C room.

[Gary]

So a common problem for us is too much dehumidification.


Too much dehumidification would be caused by low coil temp (low fan speed and/or high compressor speed).

[Abby Normal]

those mini-split fan coils are low CFM per ton, it gets foggy at Heathrow and rains a lot but when you consider room temperatures, there is not a lot of humidity in the UK.

Those mini-split fan coils run low CFM per ton and when the indoor air is dry it is mainly a sensible load. They need to wind up the air flow on them when there is no real latent load.

Pyschrometrically speaking they are a lousy choice for a server room.

[Abby Normal]

If I logged the entering/leaving condition Gary, and that is with mixed air going into the coil, perhaps it would approach the 21 you suspect, not that much of a differential with respect to the room air temperature. Coil sees mixed air

[Gary]

The difference between leaving air and room temp are what matters here, although leaving air WB temp would be more accurate.

As that leaving volume of air rises to room temp its RH drops to a predictable level.

This can be plotted as a horizontal line on a psychrometric chart.

http://www.truetex.com/psychrometric_chart.htm

We can also plot this in reverse. If we know the room temp and humidity, we can plot the coil temp @100%RH, which is presumably very close to leaving air WB temp.

[Pyr0Beast]

What control stratagy is used to maintain the coil temp at optimum temperature for dehumidification ?

The easiest one is to adjust the cooling capacity to a fixed rate, then adjust the airflow so the output air reaches 100% humidity or 2 degrees C min. at that temperature.

[Abby Normal]

The difference between leaving air and room temp are what matters here, although leaving air WB temp would be more accurate.

As that leaving volume of air rises to room temp its RH drops to a predictable level.

This can be plotted as a horizontal line on a psychrometric chart.

http://www.truetex.com/psychrometric_chart.htm

We can also plot this in reverse. If we know the room temp and humidity, we can plot the coil temp @100%RH, which is presumably very close to leaving air WB temp.

I am well versed in the chart and the air processes Gary

Now here is something to consider.

Suppose you conditioned 100% outside air so that it was cooler and dryer than the temperature/humidity you would want to maintain a space at.

You would pressurize a space with this set up and provide some means of pressure relief. ALmost like how a swamp cooler is used.

Anyways, this system could keep temperatures comfortable and it could keep humidity well under control, even though it is not removing any heat or humidity from the room air.

It is of course adding cooler and dryer air to the space, but is is not recirculating any air. Any humidity generated in the room could easily be accounted for in the mositure content of the air that was 'relieved' outdoors through the pressure relief system.

So when you have a tight structure and you draw in a samll amount of fresh air to mix in with the return air going into a coil, you can pressurize the tight structure and force air to leak out of the structure to the point of stopping natural infiltration when the system runs.

You have stopped the main humidity source from getting into your room air. The air leaking out of the building carries with it mositure which exceeds the typical internal latent gains in the space.

You end up with a space with low RH. The humidity of outdoor air has gone directly to the coil. You have elevated the heat input into the system so you will get more capacity out of your system.

Imagine a space at 75F & 50% RH, that was getting pressurized with 50 CFM. 50 CFM of air leaking out of the building at 75F & 50% RH, carries with it probably 25 to 30 pints of moisture per day, so it is similar to running a small dehumidiier.

You are usuing about the same energy, except you are controlling the fresh air first and conditioning it before it gets introduced to the space.

It is harder for a system to try and remove a lot of mositure from 50% RH air in order to account for the mositure that is carried in with infiltration.

I add 10% or so outside air down here, I pretty much have the 67WB entering condition all equipment is rated at on this side of the atlantic

[Gary]

I am well versed in the chart and the air processes Gary

I know that you are, Abby. I assumed we were kicking this back and forth for the benefit of the audience.


Now here is something to consider.

Suppose you conditioned 100% outside air so that it was cooler and dryer than the temperature/humidity you would want to maintain a space at.

You would pressurize a space with this set up and provide some means of pressure relief. ALmost like how a swamp cooler is used.

Anyways, this system could keep temperatures comfortable and it could keep humidity well under control, even though it is not removing any heat or humidity from the room air.

It is of course adding cooler and dryer air to the space, but is is not recirculating any air. Any humidity generated in the room could easily be accounted for in the mositure content of the air that was 'relieved' outdoors through the pressure relief system.

So when you have a tight structure and you draw in a samll amount of fresh air to mix in with the return air going into a coil, you can pressurize the tight structure and force air to leak out of the structure to the point of stopping natural infiltration when the system runs.

You have stopped the main humidity source from getting into your room air. The air leaking out of the building carries with it mositure which exceeds the typical internal latent gains in the space.

You end up with a space with low RH. The humidity of outdoor air has gone directly to the coil. You have elevated the heat input into the system so you will get more capacity out of your system.

Imagine a space at 75F & 50% RH, that was getting pressurized with 50 CFM. 50 CFM of air leaking out of the building at 75F & 50% RH, carries with it probably 25 to 30 pints of moisture per day, so it is similar to running a small dehumidiier.

You are usuing about the same energy, except you are controlling the fresh air first and conditioning it before it gets introduced to the space.

It is harder for a system to try and remove a lot of mositure from 50% RH air in order to account for the mositure that is carried in with infiltration.

I add 10% or so outside air down here, I pretty much have the 67WB entering condition all equipment is rated at on this side of the atlantic

Given 10% outside air with random leakage (exhaust), we might assume that 10% of the internal latent load would be exhausted, but given tight construction with controlled exhaust at the points where the latent load is generated, we might expect a much higher percentage of the internal latent load to be exhausted, perhaps approaching 100%.

[Thermatech]

You dont need to suppose & consider the possibility of this type of fresh air system.
In Japan they call it a Lossnay energy recovery system.

The Lossnay core heat exchanger can recover / transfer latent heat as well as sensible heat from incoming fresh air to outgoing extract air.
So it is an energy recovery system where as a standard plate heat exchanger is a heat recovery system as it only transfers sensible heat.

The technology regarding the Losnnay core is a bit ' complicated ' so I shall let you do your own investigations regarding this rather (as I am sure you already have) than write pages of tech stuff.

I just pulled an old Lossnay tech manual out of the archive & quote this as an example from the manual.

Outdoor fresh air
32 deg c DB, RH 70%, Absolute humidity 21.1 g/kg, enthropy 86.2 KJ/kg
Indoor air to be exausted
27 deg C DB, RH 50%, Absolute humidity 11.1 g/kg, enthropy 55.3 KJ/kg

These two airflows pass in the Lossnay core but do not mix.
The sensible & latent energy is transfered with the following result for the incomming fresh air.
Supply air
28.3 deg C DB, RH 63.1 %, Absolute humidity 15.3 g/kg, enthropy 67.4 KJ/kg.

The fresh supply air is ducted to indoor units of split systems & VRV / VRF systems which cool & dehumidify further as required for % fresh air & the internal room load.
The electronic controls for this system is fully integrated for use with VRV/VRF system controls.

The major manufactures also make this kit with a built in VRV/VRF cooling coil which can further cool/heat the supply air within the unit & then supply to VRV/VRF indoor units.

In the UK this type of fresh air heat / energy recovery is standard for all office blocks & hotel applications & some other applications but the main advantage in our cooler climate is winter pre heat of incomming fresh air & we have always found that it works very well.

For example in a hotel application each hotel room will have a small VRV / VRF ducted indoor unit in the ceiling & will have a small volume of ducted fresh air from the lossnay.

Please explain to me why this system is no good for your high humidity application other than the fact that it comes from Japan.

[Abby Normal]

Japan got the idea for heat and energy recovery ventilation from Canada

heat and energy recovery ventilation in a residential application is a nuetral pressure ventilation scheme so the home is still prone to the infiltration of humid air.

In a commercial appliaction these systems move a considerable amount of air and you can run them out of balance to create desirable positive pressure when you are in cooling mode and even a slight negative pressure in heating mode.The pressurization is typically done by running one side at 90% of the other. In a commercial application these systems can knock a huge amount of load off of the cooling system.

If you run a residential system out of balance to try to create positive pressure that stops infiltration, you end up lowering the exhaust rate too much, and it cannot receive enough humidity exchange form the fresh air you are bringing in.

So in a residential application they save you energy on ventialtion, but they leave you prone to the energy penalty associated with infiltration.

Mitsubishi was the first selling vrv on this side of the Atlantic before the "Second Coming" of Diakin. By the time you factored in the energy recovery ventilation it cost about the same as installing a chilled water system in commercial jobs.

VRV has its niche, it is just not the end all or be all of air conditioning, especially when you have serious cooling to deal with.

Editted In: the current ASHRAE 62.1 2007 standard on IAQ, mandates that you be under a positive pressure whenever you are running dehumidification. So outside of the Mojave desert, this implies you are running air conditioning

[Abby Normal]

I was going through my data Gary, air leaving the unit by 20 to 21 cooler than air entering the coil.

The leaving measurement was logged after the blower, perhaps the fan reheats the air by 1/2 a degree F so acorss the coil be 20.5 to 21.5F The SHR is high,over 90, it is a relatively flat line.

When you plot the cycle on the chart you ended up first with a line from the room point to the supply point that would account for the internal latent load of the room and sensible loads through the building envelope and generated in the room. All the load but the ventilation, return duct gains etc.

Then you draw a line between the room air and the outside air and figure out the mixed air point, and this is the air entering the coil.

Your cooling process is getting the mixed air cooled to the supply air condition.

If you correctly calculated everything, following standard procedures, you just might find that internal RH ends up lower than design. It is not necessarily because you have a super dehumidifying machine, it can be attributed to the fact that it never accounted for mositure leaving in the exfiltrating air.

I have a simple air tight structure with controlled outside air and in a tough climate, I hold a lower RH than people with variable speed split systems, dedicated dehumidifiers and I am using single stage single speed cooling system.

[Abby Normal]

In Canada, I also observed how new immigrants sought out and stuck together with their own kind.

When I relocated here, I did the same thing. People trust their fellow countrymen.

A major Law firm down here has all the partners being originally from the UK. So when they built their new office buidsling they sought out a design engineering company from the UK.

The project architect was in a panic and phoned me for what I would consider a ball park estimate on the cooling plant needed for such a building, and I was close to double what was being proposed-- construction was well under way.

They responded with a lot of points, that made sense from their prospective, the problem was the UK perspective was incorrect for here. I then suggested a few things that should really be considered, especially our ambinet dewpoint.

The cooling plant was changed and increased in capacity by close to 70%. At least these guys listened, the owner took it in the arse because it was a Bill of Quantities type of job, but still less expensive than trying to fix it after the building was finished and occupied.

I am presently diversifying away from earning the majority of my money as a designer and more into a commssioning approach on new buildings and a forensic approach on existing buildings with very similar chronic problems.

I actually am enjoying this, soemone else designed or installed the problem, and when they try and press me for whose fault the problem is, I just tend to get a little slippery and say you are paying me to resolve the problem not point a finger. I am only liable if my prescribed cures are failuers.

[Abby Normal]

If you are using the 'complicated' energy recover ventilators, also think a little more about the source of their exhaust air.

Probably in a hotel setting it draws the air to be exhausted from washrooms. People shower so the exhaust stream is a little more humid than 50% and the amount of humidity this stream can receive from the fresh air in this Lossny core will be lower. The supply air from the Lossny core is going to climb up higher than the 63 percent.

It still saves you energy, it is just you have taken the outside air from tropical to sub tropical. This air needs to be further treated, else it is a humdifier.

[Abby Normal]

I think I did my first heat recovery system in the mid 80s, my first complicated energy recover system with an enthalpy wheel around 1994. Hit the new millenium and they are all the rage in USA and Europe.

[Thermatech]

In UK the main advantage of Lossnay is for winter preheat of fresh air & in this case any extra heat from bathroom is added bonus. But in hotel rooms the bathroom is not in constant use.

The Lossnay can revert to full outside air with core bypass for free cooling when the conditions are right
which tends to be very usefull in office aplications which need lots of cooling even in low ambient conditions.

Have you found that this type of energy recovery system is of any benifit in your extreem humid climate to assist with humidity control ?
Or do you think its only usefull in more moderate humidity climates ?

[Abby Normal]

energy recovery ventialtion is useful in commercial projects, again it is possible to run them out of balance to acheive the desired pressurization.

I was in Canada for 37 years prior to relocating down here 10 years ago. I am very familiar with the heat and energy recovery ventilation including the economizer option you mention.

When the ambient dewpoint here gets down low enough not to humidify the buildings, I will call Mr. Dan in Tampa and tell him to get me a Florida Lotto ticket as hell has probably just frozen over.

[Abby Normal]

On a tough humidity challenge Gary, I do a 'moisture balance'

I first figureout the mositure into a space. Thius is the actual pounds per hour generated inside the space and contained in the ventialtion air before it is treated.

I then figure out the moisture out of the space and this is the moisture in the air being deliberately exhausted as well as air being forced to leak out of a pressurized building. The moisture content of this air is taken as the level we are trying to maintain in the space

So when I subtract mositure out from moisture in, what is left is the amount of moisture that has to come out of the condensate drain in order to maintain a a set amount of mositure in the space. What goes in must come out.

So then I look for an air conditioner, dehumidifier or combination of the two that can remove this much mositure.

I find that the cooling tonnage determined this way is lower than when you plot the whole process on a chart.

I believe when all things are perfect, you can maintian lower indoor Rh than what the chart would suggest.

[Gary]

I was going through my data Gary, air leaving the unit by 20 to 21 cooler than air entering the coil.

The leaving measurement was logged after the blower, perhaps the fan reheats the air by 1/2 a degree F so across the coil...

Also there is the coil bypass factor (air which slips between the fins without actually touching anything) which might add a degree or two... so the coil temp might be as much as 2-3 degrees lower than the measurement.

[Gary]

Keep in mind also that I am a service tech, not a system designer. I take the system as is and make it work.

If my room condition target is 75F@50%RH, I want the air leaving the coil to be 55F when the room is down to temp... and I adjust the blower speed accordingly.

The actual coil temp might be a degree or two lower, which could conceivably result in over-dehumidifying by a few percentage points given light latent load.

Generally it is well within the ballpark.

[Abby Normal]

Also there is the coil bypass factor (air which slips between the fins without actually touching anything) which might add a degree or two... so the coil temp might be as much as 2-3 degrees lower than the measurement.

well you can sort of extrapolate the coil temp tho, call it an apparatus dewpoint

[Abby Normal]

Keep in mind also that I am a service tech, not a system designer. I take the system as is and make it work.

If my room condition target is 75F@50%RH, I want the air leaving the coil to be 55F when the room is down to temp... and I adjust the blower speed accordingly.

The actual coil temp might be a degree or two lower, which could conceivably result in over-dehumidifying by a few percentage points given light latent load.

Generally it is well within the ballpark.

Sometimes you cannot do much about the cause of the humidity.

The humidity is generally from the ambient air - piss poor building envelope, supply ducts in unconditioned spaces leak, excessive use of exhaust fans, too much fresh air being used.

Sometimes you do what you can towards the root cause and maixmize what the existing system can do.

My first two steps are usually switch auto fan and drop the blower speed.

Then there is always reheat and dehumidifiers.

I like the heat pipe air handlers made in gainsville.

York has a DR series of packahged units, they will go into rehaet mode. Both compressor circuits cool the air, one circuit dumps its heat to the outdoors, the other circuit (via a solenoid) can reheat the airstream

Can blow out air roughly the same entering temperature but dry.

Lennox has an over complicated system on residential with a liquid line subcooling coil to reheat air, they play around with the condenser fan speed at the same time.

A lot of the variable speed residential units ramp up real slow so the coil runs quite cold initially.

I just believe in attacking the root problem and not all the band aids

[Gary]

I like the heat pipe air handlers made in gainsville.


Would that be the heat pipe version of the old runaround coil?

For you youngsters, the runaround coil involved two water coils, one before and one after the DX coil. Water was pumped in a loop between the two coils, thus cooling the air entering the DX coil while reheating the air after. The DX coil would run very cold, removing lots of moisture while providing minimal sensible cooling.

[US Iceman]

Would that be the heat pipe version of the old runaround coil?


A thermosyphon loop.

[Abby Normal]

Would that be the heat pipe version of the old runaround coil?

For you youngsters, the runaround coil involded two water coils, one before and one after the DX coil. Water was pumped in a loop between the two coils, thus cooling the air entering the DX coil while reheating the air after. The DX coil would run very cold, removing lots of moisture while providing minimal sensible cooling.


Yes, more like a wrap around now.Obviously no pump.

Precools air by some many degrees , coil does more latent, then reheats air by about the same amount it precooled it by

Used to use the run arounds for heat recovery out of exhaust streams, never thought about using it to enchance dehumidification

[NB34]

I agree with Abby Normals approach of keeping the moisture out of the building, but this is not always possible from a cost staindpoint. None of the walls built in southwest florida have vapor barriers because of consequences that may arrise with using them. The man you need to reference when speaking about building enclosures and moisture control is Joe Lstiburek. See attached link for vapor barrier locations and wall construction. If you have never been to seminar put on by Mr. Lstiburek I highly recommend it, you will never imagine that someone could take this subject and produce such an enjoyable seminar. I also recommend his book, Builder's Guide to Hot-Humid Climates.

[Abby Normal]

block walls, sealed attic

pretty tight

not catastrophic when it gets flooded by storm surge either

if you had a vapour barrier in Florida, it would have to be on the exterior of the wall.

It is tough to have an attic vapour barrier on the warm side of the insulation in Florida because that would mean on top of the truss chords.

Sealed attic insulated at the deck gets around this problem.

Vapour barriers are supposed to stop moisture diffussion but in essence they stop a lot of air.

With a wood framed wall, which is absolutely the worst way to build in a humid climate and in the hurricane belt, blown in foam insulation like icynene stops a lot of the air movement which allows humid air in.

It is not all that cost prohibative, it is just not conventional thinking

[Abby Normal]

Here is a unit for you Gary, custom built, it serves a night club, so there is a big internal latent load, as well as a fresh air load with a lot of moisture

An 8 row coil has four staged compressors and under a design condition will get the air down to about 52 degrees close to saturated.

There is a smaller compressor, probably about 4 tons that will pull the air down about another two degrees. The condenser coil for this small compressor is in series and reheats the air up to about 63F, the fan and motor heat the air up by another two degrees so it will blow dry 65F air if need be.

Other wise it just stages the 4 main compressors to control temperature, turns on the 4 ton dehu/reheat circuit on a rise in RH.

http://i32.photobucket.com/albums/d41/a_bee_normal/NL1.jpg

http://i32.photobucket.com/albums/d41/a_bee_normal/NL2.jpg

I have used partial hot gas reheat on a few projects that require it, it involves either a lot of people, or a lot of outside air, some even 100% outside air

I have some 2250 CFM outside air units here, in the rainy season they produce about 400 gallons of condensate a day

You can even get wall mounted Bard Units with a reheat coil and a solenoid. They use about half of the heat of rejection to reheat the air up so that the supply temperature is not much warmer than what the entering air temperature is

[Abby Normal]

You need the reheat when it becomes psychrometrically impossible for a cooling coil by itself to provide you with air at the correct dry bulb temperature and the correct mositure level.

It is for cooling processes where there is no "ADP" that will work.

You can get air at the correct dry bulb temperature, but the moisture content is too high, or you can get air at the correct mositure level, except the dry bulb temperature is much too cold.

So about all you can do is cool down the air to get the moisture content you need, then reheat it

[Abby Normal]

An example where reheat is needed, process line from the entering "mixed air" condition to the "leaving coil" condition cannot be extended to cut the saturation curve, there is no ADP possible, the process line is too steep as the latent load is big.

http://i32.photobucket.com/albums/d41/a_bee_normal/Pysch_101.jpg

Example of finding the ADP of a process, by extending the process line to the saturation curve

http://i32.photobucket.com/albums/d41/a_bee_normal/ADP_Example.jpg

You can use reheat to correct the situation. Without reheat you end up with a space at 75 to 80% RH. The line between the points labelled "Leaving Coil" and "Fan Heat" indicates the reheat needed

http://i32.photobucket.com/albums/d41/a_bee_normal/HP_Reheat.jpg

[Abby Normal]

The night club with the custom built unit, used to have about 15 mini-splits and a few smoke eaters. Place was hot as hell, humid and the staffs clothes smelt like an ashtray.

Talk about sending a boy to do a man's job. Yep VRV will take over the world alright.

I can see using inverters to get modulating discharge air control on DX and other applications, the product has it uses, it is just not the best thing since sliced bread. They are going to need an air handler with some balls that can filter the air and move significant amounts of air when needed.

[Gary]

It should be noted that reheat does not remove moisture, it extends run time.

It is the ADP which removes the moisture, but the longer it runs the more moisture is removed.

[Abby Normal]

It should be noted that reheat does not remove moisture, it extends run time.

It is the ADP which removes the moisture, but the longer it runs the more moisture is removed.

yes, the reheat keeps the system running.

Theoretically, it can give you the required dry bulb temperature you need to coincide with the supply air moitsure level needed.

ADP is sort of like how Willis Carrier rationalized how a cooling coil worked.

Some air makes contact with the coil surface be it a tube or a fin, and ends up saturated at the coil surface temperature.

Some air passes right through and does not touch the surface of a coil and is unchanged.

The supply air CONDITION you get is a mixture of the air that contacts the coil with the air that does not make contact with the coil.

The ADP is like the average surface temperature of the tubes and fins.

So when there is a big latent load there is no ADP available that will work. The only way you get the proper supply conditions is to overcool the air to get the moisture out then reheat the air so as not to overcool the space.

[Abby Normal]

In the ADP example the line segment between entering and supply would be proportional to the amount of air that contacts a coil surface.

The shorter line segment between ADP and Supply would be proportional to the amount of air that actually bypassed or avoided contacting the coil surface

All the old books inspired by Carrier like to talk about the Bypass Factor

[Gary]

An example where reheat is needed, process line from the entering "mixed air" condition to the "leaving coil" condition cannot be extended to cut the saturation curve, there is no ADP possible, the process line is too steep as the latent load is big.


I'm afraid you have lost me here. The lower limit to ADP is when the coil freezes... and this system is a long way from freezing the coil.

Lowering the blower speed and/or raising the compressor capacity would lower the ADP. Lowering the blower speed has the added advantages of extending the run time and reducing the bypass factor. Bypassing some air around the coil is another means of dropping ADP. And assuming a bottom fed coil, increasing the superheat provides yet another option (similar to bypassing air).

I am not opposed to reheat, but from an energy standpoint it should be a last resort.

[Abby Normal]

In situations where you have a large latent load, it becomes an impossible process pychrometrically for a cooling coil to condition air to have the correct temperature to deal with the sensible heat in the space as well as having a low enough moisture content to deal with the latent gains in the space. It is a case of not being able to have your cake and eating it.

In this situation your only choice is reheat, OR a chemical dessicant

There will be no possible coil ADp that will work, not even a sub zero one

[Abby Normal]

When you cannot extend to the cooling process line to cut the saturation curve, the chart is telling you that you will need reheat

[Abby Normal]

http://i32.photobucket.com/albums/d41/a_bee_normal/Pysch_101.jpg
if you extend the line from "Mixed Air" to "Leaving Coil" you cannot cut the saturation curve

What it means is you have to cool the air down so it is just about saturated at the dewpoint temperature of the "Leaving Coil" condition, and then heat it up to the Leaving coil dry bulb temperature.

[Gary]

if you extend the line from "Mixed Air" to "Leaving Coil" you cannot cut the saturation curve


If you reduce the blower speed, this lowers the leaving coil temp and reduces the bypass factor, changing the angle of that line... and extends the run time.

[Gary]

So when there is a big latent load there is no ADP available that will work. The only way you get the proper supply conditions is to overcool the air to get the moisture out then reheat the air so as not to overcool the space.

Wether the space is overcooled or not (without the reheat) would depend upon the heat load of the space and the sizing of the system.

The reheat extends the run time and nothing else. Possibly the run time doesn't need to be extended.

[Gary]

Hmmmm... it occurs to me that we are discussing a system that runs continously... 100% run time?

[Abby Normal]

If you reduce the blower speed, this lowers the leaving coil temp and reduces the bypass factor, changing the angle of that line... and extends the run time.

reheat and dehumidifiers can be band aids to fix a bad situation.


I am trying to show when physics dictate that you have to use either reheat or a desiccant.

You can force the system to dehumidifiy more, by slowing it down, you get the colder coil, you get moore moisture removed, but the problem is you get too much sensible cooling in order to get rid of the moisture, and the thermostat is satisifed quickly.

The cooling shuts off temperature is under control but humidity is still too high.

Without the reheat physics will put you in a situation of having no problem controlling temperature but you cannot control the humidity. It will sound to you like oversizing, but in some situation under sized standard commercial equipment can still control temperature without any problems, however they cannot control humidity,

[Abby Normal]

Hmmmm... it occurs to me that we are discussing a system that runs continously... 100% run time?

well when you hit the design condition, yes I would expect the system to run a couple hours straight under the peak load.

Typically when you go through and work out the required supply air condition to control temperature and humidity, the dry bulb and wet bulb temperatures are close together, even within 1F


When there is a big latent load, what will happen is there can be say a 5F difference between the required dry bulb and wet bulb. Except with a coil, when you pull a lot of mositure out, the dry bulb and wet bulb of the supply get close together. No way you will get 58 db and 53 wb or something like 59 DB and 54 WB from a coil.

Only way you get air like that is to over cool the air to have the same dewpoint as the theoretical supply air, then reheat the dry bulb temperature up.

You get in these situations and all you can do is lessen the problem until you add the reheat neede or a desiccant. It is an "Impossible Process" for a cooling coil alone.

You get a dense active occupancy like a Pentacostal Church or a night club and you come acorss this scenario

[Gary]

reheat and dehumidifiers can be band aids to fix a bad situation.


I am trying to show when physics dictate that you have to use either reheat or a desiccant.

You can force the system to dehumidifiy more, by slowing it down, you get the colder coil, you get moore moisture removed, but the problem is you get too much sensible cooling in order to get rid of the moisture, and the thermostat is satisifed quickly.


In point of fact, slowing the blower results in less sensible cooling, along with the increase in moisture removal.

I don't doubt that your strategy works very well, and the overall design is impressive, but I find the statement that the same result cannot be achieved without reheat questionable.

[Abby Normal]

You are not questioning me Gary, you are questioning the physics involved.

Should be able to connect the ADP, the required supply condition and the entering condition with a single straight line.

When you use computer programs sometimes they spit out error warnings on the big latent loads like "results out of range" , "no adp", but really it should just say "You need reheat or you will end up with high RH"

[Gary]

You are not questioning me Gary, you are questioning the physics involved.

Should be able to connect the ADP, the required supply condition and the entering condition with a single straight line.

Changing the CFM would change the required supply condition.

[Abby Normal]

Changing the CFM would change the required supply condition.

No, it will change the supply condition you will get to another one that will not work

slowing it down will increase dehumidification, drop the supply temperature.

You can slow the air down enough or perhaps you even make the coil have deeper rows plus a bigger face area, add more fins per inch, drop the coil temperature then finally you have enough mositure being removed.

The problem is, this air is now so cold the thermostat is satisified in 5 minutes and the system cycles off. A by product of getting the amount of dehumidifiaction needed is you also have way to much sensible cooling.

So it is back to what I keep trying to illustrate, from a psychrometric stand point, when the cooling and dehumidification line is steep, because of a big latent load, it can become IMPOSSIBLE for a cooling coil alone, to provide air at both the right temperature and the correct moisture content, to maintain the space at the proper condition.

Under this situation, you end up with a room at the correct temperature but very humid, or you end up with a cold meat locker. This is when reheat is an absolute necessity and not a band aid to fix a problem. You can use a desicant also, but it is other wise an Impossible Process for a cooling coil alone

[Abby Normal]

try page 5, someone else wrote it so it has to be true, wtf do I know :)

http://courses.washington.edu/me425/425%20Psychrometric%20Review%202007.pdf

[Gary]

When the airflow is reduced, the mass refrigerant flow is reduced, therefore the total heat removal is reduced.

Total heat = latent heat + sensible heat

If the coil is removing more latent heat then how can the sensible heat removal not be reduced?

The air delivered is colder but there is less of it, the end result being longer run time, not shorter run time.

[Gary]

try page 5, someone else wrote it so it has to be true, wtf do I know :)

http://courses.washington.edu/me425/425%20Psychrometric%20Review%202007.pdf

Actually, this supports my position. The SHR determines the slope of the line and lowering the blower speed changes the SHR. That's the whole point in lowering blower speed.

[Abby Normal]

When the airflow is reduced, the mass refrigerant flow is reduced, therefore the total heat removal is reduced.

Total heat = latent heat + sensible heat

If the coil is removing more latent heat then how can the sensible heat removal not be reduced?

The air delivered is colder but there is less of it, the end result being longer run time, not shorter run time.


Lets write this one off like this. You try everything you can, you get it remove as much moisture as you can, it still ends up to too humid assuming there are no temperature control problems and you fianlly add a dehumidifier or a reheat coil to solve the problem.

Sometimes you need the reheat from the get go, it is just that no one paid attention to what was truly needed.

I live somewhere humid and when I have a lot of people or a lot of outside air, it bites me in the rear if I do not check it out in the first place if there is an ADP possible or not. A cooling load program like Elite Chvac gives pyschrometric reports, and will even print out a chart. However some times the chart is incomplete and it just says "Results out of Range". What it should be doing is giving you a big warning that you need reheat, as a necessity and not a band aid.

I learned this the hard way in my OEM days. I was designing a custom unit to pressurize a laboratory with 100% outside air. The unit had multiple DX compressors, modulating gas heat, as well as a modulating steam humidifier. It had to maintain a specific pressurization level in the lab, year round with a constant temperature and RH.

I wasted days trying to circuit a coil that would give me the required summer supply condition. I played with face velocity, rows of coils, the suction pressure/temperature, the amounts of fins. I tried to overcool some air and mix it with some air that bypassed, and I could not do it. I ended up changing the sequence of operation to have some reheat and it worked.

A few years later, I was teaching some engineering students, I took over for a retiring professor. One book they gave me had the "Impossible Process" in it. It slapped me right in the face.

It all goes back to realizing that some air contacts a coil surface and is saturated at the coil surface temperature and some air bypasses the coil. How Carrier himself rationailzed that a cooling coil was basically a mixed air process.

[Abby Normal]

Actually, this supports my position. The SHR determines the slope of the line and lowering the blower speed changes the SHR. That's the whole point in lowering blower speed.

SHR is the slope or vice versa

It actually supports that in some situations it is impossible for a cooling coil by itself to do the job needed

I am not arguing how SHR of the coil lessens when you slow down air flow as this is true, think of it then as it becomes impossible for the coil SHR to match the SHR needed to maintain the space.

[Abby Normal]

his homework example works out to about 15 tons, probably somewhere around 4500 CFM.

Change the outside air condition in that home work example from 82/66 up to something like 92 db/83 wb and it becomes impossible without reheat. You would have to over cool the air down to under 50F off of that coil using an extra 1.5 to 2 tons, then reheat it by about 3.5 degrees

[Abby Normal]

post 27 in this thread gives an indication of how to come up with the required supply condition that will work

http://www.refrigeration-engineer.com/forums/showthread.php?t=6559

[Abby Normal]

Again, I am not arguing that you cannot increase how much moisture you remove by slowing down the airflow or making the coil colder. I use these steps.

I am just trying to convince a very experienced, learned and stubborn guy, that sometimes reheat is a necessity and not just a band aid

this is a tougher sell then bourdon guages at high altitude :)

[Gary]

I don't doubt that there is a point at which the SHR cannot be lowered sufficiently without freezing the coil... or situations where some minimum CFM must be maintained.

Nor do I consider reheat to be a band-aid. In some situations, it would be the preferred solution.

Hmmmm... and here I was thinking that you were the very experienced, learned and stubborn guy. :D

[Abby Normal]

it is not about freezing the coil, the limitation that creates the problem is not that the condensate is freezing

[Gary]

Then the other limitation would be a minimum CFM requirement.

[Abby Normal]

the limitation is there is no ADP

[Gary]

I'm thinking we may have to just agree to disagree on this one.

[Abby Normal]

yep, a tougher sell then a bourdon guage in Denver