High humidity comfort cooling

[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/...iew%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/...iew%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.co...ead.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.

[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