Cooling in humid climate without reheat - possible?

[Ferumbras]

Hi,

I am wondering whether it is possible to do a cooling for ambient 30 +/- 5 C and 75 +/- 5% RH to a room temperature of 24 +/- 2 C and 60 - 70% RH?

Thanks.

[Ferumbras]

Oops, I mean cooling with just cooling coil and without reheat.

Thanks.

[Sridhar1312]

Cooling and de humidification is feasible in Humid climate.
Madras in southern India which is around 35 degree c and 70% RH ambient is cooled to 24 RH not exceeding 60%.No reeat is used.

[Sridhar1312]

All the Window air conditioners made to the best of knowledge is only cooling

[nike123]

Oops, I mean cooling with just cooling coil and without reheat.

Thanks.

Yes it is. You need to control sensible to latent heat ratio by varying air quantity across evaporator or by controlling cooling capacity of refrigerant system.

[Ferumbras]

That sounds good. What's the strategy used?

I have seen a place that runs chilled water with chilled water supply at 17 C, out at 19 C but the humidity is still around 70% with temperature at 21.5 C. When the temperature setting was increased to 23.5 C, we have 97% RH measured at the sensor.

[nike123]

That sounds good. What's the strategy used?

I have seen a place that runs chilled water with chilled water supply at 17 C, out at 19 C but the humidity is still around 70% with temperature at 21.5 C. When the temperature setting was increased to 23.5 C, we have 97% RH measured at the sensor.

Here is simplest solution I could give from my head right now.

Room termostat controls compressor ON-OFF and room hygrostat controls two speed fan.

When set room temperature is reached compressor goes OFF. When set room temperature plus differential is reached compressor goes ON.
When set room humidity plus differential is reached, fan switch to lower speed (if compressor is ON). When set room humidity is reached fan switch to high speed (regardless of compressor ON-OFF condition).

[Ferumbras]

The case I am faced now is pure thermostat control. Single speed fan running at 3,740 CMH. That is the bad news.

In addition, the original design is with steam heating coil and steam humidifier. I am thinking that those 2 things are pretty much in the way and cost a fortune. That's why I am wondering whether it is entirely possible for the design to run purely on cooling coil. Problem is that I don't know the temperature of air leaving the cooling coil. A test setting room at 20.5 C seem to get humidity 70% without problem but 23.5 C setting results in over 90% RH, I suspect because at 30 C, 70% RH, the dew point is 24 C.

[nike123]

The case I am faced now is pure thermostat control. Single speed fan running at 3,740 CMH. That is the bad news.

In addition, the original design is with steam heating coil and steam humidifier. I am thinking that those 2 things are pretty much in the way and cost a fortune. That's why I am wondering whether it is entirely possible for the design to run purely on cooling coil. Problem is that I don't know the temperature of air leaving the cooling coil. A test setting room at 20.5 C seem to get humidity 70% without problem but 23.5 C setting results in over 90% RH, I suspect because at 30 C, 70% RH, the dew point is 24 C.

It is still possible to use room humidity-stat and some fan speed controller which can be bypassed with humidity-stat when humidity in room is at set value.

[cadillackid]

make sure the fan is cycking off when the compressor is not running...

having the fan run without compressor makes people feel good but will push humidity back into the room as not all the water runs off of the coil....
-Christopher

[Gary]

In order to achieve the desired humidity the coil must be cold enough to remove the moisture and the system must run long enough to do the job.

Reheat does not remove moisture, it extends run time.

In order to reach 50% humidity, the temperature of the air leaving the coil needs to be at least 11K lower than the room temperature.

You can adjust the temperature of the air leaving the coil by adjusting the fan speed.

[Abby Normal]

Hi,

I am wondering whether it is possible to do a cooling for ambient 30 +/- 5 C and 75 +/- 5% RH to a room temperature of 24 +/- 2 C and 60 - 70% RH?

Thanks.

If the question is can a cooling cool by itself take 30C air at 75% and give you a supply air stream (air leaving the coil ) of 24 C and 65% RH the answer is no, it needs reheat. There is no apparatus dew point, it is impossible

Can a coil take that warm humid outside air and maintain a room at 24C and 65% RH without reheat, the answer is yes

[Gary]

If the question is can a cooling cool by itself take 30C air at 75% and give you a supply air stream (air leaving the coil ) of 24 C and 65% RH the answer is no, it needs reheat.

Or a heat pipe/run around loop?

[Abby Normal]

Or a heat pipe/run around loop?

perhaps, may need more reheat than a passive system can do though

you have to cool the air until it has the dewpoint you want, then reheat it to get the 24C

[desA]

Or a heat pipe/run around loop?

Interesting thought. Can you explain further?

[Gary]

Interesting thought. Can you explain further?

The old runaround loop consisted of two water coils. One was placed before the evaporator and one after the evaporator. Water was pumped from one coil to the other and back in an endless loop.

The precoil cooled the air before it entered the evaporator, thus the evaporator ran at a lower temperature, increasing dehumidification.

The heat absorbed by the precoil was transferred back to the airstream by the postcoil, thus reheating the air.

The modern version of this is the heatpipe, which contains refrigerant. The refrigerant boils in the warmer coil and condenses in the cooler coil, for the same effect... without the pump. It relies entirely upon temperature difference and gravity.

[desA]

^ Thanks Gary.

[Magoo]

The problem is the supply chilled water temperature, at +17 'C leaving at +19 'C , the latent cooling/ moisture removal/ dehumidification will be next to zero. So you get half reasonable dry buld and high humidity in control space.
More water chiller capacity required.
As Gary suggested the supply chilled water temp should be in region of 10 > 12 'C, then sensible heat ratios start to get realistic, and dehumidification happens, if a budget design system it may not happen.
Check the fresh air make up volume, may be way to high.

[Gary]

The assumption here is that we are cooling 100% outdoor air.

Is that what the question is about or are we talking about recirculated air?

[Yuri B.]

Indeed, how

ambient 30 +/- 5 C and 75 +/- 5% RH

conditioned to the

23.5 C setting results in over 90% RH,

?!

[nike123]

Indeed, how

conditioned to the

?!

Maybe there is some mixing with indoor air. Or measuring instrument cannot measure more than 90% RH.

[Yuri B.]

Or a humidifier is working, the poster unawares of it, still as far as he even

don't know the temperature of air leaving the cooling coil

[nike123]

Or a humidifier is working, the poster unawares of it, still as far as he even

Wet bulb temperature (dew point) of air 30°C and 75%RH is 25°C. That mean that, by cooling that air to 25.1C, we should see 100%RH.
As you see, in this case, there is no need for humidifier.
And from posted data it is visible that we have some air mixing with drier/hotter air (probably recirculation).

[nike123]

The case I am faced now is pure thermostat control. Single speed fan running at 3,740 CMH. That is the bad news.

In addition, the original design is with steam heating coil and steam humidifier. I am thinking that those 2 things are pretty much in the way and cost a fortune. That's why I am wondering whether it is entirely possible for the design to run purely on cooling coil. Problem is that I don't know the temperature of air leaving the cooling coil. A test setting room at 20.5 C seem to get humidity 70% without problem but 23.5 C setting results in over 90% RH, I suspect because at 30 C, 70% RH, the dew point is 24 C.

For 30°C and 75%RH of outdoor air of volume of 3740 CMH to cool to 23,5°C and 60%RH you need at coil 37KW of cooling capacity. (29kW latent and 8kW sensible).

What is capacity of your chiller (if I understand correctly that chiller is cooling water)?
Also, is it all 3740 CMH fresh air or part is recirculated.

Usual temperature of chiller water is 7°C flow/ 12°C return.
Why is your such high?

[Abby Normal]

The case I am faced now is pure thermostat control. Single speed fan running at 3,740 CMH. That is the bad news.

In addition, the original design is with steam heating coil and steam humidifier. I am thinking that those 2 things are pretty much in the way and cost a fortune. That's why I am wondering whether it is entirely possible for the design to run purely on cooling coil. Problem is that I don't know the temperature of air leaving the cooling coil. A test setting room at 20.5 C seem to get humidity 70% without problem but 23.5 C setting results in over 90% RH, I suspect because at 30 C, 70% RH, the dew point is 24 C.

You have to cool the air down until it has a lower dew point than what you want to maintain in the room.

But the problem seems to be that the room will get too cold if you do this.

So you have two options.

Once the room has fallen to the temperature that you want, keep running the cooling but reheat the supply air up to room temperature, this keeps the dehumidification going without over cooling the room.

The second option is to add an independent dehumidifier.

A cooling coil alone will not work for you in this case.

I am in a humid region myself. I have to use reheat under about 3 different certain conditions.

1) When there is a high amount of outside air.

2) When I am forced to use a high air volume for filtration purposes such as a clean room or operating theatre

3) When there is a high latent load, due to a high concentration of people in the space. It is worse when the people are active like a night club, an aerobics class or a Pentacostal Church

My favourite solution involves free reheat either passive or hot gas because our deisel generated electricity is so expensive here

Here is a unit I had that was custom built, chilled water coil will cool mixed air down to 52 to 54 F range (11.1 to 12.2) and because of the high moisture content in the outdoor air here, the air off of the chilled water coil will be fog. The space dew point tends to drive towards the temperature off of the chilled water coil.

The fan and motor will reheat this air by 3F (1.7C) there is a lot of static on the system.

When it needs further dehumidification a DX coil engages and pulls the air down to 48F (8.9C)

At the same time, the condneser coil for this DX reheats the air. When combined with the fan and motor heat it blows 65F dry air (18.3F)

Basically a room thermostat turns on the DX dehuimidifier and reheat. A room dehumidistat does the same. I have to keep the rooms at 20C or 68F. I am actually monitoring the return air stream, not using room stats

Coil section you can see the upstream chilled water connections, the brown coil in the front is the DX condenser coil, there is a DX evap coil in between. The brown is heresite and anti-corrosion coating there is salt in the outdoor air here



Compressor compartment inside of the coil section, has hot gas bypass can keep the space under 40% RH if need be



Blower comparetment, BAF fan and premium efficiency motor



Moves 9000 CFM (15291 CMH) of which 2150 CFM (3653 CMH) is outside air warmer and more humid than the original poster was talking about

[Abby Normal]

it produces about 1700 litres of condensate per day

[Abby Normal]

here is an all DX version I did in 2002 for a night club

Main 8 row coil used four DX stages to pull mixed air down to 52F, on demand a 4 ton secondary DX coil was used and the condenser for the 4 ton circuit reheated the air. When combined with some fan heat it blew 64F (18C) dry air



When you use humid air, you do not have to cool it much to turn it into fog, the dry bulb and wet bulb off of a coil will be pretty much equal.

A red flag telling you that you need reheat will be that you calculate that the supply dry bulb and wet bulb are relatively far apart like more than 1.5C

[Abby Normal]

For those of you concerned about CFM per Ton, the DX version in the post above was about 150 CFM a ton

[rtamayo]

For those of you concerned about CFM per Ton, the DX version in the post above was about 150 CFM a ton

Abby, you describe an industrial dehumidifier, I believe that this system needs low energy to control of humity (in front other systems) but how it controls the temperature with other conditions of the year? you include the sensible heat of the condenser the rest of the year too. .. thank you in advanced