Lower dew point

[snow_man]

Hi ..I have a problem with a FCU ,air on is 21.2 and 52.5%rh ..air off is 14.2 and 81.6% rh .it is a new job to me and I don't know the cooling coil design ,How can I get it to dehumidify as well as cool ..Chilled water temp is 6 any thoughts ?

[mad fridgie]

Hi ..I have a problem with a FCU ,air on is 21.2 and 52.5%rh ..air off is 14.2 and 81.6% rh .it is a new job to me and I don't know the cooling coil design ,How can I get it to dehumidify as well as cool ..Chilled water temp is 6 any thoughts ?
decrease air flow or increase water flow, or both

[Emmett]

You are dehumidifying, %RH is the amount of moisture in the air compared to the amount the air can hold at that temperature, you have removed a substantial amount of moisture, however at the outlet temperature the air is nearly saturated, you will need to reheat the air in order to lower the %RH

[Gary]

You are dehumidifying, %RH is the amount of moisture in the air compared to the amount the air can hold at that temperature, you have removed a substantial amount of moisture, however at the outlet temperature the air is nearly saturated, you will need to reheat the air in order to lower the %RH

I disagree. Reheat does nothing more than extend the run time.

[goshen]

I disagree. Reheat does nothing more than extend the run time.
Hi
i agree with gary.
as is ,you are dehumidifing.
if you need to dry the air more than 45-50 rh%
you will need to use other elements to do this!
water dehumidification is usuasly used to lower humidity as i said, to max 40%, and this is done with cold water 6 degc min,and at least 8 rows on the cooling coil,using a simple fc unit in a room you should have no problem, if design is ok to keep it comfy at 50-55 %,if you were to use a big water cooled unit to dehumidify ,you would do this by lowering the temp of supply air through the coil ,and then reheating to a normal temp,if you were to use a munters unit that uses steam or electrical elemants with a rotating silica gel drum to absorb the humidity ,you would then need to cool the air supplied to the space,
good luck:)

[snow_man]

If I plot the g/kg they are the same entering as leaving ,so in my mind I am not taking any moisture out of the air or have I missed something .

[Gary]

If I plot the g/kg they are the same entering as leaving ,so in my mind I am not taking any moisture out of the air or have I missed something .

You are keeping the g/kg at that level. If you want it lower than that, you will need a colder coil.

[Gary]

I don't understand what you are trying to accomplish.

[snow_man]

The system is for soft gell tablets I have a desiccant wheel in the system I want this AHU to dehumidify a little before the air hits the wheel .The tablets are taking to long to dry .

[Gary]

What is your target temperature and humidity for the room it serves?

[snow_man]

Target temp is 20 C and 20% RH

[Gary]

Target temp is 20 C and 20% RH

I can see why you are using a dessicant. In order to get the humidity that low without it, you would need to run the coil well below freezing temperatures.

The lower the coil temperature, the more moisture is removed.

You can lower the temperature of the coil, thus removing more moisture from the air, by reducing the airflow through the coil. Slower airflow has more time to cool (and dehumidify) on the way through.

Another strategy would be to increase the water flow through the coil, increasing the heat transfer.

A third strategy would be to reduce the water temperature. Be very careful not to freeze the coil.

Let us know how this works out for you.

[Emmett]

I can see why you are using a dessicant. In order to get the humidity that low without it, you would need to run the coil well below freezing temperatures.

The lower the coil temperature, the more moisture is removed.:D

You can lower the temperature of the coil, thus removing more moisture from the air, by reducing the airflow through the coil. Slower airflow has more time to cool (and dehumidify) on the way through.

Another strategy would be to increase the water flow through the coil, increasing the heat transfer.

A third strategy would be to reduce the water temperature. Be very careful not to freeze the coil.

Let us know how this works out for you.

Gary, If you lower the coil temperature will you not have to reheat to maintain room temperature specification? and in doing so will you not also reduce the % RH in the air?
I am no expert in this area I am speaking simply from limited experience with clean rooms. what we tend to do is use either chilled water or mechanical refrigeration and lower the air temp as much as possible hence removing as much moisture as possible and then we reheat the air this often drives the RH below target so we humidify the air to our specifications. This system works very well for tight control.

[Gary]

Reheat may or may not be needed. If the room is cooled to temperature before the humidity target is achieved, then reheat can be used to extend the run time. It depends on the system and it's particular requirements.

On your system, if you dehumidified a little less (warmer coil), you could humidify a little less, getting the same results a little more efficiently.

[Emmett]

Snow_Man
have you solved your problem?

[snow_man]

I am not back on site till next week .I will try and reduce airflow to start .and let you all know how it goes ..Thanks everyone for your input .

[Emmett]

Snow_Man,
Have an update for us?