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moideen
12-03-2013, 07:41 AM
IS IT CORRECT,
The reheat coil of FAHU does not reduce the humidity, but increase the sensible load and maintaining the relative humidity by increasing the dry bulb. Same like slightly increasing the dew point temperature also.
I donít know whether my question is dumb, when increasing the dew point, increasing the possibility of condensing the water in side the air conditioned area and the chance to damage the chilled water insulation. Right now I am facing a humidity issues, it damaged entire 4 year old chilled water insulation and water is dripping everywhere. Location is island, high humid area. Maximum ambient will reach 52c and some time wet bulb will go above30c.
During the first inspection, I found the FAHU Johnson controller FX15 setting is 80%RH and leaving temp set point24c. Re-stetted to 50%RH and 22c.we are going to replace the insulation.
Your any valuable comments highly appreciated.

Moideen-dubai

TRASH101
12-03-2013, 12:08 PM
The reheat coil does change the rh. The chilled water coil air off is supposed to be set for a condition that when reheated/ mixed the humidity is of the required rh level to maintain the conditioned space. water running everywhere is a sign of poor insulation or blocked drains if before the reheat or if after then poor insulation or reheat failure/ incorrect setting. The dewpoint of the equipment surfaces in tropical conditions can be quite high.

Hope this helps but further information would be required for any more depth

moideen
12-03-2013, 12:35 PM
Thanks trash, yes today I have informed site technicians to record the RH level one week: And also planning to fix additional heater coil for the same FAHU to improve the RH level. Or else by fixing low static fan coil units it works only to decrease more humidity and not more cooling effect.

narkom
13-03-2013, 12:05 PM
High ambient humidity is not likely to damage the insulation because it comprises thousands of interconnected closed cells, each with an inherent resistance to water ingress.
I guess the insulation thickness is not enough for your area.
You may check it with
http://www.kflexusa.com/HomePages/ResourceHome.aspx?ID=5

nevinjohn
14-03-2013, 06:27 AM
IS IT CORRECT,
The reheat coil of FAHU does not reduce the humidity, but increase the sensible load and maintaining the relative humidity by increasing the dry bulb. Same like slightly increasing the dew point temperature also.
Yes, when you intoduce a reheat coil, the DBT increases. This can be well visualized if you imagine a pschyrometric chart. The point moves to the right (SENSIBLE HEATING). The RH is lowered, DBT is increased.




I donít know whether my question is dumb, when increasing the dew point, increasing the possibility of condensing the water in side the air conditioned area and the chance to damage the chilled water insulation. Right now I am facing a humidity issues, it damaged entire 4 year old chilled water insulation and water is dripping everywhere. Location is island, high humid area. Maximum ambient will reach 52c and some time wet bulb will go above30c.
During the first inspection, I found the FAHU Johnson controller FX15 setting is 80%RH and leaving temp set point24c. Re-stetted to 50%RH and 22c.we are going to replace the insulation.
Your any valuable comments highly appreciated.
Moideen-dubai
Dew point occours when DBT and WBT are same. So, basically you cannot increase the dew point, its the ambient temprature determining what the DPT is! The reason behing condensatin is two..
1. Insulation thickness is not sufficient / damaged
2. Outside air (ambient) happens to enter the conditioned space in high volume... This can cause condensation on the walls, ceilings etc...

Its always suggested to keep the building in positive pressure to achive this. Is the building too old, may the theres too much of exhaust happening in the air system!

moideen
15-03-2013, 07:55 AM
Hi nevin,

RH and absolute humidity is deferent terms. It cannot consider as single term. Yes when increase the temperature RH will maintain, but the same amount of moisture (absolute humidity) will remain after the reheating, I am talking about the dpt condition after the cooling coil. Recheck your chart, when increase the sensible heat, also increasing the dew point temperature.

nevinjohn
30-04-2013, 11:29 AM
Yes, absolute humidity will remain the same, but RH will decrease!.
To decrease absolute humidity, introduce cooling coils and make sure condensation occours at the coil!!.
This will make both RH and absolute humidity to decrease, as in the case of an air conditioner.

N.B
I wasn't reffering to absolute humidity in any of my posts mentioned above,

Hi nevin,

RH and absolute humidity is deferent terms

Where does this point come from??

Gary
30-04-2013, 02:54 PM
In order to achieve 50% RH in the room, the air temperature leaving the coil needs to be about 11K/20F below the room temperature.

nevinjohn
01-05-2013, 02:56 PM
Hi nevin,

Yes when increase the temperature RH will maintain, but the same amount of moisture (absolute humidity) will remain after the reheating,.

Wrong! When Dry Bulb Temperature (DBT) increases, with the help of a reheat coil,.. the RH goes down!

moideen
02-05-2013, 05:27 AM
Wrong! When Dry Bulb Temperature (DBT) increases, with the help of a reheat coil,.. the RH goes down!
hi nevin,RH will maintain, i mean it will comes down.but absolute humidity will be same. that why i said RH AND ABSOLUTE HUMIDITY IS DIFFERENT TERMS.

nevinjohn
02-05-2013, 12:48 PM
hi nevin,RH will maintain, i mean it will comes down..

What are you trying to say? RH will be same or it will come down? What was yor question?:D