.....and the question is?

Keith Cress
kcress - http://www.flaminsystems.com

looking the different solutions.thanks

Sounds like you're still getting outside air in the building. Need to seal all 32 floors, not just 10. Also need to review air flow patterns, make sure the building is positive all the time.
Is there a DOAS? is it working correctly?
Are any exhaust fans running when DOAS is off?

More information = better responses.

Yes, there is fresh air unit with heat recovery. Exhaust working but for kitchen, garbage room and toilet. Air flow pattern, can you explain more?

You can do a computational fluid dynamic study or you are going to have to measure airflow to know how it flows in the building.

CFD is a little overkill, so get out your velometer and start measuring.

Where is the condensate on pipe occurring? If it is on some portions and not others, look for patterns.
I have an ongoing problem building which, among other things, has condensate inside light fixtures. These fixture are a typical ceiling tile mounted fluorescent with a single plastic lens. They also happen to be in a waiting/reception room and directly adjacent to supply air diffusers. You can see the air flow pattern of the diffuser as the air crosses the light and cooling the lens to below the dew point.

I recently sat through an ASHRAE seminar about humidity control, in which the instructor detailed a problem building in southeast Asia. Turns out the building had lots of unsealed joints allowing unconditioned outside air into the building at night when the HVAC systems were shut down "to save energy". In one location, a portion of the ceiling had been taken down and through the hole, the investigator could see daylight.

To find your problem you need to thoroughly inspect every corner of the building (at 32-floors, I don't envy the task ahead of you), review design documents including sequences of operation and review operating procedures to find the source of the problem. If you have a access to a thermal camera, that can be a very useful tool. Then you can develop and implement a solution. Keep in mind also, there may be several factors at work contributing to your building's problems.

Quote (moideen)

Yes, there is fresh air unit with heat recovery. Exhaust working but for kitchen, garbage room and toilet.

Can you clarify what you got here. Is the exhaust from the kitchen, garbage and toilets providing the heat recovery or is it exhausted separately?
What kind of heat recovery unit do you have? Does it use a desiccant wheel?

You didn't mention the age of the building, is this new construction or has it been around for awhile.
If it's new, did the ductwork pass leak testing? Are there holes in the ductwork (look for loose duct insulation)?
Just throwing ideas at you here. Up to you to play Sherlock Holmes and investigate.

To avoid infiltration, the building should be tight and pressurized with about .05 CFM/SF conditioned OA through the AHU. Pressurization CFM = SA CFM - RA CFM - EXH CFM.
Also confirm AHU OA dampers are open to correct position when AHU on. I have seen AHUs with OA damper stuck shut.

How can there be a condensation on the surface of insulation which shows dew point of 25 & 20 Deg C. Whereas the space temperature has dew point of 14 Deg C . You need to check again with the Infrared sensor if the readings taken are correct.

Now I presume that the system what you are referring is in the return air path as your question is not clear.

Does this happen round the year or is it seasonal.

If you have any layout.


Thanks
Reji

Sometimes insulation can be the cause condensation. When insulating a cold surface the most important factor is a vapor barrier. Fiberglass is very porous and moisture will diffuse through it and condense on a cold pipe, then saturate the insulation and drip off. Non-porous urethane insulation is a better choice, but it is still very important to tightly seal every joint in the insulation to prevent dripping. If fiberglass pipe insulation is used it will have a vapor barrier jacket and the joints have to be sealed to prevent moisture diffusion through the insulation. The vapor barrier also prevents drying, so once dripping starts it is difficult to fix.

sun005 :I think there will have any gaps or weak section in insulation, chilled water pipe temp is 7c, condensation occur in that area even though the dew is 14c and spreading ,and weakening gradually the rest.

Compositepro : Yes, the insulation partially damaged and growth the mold. The rectification team just repaired only the affected part, and they have not used vapor barrier for the new insulation, as you said I found the form insulation is not properly sealed on joints. I think the solution is to remove all corridor insulation, drying, and entire new insulation with fiberglass with the vapor barrier.

My response was on dewpoint based on your recorded reading where you didn't mention anything on pipe temp.
However if you think there are gaps in the insulation then suggest this should be rectified and then observed. Also you need to check the thickness of the insulation based on your heat gain/loss calculation if it is adequate to avoid condensation.