I want it cold but I don't want it ALL cold - Design Issues......................................... 1

[Kinemenematics]

hi, i'm starting this thread because I'm in a design dilemma and also for others who face similar design issues.

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Just let me get excerpts from my report up here:

REPORT BRIEF
The site visit was incepted to conduct an inspection following a reported fault in an air-conditioning system. The operation of three air-conditioners (2 x 50% duty) has resulted in water condensate forming on the ceiling surfaces, which if not resolved will result in damage of equipment....

Main Air-conditioning Issue
The operation of three air-conditioners (2 x 50% duty) has resulted in water condensate forming below and above the ceilings for each of the two rooms. The severity of the condensation issue was greater in the ATC Room.

To date, equipments in the ATC Room are operating at a required controlled temperature of 22°C. the Contractor has reported that when the room’s temperature reading was 24°C, the ATC equipment began to overheat.


Certain attempts have been made by the Contractor's staff to manage and mitigate the prevailing condensation issue:

a) Thickness of condensate pipe insulation was increased, which did little to improve the situation. Drops of condensate water still formed on the insulation’s outer surface.
b) Turning off of one duty air-conditioner, which resulted in equipment overheat.
c) Placing ply-wood planks above the “roof-less” electronic equipment to prevent drips of condensate water from entering the equipment, as a temporary solution.

Other Information:
1. The ceiling panels (possibly asbestos) rested on metal (possibly galvanized iron) frames. Condensation occurred on the surfaces of these frames. Subsequently, the edges of the ceiling panels were damaged and more importantly, water condensate on these surfaces dripped onto the ATC equipment. The lower-roof directly above the M&E Services and ceiling panels were made from a ‘special’ fire-fighting composite described by the fire dept which consists of ply-wood and polymeric material:

2. The compressor units for all 6 units were isolated in a well ventilated room on the lower roofs, and it is assumed that the source of water dripping was indeed condensation and not leakage due to equipment defect.

3. Even the bolts which held the ceiling-exposed air-con units to the ceiling had condensate forming on them.

4. Layout of ATC Room:

'Special fire-fighting material'
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M&E services area (i think around 29 degrees C)
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Ceiling panels
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ATC Room, Temp = {18,22} degrees C. dimensions: 10m x 7.5m x 3m.
---------------------------------------------​

5. Equip Spec:
ATC Room: 3 ceiling exposed units (2 duty 1 standby) of ACSON A5CM 62C, capacity 55,000 btu/hr (6.41kW)
Central Control Room: 3 ceiling exposed units (2 duty 1 standby) of ACSON A5CM 50D, capacity 43,000 btu/hr (4.70kW)

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Question:
As i described the scenario above,

The room MUST be kept cold (22 degrees and below) but if it's too cold, condensation forms on the ceilings and end up damaging the equipment anyway. That's the dilemma.

What would be a feasible solution which is still relatively economical? Localising the cooling area? Setting up an exhaust fan in the M&E Services space to convect air in/out?



Thanks.

 

[MintJulep]

Condensation forms when air contacts a surface with a temperature lower than the air's dew-point.

You've told us nothing about the air dew-point or the temperature of the surfaces that condensate forms on.

 

[Kinemenematics]

Thanks for ur input MintJulep, will get to obtaining more data.

however, was the scenario a clearly painted one? Would using a dehumidifier help if say, the relative humidity was already 100% (i mean condensation is already happening, yes?)

 

[SAK9]

Need more information such as any ventilation air,its condition,dewpoint within the conditioned space(to be calculated from room dry bulb temperature and humidity readings),supply air temperature from the units etc.
It is possible that cold supply air flow from the units are directed towards metal structures taking it down below the dewpoint.

 

[MintJulep]

The bulk relative humidity of the air does not have to be 100% for condensation to form. Read and understand the first sentence of my first reply.

Sak9's guess is a good guess, but considering the lack of information it's only a guess. Same guess that I would make.

 

[chicopee]

With a psychometric chart, you can determine how close to the dew point if you knew the relative humidity since you already have the room temperature. Where is the moisture coming from? The picture showing the water damage to the suspended ceiling indicate that either the rook is leaking or you have a plugged drain line from an AC unit.

 

[DRWeig]

Here's a relevant discussion on this topic:

http://www.eng-tips.com/viewthread.cfm?qid=334316

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[Drazen]

it would be interesting if you would tell us whether condensation take place on upper or on lower surface of suspended ceiling.

if it originates on upper surface, ventilation of ceiling space might be solution. it is not impossible that ceiling area reaches over 90% RH.

 

[GMcD]

OK, if this is a sealed space, and you are recirculating conditioned air iniside it, and that air is dehumidified, where is the moisture coming from that forms the condensation? Use basic engineering logic and critical thinking.

 

[DRWeig]

Just to emphasize Drazen's comment, I have personally experienced as high as 95+ percent RH at the start of morning cool-down after night setback. Condensation was especially severe (indoor rain) on Monday mornings after being set back for the entire weekend.

The jobs were in Singapore and Hawaii. Infiltration happens.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[Drazen]

GMcD,

you should seemingly know more about vapor diffusion. "basic engineering logic" is to not make statements, at least not firm statements, if you are not informed enough.

 

[140Airpower]

kinemenematics, you do not state clearly if the 22C room air is condensing or if ceiling space air is condensing. It logically seems the latter. Is the ceiling space vented to the outside? If so that is a big problem. It is an endless source of moist air. If it is room air that is condensing, is the room air re-humidified or is AC air mixed with outside air?
Normally, air coming out of the evaporator is dehumidified and will not condense water out onto anything, but if it is mixed with outside air that is hot and humid, then it may be circulating already close to its dew point.
It is also curious that metal parts associated with the evaporator units are dripping. It means those parts are not insulated well enough from the evaporators and are cold by conduction. Note that the bottom panels on a unit may have thermal insulation inside that is wet. What can help is to replace it with insulation sealed in plastic film (bags might do).

Other than these questions, a solution can be simply much better thermal insulation of everything connected with the evaporators.

 

[Drazen]

usually, the more you try to seal ceiling space, the more likely it is that large humidity will develop.

the problem lays in practical fact that vapor seals always have some cracks, somewhere, and, on the other hand vapor pressure mechanisms always create negative pressure differentials in such places, and the outcome is that vapor is gradually entering through cracks, raising humidity indefinitely.

when air flow is allowed, such scheme is prevented as air is allowed to come in and go out. such a space should not be ventilated to the outside directly, but linked via openings with rooms at appropriate places, like restrooms or similar.

that way the problem will at least be "spread" over the whole space, and if too high dew point temperature is overall problem, it needs to be handled via AHU or space dehumidifiers.

 

[317069]

- Try to remove some tiles and watch the situation
or deal with the space above the tiles as an attic and ventilate it as they ventilate attic in houses or use HRV if possible.

 

[GMcD]

Drazen- not sure why you seem ticked off at my post. I was posting towards the original problem that Kinemenematics posted to start this chain. DrWeig and 140Airporwer are providing some good guidance and that's where I was going with the comment. The original statement of the situation is still missing a lot of pertinant information, which leads to a lot of second guessing trying to solve someone elses' engineeering issue by text on a discussion board. If the moisture is coming in through the envelope of the space being cooled because it's in a hot and humid environment, then the solution is to seal up the envelope better so the vapor transmission is reduced or eliminated rather than trying to apply reactive HVAC solutions. SAK9's post is one which should help provide further information, so let's wait and see what else can be learned.

 

[Kinemenematics]

at this point i'm still trying to gather data from that location,

what information i can provide now however:

1. yes the entire room is situated in a humid environment. DRWeig, it's located in Malaysia, neighbour to Singapore and the climate is the same. It could be that infiltration happens, will keep an eye out for that. The ATC room is sealed off from the outside humid air (except for space under entrances).

2. 140Airpower - kinemenematics, you do not state clearly if the 22C room air is condensing or if ceiling space air is condensing. It logically seems the latter. Is the ceiling space vented to the outside? If so that is a big problem. It is an endless source of moist air. If it is room air that is condensing, is the room air re-humidified or is AC air mixed with outside air?
Both the 22C room air and ceiling space air is condensing. In the 22C room (realistically around 18C), the AC units have condensate forming on its surface, and is as well causing the metal ceiling frames to become cold. In the ceiling space, i do not know if air is condensing on the upper surface of the metal frames, but it definitely is condensing on the copper pipe insulation (even after its thickness has been increased). I will double-check if that ceiling space is exposed to outside air. It didn't seem so.


3. SAK9 - the cold air has been directed away from the ceilings (albeit the AC unit themselves touching the suspended ceilings) and onto the area that requires cooling. This local area which requires cooling contains metal surfaces and equipment all over, but no condensate seems to forming on them (i think cause the equipment consistently generates heat) My guess is that the the metal ceiling frames are also losing heat through conduction.


Thank you all for trying to address the issue, with the lack of pertinent information i provide.

Cheers

ATTACHMENTS:

1. the 'area' that requires cooling.

http://files.engineering.com/getfil...2fef-4ae7-8752-d095797ea96f&file=IMAG0820.jpg

2. copper pipe insulation in ceiling space

http://files.engineering.com/getfil...b7c5-418d-ba4b-1f947d855e66&file=IMAG0827.jpg

3. could the condensers have anything to do with it? Installation clearance, clog, etc.

http://files.engineering.com/getfil...3c91-49c2-aca1-37fa7cf17ce8&file=IMAG0818.jpg

 

[Kinemenematics]

Oops. i will have to confirm that no fresh air is taken in through the AC unit. thanks.

 

[SAK9]

There are two issues at work here which may be causing the condensation.

1.Infiltration:There is no ventilation air whcih means the space is not pressurised.This leaves the door open for infiltration of humid outside air at 80% RH levels.If the equipment room is located in the core area of the building surrounded by conditioned spaces,this may not be an issue.If not there will be infiltration due to stack effect as well as temperature difference. Since the internal temperature is around 18C,the infiltration can be higher than normal.Sealing of doors and windows is definitely required.There will be infiltration through the lay in ceiling tiles as well which will not be easy to stop.It is not clear if the ceiling is insulated.Ideally the space needs to be pressurised with dehumidified air as perfect sealing is difficult to achieve

2.Equipment load variation:If the internal temepraure is set to 18C,the supply air temperature is likely to be around 10C.If the equipment load is variable,there may not be enough sensible load in the room at times and this could result in high humidity and condensation build up.Compensating heaters is one easy solution.

A data log of temperature and RH with in the space and outside for a few days as well as power draw of equipment will give a better understanding of the issues.If the equipment can be made to function at higher temperatures,that would be the cheapest and easiest solution.

 

[Kinemenematics]

SAK9,

1.
Infiltration would likely be the case. Two rooms have this condensation issue; the room with a greater exposed surface area to the external environment is experiencing more severe condensation. A dehumidifier would still be ideal yes? I need to recommend an economical solution too.



2.
by a variable heat load, you mean a constantly changing heat output from the equipment that needs to be cooled? Could you describe what a compensating heater is? I'm on Google but i can't get much info.

If the equipment can be made to function at higher temperatures, that would be the cheapest and easiest solution.
- Would the equip not overheat? The whole idea of AC in the first place was to cool the equipment in the room.

 

[SAK9]

What I meant was that you have an air conditioning system in place which is not designed to take the room down to 18C.If you want to operate it 18C, you need to make costly modifications to it.Supplying air to pressurise the space is not so simple as you need to cool and dehumidify it.I f you had known this requirement earlier,you could have designed a system with could cool the space as well as treat the outside air.
So my view is that the need for such a low temperature for equipment room needs to be questioned before doing any modifications.If you were to operate it say 25C,there would not be any condensation.Most equipment rooms such as data centres,telecom exchanges etc operate in a high temperature band of 25~28C.

Equipment heat rejection may not remain constant in many cases.If it varies a lot, it is not uncommon to provde electric reheaters that will come on during periods of low load to keep the room within an acceptable humidity range.