Low humidity in clean room

[Buxi]

Hello, All

Please advise in seeking solutions (temporary and long term) for HVAC system renovation.

System deficiency:
The HVAC system humidifies the air by steam injection before reheating coils that results in present conditions (-13 deg Celsius outside temperature and 50% outside RH) in clean room RH of 17% @ 18 deg Celsius and massive condensation of steam in the air-duct that floods production floor.

Here are some details about our HVAC system:

Clean room size: 33.5’ / 24.5’, height 8.3’
Main stream from AHU through a section of 20”/24” – 4,200 cfm
Exhaust fan 2,000 cfm while the rest is recycled trough the AHU.
Air duct temperature 14 deg Celsius

Specification:
Clean room temperature 18 deg Celsius
Clean room pressure +0.18 inch
Relative Humidity (RH%): 35% - 55%

Thank you for your time

 

[Whoops]

It looks straighforward. Have I missed something? You need a heater upstream of the humidifier, or you won’t be able to get enough water into the air. You need a humidity sensor downstream of the humidifier, to restrict it’s output to around 90 or 95%, to avoid wetting the supply duct.

If there is no heater upstream of the humidifier, then the most water you can get into the air is 100 Rh. At the winter design condition (worst case) the duct will be at
-13 degC, 100% Rh = 1.6 gm per kg of dry air, giving 15% Rh at 14degC in the supply duct and around 11% Rh at 18 degC in the room, assuming no latent gains in the room.

The humidifier should be downstream of a heater. If the supply air were heated to 14 degC before the humidifier, then you could humidify to, say 90% Rh (to avoid wetting the duct) giving you up to 70% Rh at the room temperature of 18 degC. You would, of course, control the humidifier to maintain the room Rh at the room setpoint of around 45 or 50%. I haven’t got into the recirculation.

 

[ChasBean1]

I reread and realized you're referring to an in-duct humidifier. If the humidifier is upstream of the reheat coil and the high limit humidistat is downstream of the reheat coil, this could create a saturated condition upstream of the coil while the heated air downstream of the coil indicates low humidity. This might be the problem.

If this is an Armstrong humdifier with a heated jacket, the condensate line temperature proving switch must also be set right and functional.

What is the air temperature from the central system? Is there preheat in the central system, or will there be 38°F )3°C) air due to mixing the two airstreams you mention?

Relocate the humidifier downstream of the heating coil, ensure the high limit humidistat is functional, ensure the steam condensate drain line temperature proving switch is functional (Armstrong) or there is a trap just upsteam of the control valve (Dri-Steem/Ultrasorb). If it's Dri-Steem/Ultrasorb, make sure there is no uphill turns in the drain piping. Finally, make sure air flow/pressure proving switch is functional. I would recommend putting flow proving device upstream of the humidifier so as to avoid corrosion and sticking.

 

[Buxi]

Hi, there

The temperature in the duct is 14 deg Celsius (approx. 57 F) as mentioned above. An outdoor AHU unit on the roof uses a gas furnace to pre-heat the air mixture (approx. 40% fresh air and the rest recycled air from inside the clean room)

The reheating coils just increase a bit the temperature that might be sometimes low due to dehumidification (whenever needed) achieved by mean of refrigerating coils,

Thanks

 

[Buxi]

Hi, ChasBean1

Thanks for fast reply, yes you are right about positions; the humidifier (Nortec–LIVESTEAM has a steam heating jacket) is in the air duct, upstream of the reheat coil and the high limit humidistat is downstream and is (the first humidistat) fully controlled by system and/or operator and is generally 75% lower than the room humidity reading (a second humidistat), let’s say that for 55% clean room humidity (second humidistat set up), the first shall be 40%. Both humidistats are demanding for more steam (the air humidity in the clean room is dangerously low the product is contaminated by dust particles) and the humidity is 17-19% @ 64.4 deg F (18 deg Celsius)

The temperature in the air duct before reheating coil is 57 deg F (14 deg Celsius) and after reheating coil is 64.4 deg F (18 deg Celsius).

[Note: Actually the main stream of 57 deg F (14 deg Celsius) is split into two branches, for a second clean room of 69.8 deg F (21 deg Celsius) and no humidity control. That duct is dripping too in its humidifier area]

About steam jacket of humidifiers I don’t know exactly where is the switch, but this is a nice touch, thanks, I have to make my home works again.

The last recommendations are already in process (valves, strainers and traps), about humidifier position: was there and was dripping so hard that the ceiling fell down (together with HEPA filters), flow proving exists and the corrosion too.

Thanks again

 

[quark]

Position of humidifier not seems to be the main problem. As per my calculation the main problem is with the mixing air drybulb. The mixing air DB is coming out to be

DB = 2200*18 + 2000 *(-13)/4200 = 3.23⁰C. If you check up the dew point of air at the condition
18⁰C and 35% RH it is about 2⁰C, at 40%RH it is about 4⁰C and at 50%RH it is above
6⁰C . This is well enough for condensation to take place as soon as the mixing process takes place.

If you increase the recirculation rate to 2900cfm you should not have any problem even with 50%RH room condition. Or as suggested by CB, do preheating of the ambient air.

 

[ChasBean1]

Buxi, from what I can see, the high limit humidistat is in a 64.4°F airstream while the humidifier emits steam into a 57°F airstream. With the high limit set normally, it would prevent the humidity in the duct downstream of the reheat coil from exceeding around 90%, which would be a saturated condition where the steam is emitted. I think this is the biggest problem - the humidifier (and the high limit humidistat) need to be in the warmest airsteam prior to entering the space, and they need to be in the same airstream to allow the high limit to work. Note that also the reheat discharge temperature is usually variable between no reheat (57°F) and full reheat (could be 80-130°F or more). With this configuration, the high limit is useless unless the steam injection point is in the same location as the high limit device.

The humidifier valve should open based on the room setpoint (not the supply air setpoint) and only when 1) it is proven that a sufficient volume of air is moving through the duct, and 2) the condensate line temperature is proven to be hot enough (usually >195°F). The room humidity (or return/exhaust duct humidity) setpoint is the main parameter that controls the humidifier valve position; the duct high limit is an override that modulates the humidity valve closed if in-duct conditions approach saturated.

If the system controls differently please re-post. This should typically be the way these units work. -CB