Discharge Humidity Sensor Saturated 2

[wstokes]

We have a built up airhandler with chilled water coil with setpoint at 45.0F and discharge temp of 50.0F. 100% outside air. Outdoor air temp 63.9F and 72.9%. Discharge humidity sensor reading 100.6%, our humidifier is off. The space temps are maintained at 70.0F and 40.0%. We have had issues with several airhandlers. We have a reheat after the chilled water coil which is to keep Hepa filters from getting saturated. Supposedly? After the Hepa's we have a steam humidifier to add humidity if necessary. Which is presently shut down. I am not sure why our sensor is saturated. Any ideas......

 

[quark]

Dewpoint of air corresponding to your outdoor condition is 55F and air becomes essentially saturated if you are cooling it below this temperature. Positioning the humidity sensor after reheat coil is better. Ideally, you should better keep the humidity sensor in the return duct.

Micro glass fibre(that is what HEPAs are made up of) is hydrophobic and no issues with HEPA saturation on a general note. Further, the moisture in air is in vapor form.

 

[wstokes]

We do have the spaces controlling from an exhaust humidity sensor. Which is fine but the sensor located in the supply is actually used for a high limit for the humidifier. Which seems to keep cylcing the humidifer off most of the time....

 

[AbbyNormal]

RH is a pretty difficult measurement, and even a good sensor will be off by a few points.

You say it is 100% outside air, the incoming air has a dewpoint of about 55F, you are cooling this air down to 50F so it sounds like it could be a little 'foggy' to me.

 

[AbbyNormal]

oops should have read what quark said first

 

[Whoops]

Sounds like it may be working normally.

If the CHW coil is there to de-humidify the air then, when it cools the air, some of the moisture will condense and the air stream off the coil will be saturated, 100% Rh or thereabouts.

If it's reheated then the moisture content (in gms per kg of air) will stay the same but the Rh will be lower downstream of the reheater.

The extract Rh sensor will take some time to react to an increase in supply air Rh, but the supply Rh sensor will respond almost immediately; it's very easy to find the supply sensor causing the the humidifier to hunt between 100% and off. If it's an electric humidifier, it's complicated by the intermittent blow-down & cold refill, which causes the steam to stop for some minutes during which the fall in room Rh can cause the humidifier demand to increase to 100%.

If it has electronic controls, BMS/DDC contractors will usually have a standard tried & tested strategy to download to cope with this. I can't recall the details; I believe the humidifeir output is limited by the supply sensor as the supply Rh approaches 100%.

Some Rh sensor types don't like 100% Rh and acquire a permanent large zero error. Other types can be immersed with no ill-effects.

 

[AbbyNormal]

'extract' sounds like a Brit

 

[wstokes]

Our humidifier is a Dri-Steem live steam type and our building is about nine years old. It seems that type of design kinda contradicts it self opening and closing the humidifier. This design seems like would probably serve the purpose if the sensor was placed further downstream, but our duct limits what we can do.

 

[ChasBean1]

Hi wstokes,

I’m chiming in late and they were good posts above, but it sounds like you were left with a question so I wanted to add detail:

The AHU cooling coils should discharge cold air, ~46-48°F, when the outdoor dew point is above 44.6°F (the desired dew point of the room). The reheat should then cycle to maintain space temperature, or to temper the discharge air to a higher value—depending on the attached system.

The humidifier should be disabled during dehumidification when the cooling coil removes moisture (any time the outdoor air dew point is above 44.6°F).

If the supply duct high limit humidity sensors were better, they wouldn’t likely be pegged. Real relative humidity typically drops into the low 90s with cooling coil bypass factor. Cooling coils with more fins per inch and/or more rows might have lower bypass factors (on the order of 10%). Coils with less fins per inch and/or less depth will have higher bypass factors (15-25%). The bypass factor reduces leaving dew point below the dry bulb temperature. Greater bypass means lower dew point for a given dry bulb temperature. The leaving air should therefore be less than 100% RH unless there is a 0% bypass factor. Some sensors fail however, pegging high if the humidity approaches 95% or if any water droplets form on the sensor.

During winter the reheat coil and the chilled water should not be used and should be disabled. The discharge air temperature of the AHUs should be raised and then the high limit humidity sensors (assuming they’re placed right, downstream of the humidifiers) will function as intended.

Consider modifying the program for the AHUs to do the following. This assumes an AHU serves the space directly with no additional terminal boxes with reheat:

Above 45°F OA dew point temp:
• Disable heating and humidification.
• Enable chilled water and reheat.
• Set cooling coil leaving air temperature to 47°F.
• Modulate reheat to maintain space temperature.

Below 45°F OA dew point temp and above 45°F dry bulb:
• Enable humidification to maintain a 40% exhaust
relativehumidity with a discharge air high limit of 85%
RH.
• Disable reheat.
• Use AHU heating and chilled water, without overlap, to
control the discharge air temperature setpoint on a reset
schedule that maintains the room temperature setpoint.
For example, a 68°F room may cause a 75°F discharge
setpoint, a 72°F room yields a 55°F discharge setpoint.

Below 45°F OA dry bulb temperature:
• Maintain humidification control.
• Disable chilled water and reheat.
• Use AHU heating only to maintain the discharge
temperature reset schedule based on room temperature.

Sorry – I meant for this post to be shorter.