why is there higher humidity in the SA than the RA my AHU

[WillisWashington]

we have a building with a bad humidity problem.
the building is in a neg. pres. in baton rouge, LA
the RH is near 80% through out the building
when I place the probe of the RH meter in the SA duct when the AC is off it reads the same as the conditioned space
when I turn on the AC the RH rises to 90% in the SA duct

why does the SA duct read higher RH than the RA
if in fact the chiller water coil is removing moisture

 

[quark]

Willis!

Do you have reheating arrangement after the cooling coil? If not the RH should be high because you are narrowing the difference between DBT and WBT. Once the air comes into the room it picks heat from the surroundings and DBT increases. Check along the length of the ducting and you will find decreasing RH.

The less the difference between DBT and WBT the more is RH and vice versa.

Regards,
Repetition is the foundation of technology

 

[VEEKRISH]

Willis, the reason why the RH is higher (close to 90 %) in the SA Duct is really very fundamental. If you actually plot the condition on the Psychrometric Chart you will understand why. When the Air leaves the cooling coil (true, after de-humidification), the air at the exit of the cooling couil is very close to saturation (less by the by-pass factor of the cooling coil). Hence the RH will be 90% or higher.

 

[howardeee]

I'd like to add to VEEKRISH's comment. His rationale is correct in that the RH will be higher AFTER the cooling coil than before because of the resultant temperature drop. I suggest you determine the humidity ratio, that is, the ratio of the mass of moisture to the mass of dry air. I'm sure you will find that the coil is doing its job. Good luck.

 

[ChasBean1]

Willis - the last two posts are accurate for your application. Relative humidity doesn't tell the story as it depends completely on temperature. The key issue is the amount of grains of moisture in the air - you'll find at the coil outlet that although the relative humidity is higher, some moisture has been removed at the coil. There are probably more angles to your problem than this, however, such as infiltration issues and other reasons for building relative humidity to be 80% (at standard indoor air temperature conditions). Feel free to re-post if you learn more about this issue.

 

[Islander]

Regardless of the reading you got in the supply duct, the reading at the return is the critical one imo.

IE..you know that the unit is cooling..thus removing moisture from the air..

Verify that you have an 18-20 degree drop across the coil.
Verify you are'nt pulling in an excessive amount of outside air at the ahu.

Verify the ahu is not outputting very far above the design requirements(cfm).

and most of all...find out why the building has a negative.

IE...90+ degree outside air being sucked into the building is a major problem....have a big kitchen hood or other exhaust in there? Outside air cfm's are all set?

Test and balance the job...remove the negative..have the other items i mentioned checked...should be a problem solved.