Leaving air temperature from AHU

[Mechya]

Normally the leaving air temperature from the air handling unit in a chilled water system is design at 54-56 deg F with 90-95 % RH (chilled water inlet temp is 45 deg F). My question is, Is it possible to have a leaving air temperature of 60-64 Deg F but with a lower RH value say 65-70 %. What will be the effects of this change. Thanks

 

[tombmech]

You have to look at all the variables on the Psych chart. You have selected values that appear to have the same Humidity Ratio - approx. 0.009 lbs-water/lbs-air. So, it may seem that all you're doing is putting less stress on the coil.

However, a coil does not cool with a straight line process. It cools to saturation at the existing conditions, then dehumidifies along a tangent to the saturation curve after that.

What does that mean? If you control your cooling to a LAT of 64 deg.F., you will be limiting the Humidity Ratio (increases/decreases vertically on Psych chart) to approx. 0.013 lbs-water/lbs-air, not 0.009. That could result in space Relative Humidities of over 75% at 72 deg.F. - a very uncomfortable environment.


Stated another way, you are only de-humidifying to whatever Humidity Ratio exists at 64 deg.F. At 0.009 Humidity Ratio, that results in ideal space conditions of 75 deg.F. and 50% RH. However, that will not be true at higher levels of moisture. For instance, how well can one cool a space to those conditions by only supplying 72 deg.F. air? The same situation exists if all you supply is 0.009 Humidity Ratio air. There is never any "leverage" to dehumidify the space if anything occurs to increase it.

 

[walkes]

You could reheat the air to get the higher d/a temperature but that would likely be a waste of energy.

There are a lot of variables in the discussion, system type etc. Is this for a displacement ventilation system where you want to supply at 65°F? Or an underfloor system where 55°F is too cold for the occupants? Depending on the scenario a face and bypass system may work, but you would have to look at the psychrometrics and determine whether your space conditions will meet ASHRAE-55 Thermal Comfort Conditions.

 

[Drazen]

Increasing temperature will generally decrease relative humidity even if you simply regulate it on chiller.

To achieve exactly what you want you will have to consult Molliere diagram, and do proper sizing of eventual re-heater.

If you want just one of this variables to be exact (temperature or RH) you can make regulation on your chiller until desired value is reached.