Article from IEQ Review

The selection of air handling equipment to maintain space design temperature and space design relative humidity levels is normally determined by sizing the units to meet peak load design conditions. Actually, peak load conditions occur for only a very small percentage of the time during the cooling season (usually for 2 1/2% of the time). The vast majority of system operation is at partial load.

Since the HVAC system operates at part load most of the time and space design temperature and space relative humidity level is still expected to be maintained under these conditions, partial load control of the equipment is just as important as the original selection of the equipment itself.

EDUCATIONS IN LOAD
Cooling requirements at partial load conditions may be a result of lower sensible load to the space due to less solar transmission or internal heat gain; lower latent load as a result of fewer people in the occupied space; or lower ventilation load as a result of the outside air temperature and relative humidity being less than design conditions. These reductions in load may all occur separately or in any combination with each other. Given these variables, it can be difficult to predict exactly how much room relative humidity will vary from design at part load conditions.

If the air conditioning system is to perform satisfactorily under a part load condition, an effective and efficient means of controlling capacity reduction in proportion to the reduction of the instantaneous load is required. The type of capacity control is influenced by the cooling load characteristics and the accuracy of temperature and relative humidity control level required.

Typical control methods are reheat control, on-off chilled water valve control, face and bypass control, and return air bypass control. There are other methods, but these are the most common.

REHEAT STRATEGY
Reheat control maintains the design space dry bulb temperature by substituting any decrease in the sensible load with an artificial load. As the internal latent load and/ or the outdoor latent load decreases, the relative humidity in the space will actually decrease. So it can be seen that this is by far the best control scheme for applications where latent load is always high like in schools in the southeast.
However, the reheat control strategy is restricted or prohibited (i.e., can't use renewable energy) by many codes and state and local municipalities because of its energy usage.

VALVE CONTROL
Attempting to maintain space conditions using on-off chilled water valve control usually is inexpensive but is least desirable because it allows for severe fluctuating of space temperature and space relative humidity. During the off operation, outside air is brought into the system without being dehumidified since the chilled water is shut off to satisfy the dry bulb temperature.

Units don't operate long enough with the water valve remaining open to condense sufficient moisture out of the air stream. This results in an intermittent supply of dehumidified air to the space and hence, high relative humidity levels. Additionally, the condensed moisture that remains on the cooling coil when the chilled water is turned off re-evaporates into the air stream. Both of these conditions increase the space latent load and create excessive humidity levels. This type of control is not recommended for high latent load applications. Actual field tests have shown an increase of 10% to 20% in relative humidity levels from design conditions.

MIXED AIR BYPASS
Face and bypass control is a common and accepted method of capacity control. This arrangement is often highly desirable as it affords a means of temperature control without resorting to reheat while still providing reasonably good humidity control in applications with high latent loads.

Face and bypass dampers consists of a number of opposed blade dampers installed immediately upstream of the cooling coil along with a set of dampers which can bypass the air stream around the coil. These two sets of dampers operate in tandem with each other; as one set opens the other set closes. The bypass duct is sized to have the same pressure drop as the cooling coil so that a constant air quantity can be maintained at all times during system operation.

Bypass control maintains the dry bulb temperature in the space by modulating the amount of air flowing through the cooling coil, thus varying the supply air temperature to the space. As the face dampers begin to close the bypass dampers begin to open. Some of the outside air does get diverted around the coil and mixed with air coming off the coil to obtain a supply air temperature that is proportional to the reduction in space load. Space humidity is still subject to variations but these variations are small and usually result in tolerable or acceptable conditions.

MODULATING DAMPERS
Face and bypass control results in a continuous supply of dehumidified air being supplied to the occupied spaces. However, as the load continues to reduce and the face dampers modulate toward the close position, the amount of outside air that is bypassed gets greater and greater and the amount of air that is dehumidified is reduced. Even under these conditions, reasonable relative humidity levels can be maintained in the space.

RETURN AIR BYPASS
Significantly improved relative humidity control in the space can be obtained by bypassing only the return air stream and allowing all of the outside air to pass through the cooling coil. This is a more effective use of bypass control. Bypassing return air only and dehumidifying all the outside air can result in space relative humidity control that approaches what can be obtained with reheat control.
In cases where the return air face dampers get all the way open and all outside air is going through the coil and further reduction in capacity is still required, then a second set of face and bypass dampers can be used to control the outside air portion.

CONCLUSION
In order for air conditioning systems to perform effectively under all conditions from part load to peak load - a control strategy for reducing capacity in proportion to load reductions is required. Of the control options available today, face and bypass dampers are particularly effective, especially in applications with high latent loads.

For more information, contact
Francisco Aguirre
(800) 422-7873 ext. 201
Faguirre@pureaircontrols.com