Building-pressure control.

Gravity dampers or an exhaust fan can be used to control building pressure. An engineer can subtract the amount of exhausts (such as toilet exhausts) from the minimum-outside-air quantity and estimate the extent to which the remaining outside air could pressurize the building. During commissioning, pressurization set points can be determined by taking building-pressure field measurements with known outside-air quantities.

One drawback is that the method does not truly control minimum outside air. The method uses a surrogate, such as the supply-fan capacity signal, as an approximation of total supply airflow. The controls use the surrogate signal to open the outside-air damper further as the supply fan slows, maintaining a nearly constant minimum outside airflow rather than a constant percentage of supply airflow.

Mixed-air-plenum-pressure control

Return-fan capacity can be controlled by pressure in the mixed-air plenum (Point C in Figure 1). The rest of this article will discuss how to apply the mixed-air-plenum pressure-control method.

HOW TO DO IT

A package rooftop air-handling unit (AHU) from an actual project will illustrate how mixed-air-plenum-pressure control can be implemented. The unit's characteristics include:

1. A draw-through airfoil supply fan providing 20,000 cfm at 5.32 in. wg of total static pressure.

2. An airfoil return fan providing 17,000 cfm at 1.50 in. wg of total static pressure.

3. A chilled-water cooling coil.

4. A 100-percent-outside-air economizer.

5. Supply- and return-fan variable-speed drives.

6. A minimum outside-air quantity of 4,000 cfm.

7. A winter indoor design temperature of 70°F.

8. A winter outdoor design temperature of 0°F.

9. Summer indoor design conditions of 75°F dry-bulb temperature and 55-percent relative humidity.

10. Summer outdoor design conditions of 91°F dry-bulb temperature and 73°F wet-bulb temperature.

11. A duct static-pressure sensor (Point A, Figure 1) that regulates the supply-fan variable-speed drive and controls supply-fan capacity.