Sidebar 4: Addressing the Interactive Control-Loop Problem Via a Different Design
One obvious way to eliminate the potential for control loop interaction would be to eliminate the multiple control loops and simply control the preheat coil in sequence with the chilled-water valve and economizer. This may be a viable option in some situations, but a system with a large outdoor-air fraction like AHU1 (Table A-1) in a climate zone like the one where the Student Center is located (Figure 6) will see air that is below freezing under some operating conditions. When this occurs, the systemís thermal inertia and time lags may prevent a discharge temperature based control process from reacting quickly enough to keep the air temperature downstream of the preheat coil above freezing when a major transition in operating mode occurs (a start-up for instance). The result will be freezestat trips and possibly a frozen coil. Moving the preheat coil to the minimum-outdoor-air duct would be another option, but has its own set of control issues that must be resolved. Moving the coil also introduces a significant pressure drop as well as the requirement for additional filters into the outdoor-air-intake system.