Economizer Controls
From Swikipedia
Introduction
Economizer controls are HVAC system controls that operate mixed air dampers to mix return and outdoor air to obtain air of a temperature appropriate for free cooling. Economizer controls are used during periods when outdoor air requires less cooling energy input than return air.
Brief Description
Economizers are mechanical devices intended to reduce energy consumption, or to perform another useful function like preheating a fluid. Economizer cycles typically have several different control requirements associated with them including:
The operating control strategy that governs the cycle in normal operation
Interlock strategies designed to terminate the economizer cycle when it no longer provides any useful benefit or the system it serves is not in operation
Limit control strategies designed to accommodate transient conditions such as start up or sudden load changes
Alarms designed to detect problems
The operating control of the economizer system is designed to modulate the supply and return dampers to maintain either the mixed air temperature, or in sequence with the heating and cooling coils to maintain the discharge air temperature or zone temperature. In addition to these temperature control related functions, the control of the relief dampers must also be integrated with the operation of the return and outdoor air dampers. Relief dampers need to be controlled in a manner that allows the system to deal with the extra outdoor air brought in by the economizer cycle.
In many ways, a start-up is one of the most difficult and complex operational modes for an HVAC system. Every component and control loop in the system is subjected to a significant step change in its input (going from the shut down state to the operating state). These upsets result in the control loops modulating their outputs in an effort to find a stable control point in the new operating mode. The unstable output of one control loop can become an unstable input to the other, causing instability in the second control loop in the cascade.
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