What is 'sequencing' in DDC and provide an example.

• A. Coordinating devices to operate independently without order.
• B. Starting all devices at once to maximize speed.
• C. Coordinating multiple devices to operate in a required order.
• D. Randomizing the operation of devices.

Answer: (C)

Sequencing in Direct Digital Control means coordinating multiple devices so they operate in a specific, required order, using interlocks and feedback so one action only starts after the previous conditions are met. This ensures safe startup, proper interlock protection, and smooth, efficient operation. For example, in a boiler plant the feedwater pump should start first and bring the water level to a safe point before the burner can ignite; only after ignition does the system allow the fans to run and the boiler to reach its temperature setpoint. In a chiller plant with several pumps or compressors, the control will bring the primary pump online, confirm adequate flow, then start the first chiller, and only when that reaches its setpoint will it bring in the next unit. This ordered sequence prevents inrush, protects equipment, and maintains stable process conditions.