Consulting-Specifying Engineer - February 2008 - (Page 21)

Cover Story 2. List the nature of the control signal required for each of these devices 3. For each controlled device, select the process variable 4. Define the relationship between the process variable and the controlled device 5. Identify the various parameters that are required for this relationship 6. Define the control logic required to produce the parameters 7. Determine discrete or analog conditions that apply to each process 8. Define the logic or write the sequences that describe the discrete and analog conditions necessary. Identify the devices to be controlled in each system. First, look at one building system holistically. For example, an airhandling unit may consist of the follow- ing devices to be controlled, as shown in Figure 1: • Fan start/stop & fan variable speed drive • Mixed air dampers • Cooling coil valve • Heating coil valve • Humidifier. For each of these controlled devices, the next seven steps need to be considered. List the nature of the control signal required for each of these devices. For this air handler, this list of required control signals would be: • Fan start/stop: digital output (DO) point for the fan starter circuit • VSD: analog output (AO) point • Mixed air dampers: AO point or points • Coiling coil valve: AO point • Heating coil valve: AO point. Select the process variable (Pv) for each of these control loops. For our example this list of process variables might look like this: • Fan start/stop: Pv = time, internal time schedule • VSD: Pv = static pressure, duct sensor (AI) point • Mixed air dampers: Pv = temperature, mixed air temperature sensor (AI) point • Coiling coil valve: Pv = temperature, discharge air temperature sensor (AI) point • Heating coil valve: Pv = temperature, discharge air temperature sensor (AI) point. Supply fan Cooling Heating Mixed Air Humidification Figure 1: Logic diagram for five devices controlled within an air handling unit. The cooling application shows an example of output variable from one controller provides input to another controller. In this case, discharge temperature setpoint shall be reset between 62 and 55 F as return air temperature rises from 65 to 70 F. Source: Facility Dynamics Consulting-Specifying Engineer • FEBRUARY 2008 21

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Consulting-Specifying Engineer - February 2008 Contents Viewpoint News M/E Roundtable How To Write Control Sequences Mentoring Engineers: Myths, Motivations, and Models Keep Young Electrical Engineers Grounded Protecting a Vulnerable Population Codes & Standards Case Study New Products Equipment Lifecycles Advertisers Index Green Space

Consulting-Specifying Engineer - February 2008

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