Handbook of air conditioning and refrigeration / Shan K

5.38 CHAPTER FIVE
FIGURE 5.21 Local loss coefficient C dam of air damper. (Source: ASHRAE Transactions 1988, Part I.
Reprinted by permission.)
5.8 SYSTEM ARCHITECTURE
Architecture of a Typical EMCS with DDC
Figure 5.22 shows the system architecture of a typical energy management and control system with
direct digital control (EMCS with DDC) for a medium or large building.
Operating Levels. Such an EMCS has mainly two operating levels:
1. Unit level. This level is controlled by unit controllers. A unit controller is a small and specialized
direct digital controller which is used to control a specific piece of HVAC&R equipment or device
such as a VAV box, a fan-coil unit, a water-source heat pump, an air-handling unit, a packaged
unit, a chiller, or a boiler. For HVAC&R, most of the control operations are performed at the unit
level. Since the software is often factory-loaded, only the time schedules, set points, and tuning
constants can be changed by the user. Some of the most recently developed unit controllers are also