Handbook of air conditioning and refrigeration / Shan K

5.40 CHAPTER FIVE
System Characteristics
The software in the workstation shall do the following:
● Accommodate processes as well as prioritize applications based on their input/output priority level.
● Provide system graphics including the HVAC&R equipment (such as a display of up to 10
graphic screens at once for comparison) to monitor the operating status of the system. The operator
with the proper password is able to add, delete, or change the set points, time scheduling, etc.
● Support the editing of all system applications including the generic control software provided in
the system controllers. The edited or custom programmed software shall be downloaded and executed
at one or more of the system controllers.
● Automatically save the database and restore the database that has been lost in one of the system
controllers.
● Provide scheduling, trends, totalization, alarm processing, security to view and edit data, and
system diagnostics.
Communication Network. A peer-to-peer data communication of a local-area network (LAN)
adopting either Ethernet or ARCNET will be used between system controllers and between system
controllers and PC workstations (or other system). A peer-to-peer communication means that all
system controllers or work stations have equitable access to communication resources. For the communication
subnetwork between system controllers and unit controllers, and between unit
controllers themselves, a master-slave token-passing (MS/TP) technology is often used. A system
controller also acts as a medium to provide data communication between the work station and the
unit controllers. The network technology is covered in detail in a later section.
Power Source. The temperature sensors and humidity sensors may need up to 12 V dc and
24 V ac as a power source. Many DDC units have a power source of 24 V ac or 120 V ac line voltage.
Most of the valve actuators and damper motors need a power source of 24 V ac.
Size of EMCS. The size of an EMCS depends on the number of points (or objects) that belong to
its DDC units. An EMCS of 100 points or less can be considered a small project. An EMCS that
has 1000 points or more can be considered a large project.
An architecture of EMCS incorporating DDC such as that in Fig. 5.22 has the following characteristics:
1. All DDC units are independent and stand-alone controllers. If any of the controller fails, there
is only a limited effect.
2. The system architecture shows a distributed processing model. Since most of the control operations
are performed at the unit controller level, and partly at the system controller level, such an
architecture has the advantage that it tremendously reduces the data communication between the
unit controllers and the system controller, as well as between the DDC system controller and the PC
workstation.
3. Zimmerman (1996) noted that since the introduction of DDC in the 1980s, “. ..microprocessor
and memory have declined rapidly in cost while wiring and installation costs have not
declined at the same rate.” To provide more powerful unit controllers, moving the controllers nearer
to the sensors and control devices will reduce a lot of wiring and installation costs as well as the
overall system cost.
4. If each HVAC&R piece of equipment has its own unit controller, it surely will be beneficial to
the HVAC&R equipment manufacturers to fabricate controllers and other control system components
themselves.