Handbook of air conditioning and refrigeration / Shan K

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ENERGY MANAGEMENT AND CONTROL SYSTEMS 5.39
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Unit controller
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programmable to various degrees. Sometimes the manufacturer provides a variety of preprogrammed
control sequences, such as monitoring, and diagnostics, and designers can specify the
required control sequence that best fits their designs.
2. System/building level. This level is controlled by system controllers. Since a system controller
has an onboard capacity, programmed by an operator or factory-preprogrammed software, to
execute complicated HVAC&R and other programs, they are the brain of an EMCS. Generic control
software such as scheduling, trending, alarming, diagnostics, and security is also provided in system
controllers. Generic control is covered in detail in a later section.
A system controller is used to coordinate the control operations of an HVAC&R system, such as
the coordination between the duct static pressure and the total air volume of VAV boxes in an air
system, or the sequencing of three centrifugal chillers and cooling towers in a refrigeration plant. A
system controller may interface with sensors/transmitters by means of input/output (I/O) connections
directly. Unit controllers are also configured on a separate subnetwork and connected to a
system controller.
Operator Personal Computer (PC) Workstation. The operator may interface with the EMCS
primary through an operator’s PC workstation or purpose-built device, either handheld or fixed to
the system controller. Each workstation shall consist of a PC, autodial telephone modems, and
printers. The central processing unit (CPU) shall be a minimum of an Intel 80486 and operated at a
minimum of 33 MHz. Its memory includes a minimum of 8 Mbytes RAM and 212 Mbytes hard
disk. The communication ports connected to system controllers and other control systems should be
provided. A 14-in. (356-mm) color monitor also shall be provided.
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FIGURE 5.22 System architecture of a typical large EMCS.
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