Handbook of air conditioning and refrigeration / Shan K

Generic Controls
ENERGY MANAGEMENT AND CONTROL SYSTEMS 5.59
Generic controls are usually needed by most of the systems, units (equipment), and components.
The software of generic controls is usually provided in the PC workstation and in the system controllers
except system graphical displays. The editing of the generic software usually takes place in
the PC workstation and can be downloaded to any one of the system controllers. In many EMCSs,
data and information can be monitored and collected from any point in the system and analyzed in
the working station PC and in the system controllers. Graphics displays, trending, totalization,
scheduling, alarming, etc., are examples of the generic controls. The following are features provided
by various manufacturers (or vendors).
Graphical Displays. The graphical displays provided in the PC workstation become the
showpiece of an EMCS. Some EMCS manufacturers demand that the software in the operator PC
workstation be graphically oriented. The graphical displays include the following:
● Building floor plan graphics show the selected floor plan and the space temperatures.
● Equipment graphics are provided for each major piece of equipment such as packaged unit, airhandling
unit, and chiller with status of all points.
● Schematic graphics show the detailed system drawings which can be created, modified, and saved.
● High-resolution digitized photo-quality displays are also accommodated in some EMCSs.
The graphical display system can allow a display of up to 10 graphical screens for comparison and
monitoring of system status. Graphics combined with color coding, such as temperature that is assigned
with different colors at various values, will increase viewer’s effectiveness. A graphical display
system should provide navigation from a facility map down to a specific floor plan. Associated
temperature and related parameters are provided on the floor plan. An additional “click” of the
mouse on any questionable area will cause the serving HVAC&R system with its operating information
to appear on the screen.
Speed and convenience of editing are two key issues in graphical displays. If a 486 processor is
used, graphics may take 5 to 7 s to paint. Including the navigating through 5 to 10 screens in search
of a problem, too much time will be needed. Because of the use of the Pentium chip, it is possible
to reduce the graphical response to 2 to 3 s per screen paint. The operator must create, modify, or
edit the graphics easily.
Graphical displays play a small role in increasing the effectiveness of any functional control in
an EMCS. However, they enable the user to interact more efficiently with the data and information
from the EMCS and to make proper decisions to operate the EMCS and the associated HVAC&R
system effectively and efficiently.
Trending. Trending is the ability to provide continuous track of certain parameter(s), or operating
status of a piece of equipment. The commonly used time interval for the trend log is once for every
30 min. Trending is mostly used for troubleshooting. For an example, if the output capacity of a heat
exchanger is gradually reduced, scale may form on the heat-exchange surface. Totalization is another
trend log which records the total accumulated operated time for each of the units in a plant employing
multiple units. Totalization is usually helpful for maintenance and troubleshooting purposes.
The trend graph displays the trend data in graphical form. When the user asks for a trend graph,
select the point(s). The Y axis of the graph is often automatically scaled; the X axis to indicate time
is appropriately labeled. The user can also choose the capacity of trend points, time interval between
two trend data other than the standard 30 min, and selection of both changes of value (COV)
that exceed the defined value and the timed trend. The trend graph is a powerful tool. It is very
helpful for tuning of PID control loops and analyzing operating problems.
Scheduling. Prior to the DDC systems, the use of a time clock to schedule the start and stop of
equipment automatically was most widely adopted. A DDC system usually starts and stops the