Environmental Engineering Reference
In-Depth Information
trol. This technology was used in industrial process control and even for
some building applications as early as the 1950s, but it was not until much
later that it became an acceptable technique for heating and cooling sys-
tems.
DDC is a closed loop control process that is implemented by a digital
computer. In closed loop control, a condition is controlled by sensing the
status of the condition, taking control action to ensure that the condition re-
mains in the desired range and then monitoring that condition to evaluate
if the control action was successful.
Proportional zone control is a type of temperature control. First, the
zone temperature is sensed and compared to a setpoint. When the tempera-
ture is not at the setpoint, a control action is taken to add heat or cooling to
the zone. Then, the temperature is sensed again for a new control cycle.
The control may go beyond basic proportional temperature control
and to integral or derivative control. In this case, the integral or derivative
is used to calculate the amount that the temperature is from the setpoint.
The control action is now limited to avoid overshooting the setpoint and
the oscillations that cause delays in control response. These delays can of-
ten occur with proportional control. Derivative control is often used in dy-
namic applications such as pressure control. Derivative control will mea-
sure the change of speed in the controlled condition and adjust the action
of the control algorithm to respond to this change. The use of a combined
Proportional, Integral and Derivative (PID) control loop allows the control
variable to be accurately maintained at the desired levels with very little
deviation. A combined sequence like PID can be used to integrate the con-
trol of several pieces of heating and cooling equipment to provide a more
efficient and seamless operation. Combining this type of more accurate
control with networking has been an important advance in building con-
trol.
In the mid-1980s when there was no shortage of oil, the absence of a
national energy policy resulted in a drop in the demand for energy man-
agement systems. The slower but continuous growth of these systems led
to an awareness of the benefits of computerized control. Real energy cost
reductions were noted as well as the other benefits of improved control.
These benefits include longer equipment life, more effective comfort levels
and expanded building information. The use of heating and cooling con-
trols are driven by higher energy costs and potential energy crises. These
also force a return to growth in the use of Demand Side Management. The
growing requirements of indoor air quality and related environmental re-
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