Handbook of air conditioning and refrigeration / Shan K

where K is the derivative gain. The effect of adding the derivative function K de/dt is that the
quicker the control point changes, the greater the corrective action provided by the derivative function.
Figure 5.9b shows the variation of the controlled variable for a PID control mode. As with a PI
control mode, PID control mode also has no offset once the controlled variable has reached a stable
condition, except due to instrument inaccuracy.
Compared to the PI control mode, the PID control mode, which is a combination of proportional,
integral, and derivative actions, exhibits faster corrective action and a smaller overshoot and
undershoot following an offset and a change of the controlled variable. The controlled variable is
brought to the required set point in a shorter time. However, it is more difficult to determine properly
three constants, or gains (K p, K i, and K).
Compensation Control, or Reset
Compensation control, or reset, is a type of control mode in which a compensation sensor is generally
used to reset a main sensor to compensate for a variable change sensed by the compensation
sensor. The purpose is to achieve operation that is more effective, energy-efficient, or both.
In the design of a reset mode, the first things to decide are the control point at which the main
sensor will be reset and the variable to be sensed by the compensation sensor. The main sensor,
which senses the mixed air temperature in the air-handling unit, is usually reset by a compensation
sensor that senses the outdoor temperature, as shown in Fig. 5.10.
Another task to decide on is the relationship between the variables sensed by the main and compensation
sensors—the reset schedule. Many reset schedules have a different relationship between
these two sensors at various stages. For example, in Fig. 5.10, when the outdoor temperature T o
is less than 30°F (� 1.1°C), it is within the range of stage I. No matter what the magnitude of T o is,
the set point of the mixed temperature T m is 65°F (18.3°C). When 30°F � T o � 95°F (�1.1°C �
T o � 35°C), it is in stage II. The linear relationship between the temperatures sensed by the main
and compensation sensors in Fig. 5.10 can be expressed by the following equation:
65 � 55
Tm � 65 � (5.6)
95 � 30
When To � 95°F (35°C), the set point of the mixed temperature is always 55°F (12.8°C).
Compensation control modes have been widely adopted to reset space temperature, discharged
air temperatures from air-handling units or packaged units, or water discharge temperatures from
central plants.
(To � 30)
Applications of Various Control Modes
Selection of a suitable control mode depends on
● Operating characteristics
ENERGY MANAGEMENT AND CONTROL SYSTEMS 5.15
● Process or system characteristics, such as whether the thermal capacitance should be taken into
consideration
● Characteristics of load changes
If a simpler control mode can meet the requirements (say, two-position control versus PID control),
the simpler control mode is always the first choice. Except for two-position and step control, all the
other control modes are modulation controls. Modulation control is a control mode that is capable
of increasing or decreasing a variable according to the deviation from the required value in small
increments continuously. Also note that there is a significant difference between the two-position
control, step control, floating control, and proportional control modes with an offset and the offset
free PI and PID controls.