Handbook of air conditioning and refrigeration / Shan K

Control Methods
ENERGY MANAGEMENT AND CONTROL SYSTEMS 5.7
According to the types of control signal and the different kinds of energy used to transmit the
signals, as well as whether a software is used during control operation, control methods can be
classified as direct digital, pneumatic, electric, and electronic.
Analog and Digital. There are two types of control signals: analog and digital. An analog signal
is in the form of a continuous variable. It often uses the magnitude of electric voltage or pneumatic
pressure to represent the air temperature. A digital signal is a series of on and off pulses used to
transmit information.
A conventional analog controller receives a continuous analog signal, such as a voltage or a
pneumatic signal, that is proportional to the magnitude of the sensed variable. The controller compares
the signal received from the sensor to the desired value (i.e., the set point) and sends a signal
to the actuator in proportion to the difference between the sensed value and the set point.
A digital controller, or microprocessor-based controller, receives an electric signal from
sensor(s). It converts the electric signal to digital pulses of different time intervals to represent the
signals values. The microprocessor of the digital controller performs the mathematical operations
and knowledge processing on these values. The output from the microprocessor can be either in
digital form to actuate relays or converted to an analog signal (say, a voltage or a pneumatic pressure)
to operate the actuator(s).
Direct Digital Control (DDC). A control system using DDC involves adopting a microprocessorbased
digital controller to perform mathematical operations and knowledge processing according to
the predetermined control algorithms or computer programs. The key element of DDC compared to
analog control is the software and hardware contained in the direct digital controller which expands
the control functions tremendously and adopts recently developed control logic. ADDC unit usually
has more precise sensors and uses the same type of controlled devices as other control methods.
Figure 1.2 shows an energy management and control system using DDC for an air-handling unit
in a typical floor of the NBC Tower, and Fig. 5.2 shows an EMS with DDC for a single-zone VAV
system.
Pneumatic Control. In a control system using pneumatic control, compressed air is used to operate
the sensors, controllers, and actuators and to transmit the signals. It consists of: a compressed
air supply and distribution system, sensors, controllers, and actuators. Figure 5.4 shows a typical
pneumatic control system. In Fig. 5.4, a filter is used to remove the dust particles, including submicrometer-size
particles, contained in the air. The function of the pressure-reducing valve is to reduce
the pressure of compressed air discharged from the air compressor to the required value in the
main supply line. The discharged compressed air is usually at a gauge pressure of 18 to 25 psig
(124 to 172 kPa�g), and the pressure signal required to actuate the valve or damper actuator is 3 to
13 psig (20.7 to 89.6 kPa�g).
The advantages of pneumatic control are as follows:
● The compressed air itself is inherently a proportional control signal.
● The cost of modulating actuators is low, especially for large valves and dampers.
● Pneumatic controls require less maintenance and have fewer problems.
● Pneumatic controls are explosionproof.
The disadvantages stem mainly from comparatively fewer control functions, the high cost of
sophisticated pneumatic controllers, and the comparatively higher first cost of a clean and dry,
compressed air supply for small projects.
Electric or Electronic Control. Both types of control use electric energy as the energy source for
the sensors and controllers. A control system using electric control often offers two-position on-off
control, as shown in Fig. 5.1. Switches, relays, contactors, and electromechanical devices are