Chapter hvac engineering Fundamentals: Part 1 Introduction

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Chapter Eight
Figure 8.2
Two-position control.
Figure 8.3
Modulating control.
If the output can cause the controlled device to assume any position
in its range of operation, then the system is said to
modulate
(Fig.
8.3). In modulating control, the differential is replaced by a
throttling
range
(sometimes called a
proportional band
), which is the range of
controller output necessary to drive the controlled device through its
full cycle (open to closed, or full speed to off).
Modulating controllers may use one mode or a combination of three
modes: proportional, integral, or derivative.
Proportional control is common in older pneumatic control systems.
This mode may be described mathematically by
Design Procedures: Part 6
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