Chapter hvac engineering Fundamentals: Part 1 Introduction

224
Chapter Eight
Figure 8.1
Elementary control loop.
8.2
Control Fundamentals
All control systems operate in accordance with a few basic principles.
These must be understood as background to the study of control de-
vices and system applications.
8.2.1
Control loops
Figure 8.1 illustrates a basic control loop as applied to a heating sit-
uation. The essential elements of the loop are a
sensor,
a
controller,
and a
controlled device.
The purpose of the system is to maintain the
controlled variable
at some desired value, called the
set point.
The
process plant
is controlled to provide the heat energy necessary to
accomplish this. In the figure, the process plant includes the air-
handling system and heating coil, the controlled variable is the tem-
perature of the supply air, and the controlled device is the valve which
controls the flow of heat energy to the coil. The sensor measures the
air temperature and sends this information to the controller. In the
controller, the measured temperature
T
m
is compared with the set
point
T
s
.
The difference between the two is the
error signal.
The con-
troller uses the error, together with one or more gain constants, to
generate an output signal that is sent to the controlled device, which
Design Procedures: Part 6
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.

---