Chapter hvac engineering Fundamentals: Part 1 Introduction

Design Procedures: Part 6
251
Figure 8.34
Positive positioner.
Pneumatic operators are simple pistons, driven by air pressure to
one extreme and returned by a spring when the pressure is reduced.
There are double-acting pistons driven by air in both directions for
some applications. For accurate operation when modulating action is
required—to overcome hysteresis and friction—a positive or pilot po-
sitioner is recommended (Fig. 8.34). This device is an ‘‘amplifier’’
which uses a small controller output signal change to provide full con-
trol air pressure or exhaust to move the piston. A built-in feedback
signal limits the travel to an amount proportional to the change in
the controller signal.
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.

---