Chapter 5 - Self- and System-powered Controls

Abstract

This chapter explains how self-powered and system-powered control devices work and where they are commonly used. Self-powered and system-powered controls are those that do not require an external power source such as electricity or pneumatic control air. Self-powered controls are those that draw the energy needed for their operation from the systems that they control or operate. They are commonly used on small systems or individual units where they are more convenient and less costly because they do not require an external power source such as electricity or pneumatic control air. The power source in self-powered devices is typically derived from: electrical potential generated by a thermopile; pressure resulting from expansion and contraction of a temperature-sensitive substance; pressure from the fluid being controlled. In addition system-powered control devices use supply air pressure as the power source. They are a variation on self-powered controls used on early constant air volume (CAV) regulators and variable air volume boxes and diffusers. They are famous because they do not require any external power source; system-powered controls often require that duct static pressure be relatively.