Chapter 10 - HVAC&R Controls

Abstract

This chapter outlines the fundamentals of controls and illustrates principles for providing reliable and easily maintainable control systems for a variety of air-conditioning systems. Such control is required to match system operation with the need for space conditioning. The simplest control is an ON/OFF operation typical of residential applications where, as a thermostat senses an increase in room temperature, the air conditioner is switched ON, and as the temperature falls, it is switched OFF. Most of large HVAC&R systems, however, use modulating controls, i.e., controls that vary the output of the equipment being controlled, to provide for the control of temperature, humidity, and air quality in various spaces in a building. Controls provide the critical link between HVAC design intent, HVAC&R equipment and systems, and the operating building. Controls are an integral part of the design of an HVAC&R system. They implement design intent by operating the HVAC equipment as required. Controls are instrumental in obtaining an energy-efficient building. To provide a complete system solution, an HVAC&R designer must consider how controls will make all parts of the system function. Three types of control hardware are used in HVAC&R systems—pneumatic, analog electronic, and direct digital control. Historically, pneumatic controls were used in most large building applications. They are reasonably well understood by designers and maintenance personnel, which was a compelling reason for their use. A second advantage was that pneumatic valve and damper actuators, besides being inherently modulating, are inexpensive and reliable when compared to the electric motors and gears required to produce the same control force with electric actuation. Most pneumatic control equipment, however, requires a very clean source of supply air.