6.1 Introduction

All air-handling units (AHUs) and many terminal units, if they are not self-contained, require a source of heating and/or cooling energy. This source is called a central plant, and the means by which thermal energy is transferred between the central plant and the AHU is usually a fluid conveyed through a piping system. The fluids used in HVAC practice are steam, hot or cold water, brine, refrigerant, or a combination of these. The equipment used to generate the thermal energy is described in Chap. 7. In this chapter we discuss the transport systems.

6.2 Steam

Steam is water in vapor form. Because it expands to fill the piping system, steam requires no pumping except for condensate return and boiler feed. The specific heat of water vapor is quite low, but the latent heat of vaporization is high. As a result, steam conveys heat very efficiently.

Steam may be used directly at the AHU (in steam-to-air, finned-tube coils), or a steam-to-water heat exchanger may be used to provide the hot water used in AHU coils or in radiation. Steam radiation is also employed. When used directly, steam pressures are usually 15 lb/in ² gauge or less. When used with a heat exchanger, steam pressures up to 100 lb/in ² gauge are common. Higher pressures allow smaller piping but create piping expansion and support problems. In- dustrial plants often use high-pressure steam for heating as well as for process purposes.