The focus of this chapter is on the air side of a heat exchanger on the other side of which cold water or cold refrigerant flows. How the cold water or refrigerant is produced is treated in later chapters. Most air-cooling coils consist of tubes with fins attached to the outside of the tubes to increase the area on the air side where the convection coefficient is generally much lower than on the refrigerant or water side. Refrigerant or water flows inside the tubes, and air flows over the outside of the tubes and the fins. When a refrigerant evaporates in the tubes, the coil is called a direct-expansion coil When, on the other hand, a secondary refrigerant, such as chilled water, carries away the heat, this water is chilled by an evaporator in the machine room. A chilled-water coil is shown in site. The air-conditioning systems in many large buildings use central water chillers and distribute chilled water throughout the building.