What is the primary difference between a two-way and a three-way control valve, and where are they typically used?

The primary difference between a two-way and a three-way control valve is their function in controlling fluid flow within a system. A two-way control valve has one inlet and one outlet, and it simply modulates the flow rate of fluid through it. It acts like a variable resistor, increasing or decreasing the flow depending on the control signal. A three-way control valve, on the other hand, has one inlet and two outlets (diverting) or two inlets and one outlet (mixing), and it can either divert the flow of fluid between two different paths or mix two different fluid streams into a single stream. Two-way valves are typically used in applications where the flow rate needs to be controlled to maintain a desired temperature or pressure, such as in a heating or cooling coil serving a specific zone. They are commonly used in terminal units like fan coil units or VAV boxes. Three-way valves are typically used in applications where the flow needs to be diverted or mixed, such as in a bypass loop or in a mixing application to control the temperature of a fluid stream. For example, a three-way diverting valve might be used to bypass a portion of the chilled water flow around a cooling coil when the cooling load is low, or a three-way mixing valve might be used to mix hot and cold water to maintain a desired supply water temperature.