Handbook of air conditioning and refrigeration / Shan K

Types of Control Valves
to the electric motor to drive the valve to its open or closed position, depending on whether it is
a normally open or closed valve.
It may take minutes to fully open a large valve using an electric motor valve actuator. Modern
electronic actuators use solid-state control boards to determine the speed, the action, and other
functions to meet more demanding requirements.
Pneumatic Actuators. A pneumatic valve actuator consists of an actuator chamber whose bottom
is made of a flexible diaphragm or bellows connected with the valve stem. When the air pressure in
the actuator chamber increases, the downward force overcomes the spring compression and pushes
the diaphragm downward, closing the valve. As the air pressure in the actuator chamber decreases,
the spring compresses the diaphragm, moving the valve stem and valve upward. A pneumatic valve
actuator is powerful, simple, and fast to respond.
Because of the increasing popularity of the DDC systems, there is an increasing demand for
electric actuators that can be interfaced with a DDC system.
Water control valves consist mainly of a valve body, one or two valve disks or plugs, one or two
valve seats, a valve stem, and a seal packing. Based on their structure, water control valves can be
classified into the following types:
1. Single-seated. A single-seated valve has only a single valve disk and seat, as shown in Fig. 5.15b
and c. It is usually used for water systems that need a tight shutoff.
2. Double-seated. A double-seated valve has two valve disks connected to the same valve
stem and is designed so that the fluid pressure exerted on the valve disks is always balanced.
Consequently, less force is required for the operation of a double-seated valve, as shown in
Fig. 5.15a.
3. Butterfly. A butterfly valve consists of a cylindrical body, a shaft, and a disk that rotates on an
axis, as shown in Fig. 5.15d. When the valve closes, it seats against a ring inside the body. A
butterfly valve exhibits low flow resistance when it is fully opened. It is compact and is usually
used in large water pipes.
According to the pattern of the water flow, water control valves can again be classified as twoway
valves or three-way valves. A two-way valve has one inlet port and one outlet port. Water flows
straight through the two-way valve along a single passage, as shown in Fig. 5.15a.
In a three-way valve, there are three ports: two inlet ports and one common outlet port for a
three-way mixing valve, as shown in Fig. 5.15b, and one common inlet port and two outlet ports for
a three-way diverting valve, as shown in Fig. 5.15c. In a three-way mixing valve, the main water
stream flows through the coil or boiler, and the bypass stream mixes with the main stream in the
common mixing outlet port. In a three-way diverting valve, the supply water stream divides into
two streams in the common inlet port. The main water stream flows through the coil, and the bypass
stream mixes with the main water stream after the coil. A three-way mixing valve is always located
downstream of the coil. But a diverting valve is always located upstream of the coil.
A diverting valve should never be used as a mixing valve. The unbalance pressure difference between
the two inlet ports and the outlet port at a closed position may cause disk bouncing and valve
wear when the valve disk travels between the two extremes.
Valve Characteristics and Ratings
ENERGY MANAGEMENT AND CONTROL SYSTEMS 5.27
The different types of control valves and the characteristics that are important during the selection
of these valves are as follows: