Three-Way Valves; Mixing Valve - Honeywell AUTOMATIC CONTROL Engineering Manual

VALVE SELECTION AND SIZING
— Graph C shows the relationship between heat output and
stem travel for the combined coil and equal percentage
valve. The combined relationship is close to linear. A 10
percent reduction in heat output requires the stem to close
10 percent, a 50 percent reduction in heat output requires
100%
90%
50%
0% 10%
FLOW
GRAPH A
Fig. 11. Heat Output, Flow, and Stem Travel Characteristics of an Equal Percentage Valve.

THREE-WAY VALVES

Three-way valves (Fig. 12) control the flow of liquids in
mixing or diverting valve applications (Fig. 13). The internal
design of a three-way globe valve enables it to seat against the
flow of liquid in the different applications. An arrow cast on
the valve body indicates the proper direction of liquid flow. It
is important to connect three-way valve piping correctly or
oscillations, noise, and excessive valve wear can result. Three-
way valves are typically have linear flow characteristics,
although, some are equal percentage for flow through the coil
with linear flow characteristics for flow through the coil bypass.
Ball valves are also available in a three-way configuration, while
two butterfly valves can be made to act as a three-way valve.
MIXING
VALVE
IN OUT
I
N
Fig. 12. Three-Way Valves.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
100%
50%
10%
50% 100% 0%
DIVERTING
VALVE
IN OUT
O
U
T
C2334
the stem to close 50 percent, and a 90 percent reduction
in heat output requires the stem to close 90 percent.
The equal percentage valve compensates for the
characteristics of a hot water application to provide a control
that is close to linear.
100%
90%
50%
10%
50% 90% 100% 0%
STEM TRAVEL
GRAPH B
HOT
WATER
SUPPLY
A. LOAD BYPASS IN MIXING VALVE APPLICATION
SUPPLY
B. LOAD BYPASS IN DIVERTING VALVE APPLICATION
Fig. 13. Three-Way Valve Applications.

MIXING VALVE

A mixing valve provides two inlet ports and one common
outlet port. The valve receives liquids to be mixed from the
inlet ports and discharges the liquid through the outlet port
(Fig. 12). The position of the valve disc determines the mixing
proportions of the liquids from the inlet ports.
The close-off pressure in a mixing valve equals the maximum
value of the greater inlet pressure minus the minimum value of
the downstream pressure.
EXAMPLE:
A mixing valve application has a maximum pressure of
25 psi on one inlet port, maximum pressure of 20 psi on
the other inlet port, and minimum downstream pressure of
10 psi on the outlet port. The close-off pressure is 25 psi –
10 psi = 15 psi. The application requires a mixing valve
with at least a 15 psi close-off rating. The actuator selected
must have a high enough force to operate satisfactorily.
440
10% 50% 90% 100%
STEM TRAVEL
GRAPH C
C2333
THREE-WAY
MIXING VALVE
LOAD
HOT
WATER
RETURN
BYPASS
THREE-WAY
DIVERTING VALVE
RETURN
LOAD
BYPASS
C2335A