Thermostats; Pressure Reducing Valves; Single-Pressure Reducing Valve; Two-Pressure Reducing Valve - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

The oil coalescing filter continues to coalesce and drain off
accumulated oil until solid particles plug the filter. An increase
in pressure drop across the filter (to approximately 70 kPa)
indicates that the filter element needs replacement. For very
dirty air, a 0.005 millimeter prefilter filters out large particles
and increases the life of the final filter element.

PRESSURE REDUCING VALVES

A pressure reducing valve station can have a single-pressure
reducing valve or a two-pressure reducing valve, depending on
the requirements of the system it is supplying.

Single-Pressure Reducing Valve

After it passes though the filter, air enters the PRV (Fig. 11).
Inlet pressure ranges from 415 to 1035 kPa, depending on tank
pressures maintained by the compressor. Outlet pressure is
adjustable from 0 to 175 kPa, depending on the control air
requirements. The normal setting is 140 kPa.
A safety relief valve is built into some PRV assemblies to
protect control system devices if the PRV malfunctions. The
valve is typically set to relieve downstream pressures above
165 kPa.

THERMOSTATS

Thermostats are of four basic types:
— A low-capacity, single-temperature thermostat is the basic
nozzle-flapper bleed-type control described earlier. It is
a bleed, one-pipe, proportional thermostat that is either
direct or reverse acting.
— A high-capacity, single-temperature thermostat is a low-
capacity thermostat with a capacity amplifier added. It is
a pilot-bleed, two-pipe, proportioning thermostat that is
either direct or reverse acting.
— A dual-temperature thermostat typically provides
occupied/unoccupied control. It is essentially two
thermostats in one housing, each having its own bimetal
sensing element and setpoint adjustment. A valve unit
controlled by mainline pressure switches between the
occupied and unoccupied mode. A manual override lever
allows an occupant to change the thermostat operation
from unoccupied operation to occupied operation.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
PNEUMATIC CONTROL FUNDAMENTALS

Two-Pressure Reducing Valve

A two-pressure reducing valve is typically set to pass 90 or
124 kPa to the control system, as switched by a pilot pressure.
The two-pressure reducing valve is the same as the single-
pressure reducing valve with the addition of a switchover
diaphragm and switchover inlet to accept the switchover
pressure signal. Switchover to the higher setting occurs when
the inlet admits main air into the switchover chamber.
Exhausting the switchover chamber returns the valve to the
lower setting.
The switchover signal is typically provided by an E/P relay
or a two-position diverting switch. An automatic time clock
can operate an E/P relay to switch the main pressure for a
day/night control system. A diverting switch is often used to
manually switch a heating/cooling system.
In many applications requiring two-pressure reducing
valves, a single-pressure reducing valve is also required to
supply single-pressure controllers which do not perform well
at low pressures. Higher dual pressure systems operating at
140 and 170 kPa are sometimes used to eliminate the need
and expense of the second PRV.
— A dual-acting (heating/cooling) thermostat is another
two-pipe, proportioning thermostat that has two bimetal
sensing elements. One element is direct acting for heating
control, and the other, reverse acting for cooling control.
Switchover is the same as for the dual-temperature
thermostat but without manual override.
Other thermostats are available for specific uses. Energy con-
servation thermostats limit setpoint adjustments to reasonable
minimums and maximums. Zero energy band thermostats
provide an adjustable deadband between heating and cooling
operations.
The thermostat provides a branchline air pressure that is a
function of the ambient temperature of the controlled space
and the setpoint and throttling range settings. The throttling
range setting and the setpoint determine the span and operating
range of the thermostat. The nozzle-flapper-bimetal assembly
69