Thermostats - Honeywell AUTOMATIC CONTROL Engineering Manual

in pressure drop across the filter (to approximately 10 psi)
indicates that the filter element needs replacement. For very
dirty air, a 5-micron 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 60 to 150 psi, depending on
tank pressures maintained by the compressor. Outlet pressure
is adjustable from 0 to 25 psi, depending on the control air
requirements. The normal setting is 20 psi.
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
24 psi.

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.
PNEUMATIC CONTROL FUNDAMENTALS
Two-Pressure Reducing Valve
A two-pressure reducing valve is typically set to pass 13 or
18 psi 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
20 and 25 psi 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
maintains a fixed branchline pressure for each temperature
within the throttling range (Fig. 17). The forces within the
nozzle-flapper-bimetal assembly always seek a balanced
condition against the nozzle pressure. If the setpoint is changed,
the forces in the lever system are unbalanced and the room
ambient temperature must change in a direction to cause the
bimetal to rebalance the lever system.
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ENGINEERING MANUAL OF AUTOMATIC CONTROL