Hesitation Relay; Electrical Interlocking Relays; Electric-Pneumatic Relay; Pneumatic-Electric Relay - Honeywell Pneumatic Controls Engineering Manual

Hesitation Relay

The hesitation relay is used with a pneumatic actuator in unit
ventilator applications. The output pressure goes to minimum
whenever the input pressure is below the minimum setting.
Figure 49 shows a graph of the output of a hesitation relay as
controlled by the relay knob settings (piloted from the
thermostat).
18
16
14
12
10
8
7
6
4
2
0
0 2 4 6 8 10
RELAY INPUT (PSI)
Fig. 49. Hesitation Relay Output Pressure
as a Function of Knob Setting.
The hesitation relay has an internal restrictor. Figure 50 shows
a typical application of a hesitation relay and a pneumatic
damper actuator. When the thermostat branchline pressure
reaches 1.5 psi, the relay output goes to its preset minimum
pressure. When the branchline pressure of the thermostat
reaches the setting of the hesitation relay, the thermostat
controls the damper actuator. When the thermostat branchline
pressure drops below the hesitation relay setting, the relay
holds the damper actuator at the minimum position until the
thermostat branchline pressure drops below 1.5 psi. At that
point, the hesitation relay output falls to zero.
DA ROOM
THERMOSTAT
M B
M
Fig. 50. Typical Hesitation Relay Application.

Electrical Interlocking Relays

Electrical interlocking relays bridge electric and pneumatic
circuits. The electric-pneumatic relay uses electric power to
actuate an air valve in an associated pneumatic circuit. The
pneumatic-electric relay uses control air pressure to make or
break an associated electrical circuit.
100
80
60
40
20
0
KNOB SETTING (%)
12 14 16 18
C1097
HESITATION
RELAY
P
B M
M
DAMPER
ACTUATOR
C2346

Electric-Pneumatic Relay

The electric-pneumatic (E/P) relay is a two-position, three-way
air valve. Depending on the piping connections to the ports, the
relay performs the same functions as a simple diverting relay. A
common application for the E/P relay is to exhaust and close an
outdoor air damper in a fan system when the fan motor is
turned off, as shown in Figure 51.
FAN
INTERLOCK
VOLTAGE
O
M
Fig. 51. E/P Relay Application.
When the relay coil is de-energized, the solenoid spring seats
the plunger. The normally disconnected port (X) is blocked and
the normally connected port (O) connects to the common port
(C). The connection exhausts the damper actuator which closes
the damper. When the relay coil is energized, the plunger lifts
against the tension of the spring and blocks the normally
connected port (O). Main air at the normally disconnected port
(X) connects to the common port (C) and opens the damper.

Pneumatic-Electric Relay

Figure 52 shows a simplified pneumatic-electric (P/E) relay with
a spdt switch. The P/E relay makes the normally closed contact
on a fall in pilot pressure below the setpoint, and makes the
normally open contact on a rise above a value equal to the
setpoint plus the differential. For example, with a setpoint
adjustment of 3 psi and a differential of 2 psi, the pump is
energized at pilot pressures below 3 psi and turns off at pilot
pressures above 5 psi.
TEMPERATURE
CONTROLLER
M B
M
SPRING
SETPOINT
ADJUSTMENT
Fig. 52. P/E Relay Application.
70-6925
Relays and Switches
E/P RELAY
SOLENOID
SPRING
RELAY COIL
PLUNGER
X C
EXH
OUTDOOR
AIR DAMPER
ACTUATOR
C2602
P/E RELAY
BELLOWS
N.O.
N.C.
PUMP
VOLTAGE
C
PUMP
C2384
145