Cleaver-Brooks CBH 40 - TEMPERATURE CONTROLS- Hot Water Boiler

of steam pressure. In any case, the control should
be adjusted so that it de-energizes the damper
motor, causing burner to go to low fire, at a
pressure below the limit control opening point.
When the pressure controls are properly set the
burner will maintain steam pressure within
narrow limits.
On a rise in steam pressure, the high-low fire con-
trol opens its contacts and the damper motor
returns to low fire position. The burner continues
to operate at low fire rate. If steam pressure drops,
the control will again close to energize the damper
motor and return the burner to high fire. If steam
pressure continues to rise, the boiler will remain at
low fire rate. Should the boiler pressure reach the
set point of the operating limit control, its contacts
will open to turn off the burner. When boiler
pressure drops, the operating limit control contacts
close, causing the burner to restart. It is desirable
to have the high-low fire control adjusted so that
the burner does not immediately go to high fire
upon start, but rather operates at low for a brief
period before decreasing steam pressure causes
control to close and the burner to drive to high fire.
OPERATING LIMIT PRESSURE CONTROL
(Steam) (Honeywell L404A)
Set "cut-out" (burner off) pressure on the main
scale using the large adjusting screw. Set differ-
ential on the short scale turning the small adjust-
ing screw until the indicator points to the desired
difference between cut-out and cut-in pressures.
The "cut-in" (burner on) pressure is the cut-out
pressure MINUS the differential. The cut-out
pressure should not exceed 90% of the safety valve
setting.
HIGH LIMIT PRESSURE CONTROL
(Steam) (Honeywell L404C)
Set "cut-out" (burner off) pressure on scale using
adjusting screw. The control will break a circuit to
shut off burner when pressure reaches this point.
The setting should be sufficiently above the
operating limit pressure control to avoid shut-
downs, and preferably not exceed 90% of safety
valve setting. This control requires manual reset-
ting after tripping on a pressure increase. To reset,
allow boiler pressure to return to normal and then
press the reset button.
HIGH-LOW FIRE CONTROL
(Honeywell L404A) (Steam)
Set the "cut-out" pressure-the point at which the
burner will return to low fire-on the main scale
using the large adjusting screw. This setting should
be sufficiently below the cut-out setting of the
operating limit control so that the burner will
return to the low fire position prior to shutting off
at the operating limit.
The "cut-in" pressure-the point at which the bur-
Chapter 5-Adjustment and Maintenance
ner drives to high fire-is set on the differential
scale. This setting is equal to the cut-out pressure
MINUS the amount of the differential. It should
be adjusted so that it is sufficiently below the bur-
ner "on" pressure of the operating control so that
the burner when starting will operate briefly at the
low fire position prior to advancing to high fire.
TEMPERATURE CONTROLS-
Hot Water Boiler
The temperature controls that serve as the
operating limit control and as the high-low fire
control are equipped with an adjustable dif-
ferential setting. The switch in the control opens
when the water temperature reaches a temperature
equal to the dial setting and closes at a tempera-
ture equal to the dial setting MINUS the amount
of differential. See following portion of this section
for instructions on control adjustment.
In an installation that does not require very close
control of the water temperature, the differential
setting should be adjusted as widely as possible
since this will provide less frequent burner cycling.
The operating limit temperature control should be
set slightly above the highest desired water tem-
perature and within the limits of the pressure
vessel. The high limit control should be set 5 to 10
degrees above the operating limit temperature con-
trol setting. The high-low fire control should be ad-
justed so that it is below the burner "on" tempera-
ture of the operating control.
Relative settings of the temperature controls are as
follows:
High Limit Control
Open- 190° F.
Operating Limit Control
Open- 180° F.
Close- 170° F.
High-Low Fire Control
Open- 170° F. (Low Fire)
Close- 165° F. (Hlgh Fire)
With settings similar to these, the following opera-
tional sequence occurs. On a rise in boiler water
temperature, the high-low fire control opens its
contacts at 170 degrees F. to de-energize the dam-
per motor and place the burner in its low fire
range.
If temperature decreases during low fire, the bur-
ner will return to high fire as soon as the high-low
fire control closes at its 165 degree setting As tem-
perature increases during high fire rate the burner
will be switched back to low fire when the control
opens at 170 degrees.
If temperature increases during low fire, the bur-
ner will shut down when the operating limit setting
of 180 degrees is reached. When temperature drops
back to 170 degrees the operating limit will close
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