AERCO SWDW68 U-Tube Double-Wall Heaters w/ECS - Manual (Page 26)

HE–
111− FUNCTIONAL DESCRIPTION
Rev B Mar 2013 3-6
3.3.4 Principal ECS Components
3.3.4.1 Control Box Assembly
The Control Box Assembly houses the components that monitor heat exchanger temperatures and
operate system controls. The Control Box is illustrated in Figure 3-6. The Control Box components include
the Temperature Controller, Over-temperature Switch and dc Power Supply/Voltage Regulator (not
shown).
TEMP CONTROLLER
HOLD
RUN AUTO
MAN
2408
PT.NO.
OVER TEMP SWITCH
PANEL
DOOR
TEMPERATURE
CONTROLLER
OVER-TEMPERATURE
SWITCH
Figure 3-6. Control Box Front View
Temperature Controller. The Temperature Controller processes data received from the
temperature and flow sensors. Using the PID algorithm, the Temperature Controller calculates the
optimum steam flow rate to maintain the Hot Water Output temperature within the programmed
range. The Temperature Controller translates the optimum flow rate into a 4-to-20 mA control
signal to the CXT-E Control Valve Actuator to adjust the Control Valve opening.
Over-temperature Switch. The Over-temperature Switch compares the output hot water
temperature to the programmed high temperature limit, typically 20°F above the heater setpoint. If
the measured Hot Water Output temperature exceeds the maximum setting, the Over-temperature
Switch sounds an audible alarm and activates the Over-temperature Solenoid Valve, causing the
overheated water to be dumped from the heater shell. Simultaneously, the Over-temperature
Switch cuts off dc power to the CXT-E Control Valve Actuator, shutting off the flow of steam into
the heat exchanger.
DC Power Supply/Voltage Regulator. The dc power supply within the Control Box converts the
120 VAC primary power to 12 VDC and 24 VDC to operate the Control Box components, Over-
temperature Switch, Over-temperature Solenoid Valve and the CXT-E Actuator.
3.3.4.2 DHW Outlet Dual Temperature Sensor
The DHW outlet dual temperature sensor consists of dual thermocouple sensing elements mounted in the
top head. The DHW outlet dual temperature sensor continuously monitors the service water temperature
at the Hot Water Output and transmits it simultaneously to both the Temperature Controller and the Over-
temperature Switch
in the Control Box.
3.3.4.3 Feed-Forward Mixture Temperature Sensor
The feed-forward mixture temperature sensor is installed in the Cold Water Inlet nozzle to measure the
mixture temperature of the combined input cold water and the recirculated sample of the Hot Water
Output.