Safety combustion controls are designed to shut down the burner to prevent flooding the furnace with oil after initial ignition or in the event of ignition failure or flame failure. These controls are either thermostatic (stack switches and pyrostats) or photoelectric.

Stack Switches and Pyrostats

Stack switches and pyrostats consist basically of a bimetallic helix, a mounting frame, a shaft carrying a contact mechanism, and an electric switch. The helix is connected to the mounting frame at one end and to the shaft at the other. Typical construction of a pyrostat is shown in figure 18-18.

Stack switches and pyrostats are mounted on the boiler smoke pipe or hood, with the helix protruding into the path of the combustion gases. The switch is wired into the burner electric circuit so that when the contacts are open, no current is supplied to the burner except through the Protectorelay (to permit initial ignition). When there is no heat in the boiler, the switch contacts

Figure 18-18.-Construction details of a pyrostat.

are open. When the burner is ignited, the increase in the stack temperature causes the helix to expand, and this expansion in turn causes the shaft to rotate and close the contacts. Unless the stack temperature decreases, the contacts will remain closed, permitting the operation of the burner. In the event of flame failure, the helix contracts as the stack temperature falls and causes the shaft to rotate in the reverse direction. This opens the contacts and shuts down the burner.

Photoelectric Safety Combustion Controls

Photoelectric safety combustion controls consist basically of a photoelectric cell, an amplifying unit, and a relay. These controls operate by the luminosity (light) of the flame acting upon the photoelectric cell. The photo-electric cell makes a contact within itself when light rays impinge upon it. The amplified current of the photoelectric cell operates the relay, which is wired into the burner control circuit, causing the relay to close the burner circuit when the flame is established and to break the burner circuit in the event of flame failure.