CHAPTER 9 Boiler Combustion Controls

Question 1

Thermocouple

Answer 1

Consists of two dissimilar metals welded together at one end that connect to a electric circuit at a cold junction. The temp diff from the pilot to the cold junction is enough to send a small electric signal that goes into a transformer to boost the signal and keep a fuel valve open so the burner can light. You can also use a number of thermocouple in series to boost the current instead of using a transformer this is called a thermo pile.

Question 2

Flame rod

Answer 2

A flame rod uses the principle that fire can conduct electricity. The rod is put into the pilot fire and uses the rod as one electrode the burner as the second and the flame as the electric medium. As long as the flame is lit the circuit is closed and the current runs to a relay that boost the current to keep a valve open. Used only in gas burners if used with oil the carbon deposits will affect the operation of the rod

Question 3

Photo electric cell (flame scanner)

Answer 3

Infrared- contains a lead sulfide cell that reacts to infrared rays by decreasing its electrical resistance with the more infrared rays it sees. When the resistance decrease it allows current to flow through it enough to keep the main fuel valve open. The scanner must be kept far enough away that it doesn’t get hotter then 50C some are used to sense both the pilot and main flame

Question 4

Ultraviolet scanner ( also a photoelectric cell)

Answer 4

Works the same as the infrared but sense UV light instead and is mounted as close to the flame as possible keeping it under 100C care needs to be taken to ensure it can’t see the spark that lights the pilot as it will react to that as well.

Question 5

Maintenance of flame safeguard

Answer 5

The scanner should never get so hot you can’t hold it In your hand. The scanner lens should be cleaned at regular intervals. The scanner sight tube should not be obstructed and flame rods should be checks regular for build up or deposits that will affect there operation no maintenance other then blowing out the dust in the control box is needed on the programming controls