CHAPTER 8 Hearing Boiler Operation

Question 1

Boiler on off control

Answer 1

Burner fires at a constant rate until the upper press is met at which point the burner shuts off this is the simplest control and common on low capacity boilers. Only issue is boiler will not operate at max efficiency unless continuously fired.

Question 2

High low fire control for a burner

Answer 2

Boilers may have a high and low foreign burner that fires depending on steam demand. The boiler should only be started in low fire so a manual high low switch is installed to ensure that the high fire doesn’t kick in when starting a cold boiler. As the boiler runs on low fire if the pressure drops lower then the cut in point of the low fire the high fire burner will start thus raising the pressure to just below the cut out point of the low fire burner.

Question 3

On off control

Answer 3

The pressure at which the switch operates is adjusted by a pressure adjustment spring the use of a second spring for differential control is also installed. This units should be mounted perfectly upright to ensure proper operation. There will be a leveling indicator in the housing to ensure it is upright. These use a bellows attached to a lever system that operates a mercury switch.

Question 4

Modulating burner control

Answer 4

Adjust the fuel supply to the burner and the air supply at the same time. Works similar to a on off control where the bellows senses the pressure and adjusts a lever system that moves sliding contacts in a resistance coil that in turn causes a electric motor adjust the fuel and air supply based on the pressure demand. This control should be used with a limit control to stat and stop the burner if the pressure gets to high.

Question 5

High limit control (high STEAM pressure fuel cut off)

Answer 5

A auto fired boiler not under constant supervision must have a high limit control that shuts the boiler fuel off if the pressure exceeds 15 psi this control must be connected to the boil with no stop valves between it and the boiler and protected by a siphon that maintains a water seal. Once this control activates it locks the controls out so the boiler does not start auto but needs to be man reset. If this happens the operations control is probably messed up and should be repaired or replace before the high limit control is reset. The cut out pressure of the high limit control should be no more then 90%of the PRV setting

Question 6

Operating controls for HOT WATER boilers

Answer 6

Pretty close to the same design as steam boilers but have a sensing bulb connected to a capillary tube that is filled with a expanding liquid or liquid vapor mix. As the temp rises the liquid expands and controls either a on off or modulating switch. The sensing bulb is placed in a thermowell usually in the hot water outlet. A aqua stat is connected directly where the temp is being sensed as it has no capillary tube. The code requires a high limit control on hot water boilers that cuts off the fuel supply before the temp reaches 121C