A pilot burner is a device used in a gas-fired appliance to light the main gas burners and generate sufficient millivoltage to operate a thermocouple or thermopile pilot safety shutoff device. Modern pilot burners are designed to burn continually in order to provide an automatic ignition of the burners when the main gas supply is turned on. The pilot burners in common use today can be divided into aerated and nonaerated types.
An aerated pilot (see Figure 5-55) is one that injects primary air through an air intake opening into the gas stream. The air and gas are mixed before burning.
An aerated pilot burner produces a very stable flame. For this reason, these pilots are often used where the pilot location is particularly inaccessible. Although the flame produced by an aerated pilot burner is more stable than one produced by a nonaerated type, an aerated pilot burner does have some disadvantages. An important one to remember is the tendency for the small primary air openings to clog with lint and dirt. Frequent cleaning is required, particularly when these pilots are used in areas having a large amount of foreign material in the air.
A nonaerated pilot (see Figure 5-56) does not inject primary air. As a result, the air and gas are not premixed, and the combustion process must be completed with secondary air only. This results in a less-stable flame than the one produced by an aerated pilot. On the credit side, a nonaerated pilot requires less maintenance than an aerated pilot. For this reason, the nonaerated pilot burner is usually preferred by most utility companies.
A pilot burner assembly consists of the pilot bracket, pilot orifice, primary air intake, lint screen, mixing chamber, pilot ports, and pilot hood.
The pilot bracket is a device used to mount the pilot in a fixed relationship to the burner. Some pilot brackets also contain means for mounting the thermocouple or pilot generator so that the hot junction is located directly in the path of the pilot flame (see Figure 5-57).
The pilot ports are the openings through which the gas (in nonaerated pilot burners) or the gas and air mixture (in aerated pilot burners) passes before burning. The gas and air are premixed in the mixing chamber of an aerated pilot. The air is injected into the mixing chamber of an aerated pilot through a hole or opening called the primary air intake. The amount of primary air can be controlled by adjusting an air shutter that covers the primary air intake opening.
A lint screen is generally used to remove lint, dirt, and other contaminants from the primary air before it enters the primary air opening. In some pilots, the lint and other particles are burned before entering the primary air intake. An incinerated pilot is an aerated pilot in which the primary air passes adjacent to the flame area where the particles are burned out of the air before it enters the primary air intake.
Impurities are also found in the pilot gas. These impurities can be removed by installing a pilot gas filter in the line upstream from the pilot adjustment means in the control. The pilot gas filter is expected to operate several years without service. Clogging of the filter will be indicated by shortened pilot flames, which will result in improper pilot operation. Shortened pilot flames can also be caused by pilot tube stoppage or a dirty pilot orifice. These possibilities should be checked before removing the filter. If the filter should become clogged, replace the entire filter rather than just the filtering medium.
The pilot orifice is a removable component in the pilot that contains precisely sized openings that control the admission of gas to the pilot. Pilot orifices are either of the spud or insert type. A spud orifice screws into the bottom of the pilot burner. It is both an orifice and a fitting combined into a single unit with threads at either end (see Figure 5-58). An insert orifice must be held in position by a separate fitting (see Figure 5-59). The pilot gas line (tubing) is connected to the bottom of the pilot orifice.