Gas-fired burners can use either natural gas or liquefied propane gas as a fuel. The composition of gas fuels, which possess fewer carbon molecules than fuel oils, makes gas burners cleaner burning than oil burners. Gas-fired equipment gives the best service in heavy-duty hot water cleaning applications where long periods of continuous cleaner usage are called for.
Gas Burner Types
Basically, two main types of gas burners are found in high pressure cleaning applications, natural-draft gas burners and forced draft gas burners.
Natural-draft gas burners have no external moving parts. The lack of moving parts is due primarily to the fact that LP and natural gas do not have to be delivered under high pressure and vaporized for combustion. Since they are gasses at room temperature the fuels mix better with air and generally do not require forced air for combustion.
Forced-draft gas burners suit applications where natural air delivery or ventilation may not be sufficient. These burners use a fan to deliver air to the combustion chamber and are equipped with on demand electronic ignition.
Some Delay In Firing Should Be Expected
Because of the nature of the fuel and the manner in which the fuel is supplied to the combustion area, gas burners do not respond to a call for flame from burner controls as rapidly as do oil burners. With any gas burner, there is a delay before flame is established. An oil-fired burner, however, will respond almost instantaneously with fire in the combustion area to a demand for heat from burner controls.
Consequently gas-fired hot water equipment calls for a different operation procedure than does oil-fired equipment. Once a gas burner is turned on at the burner switch, the trigger gun should be kept open for as long a period of washing as possible. The reason: each time the gas burner shuts off, it takes from several seconds to half a minute for the burner to restart.
Since shutting water flow off at the gun causes the flow switch or other burner control to cut off the burner, each time the trigger is released and depressed again, the burner shuts down completely and then begins a fairly lengthy restarting cycle.
A Few Seconds To Half A Minute
For natural-draft burners, restarting will take a few seconds to allow the gas valve to react and a flow of gas to reach the combustion area. It also takes a few seconds for shutdown to occur because gas already in the line must be burned.
If a forced-draft burner with an automatic purge cycle is used, the delay before resumed burner firing may be more than 30 seconds overall.
Demands For Firing Can Come Faster Than The Burner Can Respond
Releasing and re-depressing the trigger several times in even moderately quick succession will first break, and then reopen the burner control circuit more rapidly than the burner will respond and the burner will not fire.
Operators using gas-fired equipment should be taught to intersperse relatively long periods of washing with relatively long periods of shutdown to allow sufficient time for the burner to respond to demands for heat.
Turn the burner switch to on, keeping the trigger depressed so that water is flowing through the system. The burner will not ignite immediately. Within five to 30 seconds the burner will come on.
Keep the trigger depressed for as long as hot water is needed. Do not stop and start washing operations frequently as this will disrupt burner operation.
The gas burner with automatic ignition does not have a pilot light. When burner action is called for, as ignition spark is provided and the pilot valve is opened allowing gas flow to the pilot orifice. The ignition arc ignites the gas escaping from the pilot orifice. If a pilot flame is established, the pilot light’s heat is sensed and the main gas valve opens. If no pilot flame is established, the ignition arc will continue for 20 to 30 seconds and the system will shut down. Generally the entire system will have to be turned off to try to restart the pilot because of a safety lock-out intended to prevent various gas hazards.
A gas-fired burner with a pilot light will have a pilot generator or thermo-couple which uses the heat produced by the pilot light to generate a small amount of electricity. This electrical current, measured in millionths of a volt, is sufficient to keep open the pilot gas valve. If the pilot gas valve ceases to receive this small, heat-generated voltage, the pilot valve will close, shutting off the gas supply to the pilot and the entire gas system after the main gas valve.
Getting The Pilot Generator Up To Temperature
The pilot generator is the reason you have to hold the pilot gas valve button down manually for two to three minutes whenever the pilot light is re-lit. The valve must be held open manually until the millivolt pilot generator is hot enough to generate enough voltage to hold the valve open automatically. This prevents the gas valve from opening and allowing gas flow into the combustion chamber without an ignition source should the pilot light go out.
The Pilot Light In Action
The pilot flame is used as a continuous ignition source and to generate a very small amount of electricity.
This heat-generated voltage keeps the pilot valve open, supplying gas to the pilot.
Should the current provided to the pilot valve cease when the pilot light or ignition source is extinguished, the valve will close. The valve will remain open so long as heat from the pilot flame is generating this small voltage.
Valves For Gas
The burner must also have a pressure regulating valve. This valve reduces pressure from gas supply tank (L) or gas line (NG) to the approximately ½ psi needed for the gas burner.
An approved on/off valve must also be provided near the equipment to turn the supply of gas on and off for safety or service purposes.
The Pressure regulator, gas valve and on/off valve may be combined in a single assembly known as a 3-in-1 or combination gas valve, or they may be installed in the system as separate components.
Most LP gas units will have a two-stage regulator system to reduce tank pressure to a pressure acceptable to the gas valve and another to fine tune or adjust gas pressure at the burner. LP gas is contained in tanks at pressures higher than the pressures of gas supplied by natural gas utilities.
Several factors, including the burner configuration and the placement of the gas ports along with the nature of the fuel, cause an approximate two-second delay between the time the burner switch is turned on and complete burner ignition.
This delay allows for the opening of the valve and the beginning of gas flow. The piping and burner ring contain air that must be expelled by the new gas. Gas pressures are low so it may take a second or two for the piping and rings to be purged so gas can reach the pilot light for ignition.
Gas remaining in the system after the valve is closed causes a similar delay when the burner switch is shut off. When the gas valve closes, the pressure pushing the gas out begins to drop and the flame gets smaller and smaller until the pressure is so low the flame will actually try to follow the gas into the gas jets. At this point the lower flammability margin of the fuel and oxygen mix is reached and the flame is suddenly extinguished. This process produces the popping and sound, called extinction pop, that is made when the gas ring flame is extinguished.
Remember: There will be a slight delay in gas burner firing after the burner switch is turned on. This delay is natural and will last no more than a very few seconds. There will be a slight delay in gas burner shutdown after the burner switch is shut off. This delay is normal. The burner is simply consuming gas remaining in the burner ring and gas pipe.
Two Safety Considerations: Natural gas is lighter than air and rises. Escaped gas will accumulate near the ceiling. LP gas is heavier than air and settle. Escaped gas will accumulate near the floor.
Adequate Fuel Supply
For a gas burner to function properly it must receive an adequate supply of gas. With natural gas this is usually no problem. With LP gas, the tank must be large enough to provide sufficient flow of gas to meet the burner’s needs. If an LP machine is operated with too small a tank, the tank will literally freeze up. For this reason, only very small LP-fired equipment can be operated from a portable LP gas tank as used with motor homes and portable gas grills.
Although extremely rare in high pressure cleaners, a primary control shuts off operation of the burner when it is not functioning properly. The cadmium cell control system uses a cadmium sulfide flame detection cell which views the flame. This photosensitive sensor increases in resistance when flame is not present and burner shutdown will result in a per-determined period of time, usually about a minute.