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Heated pressure washers and steam cleaners increase the ability of a high-pressure flow of water to break down dirt and grease. They also increase the action of most cleaning detergents. These systems are very complex, and add more potential personal injury and property damage hazards. Design of these systems requires additional components as well as experienced design personnel with knowledge of fuels, heat transfers, electronics, etc.

The burner at its simplest consists of a housing assembly, an electric motor, a fuel pump, a fan, an ignition system and a fuel nozzle. The burner itself is a component in the water heating system, which includes the coil, combustion chamber, stack, and fuel tank.

Like the pressure washer, the burner is also assembled from standard components that may be used in other applications.

The electrical system consists of two main components: an electric motor, which drives the pump and fan, and an ignition system, made up of a transformer / ignitor, which provides the 10,000 / 14,000 volts or more, needed for an adequate spark at the two ignition electrodes.

Fuel is moved to and through the nozzle, where it becomes a fine spray mixed with air for ignition, by the hydrokinetic subsystem which includes the fuel pump, valves, tubing and, of course, the nozzle.

Air is handled by the fan and the intake vents in the air ring and housing assembly, forcing a draft through the air tube to the point of ignition. In other words, for the burner to fire properly, these subsystems must consistently deliver the correct mix of spark, fuel and air to the ignition point.

Configuration

The housing assembly is the chassis that allows the other components to be mounted in proper relationship with each other.

In the standard burner configuration, a small electric motor from 1/7 hp up to 1/2 hp (in the larger commercial burners) is mounted on one side of the chassis. The motor runs at one of the standard speeds – 1725 rpm or 3450 rpm – and is connected across the assembly by a coupling to drive both the fan and the fuel pump.

The fuel pump, on the other side of the housing from the motor, is mounted on the air ring, which houses the fan. The fan is essentially a blower ring consisting of small vanes, which rotate with the motor shaft and coupling to suck air through the housing assembly into the air tube on the front of the burner where the nozzle and ignition electrodes are mounted.

A transformer, which produces the proper electric current for the ignition electrodes, is set on top of the housing assembly.

Burners can be operated with a wide range of fuels including the several grades of home heating oil, diesel fuel, or kerosene as well as other fuels including jet fuel and propane or natural gas. Diesel fuel burns hotter but not as cleanly as other fuels and the burner will require more air for an efficient burn. The heavier the oil used to fire a burner, the more heat will be produced per gallon of fuel oil burned.

Heating Coils

As a combustion chamber, a coil must contain the flame of the burner. The combustion chamber is sized for the flow of water and the amount of the BTUs pumped into the chamber.

BTU stands for British Thermal Unit. One BTU is required to raise one pound of water one degree. This is for standing water. When the water is flowing, the BTUs required are raised substantially.

Temperature rise is the amount of heat transferred to the water. A temperature rise of 120° on a machine will raise moving water from a 55-degree inlet water temperature to 175 degrees. The efficiency in this formula is the efficiency of the fuel being burned. Fuel oil efficiency is 70 degrees and gas efficiency is 75 degrees. These efficiencies differ because of the various BTUs per cubic foot or gallon of fuel contained in the fuels themselves. To put all this in perspective, the average
1500 sq. ft. home uses a furnace with a rating of 175,000 BTUs. A pressure washer uses 385,000 BTUs to heat 4 gallons per minute (gpm) of water.

Timer

Some systems are equipped with a timer. Generally the timer is used to shut off the system automatically if the spray gun is left closed with water diverted to bypass for a preset length of time. The timer shuts off the electrical system.

Thermostat

The thermostat responds to water temperature to turn the burner on or off, to maintain a preset temperature. Thermostats are essentially electrical switches operated by changes in temperature. Generally a thermostat has two major components, a switch and a sensor. Temperature changes affect the sensor, which in turn operates the switch. In most cases, a rise in temperature causes a physical action of some sort, which results in throwing a switch and breaks an electrical circuit. In most high pressure cleaning applications, the switch breaks the circuit. This break in the electrical circuitry cuts power to theelectrically operated solenoid valve, diverting fuel into bypass and stopping burner operation. When the thermostat’s sensor cools, the circuit is completed again and the burner resumes operation.

High Temperature Limit Switch

This safety device throws a switch if water output temperature rises to an unacceptable level. The high temperature limit switch is used on many hot water cleaners as a protection against excessive water temperature. When used as a high limit control, this switch is normally closed. The contact is broken when the switch is opened by temperature increase. It is normally mounted at the coil outlet.

Gas Burner

Gas burners can use either natural gas or liquefied propane gas (LP) as a fuel. They are cleaner burning (due to fuel composition) than oil burners and 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, the fuels mix better with air and do not require forced air for combustion either.

These burner systems are called natural draft burners instead of forced air burners. With a fuel oil system, air is forced into the combustion chamber with a fan. In gas-fired systems, air is pulled into the combustion chamber because heat causes hot air to rise, thereby pulling in air at the bottom of the combustion chamber to be burned. This means a different type of coil must be used. The basics of the coil are the same but allowances must be made to let the flue gases rise naturally out of the combustion chamber.

The first item needed on a stack for a normal draft system is a draft diverter.

A draft diverter is a critical part of the gas system. Without it, the flue gases cannot escape from the coil fast enough to pull in clean air and this causes soot in the coil.

The principle of a draft diverter is to accelerate the flue gases, causing a negative pressure in the combustion chamber and that pulls in fresh air for burning. This is accomplished in its design by diverting the hot flue gas around a diverter, mixing it with room air and causing both to be accelerated up the flue. Similar to a turbo, the hot gases are allowed to expand momentarily causing them to speed up.

If a draft diverter is not installed in a flue, the gases cannot expand even though they are trying to and actually will slow down the escaping gases causing coil sooting. LP gas is man-made gas made from natural gas. It is made up of propane and butane. It has a higher BTU rating than natural gas so it requires smaller orifices and amounts of gas. However, it is usually higher priced than natural gas.

LP gas or propane, as some people call it, has a specific gravity of 1.53 so it is heavier than air, making it very dangerous. LP will sink to the lowest level and start to pool. When the right amount mixes with air, it is very flammable and can explode with the slightest spark. LP gas also ignites at 950 degrees compared to 1200 degrees for natural gas. As with natural gas, an odor additive is put in the gas so a person can smell a gas leak quickly.

Since LP gas is heavier than air, more pressure is needed to move it or force it up in a burner. LP gas systems can run 11" W.C. to 14" W.C. for proper combustion. LP gas produces 2500 BTUs per cubic foot. To figure the cubic foot per hour in a gas line, divide the total BTUs by 2500. This is 2.5 times greater than natural gas.

Since LP gas is stored in tanks, the pressure in a tank will vary depending on the outside temperature. The hotter outside, the more vaporizing of the liquid under pressure and the greater the tank pressure. The colder it gets, the lower the pressure. Most tanks have a pressure of 130 PSI at 80 degrees. This must be dropped to 11" W.C. to be used in the burner system. Two types of systems are used to control LP gas – a single stage or a two-stage regulator system. The two different systems are used because of the pressure drop and freezing of gas lines when pulling large amounts of gas.

The single stage regulator is generally used in small appliances like home gas grills. These have small burners in them, generally only 1000 to 2000 BTUs. A single stage regulator can control less than 1 cubic foot of gas from 130 PSI to 11" W.C. and not freeze up. If you try to pull 20 CHF of gas through this regulator, the regulator will freeze up because it cannot control that much gas through it. The openings in a regulator that would control that much gas, from 130 PSI to 11" W.C., would be huge.

To overcome this problem, a two-stage system is used. The tank regulator will drop the pressure from 130 PSI to about 3 or 4 pounds of pressure. The second stage regulator will drop the pressure again from 3 pounds to 11" W.C. Large amounts of gas can be pushed at higher pressures through a smaller line over longer distances to the second regulator. The second regulator has to control the same amount of gas but not such a large pressure change, so the openings in it can be smaller in size.

In an LP system, the tanks must be sized properly. To calculate the tank size in gallons, divide the BTUs of the washer by 9150. A 400,000 BTU machine uses 4.37 gallons per hour. To run 8 hours you need a 44-gallon LP tank. Pounds rate most LP tanks. Divide the BTUs by 2150 to get the correct pound cylinder. By dividing 400,000 by 2150 you will see that you need a 186 lb tank. Go larger if needed, but not smaller. LP tanks can only be filled to 80% capacity to allow for vaporization of the liquid LP. Dropping from a 200-pound cylinder to a 100-pound cylinder means losing 40 gallons of LP gas in the cylinder size.This could be a difference between a cylinder freezing over or not.

Benefits of Gas-Fired Equipment

· Clean burning. Contaminated fuel generally not a problem.
· No external moving parts to adjust or service.
· Natural gas machines require no fuel storage.
· Equipment can be modified to use either natural or LP gas.
· Equipment can be installed in a shed where it is protected from the elements and other abuse.

Gas Burners

Now that you understand the gas differences, we can talk about the burners and regulators needed to control the flame in a combustion chamber. Natural and LP gas burners are virtually identical in design. BOTH consist of one or more cast metal rings or manifolds with threaded fittings for gas nozzles. A ring might hold several nozzles. The average number of nozzles used in a burner ring is 44.

The nozzles, generally made of brass, are machined to produce a set amount of heat energy or BTU. Each nozzle has a hollow threaded base which screws into a tapped port on the burner ring. Gas enters through the burner ring and passes through the nozzle’s orifice. The orifice sprays gas into a brass tube. This tube has air inlet openings in its sides at the base end. These openings are sized and machined to allow precisely the right amount of air to enter for efficient combustion when mixed with the flow of gas through the orifice. These openings are the primary air inlets and allow for the proper percentage of air to natural gas. Natural gas will not burn by itself. In fact it will not burn until mixed with the proper percentage of air. The usual percentage is around 14% gas to oxygen. The ring is designed so that the flames from adjoining nozzles touch or impinge on each other. This is known as an impinged jet design.

Thermopile Controls

The gas burner uses a pilot light for an ignition source. The pilot light burns all the time. As well as providing for constant ignition, this continuous ignition source prevents gas vapor buildup and reduces the possibility of a gas explosion. Continuous pilot light operation results in constant, though small, fuel consumption. The pilot light also warms the combustion chamber during the winter.

A gas burner must have a pilot generator (thermopile), which produces DC millivoltage at the end of the thermopile. This current (measured in millionths of a volt, hence millivolts) is sufficient to ignite the pilot. If the heat-generated voltage is insufficient, the pilot valve will close, shutting off gas to the pilot.

When relighting the pilot light, it is necessary to manually hold down the pilot valve button for two or three minutes, until the millivolt heats the thermopile sufficiently to hold the valve open automatically. This prevents the gas valve from opening and allowing gas flow without an ignitionsource should the pilot light go out.

Remember: The pilot flame is used 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, the valve will close. The valve will remain open so long as heat from the pilot flame is generating this small voltage.

Gas Regulators

The burner must also have a pressure-regulating valve. This valve reduces pressure from the gas supply tank (LPL) or gas line (NG) to the approximately 1/2 psi needed for the gas burner. A valve must also be provided to turn the supply of gas on and off. The pilot generator, pressure regulator and gas 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.

Electronic Ignition

The system acts on a demand for heat by a switch to open a pilot valve in the control, supplying gas to the pilot burner and also main gas to the main valve. At the same time, the ignition control supplies a spark to the pilot, which ignites the pilot gas. The solid-state ignition control provides a purge time before gas and spark are activated. After the pilot is lit, an electronic circuit proves the presence of the pilot flame using flame rectification. This turns on the main valve allowing the main gas to flow to the burner nozzles. Sparking continues for a few seconds into the main burner cycle for ignitionstability.

Thermocouple

The thermocouple has two dissimilar metals encased in it. By heating one end of the two joined metals, an electrical current is generated. This small electrical current is carried through the insulated copper wire to the gas valve. Inside the gas valve is an electromagnet connected to a valve. The current from the heated thermocouple keeps this electromagnet energized and the gas valve open. Gas can now flow freely to the pilot light. If the pilot light goes out, no heat is applied to the thermocouple and no electricity is generated so the electromagnet lets the valve close. Therefore, no gas can flow out to thepilot light or the main gas burner ring.

To operate this type of valve, called a 100% safety valve, a dial on top is pushed down and held. This opens the pilot light valve inside the regulator. Now lighting the pilot light will cause the thermocouple to heat up. Once it is generating electricity, the electromagnet keeps the valve port for the pilot and main gas valve open when the dial is released. When this dial is turned over to the "On" position, the 120-volt side of the gas valve will work.

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.