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← ↑ → When a source of heat is needed for an appliance, both gas and electricity are used. Gas is used for both clothes dryers and incinerators. The remainder of the appliances use electricity for their source of power for operation. Strictly speaking, fuel is any substance that releases heat when mixed with the proper amount of oxygen. Only those materials that ignite at relatively low temperatures, burn rapidly, and are easily obtained in large quantities at relatively low prices are considered good fuels. The value of a fuel is derived from the amount of heat released when it’s burned and the heat of combustion is measured. This value is obtained when a given amount of fuel is burned under controlled conditions. The apparatus used for this purpose is called a calorimeter. The released heat is absorbed by a definite volume of water and the rise in temperature of the water is measured. The common ratings are given in British thermal units (Btu per pound) or cubic foot of the fuel burned. When a fuel contains hydrogen, the heat given off during combustion will depend on the state of the water vapor (H formed when the hydrogen is burned. A heating value known as the higher, or gross, heating value is obtained when this vapor is condensed and the latent heat of condensation is salvaged. If this water vapor is not condensed, the latent heat of vaporization is lost; this is known as the lower, or net, heating value. The Btu rating of gaseous fuels is given per cubic foot. The gas industry uses the standard conditions of a temperature of 60°, 30 inches of mercury pressure with a saturated condition with water vapor to determine these values. There are two different types of gaseous fuels used as heat sources for appliances. The most familiar is natural gas. The second one is known as LP (liquefied petroleum) gas. NATURAL GAS Natural gas is the lightest of all petroleum products. It’s usually found where oil is found, but in some cases it’s found elsewhere. Theorists have long argued about the ex act origin of natural gas. However, most agree that natural gas was formed during the decomposition of plant and animal remains that were buried in prehistoric times. Be cause these plants and animals lived during the same period as those that are presently found as fossils, natural gas is sometimes called a fossil fuel. Both natural gas and petroleum gas are mixtures of hydrocarbons. Both are considered fossil fuels and both are composed of various chemicals obtained from the hydrogen and carbon contained in prehistoric plants and animals. The gas industry may be broken down into the various areas of exploration, production, transmission, and distribution. The exploration section of the industry performs the function that its name implies. The people who are employed in exploration simply explore new areas, determine the location of the gas or petroleum field, and complete the necessary reports, purchases, and other essential duties prior to the actual drilling. When all the exploration functions are completed, the production department accomplishes the actual drilling of the well. The gas, or crude oil, is brought to the earth’s surface and is blocked at this point. The well remains in this state until the gas or oil is needed. When the need arises the transmission department receives the gas from the well at pressures ranging from 500 to 3000 pounds per square inch gauge (psig). Even with these high pressures, the resistance of the pipe and the distance covered require that booster pumps be used to transfer the gas from the well to the refinery. At the refinery, the gas passes through a drying process that removes moisture, propane, and butane. During this process most of the odor is also removed from the raw gas and an odorant is added to aid in leak detection. After the refining processes are completed, the distribution department receives the gas through measuring gates where the number of cubic feet of gas is recorded. The gas is then passed through a series of regulators that reduce the pressure in steps. The steps are necessary to prevent the regulators from freezing and becoming inoperative. The gas pressure is reduced to correspond with the requirements of one of two distribution systems, either the intermediate or the low-pressure system. In most cases, the low-pressure system is taken from the intermediate sys tem. The intermediate distribution system maintains pressures ranging from 18 to 20 psig, while the low-pressure system has a pressure of approximately 8 ounces. The low- pressure system is generally used when cast-iron pipe is used for distribution. Cast-iron pipe does not seem to hold the gas as well at higher pressures as does the copper or polyethylene pipe used in the intermediate systems. When the intermediate system is used, the gas pres sure is reduced from 18 to 20 psig to 44 ounces where it enters the house. Both systems use gas meters at this point, but the low-pressure system does not require a regulator. Even though the 8 ounces of pressure in the low-pressure distribution system is higher than that required in the meter loop (the meter loop consists of all the components from the main line through the meter and regulator, if used), no regulator is needed because of the pressure drop through the meter loop. This pressure drop occurs because gas does not travel in a straight line but rolls instead, reducing the flow of gas by about 10 cubic feet for each turn. This rolling action also brings about the need for straightening vanes in the line directly ahead of the gas meter. If these vanes were omitted, the meter would not measure the flow of gas correctly. On leaving the meter loop, the gas flows into the house piping. Most of the appliances used are manufactured to operate on 44 ounces of gas pressure; however, natural- gas furnaces are built to operate on 34 inches of water column of gas pressure in the furnace manifold. This requires an additional regulator at the furnace. The measurement of small pressures requires a manometer, a U-tube, or a manifold gas pressure gauge. A pressure indicated by a water column is a very small amount. A pressure of 1 psi will support a column of water 2.31 feet high, or 27.7 inches. A pressure as low as 0.05 psi will support a column of water 1.39 inches high (27.7 x 0.05). When the pressure is measured inside a pipe that is slightly higher than atmospheric pressure, the water column in the U-tube will be depressed in one leg and pushed up in the other. The height of the water column in the U-tube is 3.2 inches, which corresponds to a pressure of 0.116 psi inside the pipe to which the tube is connected. If there were no pressure difference between the inside and the outside of the pipe, the water in both columns would stand at exactly the same level. The pressures which are measured in gas appliance manifolds are always measured with a water gauge, which indicates the pressure in inches of water column. The use of inches of water column when measuring low pressures eliminates the need for converting measurements made with a water gauge to pounds per square inch. Natural gas has a specific gravity of 0.65, an ignition temperature of 1100°F, and a burning temperature of 3500°F. One cubic foot of natural gas will emit from 900 to 1400 Btu per cubic foot, with the greater amount of natural gas used emitting approximately 1100 Btu per cubic foot. The Btu content of natural gas will vary from area to area. The local gas company should be consulted when the exact Btu con tent is desired. Natural gas is made up of 55 to 98 percent methane (CH4) 0.1 to 14 percent ethane (C3H8) and 0.5 percent carbon dioxide (CO) It requires 15 cubic feet of air per cubic foot of gas for proper combustion. It’s lighter than air. Because methane and ethane have such low boiling points (methane —258.7°F and ethane —127.5°F, natural gas re mains a gas under the pressures and temperatures encountered during its distribution. Because of the varying amounts of methane and ethane, the boiling point of natural gas will vary according to the mixture. LIQUEFIED PETROLEUM (LP) GAS Liquefied petroleum is both butane and propane, and in some cases it’s a mixture of the two. These two fuels are refined natural gases and were developed for use in rural areas. They are transported by truck and stored in containers specifically made for LP gas installations. Liquefied petroleum is a liquid until the vapor is drawn off. When liquefied petroleum is extracted from raw gas at the refinery, it’s in the liquid state, under pressure, and remains in this state during storage and transportation. Only after the pressure is reduced, does liquefied petroleum become a gaseous fuel. LP gas has at least one definite advantage in that it’s stored in the liquid state and thus the heat is concentrated. This concentration of heat makes it economically feasible to provide service anywhere that portable cylinders can be used. The fact that 1 gallon of liquid propane becomes 36.31 cubic feet of gas when evaporated illustrates its feasibility. When LP gas is stored in a container, it’s both in the liquid and gaseous state. To make the action of LP gas more easily understood, let us review briefly the boiling point and pressures of water. When the pressure cooker is first filled with water with no heat applied and the top remaining off, there is no pressure on the surface of the water. However, when the top is put securely in place and heat is applied, the pressure will begin to rise after the boiling point of the water is reached. As more heat is applied, more pressure will be created above the water by the evaporating liquid. When a constant temperature is maintained, a corresponding pressure will also be maintained. Likewise, when the pressure is reduced, the boiling point is reduced. Fgr. 2 LP gas storage tank. Vapor; Liquid propane Fgr. 3 Pressure cooker. Tube Fgr. 1 U-tube manometer---Open to Atmosphere; Glass Tube; Liquid in Tube Fgr. 4 Electrical distribution. If we apply this principle to .LP gas in a storage tank, it can be readily seen that as vapor is withdrawn from the tank, more liquid will evaporate to replace that which was withdrawn. We must remember that each liquid had its own boiling point and pressure. Butane (C4H10) like propane (C3H8) and natural gas, is placed in the hydrocarbon series of gaseous fuels, because it’s composed of hydrogen and carbon. Butane has a boiling point of 31.1°F, a specific gravity of 2, and a heating value of 3267 Btu per cubic foot of vapor. It requires 30.97 cubic feet of air per cubic foot of vapor for proper combustion. At sea level it has a gauge pressure of 36.9 pounds at 100°F. Butane expands to 31.75 cubic feet of vapor per gallon of liquid. It’s heavier than air. The ignition temperature is approximately 1100°F and the burning temperature is 3300°F. Propane has a boiling point of — 43.8°F, a specific gravity of 1.52, and a heating value of 2521 Btu per cubic foot of vapor. It requires 23.82 cubic feet of air per cubic foot of vapor for proper combustion. At sea level it has a gauge pressure of 175.3 pounds at 100°F. Propane expands to 36.35 cubic feet of vapor per gallon of liquid. It’s also heavier than air. It has an ignition temperature of 1100°F and a burning temperature of 2975°F. Table 1 LP gas vapor pressures. Temp. °F; Propane; Butane When we study the physical properties of LP gases, we can see that each has both good and bad properties. These properties should be given a great deal of consideration when determining which fuel to use for any given application. The two characteristics deserving the most consideration are the Btu content and the vapor pressure. When considering these fuels, the pressure is the major limiting factor, especially in colder climates. As we study the table, we can see that when the temperature of liquid butane reaches 30°F or lower, there is no pressure in the tank. Therefore, butane would not be suitable as a fuel at these lower temperatures without some source of heat for the storage tank. This source of heat may be steam or hot-water pipes around the tank, electrical heaters around the tank, or even having the tank buried in the ground. However, these all add to the initial cost of the equipment. If we look at the pressures of propane, we see that they are suitable throughout a wide range of temperatures. Therefore, from the pressure standpoint, propane would be the ideal fuel. On the other hand, the lower Btu rating makes it less desirable than butane. To overcome this dilemma, the two fuels may be mixed to obtain some of the desirable characteristics of each gas. An example of this may be a mixture of 60 percent butane and 40 percent propane. At a temperature of 30°F, the mixture will have a vapor pressure of approximately 24 psig and a heat content of 2950 Btu per cubic foot. Since these fuels are usually mixed before delivery to the local distributor, it’s difficult to know exactly what the tank pressure and Btu content are. As long as there is enough pressure in the storage tank to allow 11 inches of water column of pressure to enter the house piping, there is little or nothing a service technician can do. Transformer bank; User substation; Generating station; Domestic user Before becoming too deeply involved in working with LP gases, state and local authorities should be consulted. Some states maintain strict control over the personnel working with these fuels. ELECTRICITY Electricity is not a new phenomenon; its existence has been known for centuries. The early applications of the heat- producing ability of electricity were limited. It was used only in a few specialized areas, mainly industrial processes and as portable heaters to help supplement inefficient heating systems. Today, electricity is used in almost every appliance and at a cost that most people can afford. Electrical power is generated at the utility company’s generating station. As it leaves the generating station, it passes through a bank of transformers to increase the volt age. From the transformer bank, the electricity is distributed to user substations. The voltage is reduced at the substation by another bank of transformers to a voltage that can be used by commercial manufacturing plants. The voltage is again reduced by the building’s current transformer and is carried through the meter loop to the disconnect switch. From the disconnect switch, the electricity is distributed through the house wiring to the various appliances and electric heating units. The major users of electricity are electric ovens, ranges, countertop cooking units, electric motors, electric clothes dryers, and other types of resistance heaters. There are three types of resistance heating elements: (1) the open wire, (2) the open ribbon, and (3) the tubular-cased wire. An electric resistance heating element can be defined as: An assembly consisting of a resistance wire, insulated supports, and terminals for connecting the electrical supply wire to the resistance wire. Resistance heating will convert electrical energy into heat energy at the rate of 3412 Btu per kilowatt (1000 watts). Theoretically, electric heating elements are 100 percent efficient; that is, for each Btu input to the heating element, 1 Btu in usable heat is recovered. The open-wire heating elements are usually made of nichrome wire, which is wire made from nickel and chromium, but without iron, wound in a spring-like shape and mounted in ceramic insulators to prevent electrical shorting to the metal frame. The open-wire elements have a longer life than the others because they release all the heat directly into the air stream and, thus, operate cooler. The tubular-cased element is the one that is used in electric cooking surfaces and ovens. The nichrome wire is placed inside a tube and insulated from it by magnesium- oxide powder. Thus, a tubular-cased heating element won’t require external insulation as do the other two resistance elements. It’s also less efficient because of the energy loss caused by the extra material that the heat must pass through before reaching the air or the utensil being heated. It is, however, safer to use because of the interior insulation used in its manufacturing process. The tubular-cased elements have a shorter life than either of the other two types of elements because of their higher operating temperature. The control of these elements is more difficult than the others because of the extra material involved. Ceramic insulator; Nichrome wire; Mica insulators (a) Open wire element; Magnesium Nichrome ribbon wire oxide powder tube; Tubular cased element Fgr. 5 Resistance elements. COMBUSTION The available energy contained in a fuel is converted to heat energy by a process known as combustion. Combustion may be defined as the chemical reaction of a substance with oxygen resulting in the evolution of heat and some light. There are three basic requirements for combustion: sufficiently high temperatures, oxygen, and fuel. See Fgr. 6. When the air-fuel mixture is admitted to the combustion chamber, some means must be provided to bring the mixture to its flash point. This is usually done by some type of ignition device. If, for any reason, the temperature of the gas—air mixture is reduced below its flash point, the flame will automatically go out. For example, if the temperature of a mixture of natural gas and air is reduced below its flash point of 1000°F, there will be no flame. Fgr. 6 Basic combustion requirements. Open ribbon element; Fuel Table 2 Fuel gas limits of flammability. --Gas; Upper Limit; Lower Limit-- Methane; Ethane; Natural; Propane; Butane; Manufactured Also, an ample supply of properly distributed oxygen must be supplied. The oxygen requirements governing the combustion process will vary with each different fuel. They also depend on whether or not the fuel and air are properly mixed in the correct proportions. The third requirement for combustion is the fuel. The properties of fuel were discussed earlier in this section. The physical properties of each fuel must be considered when determining its requirements for combustion. All of the basic requirements for combustion must be met or there will be no combustion. An important factor to keep in mind when making adjustments involving gaseous fuels is the limits of flammability, which are stated in percentages of the gas in the air of a mixture that would allow combustion to take place. To simplify this, if there is too much gas in the air, the mixture will be too rich to burn. If there is too little gas in the air, the mixture will be too lean to burn. The upper and lower limits are shown for the more common gaseous fuels. Complete combustion can be obtained only when all of the combustible elements are oxidized by all of the oxygen with which they will combine. The products of combustion are harmless when all of the fuel is completely burned. These products are carbon dioxide (CO) and water vapor The rate of combustion, or burning, depends on three factors: 1. The rate of reaction of the substance with the oxygen. 2. The rate at which the oxygen is supplied. 3. The temperature due to the surrounding conditions. All the oxygen supplied to the flame is not generally used. This is commonly called excess oxygen, or excess air. This excess oxygen is expressed as a percentage, usually 50 per cent, of the air required for the complete combustion of a fuel. For example, natural gas requires 10 cubic feet of air for each cubic foot of gas. When 50 percent excess air is added to this Fgr., the quantity of air supplied is calculated to be 15 cubic feet of air for each cubic foot of natural gas. There are several factors governing the excess-air requirements: the uniformity of air distribution and mixing, the direction of gas flow from the burner, and the height and temperature of the combustion area. Excess air constitutes a loss and should be kept to a minimum. However, it cannot usually be less than 25 to 35 percent of the air required for complete combustion. Excess air has both good and bad effects in the combustion process. It’s added as a safety factor in case the 10 cubic feet of required air is reduced for some reason, such as dirty burners, improper primary air adjustments, or a decrease in the supply of primary air. The adverse effect is that the nitrogen in the air does not change chemically and tends to reduce the burning temperature and the flue-gas temperature, thereby reducing the efficiency of the heating equipment. The air supplied for combustion contains about 79 percent nitrogen and 21 percent oxygen. Fgr. 7 Elements of combustion The products of combustion created when 1 cubic foot of natural gas is completely burned are 8 cubic feet of nitrogen, 1 cubic foot of carbon dioxide, and 2 cubic feet of water vapor. These products are harmless to human beings. In fact, carbon dioxide is the ingredient added to water that makes soft drinks fizz. The by-products of combustion are carbon monoxide—a deadly product; aldehyde—a colorless, inflammable, volatile liquid with a strong pungent odor and an irritant to the eyes, nose, and throat; keytones—used as paint re movers; oxygen acids; glycols; and phenols. These by products are harmful and must be safeguarded against by the proper cleaning and adjustment of the heating equipment. Probably the most important step in maintaining good combustion is the proper adjustment of the ratio of primary air to secondary air. QUIZ: 1. Define a fuel. 2. What gas, that is used for a heating fuel, is the lightest? 3. Of what are both natural gas and petroleum gases mixtures? 4. On most natural gas distribution systems, what is the pressure of the gas entering the residence? 5. How high will 1 psi support a column of water? 6. Name the instruments that are used to measure gas pressure. 7. What is the ignition temperature of natural gas? 8. Name the major methods of using electricity. 9. Define combustion. 10. Name the three things that determine the rate of combustion. Home top of page |