Heat Engineering

the branch of technology concerned with the production and use of heat in industry, agriculture, transportation, and the home.

Heat production. The principal sources of heat today (1970’s) are the fossil fuels, which give off heat when burned. These fuels may be solid, liquid, or gaseous. Among the more common solid fuels are coals (lignites, anthracites), combustible shales, and peat. Petroleum is a natural liquid fuel, but it is seldom used directly to produce heat. Instead, it is refined to produce gasoline for automotive and piston aircraft engines, kerosine for jet engines and certain types of piston engines, and various types of diesel fuel and mazut, used chiefly in nonnuclear thermal power plants. The most important gaseous fuel is natural gas, which consists of methane and other hydrocarbons (seeGAS FUELS.) On a smaller scale, wood (firewood, scrap wood) also serves as a fuel. Methods are now being developed to burn industrial and domestic waste materials for purposes of both disposal and heat generation.

The most important characteristic of a fuel is the specific heat of combustion. The concept of a standard fuel, having a heat of combustion of 29,308 kilojoules/kg (7,000 kilocalories/kg) is used for comparative calculations.

Various types of apparatus, such as furnaces, stoves, and combustion chambers, are used for fuel combustion. Fuel is burned in furnaces and stoves at a pressure close to atmospheric with air as the oxidizing agent. In combustion chambers, the pressure may be higher than atmospheric and oxygen air or air enriched with oxygen may serve as the oxidant.

Theoretically, a stoichiometric amount of oxygen is required for the combustion of a fuel. For example, when burning methane (CH₄) the following reaction occurs: CH₄ + 2O₂ = CO₂+ 2H₂O. It follows from this equation that 2 kilomole (16 kg) of CH₄ requires 2 kilomoles (64 kg) of O₂; that is, 1 kg of CH₄ requires 4 kg of O₂. In practice, however, a somewhat greater amount of oxidant is necessary for complete combustion. The ratio of the actual amount of oxidant (air) used for combustion to the theoretical amount is called the oxidant excess factor a. When a fuel is burned, its chemical energy is converted to the internal energy of the combustion products, as a result of which the products become hot. The temperature that would be acquired by these products if no heat were lost (adiabatic process) is known as the theoretical combustion temperature; this temperature is a function of the type and initial temperature of the fuel and oxidant and of the oxidant excess factor. For the majority of natural fuels (where air is the oxidant), the theoretical combustion temperature is 1500°–2000°C; it is increased by preheating of the fuel and oxidant. The maximum theoretical temperature is achieved when the oxidant excess factor α ≈ 0.98.

Since heat is withdrawn from the burning fuel in furnaces, the temperature of the combustion products is below the theoretical value.

Coal is usually burned in furnaces. When relatively small quantities of fuel are required, laminar combustion fireboxes are used, where lumps of coal are burned on a grate through which air is blown. For burning larger quantities of coal (hundreds of tons per hour), chamber furnaces are used. Here, coal that has first been pulverized to particle sizes of 50–300 micrometers is mixed with air and fed into burners. Mazut furnaces and gas furnaces are similar to pulverized-coal furnaces but have different burner and nozzle designs.

Since the mid-1900’s, nuclear fuel has joined organic fuel as a source of heat. The uranium isotope ²³⁵U, which makes up approximately 0.7 percent of the content of natural uranium, is the principal type of nuclear fuel. During the fission of 1 kg of ²³⁵U, approximately 84 × 10⁹ kilojoules (20 × 10⁹ kilocalories) of energy are liberated, mainly as the kinetic energy of fission fragments and neutrons. This energy is converted within a nuclear reactor to heat, which is then removed by a coolant. In almost all reactors (1970’s), the nuclear chain reaction is maintained by thermal neutrons. However, breeder reactors, which involve fast neutrons, are becoming increasingly common. Here, ²³⁸U and ²³²Th can be used as fuel to produce not only heat but also the other nuclear fuels ²³⁹Pu and ²³³U. Typical coolants for reactors using thermal neutrons are water, heavy water, and carbon dioxide; in fast-neutron reactors, they are liquid sodium and inert gases.

In addition to organic and nuclear fuels, geothermal and solar energy have been found to have practical value in heat generation. Geothermal energy manifests itself in hot groundwater, which often comes to the surface in regions of volcanic activity, and in the general temperature increase with depth inside the earth. This temperature increase is expressed by the geothermal gradient, numerically equal to the temperature rise in degrees per 100 m of depth; for depths accessible to direct measurement, the gradient averages 0.03°C/m. While the heat from hot springs is already being put to use (the 5-megawatt geothermal electric power plant built in the USSR in the valley of the Pauzhetka River in 1966), the possibility of using heat from the earth’s interior is as yet (1975) only being studied.

The sun, which sends an energy flux of 1.8 × 10¹⁷ watts to the earth, is a tremendous source of heat. However, the density of solar energy at the earth’s surface is low, amounting to only 1 kilowatt/m². Systems and equipment for collecting solar radiation on a large scale that meet both technical and economic requirements have not yet been developed. But in many regions solar energy is being used to distill water and to heat water for agricultural (hotbeds, greenhouses) and household needs; in some cases, it is used in the production of electric power.

Of great importance in view of the need to conserve natural fuels is the use of secondary heat sources. These sources include the hot exhaust gases of metallurgical furnaces or internal-combustion engines whose heat is utilized in waste-heat boilers.

Use of heat. The heat produced by various methods can either be used directly in certain production processes (heat consumption) or converted into another form of energy (thermal power engineering). The objectives and methods of the branch of heat engineering concerned with the consumption of heat are manifold. Heating is used extensively in metallurgy. For example, pig iron is obtained from iron ore in a blast furnace where the iron oxide is reduced by carbon at a temperature of approximately 1500°C; heat is liberated by burning coke. Steel is produced from pig iron in open-hearth furnaces at a temperature of approximately 1600°C, obtained mainly by the combustion of liquid or gaseous organic fuel. When steel is produced in a converter, oxygen is blown into the pig iron, and the necessary temperature is created by oxidizing the carbon contained in the pig iron. In foundry work, the heat required to maintain the necessary temperature in a furnace is generated either through the combustion of fuel, usually gas or mazut, in the furnace or through electric power.

Heating to specified temperatures is required in most processes in chemical technology and food processing. Heat is supplied or removed in heat exchangers, autoclaves, dryers, evaporators, stills, fractionating columns, and reactors with the aid of heat-transfer agents. If it is necessary to maintain a fairly high temperature in the equipment, the combustion products of an organic fuel can themselves be the heat-transfer agent. However, in most cases, the agent is an intermediary, either removing and transferring heat from the fuel’s combustion products to some other substance in the process or removing heat from this substance and transferring the heat to another part of the equipment or to the surrounding medium. Typical heat-transfer agents include water and steam, certain organic substances, for example, Dowtherm, and organosilicon compounds, mineral oils, fused salts, liquid metals, air, and various gases.

The structural designs of heat exchangers are extremely diverse, reflecting differences in purpose, temperature level, and type of heat-transfer agent. Depending on the principle of operation, exchangers are classified as recuperative, regenerative, or contact. In recuperative heat exchangers, heat is transferred from one substance (the heat-transfer agent) to another through a solid, usually metal, wall. In regenerative exchangers, heat is absorbed and subsequently released by a special filling that comes into contact first with a body giving off heat and then with a body that is to be heated. In contact exchangers, heat is transferred by the direct contact of substances. The most common are the shell-and-tube recuperative heat exchangers, where one of the heat-transfer agents flows inside tubes, and the other through the space between the tubes and an outer shell. The main characteristics of recuperative heat exchangers are the area of the surface for the exchange of heat and the heat-transfer coefficient, which represents the amount of heat transferred through 1 m² of the heat-exchange surface when the temperature difference between the heat-transfer agents is 1°C. For a given heat exchanger, this coefficient will depend on the types, parameters, and rates of flow of the heat-transfer agents.

A substantial portion of the heat produced during the colder part of the year is for household consumption; that is, it compensates for heat losses through the walls of buildings and losses involved in ventilation. Heat and electric power plants and centralized boiler rooms provide heat for the home in most cities of the USSR. The boilers at these plants and rooms heat water that is then sent to the home to supply heat. Home heaters can take the form of radiators or of pipes mounted in wall panels.

Certain buildings are equipped to produce their own heat. A hot-water boiler installed in the basement heats water that circulates naturally through the building’s heaters. In rural areas, stoves are used to heat homes, and in areas where electric energy is cheap, electric heating involving electric space heaters is sometimes used. From a theoretical point of view, direct space heating with electric energy is inefficient because with, for example, heat pumps it is possible to produce more heat than could have been produced by the electricity consumed. In this case, heating will include both the amount of heat that is equivalent to the expenditure of electric energy and a certain amount of heat that is extracted from the environment and “raised” to a higher temperature level. However, heat pumps have not become common because of their high cost.

Mechanical work is obtained from heat by using heat engines—the principal power units of factories, transport vehicles, and other installations run on heat. Heat is converted into electric power by, for example, magnetohydrodynamic generators and thermoelectric generators. As of the mid-1970’s, approximately 30 percent of world heat production was being used to generate electric power.

Theoretical principles of heat engineering. The processes of generating and using heat are based on theoretical principles in heat engineering, that is, on engineering thermodynamics and heat transfer.

Thermodynamics concerns itself with the properties of macroscopic systems in a state of thermodynamic equilibrium and with the processes of transition between these states. An equilibrium state is completely described by a few physical parameters. For example, the state of a homogeneous liquid or gas is determined by any two of the three quantities of temperature, volume, and pressure (seeCLAPEYRON EQUATION, VAN DER WAALS’ EQUATION). The energy equivalence of heat and work is established by the first law of thermodynamics. The second law of thermodynamics determines the irreversibility of macroscopic processes that proceed at a finite rate; it limits the maximum efficiency possible in converting heat into work.

Heat transfer concerns itself with processes of heat exchange between heat-transfer agents through a dividing space or wall and across an interface. In heat engineering equipment, heat can be transferred by radiant heat exchange, convection, and heat conduction.

Radiant heat exchange is typical of furnaces and combustion chambers, as well as of certain stoves. The total energy radiated by any body is proportional to the fourth power of the body’s temperature. At a given temperature, a blackbody emits the most energy. Actual bodies are characterized by their emissivities (total or spectral), which represent the portion of the energy of an ideal blackbody that a given body radiates (over the entire wavelength range or in a narrow band) at the same temperature. The total emissivity of solid bodies usually lies in the range 0.3–0.9. Gases at normal temperatures have a very low emissivity, which, however, increases with the thickness of the radiating layer.

Heat exchange by convection is carried out through the flow of matter in liquids, gases, and free-flowing media. The heating or cooling of liquids and gases in various heat-engineering apparatus occurs through convection, as in hot-blast stoves and the economizers in steam boilers. Heat exchange by convection is most characteristic of processes where a solid wall comes into contact with the turbulent flow of a liquid or gas. Here, the heat is transferred either to or from the wall by the turbulent agitation of the flow. The intensity of this process is given by the heat-transfer coefficient (see alsoCONVECTIVE HEAT EXCHANGE).

Heat exchange by conduction is typical of solid bodies and of the laminar flow of liquids and gases (seeLAMINAR FLOW) in contact with a solid wall. In this case, heat is transferred by a microscopic process of energy exchange between the molecules or atoms of a body. In practice, the heat-transfer process is often caused by the joint effect of the above-mentioned types of heat exchange.