heat capacity Ratio of heat absorbed by a material to the change in temperature. It is usually expressed as calories per degree in terms of the amount of the material being considered. Heat capacity and its tempera¬ ture variation depend on differences in energy levels for atoms. Heat capacities are measured with a calorimeter and are important as a means of determining the entropies of materials. See also specific heat.
heat exchanger Any of several devices that transfer heat from a hot to a cold fluid. In many engineering applications, one fluid needs to be heated and another cooled, a requirement economically accomplished by a heat exchanger. In double-pipe exchangers, one fluid flows inside the inner pipe, and the other in the annular space between the two pipes. In shell-and-tube exchangers, many tubes are mounted inside a shell; one fluid flows in the tubes and the other flows in the shell, outside the tubes. Special-purpose devices such as boilers, evaporators, superheaters, con¬ densers, and coolers are all heat exchangers. Heat exchangers are used extensively in fossil-fuel and nuclear power plants, gas turbines, heating and air conditioning, refrigeration, and the chemical industry. See also COOLING SYSTEM.
heat exhaustion or heat prostration Response of the body to excessive heat. The body temperature rises moderately and heavy perspi¬ ration persists. Heat exhaustion results from inadequate water and salt intake and can lead to dehydration and collapse. It may progress to heat¬ stroke if not treated by lying down in a cool place and drinking fluids, preferably water with salt added.
heat pump Device for transferring heat from a substance or space at one temperature to another at a higher temperature. It consists of a com¬ pressor, a condenser, a throttle or expansion valve, an evaporator, and a working fluid (refrigerant). The compressor delivers vapourized refriger¬ ant to the condenser in the space to be heated. There, cooler air condenses
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heat-treating ► heavy spar I 853
the refrigerant and becomes heated during the process. The liquid refrig¬ erant then enters the throttle valve and expands, coming out as a liquid- vapour mixture at a lower temperature and pressure. It then enters the evaporator, where the liquid is evaporated by contact with the warmer space. The vapour then passes to the compressor and the cycle is repeated. A heat pump is a reversible system and is commonly used both to heat and to cool buildings. It operates on the same thermodynamic principles as REFRIGERATION.
heat-treating Changing the properties of materials such as metals or glass by processes involving heating. It is used to harden, soften, or modify other properties of materials that have different crystal structures at low and high temperatures. The type of transformation depends on the temperature that the material is heated to, how fast it is heated, how long it is kept heated, what temperature it is first cooled to, and how fast it is cooled. For example, quenching hardens steel by heating it to high tem¬ peratures and then quickly immersing it in room temperature oil, water, or salt brine to “freeze” the new crystal structure; in cryogenic treatments the cooling bath ranges from -180 to -70 °C (-300 to -100 °F), and it is often used in treating high-carbon and high-alloy steels. The two main approaches to softening a metal (to restore its ductility) are annealing, in which its temperature is slowly raised, held for some time, and slowly cooled, and tempering, in which it is slowly heated in an oil bath and held for some hours.
Heath, Sir Edward (Richard George) (b. July 9, 1916, Broad- stairs, Kent, Eng.—d. July 17, 2005, Salisbury, Wiltshire) British politi¬ cian, prime minister of Britain (1970-74). He held various government positions after being elected to Parliament in 1950, and after the Conser¬ vative defeat in 1964 he became a major opposition figure. As prime min¬ ister, he faced the crisis of violent conflict in Northern Ireland, over which he imposed direct British rule in 1972, and won French acceptance of British entry into the European Economic Community. Unable to cope with Britain’s mounting economic problems, chiefly rising inflation and unem¬ ployment and crippling labour strikes, he was succeeded as prime min¬ ister by Harold Wilson in 1974 and replaced as party leader by Margaret Thatcher in 1975. Heath, who was knighted in 1992, remained in the House of Commons until 2001.
heath family Family Ericaceae, made up mostly of shrubs and small trees, including azaleas, rhododendrons, mountain laurel, blueberries, and the low evergreen shrubs of the genus Erica (see erica). A large percent¬ age of the family’s approximately 110 genera and 4,000 species are cul¬ tivated. Members are widely distributed, extending into the subarctic and along mountain chains through the tropics. They are often evergreen spe¬ cies that thrive on open, barren land with usually acid and poorly drained soils. See also heather.
Heathcoat, John (b. Aug. 7, 1783, Duffield, Derbyshire, Eng.—d. Jan. 18, 1861, Tiverton, Devon) British inventor. The lacemaking machine he patented in 1809, the most complex textile machine then in existence, produced an exact imitation of handmade pillow lace. His lace factory was destroyed by Luddites in 1816. He later developed means for orna¬ menting net in the course of manufacture, for making ribbons as well as plaited and twisted net, and for winding raw silk from cocoons.
heather or Scotch heather Low evergreen shrub (Calluna vulgaris ) of the heath family, widespread in western Europe and Asia, North America, and Greenland. It is the chief vegetation on many wastelands of northern and western Europe. C. vulgaris is distinguished from true heaths, which are sometimes loosely called heather, by the lobes of its calyx (see flower), which conceal the petals; in true heaths the petals cover the calyx. Scotch heather has purple stems, close-leaved green shoots, and feathery spikes of bell-shaped flowers. It has various economic uses: large stems are made into brooms, shorter ones are tied into bundles that serve as brushes, and long trailing shoots are woven into baskets.
heating Process of raising the temperature of an enclosed space. Heat can be delivered by convection, radiation, and thermal conduction. With the exception of the ancient Romans, who developed a form of central heating, most cultures relied on direct heating methods such as fireplaces and stoves. Central heating, adopted for use again in the 19th century, is a method of indirect heating: heat is produced away from the occupants and then conveyed to them. In warm-air heating, air heated by a furnace rises through ducts to rooms above, where it is emitted through grills. In hot-water systems, a pump circulates water from a boiler through a sys¬ tem of pipes to radiators or convectors in rooms. In steam systems, steam
is generated in the boiler and led to radiators through pipes. The high temperature of the steam makes it hard to control, and steam heating has been largely superseded. A common type of electric heating system con¬ verts electric current to heat by means of resistors that emit radiant energy. See also radiant heating, solar heating.
heatstroke Debility caused by exposure to heat and humidity, usually for many hours, called sunstroke when caused by direct sunlight. Body temperature is 106-110°F (41^J3°C) or higher. Perspiration almost stops, leading to the rapid temperature rise, collapse, and coma. Cooling with ice-water baths or packs, with massage to promote circulation, is urgent to save the victim’s life. Even after body temperature drops, circulatory disorder and brain damage may cause death. See also heat exhaustion.
heaven Dwelling place of God or the gods and the abode of the blessed dead. The term also refers to the celestial sphere, the place of the sun, moon, planets, and stars and the source of light, which symbolizes good. For later Judaism and Chris¬ tianity, heaven is the destination of the faithful after a general resurrec¬ tion of the dead, in contrast to hell, the place of punishment for the wicked. Islam has a similar belief. In Chinese religion, heaven is equated with the divine will, which guides the operation of all physical and moral laws. In some Mahay ana Bud¬ dhist sects, heaven is a paradise for those who have received the saving grace of Amitabha.