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evaporators are all examples of heat exchangers. Heat always moves from the object or medium having the
greatest level of heat energy to a medium or object having a lower level.
Air-to-air, air-to-liquid, liquid-to-air, and liquid-to-liquid heat exchangers are commonly used with
aircraft engines.
heating element (electrical heater). A coil or a piece of resistance wire connected across a source of
electrical energy. The resistance of the element is high enough that current flowing through it produces
enough heat to cause it to glow red-hot.
heat lamp. A form of incandescent lamp that produces a maximum of infrared radiation (heat) with a
minimum of visible light rays. Heat lamps are used for curing resins, drying paint, keeping food warm, and
applying heat to parts of the human body.
heat load (air conditioning system). The amount of heat that must be added to or taken away from the
inside of an aircraft cabin, during a specified unit of time, to keep the temperature of the cabin constant.
heat pump. A device that moves heat from one location to another. An air conditioning system is a form of
heat pump. Heat from inside the aircraft cabin is absorbed by a liquid refrigerant. When the refrigerant
absorbs the heat, it evaporates, or changes into a gas. The gas is then compressed, and the compression
raises its temperature higher than that of the outside air.
The compressor forces the hot gas through the condenser, located outside the aircraft. Here, the
gas transfers some of its heat to the outside air and condenses back into a liquid. Heat in this way is
pumped from inside the aircraft cabin to the air outside the aircraft.
heat-shrinkable fabric. Certain inorganic fabrics used to cover aircraft structure. The fabric used for this
purpose is sold in its unshrunk state and is put on the structure so that it is taut, but not tight. After it is
secured to the structure with a special adhesive, it is shrunk to the correct tautness by ironing it with an
electric iron or blowing it with a special heater/blower.
After the fabric has the correct tautness, it is coated with a non-tautening dope. Organic fabrics,
such as cotton and linen, are not heat-shrinkable and are shrunk on the structure with a shrinking-type dope.
heat sink. A piece of heavy metal on which such electrical components as power transistors are mounted.
Heat from the component is transferred into the heat sink by conduction, and from there it is dissipated into
the surrounding air. A silicone grease may be spread between the component and the heat sink to help
transfer the heat.
Many heat sinks on which high-power components are mounted have fins to increase their cooling
area.
heat treatment (metal). A process by which certain types of metal are hardened or softened by heat. Most
metals containing iron can be hardened by heating them to a high temperature and quenching them in water,
brine (a solution of salt and water), or oil. These same metals can be softened by heating them to a high
temperature and allowing them to cool slowly in an oven.
Copper cannot be hardened with heat. When copper is heated red-hot and quenched in water, it
becomes soft. Metals that cannot be hardened with heat can be work hardened by bending, hammering,
rolling, or stretching. However, most of these metals can be softened by heating them and cooling them
slowly.
heat treatment (plastic resins). A process used to strengthen cemented joints in thermoplastic resins.
After the cemented joint is completed, the material is held at an elevated temperature (not high enough to
distort the material) for a specified period of time.
The heat causes solvents concentrated in the joint to diffuse (to spread evenly) throughout the
material. This diffusion decreases the concentration of the solvents in the joint and increases its strength.
Heaviside layer (atmosphere). A layer of ionized particles that surrounds the earth at a height of about 55
to 85 miles above the surface. The Heaviside layer is also called the Kennelly Heaviside layer and the
E-region of the atmosphere. The Heaviside layer reflects relatively long, low-frequency radio waves.
heavy ends (petroleum refining). The components of crude petroleum that have the highest boiling points.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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In the process of fractional distillation, the lighter products boil off first and are condensed. The heavy ends
are the parts of the petroleum that boil off last.
I
heavy hydrogen. Isotopes of hydrogen having one or two neutrons in the nucleus. Isotopes with one
neutron are called deuterium 2H, and isotopes with two neutrons are called tritium 3H.
heavy water. A compound of hydrogen and oxygen containing a larger proportion of deuterium (an isotope
　
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