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HORSEPOWER - unit of power equal to 33,000 foot-pounds per minute, equivalent to 745.7 watts.

HUMIDITY - water vapor in the atmosphere. Absolute humidity is the amount of water vapor in a given quantity of air; it is not a function of temperature. Relative humidity is a ratio of actual atmospheric moisture to the maximum amount of moisture that could be carried at a given temperature, assuming constant atmospheric pressure. The higher the temperature--other factors remaining constant--the lower the relative humidity (i.e. the drier the air).

HYDRAULIC FLUID - fluid serving as the power transmission medium in a hydraulic system. The most commonly used fluids are petroleum oils, synthetic lubricants, oil-water emulsions, and water-glycol mixtures. The principal requirements of a premium hydraulic fluid are proper viscosity, high viscosity index, anti-wear protection (if needed), good oxidation stability, adequate pour point, good demulsibility, rust inhibition (see rust inhibitor), resistance to foaming, and compatibility with seal materials. Anti-wear oils are frequently used in compact, high pressure, and high-capacity pumps that require extra lubrication protection. Certain synthetic lubricants and water-containing fluids are used where fire resistance is needed. See fire-resistant fluids.

HYDRAULIC SYSTEM - system designed to transmit power through a liquid medium, permitting multiplication of force in accordance with Pascal’s law, which states that “a pressure exerted on a confined liquid is transmitted undiminished in all directions and acts with equal force on all equal areas.” Hydraulic systems have six basic components: (1) a reservoir to hold the fluid supply; (2) a fluid to transmit power; (3) a pump to move the fluid; (4) a valve to regulate the pressure; (5) a directional valve to control the flow, and (6) a working component--such as a cylinder and piston or a shaft rotated by pressurized fluid--to turn hydraulic power into mechanical systems: they eliminate complicated mechanisms such as cams, gears, and levers; are less subject to wear; are usually more easily adjusted from control of speed and force; are easily adaptable to both rotary and linear transmission of power; and can transmit power over long distances and in any direction with small losses.

HYDROCARBON - chemical compound of hydrogen and carbon; also called organic compound. Hydrogen and carbon atoms can be combined in virtually countless ways to make a diversity if products. Carbon atoms form the skeleton of the hydrocarbon molecule, and may be arranged in chains (aliphatic) or rings (cyclic). There are three principal types of hydrocarbons that occur naturally in petroleum: paraffins, naphthenes, and aromatics, each with distinctive properties. Paraffins are aliphatic, the others cyclic. Paraffins and naphthenes are saturated; that is, they have a full complement of hydrogen atoms and, thus, only single bonds between carbon atoms. Aromatics are unsaturated, and have as part of the molecular structure at least one benzene ring, i.e. six carbon atoms in a ring configuration with alternating single and double bonds. Because of these double bonds, aromatics are usually more reactive than paraffins and naphthenes, and are thus prime carbons for chemical synthesis. Other types of hydrocarbons are formed during the petroleum refining process. Important among these are olefins and acetylenes. Olefins are unsaturated hydrocarbons with at least one double bond in the molecular structure, which may be in either an open chain or ring configuration; olefins are highly reactive. Acetylenes are also unsaturated and contain at least triple bond in the molecule. See saturated hydrocarbons, unsaturated hydrocarbons.

HYDROCARBONS (HC) EMISSIONS - substances considered to be atmospheric pollutants because the more reactive hydrocarbons (e.g., aromatics) undergo a photochemical reaction with nitrogen oxides (NOx) to form oxidants, components of smog that can cause eye irritation and respiratory problems. Motor vehicles account for about one-third of man-made hydrocarbon emissions, although automotive emission controls are reducing this amount. The greatest portion of total atmospheric hydrocarbons is from natural sources, such as pine trees. See catalytic converter, emissions (automotive).

HYDROCRACKING - refining process in which the middle and heavy distillate fractions are cracked (broken into smaller molecules) in the presence of hydrogen at high pressure and moderate temperature to produce high-octane gasoline, turbine fuel components, and middle distillates with good flow characteristics and cetane ratings. The process is a combination of hydrogenation and cracking.

HYDRODYNAMIC LUBRICATION - See full-fluid-film lubrication.

HYDROFINISHING - form of hydrogen treating in which refinery distillate, lube, and wax streams are treated with hydrogen at elevated temperatures and moderate pressures in the presence of a catalyst, to improve color and stability, and reduce sulfur content.

HYDROFORMING - a dehydrogenation process in which naphthas are passed over a solid catalyst at elevated temperatures and moderate pressures in the presence of hydrogen to form high-octane motor gasoline, high-grade aviation gasoline, or aromatic solvents. This process is a net producer of hydrogen.

HYDROGENATION - in refining, the chemical addition of hydrogen to a hydrocarbon in the presence of a catalyst, a severe form of hydrogen treating. Hydrogenation may be either destructive or non-destructive. In the former case, hydrocarbon chains are ruptured (cracked) and hydrogen is added where the breaks have occurred. In the latter, hydrogen is added to a molecule that is unsaturated (see unsaturated hydrocarbon) with respect to hydrogen. In either case, the resulting molecules are highly stable. Temperature and pressures in the hydrogenation process are usually greater than hydrofinishing.

HYDROGEN SULFIDE (H2S) - gaseous compound of sulfur and hydrogen commonly found in crude oil; it is extremely poisonous, corrosive, and foul-smelling.

HYDROGEN TREATING - refining process in which hydrocarbons are treated with hydrogen in the presence of a catalyst at relatively low temperatures to remove mercaptans and other sulfur compounds, and improve color and stability. See hydrofinishing.

HYDROPHILIC - also hydroscopic, having an affinity for water. Some polar compounds are simultaneously hydrophilic and oil soluble.

HYDROPHOBIC - the opposite of hydrophilic.

HYDROSTATIC LUBRICATION - See full-fluid-film lubrication.

HYDROSCOPIC - See hydrophilic.

HYPOID GEAR - See gear.