Air, standard Dry air at 70 degrees F, 21。11 degrees C, and  29。92 in。 Hg

barometer。 This is substantially equivalent to 0。075 pounds per cubic feet (lb/ft3)。 Specific heat of dry air = 0。24 Btu/lb-F (1。004 kJ/(kg。K)。

Aspect ratio (AR) Ratio of the width to the length; AR = W/L。 Blast gate Sliding damper。

Capture velocity Air velocity at any point in front of the hood or at the hood

opening necessary to overcome opposing air currents and to capture the contaminated air at that point by causing it to flow into the hood。

Dust Small solid particles created by the breaking up of larger particles by processes crushing, grinding, frilling, explosions, etc。 Dust particles already in existence in a mixture of materials may escape into the air through such operations as shoveling, conveying, screening, and sweeping。

Dust collector Air cleaning device to remove heavy particulate loadings

from exhaust systems before discharge to outdoors。 Usual range: loadings 0。003 grains per cubic foot and higher。

Fan class This term applies to the fan’s performance abilities。 The required fan class is determined according to the operating point of the ventilation system。 AMCA 99-2408 provides a set of five minimum performance limit standards (Class I through V) which manufactures use to apply the correct class to their fans。

FRP Fiberglass reinforced plastic used in construction of such items as boats and airplanes。 It is also used for ductwork in corrosive environments。

Fumes Small, solid particles formed by the condensation of vapors of solid materials。

Gases Formless fluids which tend to occupy an entire space uniformly at ordinary temperatures and pressures。

Gravity, specific Ratio of the mass of a unit volume of a substance to the

mass of the same volume of a standard substance at a standard temperature。 Water at 39。2 degrees F is the standard substance usually referred to for gases, dry air, at the same temperature and pressure as the gas is often taken as the standard substance。

Hood A shaped inlet designed to capture contaminated air and conduct it into the exhaust duct system。

Humidity, relative Ratio of the actual partial pressure of the water vapor in a

space to the saturation pressure of pure water at the same temperature。

Lower explosive limit Lower limit of flammability or explosiveness of a gas or (LEL) vapor at ordinary ambient temperatures expressed in

percent of the gas or vapor in air by volume。 This limit is assumed constant for temperatures up to 250 degrees F。 Above these temperatures, it should be decreased by a factor of 0。7 since explosiveness increases with higher temperatures。

Manometer An instrument for measuring pressure; essentially a U-tube

partially filled with a liquid, usually water, mercury or a light oil, so constructed that the amount of displacement of the liquid indicates the pressure being exerted in the instrument。

Micron A unit of length; the thousandth part of 1 millimeter or the millionth of a meter (approximately 1/25,000 of an inch)。

Mists Small droplets of materials that are ordinarily liquid at normal temperature and pressure。