Heat also facilitates the transmission of water through the cell walls, thereby assisting its passage from the interior to the surface of the material; it increases the vapour pressure of water, thus increasing its tendency to evaporate; and it increases the water‐vapour‐carrying capacity of the air. In the United States the unit of heat customarily used is the British thermal unit (B.t.u.), which for practical purposes is defined as the heat required to raise the temperature of a pound of water 1° F. Heat is commonly produced through the combustion of oil, coal, wood, or gas. Heating by electricity is seldom practicable because of its greater cost; but where cheap rates prevail it is one of the safest and most efficient, convenient and easily regulated methods. Direct heat, direct radiation and indirect radiation are the types of heat generally employed. Direct‐heating systems have the highest fuel or thermal efficiency. The mixture of fuel gases and air in the combustion chamber passes directly into the air used for drying. This method requires the use of special burners and a fuel, such as distillate or gas, which burns rapidly and completely, without producing soot or noxious fumes. The heater consists simply of a bare, open firebox, equipped with one or more burners, an emergency flue to discharge the smoke incidental to lighting, and a main flue, through which the gases of combustion are discharged into the air duct leading to the drying chamber. Direct‐radiation systems also are relatively simple and inexpensive and have a fairly high thermal efficiency. A typical installation consists of a brick combustion chamber with multiple flues, which carry the hot gases of combustion back and forth across the air‐heating chamber and out to a stack. The air is circulated over these flues and heated by radiation from them. The flues are made of light cast iron or sheet iron. The air‐heating chamber should be constructed of fireproof materials. The efficiency of the installation depends upon proper provision for radiation. This is attained by using flues of such length and diameter that the stack temperatures will be as low as is consistent with adequate draught. Heating the air by boiler and steam coils or radiators is an indirect‐radiation system, as the heat is transferred from the fuel to the air through the intermediate agency of steam. Such a system costs more to install and has a lower thermal efficiency than either of the other two systems. It is principally adapted to large plants operating over a comparatively long season on a variety of materials where the steam is needed to run auxiliary machinery or to process vegetables. Large volumes of air are required to carry to the products the heat needed for evaporation and to carry away the evaporated moisture. Insufficient air circulation is one of the main causes of failure in many dehydrators, and a lack of uniformity in the air flow results in uneven and inefficient drying. The fan may be installed to draw the air from the heaters and blow it through the drying chamber, or it may be placed in the return air duct to exhaust the air from the chamber. An advantage of the first installation is that the air from the heaters is thoroughly mixed before it enters the drying chamber. It has been claimed that exhausting the air from the chamber increases the rate of drying by reducing the pressure, but the difference is imperceptible in practice. Either location for the fan is satisfactory, and the chief consideration in any installation should be convenience. Close contact between the air and the heaters and between the air and the material is necessary for efficient transfer of heat to the air and from the air to the material, and to carry away the moisture. The increased pressure or resistance against which the fan must operate because of such contact is unavoidable and must be provided for, but at other points in the system every effort should be made to reduce friction. To this end air passages should be large, free from constrictions, and as short and straight as possible. Turns in direction should be on curves of such diameter as will allow the air to be diverted with the least friction. The general rule in handling air is that a curved duct should have an inside radius equal to three times its diameter. The water vapour present in air at ordinary pressures is most conveniently expressed in terms of percentage of relative humidity. Relative humidity is the ratio of the weight of water vapour actually present in a space to the weight the same space at the same temperature would hold if it were saturated. Since the weight of water vapour present at saturation for all temperatures here used is known, the actual weight present under different degrees of partial saturation is readily calculated from the relative humidity. Relative humidity is determined by means of two thermometers, one having its bulb dry and the other having its bulb closely covered by a silk or muslin gauze kept moist by distilled water. Tap water should not be used because the mineral deposits from it clog the wick, retard evaporation, and produce inaccurate readings. The wick must be kept clean and free from dirt and impurities. The two thermometers are either whirled rapidly in a sling or used as a hygrometer mounted on a panel with the wick dipping in a tube of water and the bulbs exposed to a rapid and direct current of air. The relative humidities corresponding to different wet‐ and dry‐bulb temperatures are ascertained from charts furnished by the instrument makers, or published in engineers' handbooks. As a general rule, the more rapidly the products have been dried the better their quality, provided that the drying temperatures used have not injured them. Some fruits and vegetables are more susceptible to heat injury than others, but all are injured by even short exposures to high temperatures. The duration of the exposure at any temperature is important, since injury can be caused by prolonged exposure at comparatively moderate temperatures. The rate of evaporation from a free water surface increases with the temperature and decreases with the increase of relative humidity of the air.
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