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a device for changing a substance from its gaseous (vapor) state to its liquid or solid state. condensers are widely used in chemical technology in thermal-power and refrigeration installations to condense the working substance, in evaporation equipment to obtain distillates and to separate vapor mixtures, and elsewhere. The condensation of vapor in a condenser occurs as a result of the contact between the vapor and the surface of a solid (surface condensers) or a liquid (contact condensers) having a lower temperature than the saturation temperature of the vapor at a given pressure. The condensation is accompanied by the liberation of heat previously expended to evaporate the liquid; the heat must be removed by some cooling agent.
Surface condensers are usually constructed as a bank of horizontal or vertical tubes. In this case, the cooling medium (water, brine, air) flows within the tubes and the vapor enters the space between the tubes and condenses on their outer surfaces, or vice versa. The space in which condensation occurs may be at atmospheric pressure or at higher or lower pressures. The design of surface condensers is similar to that of other surface-type heat exchangers (usually of a shell-and-tube design). condensers of this type are used when it is mandatory to preserve the purity of the condensate.
If a liquid is formed upon condensing the vapor, it drains from the heat-transfer surface under the action of gravity or it may be carried away by the moving vapor. If a solid phase is formed (for instance, ice), it is continuously or periodically removed by scrapers or other devices. If air or other gas is used as the cooling medium, the condenser surface exposed to the medium is usually equipped with fins, which are used to increase the rate of heat transfer.
In contact condensers the condensate obtained is mixed with the cooling liquid and is then withdrawn together with the liquid.
Depending on the direction of motion of the vapor and the condensate with respect to each other, there are direct-flow, counter-flow, or crossflow condensers. The condensate is usually removed from the condenser by a pump; the noncondensable gases are sucked out by a vacuum pump. To increase the surface of contact between the vapor and liquid, the liquid in a contact condenser is broken up into jets or drops (by overflow devices, spray plates, spray nozzles). The condensation of vapor then occurs on the surface of such jets or drops. Sometimes the vapor is carried into the volume occupied by the liquid and permeates the liquid in the form of bubbles, on the surface of which condensation occurs. To assure a trouble-free operation, condensers are equipped with a number of auxiliary devices, thus forming a condensing installation.
REFERENCESShumskii, K. P. Vakuumnye kondensatory khimicheskogo mashinostroeniia. Moscow, 1961.
Kirsanov, I. N. Kondensatsionnye ustanovki Moscow-Leningrad, 1965.
Kasatkin, A. G. Osnovnye protsessy i apparaty khimicheskoi tekhnologii, 8th ed. Moscow, 1971.
L. D. BERMAN
a short-focus lens or system of lenses used in optical instruments to illuminate an object being examined or projected. A condenser collects and directs toward the object rays originating at a light source, including rays that would otherwise have missed the object. As a result, the illumination of the object is sharply increased. condensers are used in microscopes, spectral instruments, and various types of projectors (slide projectors, episcopes, photographic enlargers, and so on).
The design of a condenser becomes more complex with increasing aperture. For numerical apertures up to 0.1 a single lens is used; for apertures of 0.2–0.3, a two-lens condenser; for apertures greater than 0.3, a three-lens condenser. The most common type of condenser consists of two identical plane-convex lenses whose curved surfaces are facing each other to reduce spherical aberration (see Figure 1). Sometimes the surfaces of condenser lenses have a more complex shape (a paraboloid or ellipsoid).
The resolving power of a microscope increases with an increase in the aperture of its condenser; therefore, microscope condensers are usually built as complex two-lens or three-lens systems. In microscopes and motion-picture projectors, mirror condensers and mirror-lens condensers are widely used. The aperture of such condensers can be very large; the expansion angle 2u of the light beam being condensed can be as great as 240° (Figure 2). Frequently several lenses are used in a condenser, not only be-cause of the desire to increase the aperture but also because of the necessity to provide uniform illumination of the object even in cases where the structure of the light source is nonuniform (see Figure 3).