The thermostat (q.v.) is usually of the metallic coil or the volatile liquid bulb and bellows type. In either case a change of temperature causes a motion of a contact point or of the arm of a lever sufficient to cause the make or break of an electric circuit. The latter may be direct on the motor line or it may operate a relay switch. The evaporating pressure is controlled either by means of a float valve, operated either on the condenser pressure or the suction pressure side of the system or by means of a pressure reducing (expansion) valve. The expansion valve is of the needle valve principle, using a spring loaded diaphragm. Varia tion of the suction pressure may be obtained by a manual control of the tension of the spring. The water pressure control may be from the principle of the temperature of the water leaving the condenser which acts on a bellows filled with a volatile liquid or on the principle of a spring loaded diaphragm subjected to the condenser pressure. Finally the excess pressure shut off is another loaded diaphragm so connected as to break the electric circuit should the pressure become greater than the safety limit.
The household machine uses sulphur dioxide nearly always as the refrigerant. [See REFRIGERATORS (HOUSEHOLD).] Heat Transfer.—The subject of heat transfer is of the great est importance in refrigerating industries. In cold storage plants it is necessary at times. to install a thickness of 8 in. or more of corkboard on the walls, ceilings and floors. The best insula tion for cold temperatures is one that is water-proof, that has a low value for the coefficient of heat transmission and is easily erected in place. The low value for the heat transmission is ob tained by means of small air cells in the structure as is found in cork, wood and vegetable products, mineral wool and hair felt. The insulating property of the material is obtained from the effect of the air cells which are so small as to prevent convection cur rents, and so the statement is frequently made that the material which has. the greatest number of air cells has the greatest value as an insulator. However, in refrigeration, the tendency is to ab sorb moisture from the air and to have it collect on the coldest surface. Therefore the material must be made waterproof if it is not so naturally. Saw-dust, shavings and materials of a corn pressed vegetable nature like corn stalks, sugar cane stalks, straw, etc., are suitable as heat insulators providing they are water proofed. As this is not practical in a number of cases except by the use of waterproof paper such construction is not used in the best installations.
Formula for Heat Transfer—If the value k of the coefficient of heat transfer is known in B.T.U. per square foot per one degree difference of temperature per hour then the total heat transfer per hour is found by Q= Akt. where Q is the total heat transfer in B.T.U. per hour, A is the area of the surface in square feet and t,„ is the temperature difference on the two sides of the surface. The coefficient of heat transfer (k) must be found by experiment and certain of these are given in Table III. If a built up structure is used, composed of several thicknesses of material then the f or mula usually employed is: where T is the thickness of each material in inches and C is the coefficient of con ductivity in B.T.U. per i inch thick.