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836 CHAPTER 37 1953 Guide /. the low pressure side,of the system. A .needle valve, operated through a simple lever mechanism attached to the float, permits the passage of more or less refrigerant, as the level in the receiver or the evaporator fluctuates. Such a control must be used in conjunction with a flooded evaporator, and has been applied-extensively to household refrigerators and, to some extent, in commercial and industrial, instal lations. High-Side Float Valves. A high-side float valve differs from a low-side float valve in that the float is located in a receiver or container on the high pressure side of the system. , Proper .operation again dependsupon metering of, the refrigerant through a controlled opening, depending uppn the level of the liquid refrigerant in the con tainer. Such a control lias the disadvantage that the evaporator must be placed directly adjacent to the float container, or some intermediate pressure device must be applied to prevent flashing of the refrigerant upon pressure:drop.`.: * Capillary Tubes. A capillary tube may be used as a liquid refrigerant expanding device. . Such a device consists of an extremely small bore tube (in the order of 0.04 inch in diameter) of five, to twenty feet in length. Although such a restricting device operates as a very simple means of expanding the liquid refrigerant, it has the disadvantage that no modifications are possible to adjlist the rate of expansion under various operating conditions. The bore and length of the tube, as well as the proportions of the rest of the system, are critical. It is for these reasons that its application has been limited to factory assembled domestic and commercial units. Refrigeration Control Devices In addition to automatic control of expansion of the liquid refrigerant, a completely automatic refrigeration system requires i(l) some means for on-off operation of the compressor motor, (2) control of the flow of the con densing medium,- and (3) safety devices for prevention of damage to the equipment. In addition, special controls designed for specific applications are frequently required. The various types of devices used to accomplish these purposes are so numerous that it would be impossible to describe all of their modifications. Only the general purposes and operating char acteristics of the more typical mechanisms are here discussed. Compressor Motor Controls. Two types of controls are used for intermittently starting and stopping compressors. The first of these is a pressure motor control responsive to the evaporator pressure, and the second a thermostatic motor control responsive to the temperature of the load surrounding the evaporators. In the 'first case the compressor operation is indirectly dependent, upon the temperature of !the load, and is.controlled by the refrigerant pressure at the point of control location. |.The second type is dependent upon the temperature of the load being cooled. With the pressure actuated device, the control is frequently located directly on the condensing unit, and the low pressure in the suction line or the crankcase of the compressor is used to control motor operation. Such a control usually consists of- a low pressure bellows, connected through tubing directly to the low pressure control '.source, and an electrical switch operated through linkage by the movement of the bhllows. The electrical circuit is closed on rising pressure, and opened on falling pressure. The thermostatic type of motor control is usually similar in construction tofthe pressure control, with the exception that a temperature bulb and capillary tube replace the pressure line, and the temperature bulb is located adjacent to the evaporator itself. In this case motor control is directly responsive to changes in the temperature of the load surrounding the evaporator. Frequently, a high pressure safety, cutout switch is combined with the motor control, and operates to cut off the power from the motor in case the high side pressure exceeds a predetermined limit. Solenoid Valves: Solenoid or magnetic stop valves are frequently used in re frigeration systems for control of gas and liquid flow. When applied as liquid stop valves, they are placed in the liquid line between the condenser or receiver and the evaporator; and the line is open to passage of the refrigerant only when the com pressor is in operation. When the compressor is not in operation, leakage of liquid refrigerant in the evaporator is prevented. In some cases such a magnetic stop valve is operated directly by a thermostat located at the point of the load, and the compressor motor' operation is controlled independently by a low pressure switch. Refrigeration 837 Magnetic liquid, stop valves are also widely used for the control ofi the, refrigerant flow to individual^ evaporators in a multiple evaporator system operated by one .compressor.! In some installations magnetic liquid line and suction'lihe"valves are used to isolate completely an evaporator for defrosting purposes/ A magnetic valve may be installed in a by-pass around one or more cylinders of a multiple cylinder .Compressor, .and thereby.be used to unload a compressor during starting to. reduce load. Additional applications are found in control of the circmation of brine in a secondary refrigeration system. ,, Suction Pressure Valves. Suction pressure control valves, frequently called back .pressure regulators or two-temperature valves, are sometimes placed in the suction lihe to prevent the evaporator pressure and temperature from dropping below: a pre determined level. Typical applications of such controls occur in water.cooling or <milk cooling systems, where freezing and other damage would result if the evaporator pressure dropped too low; or in multiple systems where several evaporators are supplied by. one condensing unit. Thus, different evaporators may be kept at 'different temperatures by maintaining a pressure drop between the evaporator and the suction line. ,.... Condensing Water Control. The majority of the refrigeration systems, other than fractional horsepower, use water cooled rather than air cooled condensers, since the lower condensing temperatures result in more economical operation. Automatic control of the water flow to the condenser must be maintained if water wastage is to be eliminated. Such control may be provided through, the use of either an electric solenoid water valve or by means of a pressure control valve. With a solenoid valve, the flow is two-position, either off. or on, and its operation is simultaneous with starting and stopping of the compressor motor. With a pressure operated valve, the flow is modulated and is dependent entirely upon condenser pressure'rather than condensing unit operation. Similar water valves controlled thermostatically by the temperature of the water discharged from the condenser are also available. Safety Controls. Many controls are designed not to aid in proper operation of the system, .but to prevent damage in case of improper operation. One such' safety control is the high-pressure cutoff frequently combined with the low-pressure motor control as previously described. Another safety control often used is a low; voltage cut-off which shuts down the system automatically in case the line voltage drops below a minimum value. High pressure relief valves are used for safety.purposes to prevent damage in case excessive condensing pressures are encountered.<. Refrigeration Control for Air Conditioning Equipment When refrigerating equipment is used for space cooling, two: major control problems exist: one is control of the temperature and the-other, control of the humidity. In some applications the amount of latent heat to be removed is small compared with the sensible heat. In such cases, sufficient dehumidification will usually occur without any special provisions. .In other cases, such as theaters, where the latent load is.relatively, high, the air must be cooled below its dew-point temperature, and sometimes re warmed to return it to the comfort range., Chapter 39; Automatic Control, discusses, among other topics, control systems for.unit coolers, well water and ice cooling systems, central fan systems, and all-year air conditioning systems. REFRIGERATION PIPING " The pressure drop which occurs during passage of the refrigerant through connecting piping is similar in effect to that which occurs through suction and discharge valves of the compressor. Thus, the effect of the pressure drop in the suction line between evaporator and compressor requires that a Tower pressure be maintained inside the compressor during suction- than ' is maintained in the evaporator. The pressure drop through the connect; ing piping between the compressor and the condenser requires -that a higher ` pressure be maintained inside the compressor during discharge -than in the