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726 CHAPTER 45 1965 Guide And Data Book loaded expansion valve. This arrangement can provide im proved modulation over a wide range of load and pressure drop conditions. Application A refrigeration system in which a high side float valve is used ordinarily consists of a single evaporator, compressor and condenser. The operating receiver or a liquid sump at the condenser outlet can be quite smalL A full sized receiver is required for pumping out the flooded evaporator. The high side float valve may be used to feed more than one evaporator in a system, under certain conditions, in which case addi tional control valves are required. One of the disadvantages of the high side float valve system is that the amount of sys tem refrigerant charge is critical. The use of an excessive amount of system charge will cause floodback, while an in sufficient amount of system charge will cause a reduction in system capacity. LOW SIDE FLOAT VALVES The low side float valve-performs the same function as the high side float valve, but it is connected to the low pressure side of the system. When the evaporator liquid level drops, the float opens the valve, allowing refrigerant liquid from the liquid line to flow through the valve port anH enter, directly into the evaporator or surge drum. In-another type of valve design, the refrigerant liquid flows .through the valve port, passes through a remote-feed line, and enters the evaporator through a separate connection. A typical direct-feed valve construction is shown in Fig. 18. Selection - Low sidefloat valves are selected in the same manner as the high side float valves discussed previously. Application In the low side float valve system, the refrigerant system charge is not critical. The-low side-float valve can be used in multiple evaporator systems in which some of the evaporators may be controlled by other low side float valves, and some by thermostatic expansion valves. The float valve, depending on its design, may.be mounted directly in the evaporator or surge drum, or it may be mounted in an external chamber which is connected to the evaporator or surge chamber with equalizing lines, Le., a-gas line at the top and a liquid line at the bottom. In the externally mounted type, the float valve is separated from the float chamber by a gland winch provides a quiet level of liquid in the float chamber for steady actuation of the valve. In evaporators with high boiling rates or restricted liquid and gas passages, the boiling action of.the liquid will raise the refrigerant level during operation. When the compressor stops or the solenoid suction valve closes, the boiling action of the refrigerant liquid ceases and the refrigerant level in the evapo rator drops. Under .these conditions, the high pressure liquid line supplying the low side float valve should be shut off by a solenoid liquid valve to prevent overfilling the evaporator. Otherwise,, excess refrigerant will , enter, the evaporator on the of cycle and can cause floodback when the. compressor starts or the solenoid suction valve opens. When a low aide float valve'is used, precautions must be taken to have the float in a quiet liquid level that properly falls in response to an increase in evaporator load, and rises with a decrease in evaporator load. In low temperature sys- terns, particularly, it is important that the equalizer lines be tween the float chamber and the evaporator, or between the surge drum and the evaporator, be generously sized to elimi nate any reverse response of the refrigerant liquid level in tha vicinity of the float. Where the low side float valve is located in a non-refrigerated room, the equalizing liquid and gas lines and the float chamber must be insulated in order to provide a quiet liquid level for the float. SOLENOID VALVES A solenoid valve is a valve that is closed by gravity, pres sure, or spring action, and is opened by the movement of a plunger, caused by the magnetic action of an electrically ener gized coil, or Vice versa. Figs. 19 and 20 show cross-sections of solenoid valves with their principal .components..identified. The-solenoid is composed of the coil and parts that make up .tiie magnetic circuit. The plunger, coil housing, coil plate, mid coil sleeves are made of magnetic materials. In the par-feWt type"of solenoid valve, the plunger is enclosed in a'nonmag netic enclosing tube. . Solenoid valves, being electrically actuated, may be con veniently operated in remote locations by any suitable electric switch. These valves are always fully open or fully closed; in contrast to motorized valves, .which may be designed to operate in a modulating position. They may be used to`con trol the flow of many different fluids, provided due considera tion is given to the pressures and temperatures involved, the viscosity of the fluid, and the suitability of the materials used in the valve construction. Solenoid valves can be divided into the following general types: .: . . 1. Normally-dosed adenoid taloe. A-valve in which the closure member is.moved away from the port to cause the open action when the coil is energized, e.g., a two-way solenoid valve. 2. Normatly-open adenoid tube. A valve-in which the closure member is moved to the port to cause the doting action when the coil is energized, e.g., a two-way Bolenoid valve. . ..... . '. 3. Mvltmction adenoid tube. A valve which combines in one body the action of one or more normally-open and one-or more normally-doted solenoid valves, e.g., a- three-way solenoid valve or a four-way Bolenoid valve. . Refrigerant Control Devices 727 Operation While all types of solenoid valves are in' common use, the normally-closed type is used far more extensively than the others. Therefore, the following discussion pertains to the normally-closed type, unless otherwise specifically stated. In tiie normally-dosed direct-acting solenoid,valve shown in Fig. 19, the powerof thesolenoid coil, acting on the plunger, pulls the valve pin away from and off the valve port, thereby opening it directly. Since this valve depends,solely, on the power of the solenoid coil for operation, its port axe for a given operating pressure differential is. limited by the'practical limitations of solenoid coil size. fig. 20 shows one of the medium size,'norihQUy-dosed, pilot* operated solenoid valves. In this valve, the solenoid coil, acting on the plunger, does not open the main port directly,, but, instead, merely opens the pilot port A. Pressure trapped on top of the piston B is released through the pilot port, thus dealing a pressure unbalance across the piston, forcing it up ward.and opening the main port C. When the solenoid coil is de-energized, the plunger drops and doses pilot port A. Then the pressure above and below the piston equalize' again, and the piston drops and closes the main port. .In some pilotoperated solenoid valve designs, a diaphragm, is used for the main doting member instead of a piston.. Ordinarily in medium size valves, the pitotport. is located in the main finding member. In the large size valves, where the move ment of the doting member is greater, it is frequently-neces sary to locate the pilot port at a point remote from the main doting member, as a matter of practical design. Such pilot-operated valves depend on a certain minimum pressure drop across the valve (approximately 0.5 psi or Rg. 21 .... Normally-Closed Pilot-Operated Solenoid Valve with the Hammer-Blow and Mechanically linked Piston-Pin Plunger Features more) to hold'the piston or main closure member in the open position. Where it is desirable to keep the valve open without this pressure drop penalty, such as on refrigeration suction lines, the piston may be mechanically linked to the solenoid valve pin and plunger, as shown in Fig. 21. Hie opening and closing actions are the same as before. However, advantage is taken of the increased pulling force of the plunger, as it approaches its stop position in the coil,' to hold the piston in the open position, without requiring valve pressure drop for this action. In order to obtain the maximum operating pressure differ ential for a given solenoid pulling power/many valve plungers are constructed so that the plunger is free to gain momentum before it knocks the valve pin out of the valve port with an impact or a hammer-bloto effect. Solenoid valves with the hammer-blow feature must have the full rated voltage, within the eustomary tolerance of + 10 to -- 15 percent, applied instantaneously to their coils so that the valves will open under rated conditions. Otherwise they will fail. If, after this valve is in the open position, the line voltage drops below the hold-in voltage value, but not to zero, the plunger will drop and the valve will close. After the line.voltage builds up again, the valve will not reopen, whether it is used on alternating or direct current, because the hammer-blow effect is lost. However, in the case of the alternating current valve, the coil will overheat and may burn out under this condition, because the inherent high inrush current continues to flow through tiie ooil when the plunger is not pulled all the way into the coil to close the air gap. Short-stroke solenoid valves, in which the valve pin is an in tegral part of the plunger, can bo designed to open fully at rated voltage, -+ 10 to -- 15 percent, whether the voltage is applied gradually or instantaneously. Their alternating current coils can be designed against bum-out, even .though they may be subjected to low voltage. However, such con struction tends to have a tower maximum operating pressure differential than the hammer-blow design.