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724 CHAPTER 45 1965 Guide And Data;Book Multiple System est unit. With such a system, the compressor may be con trolled by a low-pressure switch or by thermostats instelWi in the individual units. The evaporator pressure regulator, with internal pilot pos tage, receives its source of pressure for pilot operation at the regulator inlet connection, whereas the regulator with the external pilot connection not only permits a choice of the pressure source for pilot operation, but also permits the use of a remote pressure pilot or pilot valves for other purposes. If the regulator inlet pressure is unsteady and adversely affecting the regulator performance, a steadier source of pres sure than that' which is available at the regulator inlet may be obtained for the pilot by connecting the external pilot line to a surge drum or an enlarged section of suction tin**, up stream from the regulator. The use of a remote pressure pilot instAllpH in the external pilot line: facilitates adjustment of the pressure setting when the regulator must be installed in an imwegahla location. A pressure pilot with a pneumatic connection permits reset ting and controlling of the'evaporator pressure and tempera ture by. a pneumatic control system. ' In Fig. 15 a pilot solenoid valve installed in the external pilot line enables the regulator to function as a suction stop valve as well as an evaporator pressure regulator. This feature is' particularly useful on a flooded evaporator to prevent pumping-out of the evaporator when the thermostat is satis fied and the compressor continues operating to cool other evaporators. A dual-pressure regulator may be attained by using two pressure pilots; the lower pressure'pilot is piped-through, a pilot solenoid valve, and, when energized, controls the lower pressure. The higher pressure, which occurs when tire pilot solenoid valve is de-energized, is generally used for'a warmer evaporator requirement, or for pressure above 32 F saturation,, for defrosting/ - Temperature-operated regulator. In some instances, it is de sirable to control the air or liquid entering or leaving the evaporator at a constant temperature by modulating the evaporator pressure and temperature in accordance with the load demand in such a way that the evaporator temperature is decreased during heavy loads and is increased during lighter loads. This can be accomplished with an evaporator pressure regulator modified in one of the following ways: 1. By using a temperature-actuated pilot in the external pilot fine in place of the pressure pilot, and having the bulb of the tem perature pilot placed where it will respond to the" temperature of theair or liquid being.cooled. The temperature pilot will rnMn- Iate the'position'of the regulator according to too load require-' meat, and thereby maintain the air or Kquid being,oooled at a constant temperature. . 2, By'using a pneumatic thermostat arid a pressure pilot' with a pneumatic connection so that the evaporator pressure and'tem perature are raised by an increase in air pressure supplied to the pressure pilot at reduced loads, and are lowered by a decrease in air,pressure supplied to the pressure pilot at increased as required to. maintain the temperature of the bulb of the pneu matic thermostat which is placed to respond to the temperature of the air or liquid being cooled. Thus, the air or liquid ho-rig cooled is maintained at a constant temperature. , 3. By, using-a pressure pilot that is adaptable to a reversible electric motor drive which resets the .pressure pilot and a po tentiometer type thermostat to control the reversible electric motor. The evaporator pressure and temperature are raised at re duced loads and lowered at increased londu as required to tnain- . tain the temperature of the bulb.(of the thermostat) which is placed to respond to the temperature of the air or liquH hoiwg tooled: The air or liquid being cooled ia thus at a con stant temperature. Y:; In 'addition, to the normal applications, these regulators, when equipped with high pressure pilots and suitable valve seat material, dr a higher range pressure'spring'such as in the direct acting regulator, are used in several methods of appli cation in refrigeration systems, having air-cooled condensers. They'are used for maintaining low-limit control of the com pressor discharge, the liquid line pressure,' or:both, during cold weather operation. SUCTION PRESSURE REGULATORS The 'suction pressure regulator (hold-back valve or crank case pressure regulator) limits the compressor suction pres sure (regulator outlet pressure) to a maximum value. This .type of, regulator should be used in the suction line at the compressor bn any refrigeration installation where ,the liquid expansion valve.cannot limit the suction pressure, and where compressor motor overload.would otherwise,exist because of: 1.. Excessive starting load. .. -,, . . 2. Excessive suction pressure following the defrost cycle. .3.' Prolonged operation at excessive suctkm'pressuro. 4. Low voltage and high auction pressure conditions. ' Refrigerant Control Devices 725 Operation ' The direct-acting suction pressure regulator is designed' with the principle of operation as is used in the constant pressure expansion valve. ,. *^The externally pilot-operated suction pressure regulator em ploys a constant pressure expansion valve for the pilot valve. jn the internally pilot-operated suction pressure regulator, Fig 16, the compressor suction pressure P\ is transmitted through passage A to the space beneath the diaphragm, where it exerts its pressure against the main spring pressure on top of the diaphragm. When the. compressor suction pressure drops'below' the value of the main spring pressure, the dia phragm moves downward, pressing pin in an opening direc tion with respect to pilot port C, allowing the pressure Pi in the spa'*' above the piston to be reduced toward the Value,of Pi by flowing into passage A. Since the pressure Pi oh the up^ stream or evaporator side of the piston is greater than the'coin- pressor suction pressure Pi, the differential pressure across the piston moves it in an opening direction. ' When the compressor suction pressure Pi exceeds the main spring pressure, the diaphragm is moved, upward, allowing the pilot valve spring to move pin in'a closing direction. This reduces the escape of gas from on top of the piston'. With the pilot port throttted, the pressure builds up bn top of the piston through a bleed hole in the piston or through the around the piston causing Pt to approach the value of Pa, thusi pressure differential across the piston moves it inVclosing direction. ' Selection The suction pressure regulator should be selected for the required capacity and system refrigerant, at the required regulator inlet pressure, and the lowest pressure drop 'across^ the regulator for economical compressor operation. . -- Application -In addition to the normal applications, these regulators, when equipped with high-pressure pilots, suitable-valve seat material or a higher range pressure spring such as in the direct acting regulator are used in several methods of applica tion in refrigeration systems with air-cooled condensers. They are used for maintaining low-limit control of the compressor discharge or liquid line pressure, or both, during cold weather operation. : In addition, such modified regulators may be used to bypass compressor discharge gas in refrigeration systems as described in the Application Section for the Constant Pressure Expan sion .Valve. CONDENSER PRESSURE REGULATORS - The condenser pressure regulating valves and condenser bypass valves, used for controlling cold weather operation of remote (roof mounted) air-cooled condensers,' are con structed and operate similarly to evaporator pressure regu lators and suction pressure regulators. (See Figs* 13, 14, and 16.) The valves are often identical except for the pressure spring, or high pressure pilot, and seat material if it is other than metal. These valves are responsive to compressor dis charge pressure or condenser inlet or outlet pressure, depend ing on the location of the valve in the condenser circuit. HIGH SIDE FLOAT VALVES Operation A high side float valve controls .the mass flow rate of re frigerant HqyjH entering the evaporator so that it equals the rate at which the refrigerant gas is pumped from the evapo rator by the compressor. A cross-section of a typical valve construction is shown in Fig. 17. The refrigerant liquid flows from the condenser into the high ride float valve body where it raises the float and thereby moves the valve pin in an opening direction. Thus the refrigerant liquid is per mitted to pass through the valve port, expand and flow into the evaporator. Most of the system refrigerant charge is con tained in the evaporator at all times. Selection - For acceptable performance, the high side float valve should be selected for the system refrigerant and a rated capacity that is neither excessively large nor too small. The orifice in the valve is for the maximum required capacity with the minimum pressure drop across the valve. The valve operated by the float may be a oeedle-and-eest'construction, a butter fly valve, a teiumwl double-ported valve, or a sliding gate or spool'valve. The internal bypass vent tube allows installation of the high'ride float valve near the evaporator and above the condenser,1 without danger of the float valve becoming gas bound. Some large' capacity valves use a high ride float valve for pilot operation of a diaphragm or piston type spring-