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Section 8CONTROL DEVICES, ELECTRICALBoth electrical and pneumatic controls are widely usedfor air conditioning and refrigeration system control.Pneumatic controls are primarily used on large centralsystems, while electric controls are used on applicationsof all sizes. Since electric controls are used almostexclusively in the commercial refrigeration field, thismanual will cover only electric controls.CONTROL DIFFERENTIALThe basic function of most electrical control devices isto make or break an electric circuit which in turn controlsa contactor, a solenoid coil, or some other functioningpart of the system. Controls are available which maymake or break a circuit on either a rise or fall in pressureor temperature. The type of action required dependson the function of the control and the medium beingcontrolled.voltage to the control circuit voltage, usually 24 volts.LOW PRESSURE AND HIGH PRESSURECONTROLSA low pressure control is actuated by the refrigerantsuction pressure, and normally is used to cycle thecompressor for capacity control purposes, or as a lowlimit control. The low pressure control often is used asthe only control on small systems which can toleratesome fluctuations in the temperature to be maintained.The standard low pressure control makes on a rise inpressure, and breaks on a fall in pressure.The point at which a control closes a contact and makesa circuit is called the cut-in point. The point at which thecontrol opens the switch and breaks the circuit is calledthe cut-out point. The difference between the cut-in andcut-out points is known as the differential.A very small differential maintains close control butcan cause short cycling of the compressor. A largedifferential will give a longer running cycle, but mayresult in fluctuations in the pressure or temperaturebeing controlled, so the final operating differential mustbe a compromise.The differential may be either fixed or adjustable,depending on the construction of the control. Adjustmentof controls varies depending on the type and themanufacturer. On some controls, both the cut-in andcut-out points may be set at the desired points. On manypressure controls, the differential can be adjusted, andthis in turn may affect either the cut-in or the cut-outpoint.LINE VOLTAGE AND LOW VOLTAGE CONTROLSLine voltage controls are designed to operate on thesame voltage as that supplied to the compressor. Both110 and 220 volt controls are quite commonly used,and 440 volt controls are available but are seldomused due to the danger from high voltage at the wiringconnections.Local codes often require low voltage controls, and acontrol circuit transformer may be used to reduce lineA high pressure control senses the compressordischarge pressure, and is normally used to stop thecompressor in case of excessively high pressures.Since the allowable pressure limit varies with differentrefrigerants, the proper high pressure control for therefrigerant in the system must be used. A high pressurecontrol makes on a fall in pressure and breaks on arise in pressure. Either manual reset or automatic resetcontrols are available, the choice depending on thedesired system operation.Dual pressure controls are comprised of a low pressureand a high pressure control mounted in a single housingwith a single switch operated by either control.© 1967 Emerson Climate Technologies, Inc.All rights reserved.8-1
CONDENSER FAN CYCLING CONTROLIn order to maintain air cooled condensing pressuresat a satisfactory level during low ambient conditions,a condenser fan pressure control is frequently used.The control acts to break the circuit to the condenserfan on a drop in condensing pressure and makes thecircuit on a rise in condensing pressure. Since this isthe reverse of the action on a normal high pressurecontrol, this is often described as a reverse acting highpressure control.THERMOSTATSA thermostat acts to make or break a circuit in responseto a change in temperature. There are numerous typesof thermostats ranging from a simple bimetallic switch tomultiple switch controls operating from remote sensingbulbs. Thermostats may have a fixed control point ormay have variable adjustments.Normally a cooling thermostat will make on a rise intemperature and break on a fall in temperature, whilea heating thermostat will make on a fall in temperatureand break on a rise.OIL PRESSURE SAFETY CONTROLSpecial pressure controls have been developed toprotect the compressor against loss of oil pressure. Thecontrol is actuated by the difference in pressure betweenthe outlet oil pressure of the oil pump and crankcasepressure. Since the inlet pressure of the oil pump isalways crankcase pressure, the net difference in thetwo pressures is the net lubrication oil pressure.Oil pressure safety controls are available with bothadjustable and non-adjustable control settings, but thenon-adjustable type is preferred to avoid difficultiesarising from improper field adjustment.If the oil pressure falls below safe limits, the controlbreaks to stop the compressor. As an added refinement,a time delay circuit is incorporated to delay the actionof the control for a period up to 2 minutes to allow thecompressor to establish oil pressure on start-up withoutnuisance tripping.TIME CLOCKSFrequently it is desirable to stop the compressoroperation for a period of time to allow defrosting. In orderto insure that this is done regularly at convenient times,a time clock can be used to either make or break wiringcircuits at preset time intervals. Clocks are available forboth 24 hour and 7 day cycles, and the defrost intervaland time of initiation and termination can be adjustedas desired.Various types of defrost control circuits are commonlyused, such as time initiated, time terminated; timeinitiated, temperature terminated; or time initiated,pressure terminated. Normally on circuits with pressure ortemperature termination, an overriding time termination is8-2© 1967 Emerson Climate Technologies, Inc.All rights reserved.
Page 3 and 4: Part IIREFRIGERATION SYSTEM COMPONE
Page 5 and 6: Section 4COMPRESSORSThe compressor
Page 7 and 8: CROSS-SECTIONAL VIEW OF COPELAMETIC
Page 9 and 10: When the piston starts down on the
Page 11 and 12: COMPRESSORS WITH UNLOADERSIn order
Page 13 and 14: flow by means of louvers is also an
Page 15 and 16: Figure No.20 shows parallel circuit
Page 17 and 18: and the total pressure existing in
Page 19 and 20: if the evaporator temperature is to
Page 21 and 22: In modern refrigeration practice, a
Page 23 and 24: CAPILLARY TUBE SELECTION R-22HIGH T
Page 25 and 26: CAPILLARY TUBE SELECTION R-12MEDIUM
Page 27 and 28: CAPILLARY TUBE SELECTION R-502LOW T
Page 29 and 30: multiple evaporators are used. On l
Page 31 and 32: which may be caused by pulsations i
Page 33: WATER REGULATING VALVESOn water coo
Page 37 and 38: 1. The continuous VA (volt-amperes)
Page 39 and 40: A. Continuous VA requirement2 Conta
Page 41 and 42: The desired pulley size or the resu
Page 43 and 44: un winding is wound with relatively
Page 45 and 46: TWO PHASE MOTORSTwo phase power is
Page 47 and 48: 1 = 1 + 1MFD total MFD1 MFD2For exa
Page 49 and 50: voltage torque, and star-delta star
Page 51 and 52: 10-6© 1967 Emerson Climate Technol
Page 53 and 54: Regular Copeland® brand motor prot
Page 55 and 56: contact prematurely due to shock or
Page 57 and 58: The effect of an open phase in the
Page 59 and 60: vulnerable to liquid damage such as
Page 61 and 62: REFRIGERATION GAUGESPressure gauges
Page 63 and 64: © 1967 Emerson Climate Technologie

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