GB1142410A - Defrost control means for refrigerating systems - Google Patents

Abstract

1,142,410. Defrosting. RANCO INC. 22 March, 1967 [10 May, 1966], No. 13557/67. Heading F4H. [Also in Division G1] A refrigeration system in which air is forced over the evaporator is defrosted in response to a change in the relative resistances of two selfheating resistors, which change occurs when the air flow rate falls because of frost accumulation on the evaporator. As shown, the resistors 81, 82 are connected in a circuit which includes a D.C. source 62 and the solenoid 54 of a doublethrow relay switch 55, the latter being adapted to energize either a reversing valve 44 for hotgas defrosting or the air circulating fans 36. Solenoid 54 forms part of the output circuit of an amplifier including a PNP transistor 67, the input of the amplifier comprising an NPN transistor 71. The base of the latter is connected to a junction 83 which is normally connected via a switch 92, also controlled by solenoid 54, to the junction 80 between resistors 81, 82. Resistors 81, 82 are respectively exposed to and shielded from the air flow so that the resistance of 82 is normally substantially greater than that of 81 and the base of transistor 71 is negatively biased so that it is held off. Junction 83 is also connected via diode 96 and rheostat 90 with a thermistor 91 which is in contact with the evaporator and has a resistance which decreases on rise in temperature. As the air flow rate falls, resistor 81 breaks up, junction 80 and the base of transistor 71 become more positive and transistors 71 and 67 turn on. Solenoid 54 is thus energized and causes switch 55 to initiate defrosting and switch 92 to move to contact 94 so putting resistances 84, 85 in parallel and increasing the forward bias on the base of transistor 71. When the evaporator temperature increases as a result of completion of defrosting the resistance of thermistor 91 is sufficiently small to cause negative bias at the base of transistor 71 so that transistors 71, 67 turn off and switches 55, 92 return to their original positions. Initially the resistance of thermistor 91 remains sufficiently small to maintain the negative bias on transistor 71 and so prevent resistors 81, 82 from initiating a false defrost cycle. The refrigeration system is incorporated in an open-front display case (Figs. 1 and 2, not shown) and resistors 81, 82 are located in bores in an insulating block located in an air flow duct downstream of the evaporator, the bore accommodating resistor 82 being closed at its downstream end (Fig. 6, not shown).