| 摘要 |
<p>904,711. Refrigerating. PARNALL & SONS Ltd. Dec. 22, 1959 [Jan. 2, 1959], No. 131/59. Class 29. [Also in Group XXXVI] In a refrigerating system having an evaporator associated with a fan for circulating air thereover and an electric heater for defrosting, the latter is energized in response to a decrease in the airflow past the evaporator due to accumulation of ice thereon. The heater is controlled by a relay in the output circuit of a Wheatstone bridge circuit, two arms of which are constituted by thermistors R5, R7 disposed in a casing a through which flows part of the air cooled by the evaporator. Thermistor R5 is disposed downstream of a resistance heater R2 and, when the air flow is normal, the bridge is balanced. When ice accumulates, the air flow drops and the temperature of R7 remains the same whereas that of R5 rises, thus putting the bridge out of balance and energizing the defrosting heater. At the same time the fan is stopped, the resistance heater R2 is de-energized and one arm of the bridge is replaced by a thermistor responsive to evaporator temperature. When defrosting is completed, this thermistor rises in temperature to bring the bridge back into balance and restore the system to its refrigerating condition.</p> |
