Dehumidifier Control System

The present invention relates to dehumidifier control systems which prevent the buildup of ice on the refrigerant coils in the dehumidifier.

Home dehumidifiers typically consist of a humidistatically controlled compressor for circulating a refrigerant, such as Freon (Trademark of E. I. duPont de Nemours & Co., Inc.), through sets of evaporator and condenser coils. An electric fan, also controlled by the humidistat, blows ambient air over the coils which causes moisture in the air to condense on the cold evaporator coils. The condensation then either drips into a pan or flows through a drain hose.

When a dehumidifier is in a cold environment, such as may be encountered in an unheated basement, if the ambient temperature falls low enough (e.g. below 65.degree. F.), the evaporator coil surface temperature may drop below the freezing point. Ths causes ice to form on the evaporator coil creating an insulating layer which interferes with the dehumidification. Eventually, the ice builds up to the point where the air flow through the coil becomes completely blocked at which time the dehumidification process ceases. The dehumidifier then must be manually shut off in order for the temperature of the evaporator coil to rise above the freezing point allowing the ice to melt.

The control system for a dehumidifier includes a humidistat switch for applying power to the dehumidifier when the humidity in the ambient air is above a predetermined level. Connected to the humidistat is an electric fan which operates whenever the humidistat switch is closed. Also connected to the humidistat switch is a compressor control circuit consisting of the series connection of a thermostat switch and the compressor motor. A separate time actuated switch is also included for periodically providing a conductive path in parallel to the thermostat switch.

One of the objects of the present invention is to provide a means for preventing ice buildup on the evaporator coils of a dehumidifier.

Another object of the present invention is to periodically cycle the operation of the compressor of the dehumidifier while continuing to blow air across the evaporator coils to prevent ice buildup when the ambient air drops below a given temperature.

A further object is to increase the efficiency of the dehumidifier by limiting the formation of an insulating layer of ice on the evaporator coils.

The FIGURE is a schematic diagram of the dehumidifier incorporating the present invention.

As shown in the accompanying drawing, a dehumidifier, generally designated as 10, includes two terminals 12 and 14 to which 120 volts of alternating electric current is applied to provide power to the dehumidifier. One of the power terminals 12 is connected to one contact of a humidistat switch 16. The humidistat operation is adjustable so that switch 16 will close when the humidity in the ambient air is above a predetermined level. A second contact of the humidistat switch 16 is connected to a first terminal 17 of the motor for an electric fan 18. A second terminal 19 of the fan 18 is connected to the other power terminal 14.

Connected in parallel with the fan 18 is the dehumidifier compressor circuit including motor 20 for driving a compressor 34. The compressor motor 20 has one terminal connected to the second power terminal 14 and to the second terminal of the fan 18. Another terminal of the compressor motor 20 is connected to node 22. Compressor 34, when driven by motor 20, causes a refrigerant to flow through evaporator and condensor coils 30 and 32 respectively.

A thermostat switch 24 is connected between the first terminal 17 of the electric fan 18 and node 22. The thermostat switch 24 is adjustable so that the switch will open when the ambient air drops below a selected temperature. Alternatively a thermostat switch with a fixed temperature setting can be used.

A timer 26 has a switch 29 connected in parallel with the thermostat switch 24. The timer 26 cycles its switch 29 through periods of conductive and nonconductive states as described below. The timer periods may be fixed or variable. Although the timer 26 may be any of several types fulfulling this function, the instant timer has a motor 28 connected in parallel with the fan 18. The timer motor 28, which is activated whenever the humidistat switch 16 applies power to the electric fan 18, drives a mechanical coupling to open and close the timer switch 29. As schematically illustrated in the drawing, the motor 28 drives a cam wheel 25 with a cam follower 27 connected to the timer switch 29.

Under a condition of high humidity in the ambient of the dehumidifier 10, the humidistat switch 16 will close, turning on the electric fan 18 and blowing ambient air through an evaporator coil 30 connected to the compressor 34. Assuming initially that the temperature of the ambient air is relatively warm, the thermostat switch 24 will be closed supplying power to the compressor motor 20. This drives the compressor 34 which circulates the refrigerant through the condenser and evaporator coils 32 and 30, respectively. In this situation the timer motor 28 is activated, periodically opening and closing the switch 29. However, since the thermostat 24 provides a parallel conductive path to that of the timer switch 29, the compressor motor 20 runs continuously.

A second situation exists when the humidity is above the preset level so that humidistat switch 16 is closed, but the temperature has dropped below the setting of the thermostat switch, e.g. below 65 degrees Fahrenheit. In this case thermostat switch 24 will be open. As long as the temperature remains below the temperature setting of thermostat switch 24, the operation of the compressor motor 20 will be controlled by the timer 26. In this mode, the timer motor 28 cycles the switch 29 through open and closed states periodically disconnecting the power to the compressor motor 20 to stop the flow of refrigerant through the coils 30 and 32. For example, the timer 26 may cause the compressor motor to run for fifteen to twenty minutes and then turn off for a period of forty-five to sixty seconds. Although a shorter off period may be adequate to prevent ice buildup, the off period must be sufficiently long so that pressures throughout the refrigerant system bleed to a uniform level. If the pressures do not equalize within the system, the compressor may be overtaxed upon restarting at the end of the off portion of the control cycle. The off time may be selected for a given environment so that the temperature of the evaporator coil 30 never rises above the dew point. This results in the dehumidification process continuing even when ice is melting. During these hiatuses in the compressor operation, fan 18 continues circulating relatively warm air over the coils 30 and 32 to melt any ice on the evaporator coil 30.

The present dehumidifier operates intermittently when the ambient temperature falls below the point where normal air flow will not prevent ice buildup on the coils. This condition is sensed by the thermostat switch 24 which responds by permitting the timer switch 29 to control the compressor operation. Although simply turning off the compressor 34 for a long period of time would eventually allow the ice to melt, the melting period is shortened by leaving the fan 18 operating.