26 CIRCULAR 619, U. S. DEPARTMENT OF AGRICULTURE 



compression spring closes the valve in the thermostat, cutting off 

 the supply of air from the air compressor. This opens the air-escape 

 port and allows the compression spring in the diaphragm valve to 

 open it so that steam may again flow into the radiators. By use of 

 a key, adjustments may be made for any desired temperature within 

 the operating range of the disk. The regulator is provided with 

 three interchangeable disks good for an operating range of 110° to 

 135°, 135° to 160°, and 160° to 190° F., respectively. By this means 

 the air is held at any predetermined temperature with a variation of 

 not more than 1° plus or minus, which gives a practically constant 

 temperature in all six of the drying compartments. 



The temperature may be varied in the compartments by setting 

 the damper in the port at the bottom of the compartment for a 

 larger or smaller opening. This varying of the temperature is brought 

 about by the pull of the fan on the air flowing through the opening. 

 With a large opening more air passes through the compartment than 

 with a smaller opening, and a higher temperature results, as the greater 

 volume of air carries a greater number of heat units and compensates 

 for the refrigerating effect of evaporation on the air. 



The air is humidified and dehumidified by carefully regulating a set 

 of dampers, either by hand or by an automatic device. The de- 

 hydrator is equipped with a hygrostat which regulates the amount of 

 compressed air admitted to the damper motor. A high humidity 

 saturates the wooden member of the hygrostat and deflates the motor 

 diaphragm, opening the pair of dampers, one permitting the admission 

 of fresh atmospheric air and the other permitting an equivalent amount 

 of saturated air to exhaust to the atmosphere. When this exchange of 

 air has reduced the amount of moisture present in the mixture to the 

 desired percentage of relative humidity, the hygrostat passes air to the 

 motor diaphragm, inflating it. This closes the dampers and effects 

 complete recirculation of the air. The moisture given off by the fruit 

 or vegetables may be the sole source of vapor for conditioning the air 

 under this automatic control. 



The action of the hygrostat is direct, so that the movement of the 

 dampers is positive in opening and closing. Moreover, the amount 

 of air passed by the dampers is proportioned so that the change in 

 humidity is so gradual that any desired relation may be obtained. 



One feature in which this dehydrator differs from any other is in its 

 adaptability to the blanching operation so necessary in drying vege- 

 tables. Perforated pipes which discharge steam through small nozzles 

 are located in each compartment. These pipes are supplied with 

 steam by a 1-inch pipe. There is a valve to control the flow of steam 

 to each compartment. Products to be blanched are put into the 

 compartment, the valves at the top of the compartment are closed, 

 and the port in the bottom is closed. Then the steam valve is opened 

 and the steam flows through the nozzles, filling the compartments and 

 thoroughly blanching tne products. The valves at the top of the com- 

 partment and the port in the bottom are then opened, and the circula- 

 tion of air promptly clears the vapor from the compartment, and 

 drying proceeds. 



In drying the spinach described by Nichols and Powers (9) , it was 

 first blanched in the chambers of the dehydrator by blowing steam at 

 12 pounds gage into the closed compartment until the temperature of 

 the spinach on the trays reached 180° F., as indicated by a recording 



