This invention relates to a cold-air generating device, and in particular, to a cold-air generating device which employs a plurality of evaporators and compressors, thereby improving the evaporator heat exchange efficiency and making it possible to reduce the level of starting current needed.
An example of conventional cold-air generating device of this type is disclosed in Japanese Utility-Model Publication No. 48-22155.
The conventional device disclosed therein is equipped with a refrigerant compressor, a condenser, a liquid tank, an evaporator, a reheater, a capillary tube and an electromagnetic valve for changing cycles. In its refrigeration cycle, which is designed to enable switching between dehumidification and air-conditioning operations, a by-pass circuit is formed in parallel with the liquid tank so that, during dehumidification, the refrigerant will be conveyed to the reheater instead of being passed into the liquid tank.
It has been the practice with conventional cold-air generating devices of the above-described type to use a blowing compressor when the air is to be refrigerated down to a temperature of 0.degree. C. or below (-20.degree. C. to -80.degree. C.).
Such a compressor, however, involves noise and vibration, and requires additional capacity, which makes the cold-air generating device unsuited for medical use.
In regard to power capacity, it may be noticed that, given a limited power source of AC100 V, the output of commercially available blowing compressors is 0.75 KW or less.
If a blowing compressor with an output of 0.75 KW is used, the starting current required will be 50 A or so. Consequently, a dedicated power source will have to be provided in order to start this blowing compressor. This requires extra electrical work.
If such extra electrical work is not to be performed, the output of the blowing compressor must be below 0.4 KW. In the case of a commercially available blowing compressor with an output of 0.4 KW, the airflow volume is a little less than 50 l/min, a value which is too low for use in a medical apparatus for cryotherapy.
To refrigerate air down to a temperature of 0.degree. C. or below (-20.degree. C. to -80.degree. C.), a single evaporator is not enough because almost all the water contained in the air will then turn into ice and adhere to the surface of the evaporator. If air at a temperature of 20.degree. C. and with a relative humidity of 60% is cooled down to -30.degree. C., about 10 g of ice per 1 m.sup.3 air adheres to the evaporator surface. If the air is allowed to flow at a rate of 100 l/min, 60 g of ice per hour will adhere to the surface. Accordingly, if the air is allowed to flow for a long time, the smoothness of the flow will be impaired, resulting in the evaporator providing a poor heat exchange efficiency.
Besides this, provision of a single evaporator requires that its size be considerably large; the refrigerating compressor should also be large. Since only one refrigerating compressor is provided, the current required for starting it is relatively large as compared with the case where two refrigerating compressors are provided (the starting current that flows is about four or five times as large as the rated current), and poor starting may be the result if the power capacity is relatively small. If starting problems occur, some extra electrical work has to be conducted for the purpose of raising the power capacity.