This invention relates to an evaporator of a refrigerating apparatus in which a phenomenon of frost formation tends to occur.
A refrigerating apparatus of the low temperature type used for preserving foodstuffs in cold storage includes an evaporator constituting a part of the refrigeration cycle for causing boiling and evaporation of a refrigerant. The evaporator has a surface temperature of below 0.degree. C. when the refrigerating apparatus is in operation, and there is a tendency of frost formation on the surface of the evaporator as air is cooled. This has made it necessary to provide the refrigerating apparatus with defrosting means in addition to cooling means including a combination of refrigerant tubing and fins. The defrosting means includes a defrosting heater for avoiding a reduction in the amount of cold air and for preventing a reduction in cooling capabilities due to frost formation.
Because of the need to incorporate a defrosting means, an evaporator of a refrigerating apparatus is more complex in overall construction and higher in production cost than an ordinary heat exchanger, and difficulties are encountered in servicing the evaporator due to deterioration of and damage to the heater.
An evaporator of the prior art that has been used popularly will be described by referring to FIGS. 1 and 2. As shown, fins 1 include long fins 1a and short fins 1b and are formed with U-shaped notches 1c for inserting refrigerant tubing and U-shaped notches 1d for inserting a heater. The refrigerant tubing 2 is in the form of a serpentine tube, and the heater 3 includes a heater tube 3a for enclosing a heater wire 3b. 4 designates a duct. In assembling these parts into an evaporator, the long fins 1a and short fins 1b are arranged alternately in parallel relation in such a manner that the end portions of the fins 1 from which air currents flow into the evaporator are staggered. Then the serpentine refrigerant tubing 2 and heater 3 are inserted in the U-shaped notches 1c and U-shaped notches 1d respectively, and the assembly is mounted in the duct 4.
A modification of the evaporator shown in FIGS. 1 and 2 that has also been popular in the past will be described by referring to FIGS. 3 and 4. Fins 5 include long fins 5a and short fins 5b and are formed with collars 5c for inserting refrigerant tubing 6 consisting of straight tubes 6a and U-shaped tubes 6b connected together to form a serpentine tube. A heater generally designated by the reference numeral 7 includes a heater tube 7a enclosing a heater wire 7b.
In assembling these parts into an evaporator, the long fins 5a and short fins 5b are arranged in the same manner as described by referring to FIGS. 1 and 2, and the straight tubes 6a are inserted in the collars 5c. The straight tubes 6a are connected together by the U-shaped tubes at opposite ends of the straight tubes 6a so that the flow path through the refrigerant tubing 6 is in serpentine form. Then the heater 7 is fitted to the fins 5 on either side of the refrigerant tubing 6, and the assembly is arranged in the duct 4.
The evaporator of the prior art shown in FIGS. 1 and 2 has the disadvantages that the heat transfer area of each fin 1 is greatly reduced because the refrigerant tubing inserting notches 1c of a large size are formed therein by stamping, and that there is high contact thermal resistance between the refrigerant tubing 2 and fin 1 due to a reduced area of contact therebetween, thereby impairing the heat transfer function of the fins 1.
In the modification shown in FIGS. 3 and 4, the U-shaped tubes 6b are joined to the straight tubes 6a by welding. This makes it necessary to perform additional operations manually, and the refrigerant might leak through the welds when the resistance offered to the flow of the refrigerant increases or when the welds are defective or develop corrosion.
In these two types of evaporators, the fins are continuous in their main portions from the end thereof at which air currents flow into the evaporator to the end thereof at which the air currents leave the evaporator, and the fins have a high central value for the heat transfer area. Because of this, temperature boundary layers would develop on the air current exit end of the evaporator, thereby greatly reducing the mean heat transfer rate.