Abstract:
A screw compressor has a screw rotor and a cylinder having an economizer port. The economizer port communicates with a compression chamber formed between the screw rotor and the cylinder. The economizer port is configured and arranged to jet a refrigerant into the compression chamber before closing the compression chamber. Accordingly, the economizer is in communication with the compression chamber while the inner pressure of the compression chamber is low.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. National stage application claims priority under 35 U.S.C. §119(a) to Japanese Patent Application No. 2003-316469 filed in Japan on Sep. 9, 2003, the entire contents of which are hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to, for example, a screw compressor for compressing refrigerant and a freezer using the screw compressor. 
     BACKGROUND ART 
     A conventional screw compressor is composed of, as shown in a developed view of  FIG. 7 , a screw rotor  40 , a pair of gate rotors  44  (only one rotor is shown) engaging with the screw rotor  40  interposed therebetween, and a cylinder  41  for housing the screw rotor  40  in a rotatable state (see Patent Document JP 11-248264 A). The screw rotor  40  rotates in an arrow A direction. 
     A compression chamber  43  is formed between the screw rotor  40  and the cylinder  41 . More specifically, the compression chamber  43  is tightly closed by engagement between a screw groove  40   a  of the screw rotor  40  and a tooth section  44   a  of the gate rotor  44 . 
     The cylinder  41  has an economizer port  42  for jetting a refrigerant into the compression chamber  43 . 
     The economizer port  42  does not yet communicate with the compression chamber  43  immediately after closing the compression chamber  43 . The economizer port  42  opens the compression chamber  43  after starting to compress the refrigerant, as shown by a dotted line in  FIG. 2 . 
     In the conventional screw compressor, the economizer port  42  communicates with the compression chamber  43  when the inner pressure of the compression chamber  43  is high after starting to compress the refrigerant. Therefore, a pressure in the economizer port  42 , which communicates with the compression chamber  43 , is also high (e.g., the average pressure in the economizer port  42  becomes about 7 kg/cm 2 , as shown by the dotted line in  FIG. 2 ). This decreases the amount of the refrigerant jetted from the economizer port  42 . Consequently, it becomes impossible to fulfill a cooling effect by the refrigerant and to make best use of the economizer effect. 
     SUMMARY OF INVENTION 
     Subjects to be Solved by the Invention 
     It is an object of the present invention to provide a screw compressor making the best use of economizer effect obtained by increasing the amount of a refrigerant jetted from an economizer port to enhance a cooling effect, and a freezer using the screw compressor. 
     Means for Solving the Subjects 
     In order to achieve the above-mentioned object, the present invention provides a screw compressor, comprising: 
     a screw rotor; 
     a cylinder for housing the screw rotor; and 
     an economizer port provided in the cylinder and communicating with a compression chamber formed between the screw rotor and an inner face of the cylinder, wherein the refrigerant is jetted into the compression chamber before closing the compression chamber. 
     According to the screw compressor in the present invention, before the compression chamber is closed, the economizer port communicates with the compression chamber formed between the screw rotor and the inner face of the cylinder. Therefore, it is possible for the economizer to communicate with the compression chamber when the inner pressure of the compression chamber is low before starting to compress the refrigerant. This makes it possible to increase the amount of the (vapor and liquid two-phase) refrigerant jetted form the economizer port. Therefore, the cooling effect is obtained by the refrigerant from the economizer port, which makes it possible to make the best use of the economizer effect and enhance the capacity. 
     In one embodiment of the present invention, the compression chamber is closed before the refrigerant jetted from the economizer port leaks to a low pressure side of the screw rotor. 
     According to the screw compressor of this embodiment, the refrigerant jetted from the economizer port does not leak to the low pressure side of the screw rotor. Thereby, the suction amount of the refrigerant on the low pressure side of the screw rotor is prevented from decreasing and therefore deteriorating the efficiency. 
     In one embodiment of the present invention, the economizer port has a shape along a length direction of a vane of the screw rotor. 
     According to the embodiment, it is possible to swiftly open and close the economizer port and therefore to further decrease in the inner pressure of the economizer port because the economizer port has a shape along the length direction of a vane of the screw rotor. It is also possible to increase the opening area of the economizer port. This allows increase in the amount of the refrigerant jetted from the economizer port. 
     In one embodiment of the present invention, a width of the vane of the screw rotor becomes gradually larger from a central section of the screw rotor toward at least one end side, and a width of the economizer port in an axis direction of the screw rotor becomes larger toward the end side where the width of the vane is larger. 
     According to the screw compressor in the embodiment, the width of the economizer port becomes larger toward the end side where the width of the vane is larger, which makes it possible to open and close the entire length of the economizer port at the same timing. Thereby, the economizer port is more swiftly opened and closed, which achieves further enhancement of the capacity. 
     In one embodiment of the present invention, the economizer port is closed by the vane. 
     According to the screw compressor in the embodiment, the economizer port is closed by the vane, so that the adjacent compression chambers do not communicate with each other through the economizer port. This results in enhancement of the compression efficiency. 
     The present invention also provides a freezer, comprising: 
     the screw compressor ( 1 ) according to claim  1 ; 
     a condenser ( 2 ); 
     an expansion section ( 3 ); and 
     an evaporator ( 4 ), 
     wherein the screw compressor ( 1 ), the condenser ( 2 ), the expansion section ( 3 ) and the evaporator ( 4 ) are connected in sequence, 
     further comprising: 
     a sidestream path ( 31 ) diverging from a mainstream path ( 30 ) between the condenser ( 2 ) and the expansion section ( 3 ), and communicating with the economizer port ( 12 ); 
     a supercooling expansion section ( 32 ) provided on the sidestream path ( 31 ); and 
     a supercooling heat exchanger ( 33 ) for executing heat exchange between a refrigerant on an outlet side of the supercooling expansion section ( 32 ) and a refrigerant in the mainstream path ( 30 ). 
     According to the freezer in the present invention, the presence of the screw compressor in the present invention increases the amount of the refrigerant jetted from the sidestream path (the supercooling heat exchanger), which makes it possible to enhance the efficiency of the supercooling heat exchanger. Thus, it is possible to increase the degree of liquid supercooling (SC) of the refrigerant immediately before the expansion section, and therefore to enhance the refrigeration capacity. Moreover, reduction in product size and cost can be achieved by downsizing of the supercooling heat exchanger. 
     EFFECTS OF INVENTION 
     According to the screw compressor in the present invention, the economizer port communicates with the compression chamber before being closed, which enables the economizer port to communicate with the compression chamber when the inner pressure of the compression chamber is low. Consequently, it is possible to increase the amount of the refrigerant jetted from the economizer port and to obtain the cooling effect by the refrigerant. 
     According to the screw compressor in one embodiment, the refrigerant jetted from the economizer port does not leak to the lower pressure side of the screw rotor. This prevents the suction amount of the refrigerant on the low pressure side of the screw rotor from decreasing and therefore deteriorating the efficiency. 
     According to the screw compressor in one embodiment, the economizer port has a shape along the length direction of the vane of the screw rotor, which makes it possible to quicken opening and closing of the economizer port and decrease the inner pressure of the economizer port. It is also possible to increase the opening area of the economizer port and increase the amount of refrigerant from the economizer port. 
     According to the screw compressor in one embodiment, the width of the economizer port becomes larger toward the end side where the width of the vane is larger, which makes it possible to open and close the entire length of the economizer port at the same timing. Therefore, faster opening and closing of the economizer port is possible, which achieves further enhancement of the capacity. 
     According to the screw compressor in one embodiment, the economizer port is closed by the vane, so that the adjacent compression chambers do not communicate with each other through the economizer port, resulting in enhancement of the compression efficiency. 
     According to the freezer in the present invention, the screw compressor in the present invention makes it possible to increase the amount of the refrigerant jetted from the sidestream path (the supercooling heat exchanger), which results enhancement in the efficiency of the supercooling heat exchanger. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a simplified plane development view showing a screw compressor in one embodiment of the present invention; 
         FIG. 2  is a screw compressor indicator diagram showing the comparison between a screw compressor in the present invention and a conventional screw compressor; 
         FIG. 3  is plane view showing an economizer port in another embodiment; 
         FIG. 4A  is a plane view showing an economizer port in still another embodiment; 
         FIG. 4B  is a is plane view showing an economizer port in yet another embodiment; 
         FIG. 5  is a simplified block diagram showing a freezer in one embodiment of the present invention; 
         FIG. 6  is a Ph diagram showing the comparison between a freezer in the present invention and a conventional freezer; and 
         FIG. 7  is a simplified plane development view showing a conventional screw compressor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will now be described with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  shows a simplified plane development view of a screw compressor according to one embodiment of the present invention. The screw compressor, which is a so-called single screw compressor, is composed of a screw rotor  10 , a pair of gate rotors  14  (only one rotor is shown) engaging with the screw rotor  10  interposed therebetween, and a cylinder  11  for housing the screw rotor  10  in a rotatable state. The screw rotor  10  rotates in an arrow A direction. 
     A compression chamber  13  is formed between the screw rotor  10  and the inner face of the cylinder  11 . More specifically, the compression chamber  13  is tightly closed by engagement between a screw groove  10   a  of the screw rotor  10  and a tooth section  14   a  of the gate rotor  14 . 
     The cylinder  11  has an economizer port  12  for jetting a refrigerant to the compression chamber  13 . 
     The economizer port  12  communicates with the compression chamber  13  before the compression chamber  13  is closed. In other words, the economizer port  12  communicates with the screw groove  10   a  before the start of compressing a refrigerant in the screw groove  10   a . Herein, the tightly closed state of the compression chamber  13  refers to the state in which the screw groove  10   a  is closed with the tooth section  14   a  to prevent the refrigerant from leaking. 
     According to the thus-structured screw compressor, the economizer port  12  communicates with the compression chamber  13  before the compression chamber  13  is closed. Therefore, The economizer port  12  communicates with the compression chamber  13  when the inner pressure of the compression chamber  13  is low before compression of the refrigerant has not yet started. Thereby, the inner pressure of the economizer port  12  is decreased to the utmost. 
     Particularly, as shown by a solid line in a screw compressor indicator diagram in  FIG. 2 , the economizer port  12  is opened before the compressing operation starts in the compression chamber  13  and is earlier closed. In short, the opening timing of the economizer port  12  is set to be the timing at which the inner pressure of the compression chamber  13  is lower than that in the conventional example shown by a dotted line. This makes it possible to decrease the average pressure of the economizer port  12  to about 6 kg/cm 2 . 
     Thus, the amount of the refrigerant jetted from the economizer port  12  can be increased, and therefore the cooling effect by the refrigerant from the economizer port  12  can be fulfilled. 
     In this case, the compression chamber  13  is closed before the refrigerant jetted from the economizer port  12  starts to leak to the low pressure side of the screw rotor  10 . In other words, the opening timing of the economizer port  12  is set to be a threshold timing, at which the refrigerant from the economizer port  12  will not leak to the low pressure side of the screw rotor  10  before the start of compression in the compression chamber  13  even if the economizer port  12  has opened in advance. This timing is determined by elements such as flow velocity of the refrigerant. 
     Thereby, deterioration of efficiency due to decrease in the amount of the incoming refrigerant from the low pressure side of the screw rotor  10  is prevented. 
     The economizer port  12  should preferably be fully opened to the compression chamber  13  by the start of the compressing operation of the compression chamber. 
     As shown in  FIG. 1 , the economizer ports  12  are formed along the length direction of a vane lob of the screw rotor  10 . More specifically, the economizer port  12  is composed of two holes  20 ,  20 , which are placed along the length direction of the vane  10   b.    
     The width of the vane  10   b  becomes gradually larger from a central section of the screw rotor  10  toward at least one (high pressure side) of end sides. It is to be noted that the right-hand side in the drawing is a discharge-side end of the screw rotor  10 . 
     This structure allows swift opening and closing of the economizer port  12 , and therefore allows further decrease in the inner pressure of the economizer port  12 . This structure also allows the opening area of the economizer port  12  to be increased, and therefore allows the amount of the refrigerant jetted from the economizer port  12  to be increased. 
     Moreover, the economizer port  12  is closed by the vane  10   b . Therefore, it is impossible for the adjacent compression chambers  13 ,  13  to communicate with each other via the economizer port  12 , which enhances compression efficiency. 
     The number of the holes may be three as shown in  FIG. 3  or may be four or more. Moreover, though unshown, the economizer port  12  may be composed of one long hole. 
     Second Embodiment 
     Next, a screw compressor according to another embodiment of the present invention is shown in  FIG. 4A  and  FIG. 4B . In the first embodiment, the width of the economizer port  12  in axis direction of the screw rotor  10  is uniform. On the other hand, the width of the economizer port  12  in  FIG. 4A  and  FIG. 4B  is larger toward the end side where the width of the vane  10   b  is larger. 
     Specifically, as shown in  FIG. 4A , the size of four holes  20  constituting the economizer port  12  becomes larger in sequence toward the end side of the screw rotor  10 . 
     Moreover, as shown in  FIG. 4B , the size of the long hole  21  constituting the economizer port  12  becomes gradually larger toward the end side of the screw rotor  10 . More particularly, the long hole  21  has deformation of an elliptic shape. 
     Thus, the economizer port  12  can be opened and closed over the entire length thereof at the same timing, which allows faster opening and closing of the economizer port, thereby achieving further enhancement of the capacity. 
     Third Embodiment 
     Next, a freezer according to one embodiment of the present invention is shown in  FIG. 5 . In the freezer, the screw compressor  1  in the present invention, a condenser  2 , an expansion section  3  and an evaporator  4  are connected in sequence like a ring so as to form a refrigeration cycle with use of a refrigerant. Expansion valves and capillary tubes, for example, are used as the expansion section  3 . 
     Description will be given of the refrigeration cycle. A vapor-phase refrigerant discharged in the screw compressor  1  is deprived of heat by the condenser  2  and attains a liquid phase. This liquid-phase refrigerant is decompressed by the expansion section  3  and attains two phases of vapor and liquid. Then, the two-phase refrigerant (humid gas) is given heat in the evaporator  4  and attains a vapor phase. This vapor-phase refrigerant is sucked and pressurized in the screw compressor  1  before being discharged again by the screw compressor  1 . 
     The freezer has a sidestream path  31  which diverges from a mainstream path  30  located between the condenser  2  and the expansion section  3 , and communicates with the economizer port  12  in the screw compressor  1 . The mainstream path  30  and the sidestream path  31  are formed from pipes. 
     On the sidestream path  31 , there are provided a supercooling expansion section  32  and a supercooling heat exchanger  33  for executing heat exchange between the refrigerant on the outlet side of the supercooling expansion section  32  and the refrigerant in the mainstream path  30 . Expansion valves and capillary tubes, for example, are used as the supercooling expansion section  32 . 
     In  FIG. 5 , the sidestream path  31  diverges from the mainstream path  30  on the downstream side of the supercooling heat exchanger  33 . However, the sidestream path  31  may diverge from the mainstream path  30  on the upstream side of the supercooling heat exchanger  33 . 
     Description is now given of the operation of the supercooling heat exchanger  33 . A liquid-phase refrigerant coming from the condenser  2  into the mainstream path  30  is distributed to the sidestream path  31 . The liquid-phase refrigerant in the sidestream path  31  is decompressed in the supercooling expansion section  32  to be a two-phase refrigerant formed of vapor and liquid. This two-phase refrigerant draws heat from the liquid-phase refrigerant in the mainstream path  30  via the supercooling heat exchanger  33  to be a vapor-phase refrigerant. This vapor-phase refrigerant is sucked by the screw compressor  1 . In this case, the liquid-phase refrigerant in the mainstream path  30  is cooled via the supercooling heat exchanger  33 . 
     According to the thus-structured freezer, the screw compressor  1  of the invention increases the amount of the refrigerant jetted from the sidestream path  31  (the supercooling heat exchanger  33 ), which makes it possible to enhance the efficiency of the supercooling heat exchanger  33 . 
     Thereby, the degree of liquid supercooling (SC) of the refrigerant immediately before the expansion section  32  can be increased and therefore the refrigeration capacity can be enhanced. Moreover, downsizing of the supercooling heat exchanger  33  allows reduction in product size and cost. 
     Specifically, since the freezer of the present invention shown by thick lines in  FIG. 6  is optimized in shape and layout of the economizer port  12 , the freezer makes the degree of liquid supercooling (SC) larger than the conventional freezer shown by dotted lines, as shown in  FIG. 6 . Thereby, the refrigeration capacity is enhanced. 
     It should be noted that the present invention is not limited to the above-stated embodiments, and that design may be changed within the scope of the present invention. For example, the present invention may apply to a twin screw compressor, as a screw compressor of the invention, which forms a compression chamber by engagement of a pair of male and female rotors, besides the single screw compressor.