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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Phase of International Application No. PCT/CN2010/071788, which was filed on Apr. 15, 2010, and which claims priority to and the benefit of Chinese Patent Application No. 201010101730.2, filed on Jan. 28, 2010, and the disclosures of which are hereby incorporated herein by reference in their entireties. 
     TECHNICAL FIELD 
     The present invention relates to a forced cooling system for circulating medium during drilling, in particular to a forced cooling circulation system for low temperature mud sampled in natural gas hydrate drilling, the system being also configured as a forced cooling circulation system for high temperature mud obtained in petroleum and natural gas drilling, continental scientific deep drilling, and geothermal well deep drilling. 
     BACKGROUND ART 
     Exploitation of natural gas hydrates involves, first, obtaining a core sample of natural gas hydrates by drilling and sampling, analyzing the core sample, and assessing hydro-geological parameters such as storage of the natural gas hydrates, and occurrence, scale and property of ore beds of the natural gas hydrate. Thus, drilling and sampling are the most direct measures for exploitation of natural gas hydrates. Natural gas hydrates exists in sedimentary strata with the temperature thereof is 0 to 10° C. and the pressure thereof is higher than 10 MPa, and the natural gas hydrates dissociates in the condition that the temperature of the strata containing the natural gas hydrates increase or that the pressure of the strata decrease. Substantial heat is generated by a drill bit. In this way cutting rocks during drilling and sampling construction, and heat is generated by friction of a drilling tool and a wall of a drilling hole, both of which cause the temperature at the bottom of the hole to increase. The temperature of drilling mud increases as the heat is transferred to the mud. The increasing of the mud temperature will lead to the natural gas hydrates to dissociate during drilling the core of the natural gas hydrates, which makes it possible that no in-situ hi-fi core sample of the natural gas hydrates be obtained. This may not only affects the assessment of the storage of ore bed, but also cause accidents in the drilling hole and damage drilling equipments used. Consequently, the temperature of low temperature mud used for drilling must be controlled and should be generally maintained in the range of −3° C. to 3° C., in order to ensure the stability of the natural gas hydrate stratum and core during drilling. 
     At present, techniques for cooling mud have been developed. The ground temperature is up to 350° C. and the temperature of the returned mud is up to 60 to 111° C. during drilling in hot water layer of deep geothermal well. The highest ground temperature in WD-1A well in Kakkonda, Japan is up to 500° C., and the mud with high temperature causes severe corrosions to drilling tools and tubes and scalds operation staff easily. Lengthening circulating path of mud channels is generally adopted for cooling the mud, so that the returned mud can be cooled down naturally during the circulation. Another method adopted is adding ice to a mud pond to lower the mud temperature, and mud cooling devices can be deployed if necessary. The designed mud cooling devices include: a cooling tower mounted to the mud pond, and a power fan mounted near a vibrating screen for forced cooling. However, all the above techniques are for high temperature mud, and they are not suitable for cooling low temperature mud sampled during natural gas hydrates drilling, the mud temperature of which should be controlled within the range of −3° C. to 3° C. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to overcome the drawbacks existed in the prior art and provides a forced cooling circulation system for drilling mud which is configured for continental frozen soil layer drilling construction and ocean drilling construction, and particularly configured for cooling natural gas hydrates drilling mud obtained in natural gas hydrates drilling and sampling construction, the system also being suitable for cooling the high temperature mud obtained in geothermal well deep drilling, continental scientific deep drilling, and petroleum and natural gas deep drilling. 
     The above object of the invention is achieved by the technical solutions disclosed below. 
     In a forced cooling circulation system for drilling mud, an output end of the refrigeration unit is in connection with an input end of the refrigerant tank via a first valve, an output end of the refrigerant tank is in connection with an input end of the refrigeration unit via a third valve and a refrigeration unit pump, another output end of the refrigerant tank is in connection with an input end of the coaxial convection heat exchanger via a first temperature sensor, a fourth valve, a refrigerant tank pump and a second temperature sensor, an output end of the coaxial convection heat exchanger is in connection with the mud pond via a fourth temperature sensor, another input end of the refrigerant tank is in connection with another output end of the coaxial convection heat exchanger via a second valve and a third temperature sensor, and another input end of the coaxial convection heat exchanger is in connection with the mud pond via a fifth temperature sensor and a mud delivery pump; wherein a sixth temperature sensor is provided in the mud pond, a seventh temperature sensor is in connection with an output end of a mud pump extending to the mud pond, and an eighth temperature sensor is provided in a mud circulation channel from an output end of the mud pump returning to the ground, and wherein the first temperature sensor, the second temperature sensor, the third temperature sensor, the fourth temperature sensor, the fifth temperature sensor, the sixth temperature sensor, the eighth temperature sensor and the seventh temperature sensor are in connection in parallel to an inspection instrument, and the inspection instrument is configured for displaying temperature values at all measuring points of the temperature sensors so that parameters related to the system can be adjusted based on the temperature values. 
     Heat exchange tubes of the coaxial convection heat exchanger are disposed in a two-layer configuration or in a multiple-layer configuration, in which an inner tube is fitted within an outer tube, the inner tube is coaxial with the outer tube, and an annular gap formed between these two tubes is configured as a circulation passage for refrigerant or mud, the annular gap being closed at two ends thereof; the inner tube is configured as a circulation passage for mud or refrigerant, the circulating mud and refrigerant flowing conversely so as to form counter flow heat exchange; the inner tubes are communicated with each other via flanges and U-shaped bellows, the outer tubes are communicated with each other via short tubes and flanges, there are also flanges provided between the short tubes, and a support is welded to the outer tubes to define a distance between two outer tubes; a mud or refrigerant inlet and a mud or refrigerant outlet are provided on the same end of the mud convection heat exchanger, a refrigerant or mud inlet and a refrigerant or mud outlet are provided on the same side of the coaxial convection heat exchanger and communicated with the outer tubes, and an outer wall of the outer tubes is coated with a thermal insulation layer. 
     The thermal insulation layer comprises a four-layer structure composed of, from inside to outside, a layer of thermal insulation paint, polyurethane foams, a rigid thermal insulation material and a tinfoil in sequence. The thermal insulation paint is configured as an oil-based double-component thermal insulation primer, a layer of thermal insulation paint for oil tank or a layer of aqueous thermal insulation paint. The rigid thermal insulation material is preferably configured as a rigid rubber or a rigid polyurethane foam tile. The inner tube has a smooth surface as an inner wall, and the refrigerant is aqueous glycol solution or other low temperature resistant materials. 
     Benefits of the invention are embodied in the following aspects: the forced cooling circulation system for drilling mud, upon test, has a good heat exchange effect, can be able to cool mud quickly, can dynamically maintain the temperature of the mud within defined temperature range, and operate simply. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a structural illustration of a forced cooling circulation system for drilling mud; 
         FIG. 2  is a top view of a coaxial convection heat exchanger  12  of  FIG. 1 ; 
         FIG. 3  is a front view of the coaxial convection heat exchanger  12  of  FIG. 1 ; 
         FIG. 4  is a structural illustration of heat exchange tubes of the coaxial convection heat exchanger  12  of  FIG. 1 ; 
         FIG. 5  shows layout of ports of tubes of a refrigerant tank  4  of  FIG. 1 . 
     
    
    
     REFERENCE LISTS 
       1  refrigeration unit,  2  refrigeration unit pump,  3  first valve,  4  refrigerant tank,  5  second valve,  6  third valve,  7  first temperature sensor,  8  fourth valve,  9  refrigerant tank pump,  10  second temperature sensor,  11  third temperature sensor,  12  coaxial convection heat exchanger,  13  sixth temperature sensor,  14  fifth temperature sensor,  15  mud delivery pump,  16  sixth temperature sensor,  17  mud pond,  18  mud pump,  19  eighth temperature sensor,  20  seventh temperature sensor,  21  drilling hole,  22  inspection instrument,  23  inner tube,  24  insulation layer,  25  outer tube,  26  U-shaped bellow,  27  short tube,  28  support,  29  flange,  30  mud inlet or refrigerant inlet,  31  refrigerant outlet or mud outlet,  32  refrigerant inlet or mud inlet,  33  mud outlet or refrigerant outlet. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A further detail description to the invention will be given now in combination with the drawings and examples. 
     Example 1 
     A forced cooling circulation system for drilling mud is provided, in which an output end of its refrigeration unit  1  is connected with a refrigerant tank  4  via a first valve  3 , an output end of the refrigerant tank  4  is connected with an input end of the refrigeration unit  1  via a third valve  6  and a refrigeration unit pump  2 , another output end of the refrigerant tank  4  is connected with an input end of a coaxial convection heat exchanger  12 , In this way a refrigerant inlet  32 , via a first temperature sensor  7 , a fourth valve  8 , a refrigerant tank pump  9  and a second temperature sensor  10 , an output end of the coaxial convection heat exchanger  12 , In this way a mud outlet  33 , is connected with a mud pond  17  via a fourth temperature sensor  13 , an input end of the refrigerant tank  4  is connected with another output end of the coaxial convection heat exchanger  12 , In this way a refrigerant outlet  31 , via a second valve  5  and a third temperature sensor  11 , and an input end of the coaxial convection heat exchanger  12 , In this way a mud inlet  32 , is connected with the mud pond  17  via a fifth temperature sensor  14  and a mud delivery pump  15 . A sixth temperature sensor  16  is provided in the mud pond  17 , a seventh temperature sensor  20  is connected with an output end of a mud pump  18  extending to the mud pond, and an eighth temperature sensor  19  is provided in a mud channel returning back to the ground. The first temperature sensor  7 , the second temperature sensor  10 , the third temperature sensor  11 , the fourth temperature sensor  13 , the fifth temperature sensor  14 , the sixth temperature sensor  16 , the eighth temperature sensor  19  and the seventh temperature sensor  20  are in a parallel connection to an inspection instrument  22 . The inspection instrument is configured for displaying temperature values at all measuring points of the temperature sensors, so that parameters related to the system can be adjusted based on the temperature values. 
     The coaxial convection heat exchanger is disposed in a double-layer configuration, in which an inner tube  23  and an outer tube  25  are straight segments with the same length. The inner tube  23  is fitted within the outer tube  25  and the inner tube  23  is coaxial with the outer tube  25 , constituting a set of coaxial tubes. The coaxial tubes in different sets are arranged in parallel, and the inner tubes  23  of the coaxial tubes in adjacent two sets are communicated with each other via a U-shaped bellow  26  and a flange  29 . An annular gap is formed by the outer tube  25  and the inner tube  23 , and the annular gap of the coaxial tubes in each set is closed at two ends thereof. A short tube  27  is welded to the outer tube  25  at one side of the outer tube  25  and is communicated with a short tube  27  which is welded to the outer tube  25  of the coaxial tubes in a neighboring set via a further flange  29 . The coaxial tubes in these two sets are connected with each other at the other end by means of a support  28 . The support  28  and the short tube  27  have the same length. The support  28  defines a distance of the outer tubes  25  in the adjacent two sets to keep the outer tubes  25  parallel. An outer surface of the outer tubes  25 , an outer surface of the short tube  27  connecting the outer tubes  25 , and an outer surface of the U-shaped bellow  26  are each coated with an insulation layer  24 . The insulation layer  24  has an innermost layer which is formed as an oil-based double-component thermal insulation primer applied onto the outer tubes  25 , and, from inside to outside, polyurethane foams, a rigid rubber and a tinfoil are wrapped in sequence. The mud inlet  30  and the mud outlet  33  are provided at a first side of the same end of the coaxial tubes in two layers respectively, and the refrigerant inlet  32  and the refrigerant outlet  31  are provided at a second side of the same end of the coaxial tubes in two layers respectively, the second side being different from the first side. The mud inlet and the refrigerant outlet are located at two neighboring sides of the coaxial tubes in one layer, and the mud outlet and the refrigerant inlet are located at two neighboring sides of the coaxial tubes in the other layer. The circulating medium in the inner tube  23  is mud, and the circulating medium flowing in the annular gap formed by the outer tube  25  and the inner tube  23  is refrigerant, these two media flowing conversely so as to form counter flow heat exchange. All heat exchange tubes are connected together and fixed to a chassis which is configured as a steel structure, and transported to a construction site when required. The mud in the mud pond  17  is delivered into the coaxial convection heat exchanger  12  via a mud delivery pump  15 , and returned to the mud pond  17  after cooled. In this way, the mud in the mud pond  17  is cooled at the coaxial convection heat exchanger  12  by continuously circulating, and the cooled mud is delivered into a drilled hole  21  via a mud pump  18  in a drill. 
     A working process of the forced cooling circulation system for drilling mud is as follows: the refrigerant in the refrigerant tank  4  is delivered into the refrigeration unit  1  via the third valve  6  and the refrigeration unit pump  2 , is returned to the refrigerant tank  4  via the output end of the refrigeration unit  1  and the first valve  3  after cooled by the refrigeration unit  1 , and is then delivered to the coaxial convection heat exchanger  12  via the first temperature sensor  7 , the second valve  8 , the refrigerant tank pump  9  and the second temperature sensor  10 . Then, heat exchanging is performed to the mud in the coaxial convection heat exchanger  12 . The heated refrigerant by heat exchanging is returned to the refrigerant tank  4  via the third temperature sensor  11  and the second valve  5  and is mixed with the refrigerant cooled by the refrigeration unit  1 , during which heat exchanging occurs. The resulted refrigerant is returned to the refrigeration unit  1  via the third valve  6  and the refrigeration unit pump  2  and is cooled again. The process is repeated. The cooled mud is delivered to the mud pond  17  via the fourth temperature sensor  13 , and is delivered to the bottom of the hole via the mud pump  18 , the seventh temperature sensor  20 , a tap and a drill pipe, so as to lower the temperature of a drill bit and a protection wall. After lowering the temperature of the drill bit and the protection wall, the mud is returned to the ground via an annular gap between the drill pipe and a wall of the hole, and then moved to the mud pond  17  via the eighth temperature sensor  19  and the mud channel. The cuttings carried with the mud deposits in the mud pond  17 , and after this, the mud is delivered to the coaxial convection heat exchanger  12  via the mud delivery pump  15  to be cooled by heat exchanging. The resulted mud is delivered to the bottom of the hole via the mud pump  18 , the seventh temperature sensor  20 , the tap and the drill pipe, so as to lower the temperature of the drill bit and the protection wall. The process is repeated. 
     During the process of mud cooling by the forced cooling circulation system for drilling mud, the datum detected by the first temperature sensor  7 , the second temperature sensor  10 , the third temperature sensor  11 , the fourth temperature sensor  13 , the fifth temperature sensor  14 , the sixth temperature sensor  16 , the eighth temperature sensor  19  and the seventh temperature sensor  20  are displayed in real-time on a screen of the inspection instrument  22 . 
     Example 2 
     A forced cooling circulation system for drilling mud is provided, in which an output end of its refrigeration unit  1  is connected with a refrigerant tank  4  via a first valve  3 , an output end of the refrigerant tank  4  is connected with an input end of the refrigeration unit  1  via a third valve  6  and a refrigeration unit pump  2 , another output end of the refrigerant tank  4  is connected with an input end of a coaxial convection heat exchanger  12 , In this way a refrigerant inlet  32 , via a first temperature sensor  7 , a fourth valve  8 , a refrigerant tank pump  9  and a second temperature sensor  10 , an output end of the coaxial convection heat exchanger  12 , In this way a mud outlet  33 , is connected with a mud pond  17  via a fourth temperature sensor  13 , an input end of the refrigerant tank  4  is connected with another output end of the coaxial convection heat exchanger  12 , In this way a refrigerant outlet  31  via a second valve  5  and a third temperature sensor  11 , and an input end of the coaxial convection heat exchanger  12 , In this way a mud inlet  32 , is connected with a mud pond  17  via a fifth temperature sensor  14  and a mud delivery pump  15 . A sixth temperature sensor  16  is provided in the mud pond  17 , a seventh temperature sensor  20  is connected with an output end of the mud pump  18  extending to the mud pond, and the eighth temperature sensor  19  is accommodated within a mud channel returning to the ground. The first temperature sensor  7 , the second temperature sensor  10 , the third temperature sensor  11 , the fourth temperature sensor  13 , the fifth temperature sensor  14 , the sixth temperature sensor  16 , the eighth temperature sensor  19  and the seventh temperature sensor  20  are in parallel connection with an inspection instrument  22 . 
     The coaxial convection heat exchanger is disposed in a multiple-layer configuration, in which an inner tube  23  and an outer tube  25  are straight segments with the same length. The inner tube  23  is fitted within the outer tube  25 , the inner tube  23  is coaxial with the outer tube  25 , and an annular gap is formed by the outer tube  25  and the inner tube  23 , constituting a set of coaxial tubes. The annular gap of the coaxial tubes in each set is closed at two ends thereof. Whether the coaxial tubes in different sets are arranged in a planar relationship or in a vertical relationship, the inner tubes  23  of the coaxial tubes in adjacent two sets are communicated with each other via a U-shaped bellow  26  and a flange  29 . A short tube  27  is welded to the outer tube  25  at one side of the outer tube  25  and is communicated with the short tube  27  welded to the outer tube  25  of the coaxial tubes in a neighboring set via a further flange  29 . The coaxial tubes in adjacent two sets are connected with each other at the other end by means of a support  28 . The support  28  and the short tube  27  have the same length. The support  28  defines a distance of the outer tubes  25  in the adjacent two sets to keep the outer tubes  25  parallel. An outer surface of the outer tubes  25 , an outer surface of the short tube  27  connecting the outer tubes  25 , and an outer surface of the U-shaped bellow  26  are each coated with an insulation layer  24 . The insulation layer  24  has an innermost layer which is formed as a layer of thermal insulation paint for oil tank applied onto the outer tubes  25 , and, from inside to outside, polyurethane foams, a rigid polyurethane foam tile and a tinfoil are wrapped in sequence. The mud inlet  30  on the coaxial tubes of a third layer and the mud outlet  33  on the coaxial tubes of a second layer are communicated with each other via a U-shaped bellow  26  and a flange  29 , the refrigerant outlet  31  on the coaxial tubes of the third layer and the refrigerant inlet  32  on the coaxial tubes of the second layer are communicated with the short tube  27  welded to the outer tube  25  of the third layer via a further flange  29 , and the same applies to a fourth layer, a fifth layer till the Nth layer. The refrigerant outlet  31  is welded onto a side of the outer tube  25  of the coaxial tubes of a last layer, and the mud outlet  33  is provided at the same end of the coaxial tubes of the last layer as the refrigerant outlet  31 . The mud inlet  30  and the refrigerant outlet  31  are located at two neighboring sides of the coaxial tubes, and the mud outlet  33  and the refrigerant inlet  32  are located at two neighboring sides of the coaxial tubes. The circulating medium in the inner tube  23  is mud, and the circulating medium flowing in the annular gap formed by the outer tube  25  and the inner tube  23  is refrigerant, these two media flowing conversely so as to form counter flow heat exchange. All heat exchange tubes are connected together and fixed to a chassis which is configured as a steel structure, and transported to a construction site when required. The mud in the mud pond  17  is delivered into the coaxial convection heat exchanger  12  via a mud delivery pump  15 , and returned to the mud pond  17  after cooled. In this way, the mud in the mud pond is cooled at the coaxial convection heat exchanger  12  by continuously circulating, and the cooled mud is delivered into a drilled hole  21  via a mud pump  18  in a drill. 
     A working process of the forced cooling circulation system for drilling mud is as follows: the refrigerant in the refrigerant tank  4  is delivered to the refrigeration unit  1  via the third valve  6  and the refrigeration unit pump  2 , is returned to the refrigerant tank  4  via the output end of the refrigeration unit  1  and the first valve  3  after cooled by the refrigeration unit  1 , and is then delivered to coaxial convection heat exchanger  12  via the first temperature sensor  7 , the second valve  8 , the refrigerant tank pump  9  and the second temperature sensor  10 . Then, heat exchanging is performed to the mud in the coaxial convection heat exchanger  12 . The heated refrigerant by heat exchanging is returned to the refrigerant tank  4  via the third temperature sensor  11  and the second valve  5  and is mixed with the refrigerant cooled by the refrigeration unit  1 , during which heat exchanging occurs. The resulted refrigerant is returned to the refrigeration unit  1  via the third valve  6  and the refrigeration unit pump  2  and is cooled again. The process is repeated. The cooled mud is delivered to the mud pond  17  via the fourth temperature sensor  13 , and is delivered to the bottom of the hole via the mud pump  18 , the seventh temperature sensor  20 , a tap and a drill pipe, so as to lower the temperature of a drill bit and a protection wall. After lowering the temperature of the drill bit and the protection wall, the mud is returned to the ground via an annular gap between the drill pipe and a wall of the hole, and then moved to the mud pond  17  via the eighth temperature sensor  19  and the mud channel. The cuttings carried with the mud deposits in the mud pond  17 , and after this, the mud is then delivered to the coaxial convection heat exchanger  12  via the mud delivery pump  15  to be cooled by heat exchanging. The resulted mud is delivered to the bottom of the hole via the mud pump  18 , the seventh temperature sensor  20 , the tap and the drill pipe, so as to lower the temperature of the drill bit and the protection wall. The process is repeated. 
     During the process of mud cooling by the forced cooling circulation system for drilling mud, the datum detected by the first temperature sensor  7 , the second temperature sensor  10 , the third temperature sensor  11 , the fourth temperature sensor  13 , the fifth temperature sensor  14 , the sixth temperature sensor  16 , the eighth temperature sensor  19  and the seventh temperature sensor  20  are real-time displayed on a screen of the inspection instrument  22 . 
     Example 3 
     A forced cooling circulation system or a drilling mud is provided, in which an output end of its refrigeration unit  1  is connected with a refrigerant tank  4  via a first valve  3 , an output end of the refrigerant tank  4  is connected with an input end of the refrigeration unit  1  via a third valve  6  and a refrigeration unit pump  2 , another output end of the refrigerant tank  4  is connected with an input end of a coaxial convection heat exchanger  12 , In this way a refrigerant inlet  30 , via a first temperature sensor  7 , a fourth valve  8 , a refrigerant tank pump  9  and a second temperature sensor  10 , an output end of the coaxial convection heat exchanger  12 , In this way a mud outlet  31 , is connected with a mud pond  17  via a fourth temperature sensor  13 , an input end of the refrigerant tank  4  is connected with another output end of the coaxial convection heat exchanger  12 , In this way a refrigerant outlet  33  via a second valve  5  and a third temperature sensor  11 , and an input end of the coaxial convection heat exchanger  12 , In this way a mud inlet  32 , is connected with the mud pond  17  via a fifth temperature sensor  14  and a mud delivery pump  15 . A sixth temperature sensor  16  is provided in the mud pond  17 , a seventh temperature sensor  20  is connected with an output end of a mud pump  18  which is connected to the mud pond, and an eighth temperature sensor  19  is provided in a mud channel returning to the ground. The first temperature sensor  7 , the second temperature sensor  10 , the third temperature sensor  11 , the fourth temperature sensor  13 , the fifth temperature sensor  14 , the sixth temperature sensor  16 , the eighth temperature sensor  19  and the seventh temperature sensor  20  are in a parallel connection to an inspection instrument  22 . 
     The coaxial convection heat exchanger is configured such that an inner tube  23  and an outer tube  25  are straight segments with the same length. The inner tube  23  is fitted within the outer tube  25  and the inner tube  23  is coaxial with the outer tube  25 , constituting a set of coaxial tubes. The coaxial tubes in different sets are arranged in parallel, and the inner tubes  23  of the coaxial tubes in adjacent two sets are communicated with each other via a U-shaped bellow  26  and a flange  29 . An annular gap is formed by the outer tube  25  and the inner tube  23 , and the annular gap of the coaxial tubes in each set is closed at two ends thereof. A short tube  27  is welded to the outer tube  25  at one side of the outer tube  25  and is communicated with the short tube  27  welded to the outer tube  25  of the coaxial tubes in a neighboring set via a further flange  29 . The coaxial tubes in these two sets are connected with each other at the other end by means of a support  28 . The support  28  and the short tube  27  have the same length. The support  28  defines a distance of the outer tubes  25  in the adjacent two sets to keep the outer tubes  25  parallel. The refrigerant inlet  30  and the refrigerant outlet  33  are provided at the same first side, and the mud inlet  32  and the mud outlet  31  are provided at the same second side. The refrigerant inlet  30  and the mud outlet  31  are located at two neighboring sides, and the refrigerant outlet  33  and the mud inlet  32  are located at two neighboring sides. The circulating medium in the inner tube  23  is refrigerant, and the circulating medium flowing in the annular gap formed by the outer tube  25  and the inner tube  23  is mud, these two media flowing conversely so as to form counter flow heat exchange. All heat exchange tubes are connected together and fixed to a chassis which is configured as a steel structure, and transported to a construction site when required. The mud in the mud pond  17  is delivered into the coaxial convection heat exchanger  12  via a mud delivery pump  15 , and returned to the mud pond  17  after cooled. In this way, the mud in the mud pond  17  is cooled at the coaxial convection heat exchanger  12  by continuously circulating, and the cooled mud is delivered into a drilled hole  21  via a mud pump  18  in a drill. 
     A working process of the forced cooling circulation system for drilling mud is as follows: the refrigerant in the refrigerant tank  4  is delivered into the refrigeration unit  1  via the third valve  6  and the refrigeration unit pump  2 , is returned to the refrigerant tank  4  via the output end of the refrigeration unit  1  and the first valve  3  after cooled by the refrigeration unit  1 , and is then delivered to the coaxial convection heat exchanger  12  via the first temperature sensor  7 , the second valve  8 , the refrigerant tank pump  9  and the second temperature sensor  10 . Then, heat exchanging is performed to the mud in the coaxial convection heat exchanger  12 . The heated refrigerant by heat exchanging is returned to the refrigerant tank  4  via the third temperature sensor  11  and the second valve  5  and is mixed with the refrigerant cooled by the refrigeration unit  1 , during which heat exchanging occurs. The resulted refrigerant is returned to the refrigeration unit  1  via the third valve  6  and the refrigeration unit pump  2  and is cooled again. The process is repeated. The cooled mud is delivered to the mud pond  17  via the fourth temperature sensor  13 , and is delivered to the bottom of the hole via the mud pump  18 , the seventh temperature sensor  20 , a tap and a drill pipe, so as to lower the temperature of a drill bit and a protection wall. After lowering the temperature of the drill bit and the protection wall, the mud is returned to the ground via an annular gap between the drill pipe and a wall of the hole, and then moved to the mud pond  17  via the eighth temperature sensor  19  and the mud channel. The cuttings carried with the mud deposits in the mud pond  17 , and after this, the mud is then delivered to the coaxial convection heat exchanger  12  via the mud delivery pump  15  to be cooled by heat exchanging. The resulted mud is delivered to the bottom of the hole via the mud pump  18 , the seventh temperature sensor  20 , the tap and the drill pipe, so as to lower the temperature of the drill bit and the protection wall. The process is repeated. 
     During the process of mud cooling by the forced cooling circulation system for the drilling mud, the datum detected by the first temperature sensor  7 , the second temperature sensor  10 , the third temperature sensor  11 , the fourth temperature sensor  13 , the fifth temperature sensor  14 , the sixth temperature sensor  16 , the eighth temperature sensor  19  and the seventh temperature sensor  20  are real-time displayed on a screen of the inspection instrument  22 .

Summary:
A forced cooling circulation system for drilling mud, which includes a refrigeration unit ( 1 ), a secondary refrigerant tank ( 4 ), a coaxial convection heat exchanger ( 12 ) for mud and a mud pond ( 17 ), is disclosed. The refrigeration unit ( 1 ) is in connection with the secondary refrigerant tank ( 4 ) and the coaxial convection heat exchanger ( 12 ) for mud via a pump ( 2 ), and the coaxial convection heat exchanger ( 12 ) for mud is in connection with the mud pond ( 17 ) via a pump ( 15 ) and pipelines. Heat exchange tubes of the coaxial convection heat exchanger ( 12 ) for mud are disposed as a double-layer structure or a multi-layer structure, and the inner heat exchange tubes ( 23 ) are mounted inside of the outer heat exchange tubes ( 25 ). The secondary refrigerant or the mud is circulated in the annular space between the inner heat exchange tubes ( 23 ) and the outer heat exchange tubes ( 25 ), and the mud or the secondary refrigerant is circulated in the inner tubes ( 23 ). The flow of the circulated mud is opposite to that of the circulated secondary refrigerant, and insulation material ( 24 ) is painted on the external wall of the outer tubes ( 25 ).