Design method for boiler for concentrates preparation device Field of the Invention The invention relates to a design method for the concentrates preparation device, especially to a design method for the boiler for 5 the concentrates preparation device using low pressure and low boiling point. Description of prior art The boiling point of water decreases with the pressure, so water can 10 be boiling with low temperature under low pressure, the boiling water changes liquid water into water vapor, mist water vapor condenses on the interface between cool and hot, a lot of mist collcets together to form waterdrop, and a lot of water drop collects together to form water flow, whether we can use the bove principle 15 to lower the moisture content of the mixture so as to increase the concentration. Summary of the present invention The object of the invention is to provide a design method for the 20 boiler for the concentrates preparation device so as to solve the above problem. Therefore, the invention provide a design method for boiler for the concentrates preparation device, the method comprises 25 arraning the boiler with a sealing structure, and at the bottom portion thereof is provided with a feeding opening, a discharging opening and a heating unit; providing a sampling unit near to the bottom portion of the 1/16 sidewall of the boiler; providing the upper cover of the boiler with a condensing unit. Specifically, providing a collecting portion of the condensing unit 5 inside the upper cover; providing the collecting portion with a bar confluence rod fitting well with the interior crow surface of the cover body and a confluence ring fitting well with the interior circumferential surface of the cover body; 10 arranging the opening of the confluence ring toward the top portion of the cover body; arranging for the confluence ring and the cover body to define a confluence chamber; arranging for the tail end of the bar confluence rod to extend 15 to the confluence chamber communicated with the outside through the output duct. Providing the bar confluence rod with an abutment surface corresponding to the interior crow surface of the cover body; symmetrically arranging two first arcuate surfaces opposed 20 invagination to the two side surfaces vertical to the abutment surface; arranging two second arcuate surfaces opposed invagination to the top surface parallel to the abutment surface; forming an overflow prevention edge between the first and 25 second arcuate surfaces; providing a first slope and a second slope near to the top end of the bar confluence rod; arranging for the angle made with the first and second slopes to be an obtuse angle; 30 arranging for the second slope to slope from the top end of the bar confluence rod to the abutment surface; arranging for the first slope to slope from the top end of the bar confluence rod to the abutment surface; arranging for the second arcuate surface to extend to the 2/16 second slope, arranging for the first arcuate surface to extend to the first slope. Arranging for the single-side section of the confluence ring to be a form of letter "L", the bottom edge of the "L" is vertical to the 5 interior circumferential surface of the upper cover, the vertical edge of the "L" is parallel to the interior surface of the upper cover. Locating the cooling portion of the cooling unit outside the upper cover; providing the cooling portion with a plurality of the ringlike 10 sheets coaxial with one another, the ringlike sheet is arranged along the crow surface of the cover body, the axis of the ringlike sheet is coaxial with the axis of the cover body, the bottom portion of the ringlike sheet fits well with the exterior crow surface of the cover body, the top and bottom portions of the condensing unit are 15 respectively provided with a water inlet duct and a water outlet duct. Specifically, providing the sampling unit with a base body, a movable rod, a limiting pin, a sampling chamber, a sampling bore, an axle 20 sleeve and an isobaric chamber; arranging for the sampling chamber to pass through the upper and lower surfaces of the base body; arranging for the isobaric chamber to be a cylindrical blind bore; 25 arranging for the isobaric chamber to passes through the front surface of the base body and to passes through the front and back surfaces of the sampling chamber. Arranging for the axle sleeve has an interference fitting with the isobaric chamber; 30 hermetically slidingly matting the movable rod with the axle sleeve; fastening the limiting pin on the movable rod; matting the limiting pin with the front surface or the back surface of the sampling chamber; providing a gas permeable passage, of which the head and tail 3/16 ends respectively communicate with the isobaric chamber and the outside, inside the base body. Specifically, arranging the interior duct opening of the output duct 5 near to the bottom portion of the confluence chamber and far below the top end of the vertical edge of the "L"; fixedly connecting the bottom portion of the water inlet duct with the top portion of the cover body; providing the bottom portion of the water input duct with a 10 plurality of water outlet openings. Specifically, the distance from the top portion of the ringlike sheet to the bottom surface of the cover body is tapering such as to decrease with increasing self diameter. 15 Profitably, arranging the movable rod at the withdraw position and arranging the sampling bore outside the base body when the limiting pin mates with the front surface of the sampling chamber; arranging the movable rod at the insertion position and 20 arranging the sampling bore inside the sampling chamber when the limiting pin mates with the back surface of the sampling chamber. Profitably, arranging for the gas permeable passage to extend to the front surface of the base body; 25 arranging the sampling chamber of the sampling unit inside the boiler, and arranging the handle outside the boiler. Advantageous Effects When the sampling unit takes samples of concentrates from the 30 boiler, it is no necessary to stop concentration process or to open the upper cover of the boiler. Concentrates in the boiler can be taken out through the sampling bore of the sampling unit, the movable rod can take samples freely with the help of gas permeable passage for keeping the pressure balance of ends of the movable 4/16 rod. The cooling portion of the condensing unit consists of a plurality of ring like sheets arranged coaxially and step-shaped, such 5 arrangement enlarges the connecting area between the upper cover of the boiler and the cooling liquid so as to increase the cooling efficiency. The first and second arcuate surfaces arranged to the bar 10 confluence rod enlarge the interface between hot and cool so as to incarese the condensing efficiency. The overflow prevention edge arranged to the bar confluence rod makes condensed water drop slide along the first and second 15 arcuate surfaces into the confluence chamber so as to prevent condensed water drop directly from dropping into the concentrates, such that the density of concentrates is reduced. Brief description of the drawing 20 In the following, the invention will be described in greater detail by means of some embodiments with reference to the accompanying drawings, in which Fig.1 is a 3d-drawing of the concentrates preparation device; Fig.2 is a 3d-drawing of the boiler in Fig.1; 25 Fig.3 is an exploded 3d-drawing of the boiler; Fig.4 is a 3d-drawing of the boiler without the upper cover in Fig.3; Fig.5 is a top view of the upper cover of the boiler in Fig.3; Fig.6 is a 3d-drawing of the upper cover in Fig.5; Fig.7 is a 3d-drawing of the upper cover from another view angle in 30 Fig.5; Fig.8 is an exploded 3d-drawing of the upper cover in Fig.5; Fig.9 is a sectional view of Fig.5 along A-A direction; Fig.10 is a sectional view of Fig.5 along B-B direction; 5/16 Fig.11 is a detailed view of the part I in Fig.12; Fig.12 is a 3d-drawing of a single bar confluence rod in Fig.8; Fig.13 is a detailed view of the part II in Fig.12; Fig.14-18 are 3d-drawings of the sampling unit in Fig.3, wherein the 5 movable rod is at different positions; Fig.19 is an exploded 3d-drawing of the sampling unit in Fig.3; Fig.20 is a sectional 3d-drawing of the sampling unit in Fig.3; Fig.21-23 are sectional front views of the sampling unit, wherein the movable rod is at different position; 10 Fig.24 is a sectional 3d-drawing of the accumulator unit in Fig.1; Fig.25 is a detailed view of the accumulator unit in Fig.24; Fig.26 is an exploded 3d-drawing of the second and third water valves in Fig.25. 1. the boiler; 11. the upper cover; 12. the heating unit; 15 13. the sidewall; 14. the sampling unit; 15. the feeding opening; 16. the discharging opening;.1101. the cover body; 1102. the confluence ring; 1103. the bar confluence rod; 1104. the ring like sheet; 1105. the output duct; 1106. the water inlet duct; 1107. the water outlet duct; 20 1108. the gas inlet duct; 1109. the water outlet opening; 1103a. the abutment surface; 1103b. the first arcuate surface; 1103c. the second arcuate surface; 1103d. the overflow prevention edge; 1103e. the second slope; 1103f. the first slope; 1103g. the third slope; 1401. the base body; 25 1402. the limiting pin; 1403. the movable rod; 1404. the sampling bore; 1405. the sampling chamber; 1406. the handle; 1407. the shaft sleeve; 1408. the gas permeable passage; 1409. the isobaric chamber; 2. the accumulator unit; 20. the connecting block; 30 21. the barrel; 22. the fourth water valve; 23. the third water valve; 24. the second water valve; 25. the first water valve; 26. the water level tagging pipe; 27. the fourth duct; 28. the third duct; 29. the second duct; 210. the first duct; 201. the circular shaft; 3. the cooling unit; 6/16 4. the depressurization unit. Detailed description of the preferred embodiment In order to distinguish between the interior duct opening and the 5 exterior duct opening, the duct opening which is inside the container is called the interior duct opening, the duct opening which is outside the container is called the exterior duct opening. Referrign to Fig.1, it is a concentrates preparation device of the 10 invention, the device comprises a boiler 1,an accumulator unit 2, a cooling unit 3 and a depressurization unit 4; the output duct 1105 of the confluence chamber of the boiler 1 communicates with the second water valve 24 of the accumulator unit 2, the water outlet duct 1107 of the cooling portion of the boiler 1 communicates with 15 the input end of the cooling unit 3 through the water valve, the water inlet duct 1106 of the cooling portion of the boiler 1 communicates with the output end of the cooling unit 3 through the water valve, the third water valve 23 of the accumulator unit 2 communicates with the input end of the depressurization unit 4 20 through the duct, the first and fourth water valves 25,22 of the accumulator unit 2 communicate with the outside. Referring to Fig.2-4, the sidewall 13 near to the bottom portion of the boiler 1 is provided with a sampling unit 14, the upper cover 11 25 of the boiler 1 is provided with a condensing unit, the boiler 1 has a sealing structure, a feeding opening 15, a discharging opening 16 and a heating unit 12 are arranged to the bottom portion of the boiler 1. 30 Referring to Fig.5, 6, 9 ,10, the cooling portion of the condensing unit is located outside the upper cover 11, the cooling portion includes a plurality of ring like sheets 1104 arranged coaxially, and the ring like sheet 1104 are arranged along the crow surface of the 7/16 cover body 1101, the axis of the ring like sheet 1104 is coaxial with the axis of the cover body 1101, the bottom portion of the ring like sheet 1104 fits well with the exterior crow surface of the cover body 1101, the distance from the top portion of the ring like sheet 1104 5 to the bottom surface of the cover body 1101 is tapering such as to decrease with increasing self diameter. The top ring of the ring like sheet 1104 is provided with a water inlet duct 1106, of which the bottom portion fixedly connects with the top portion of the cover body 1101, the bottom portion of the water inlet duct 1106 is 10 provided with a plurality of water outlet openings 1109; the bottom ring of the ring like sheet 1104 is provided with a water outlet duct 1107, water with low temperature flows into the top ring through the water inlet duct 1106, water overflows into the next ring bellow the top ring when the top ring is full of water with low temperature, 15 water with low temperature sequentially fills the rings one after one, finally water goes out of the water outlet duct 1107 of the bottom ring, obviously, the temperature of the top ring is higher than the temperature of the bottom ring. 20 Referring to Fig.7, 8, 9, 10, the collecting portion of the condensing unit is located inside the upper cover 11, the collecting portion includes a bar confluence rod 1103 fitting well with the interior crow surface of the cover body 1101 and a confluence ring 1102 fitting well with the interior circumferential surface of the cover 25 body 1101, the opeing of the confluence ring 1102 is toward the top portion of the cover body 1101, the confluence ring 1102 and the cover body 1101 form a confluence chamber, the tail end of the bar confluence rod 1103 extends into the confluence chamber, the single section of the confluence ring 1102 is a form of letter "L", the 30 bottom edge of the "L" is vertical to the interior circumferential surface of the upper cover 11, the vertical edge of the "L" is parallel to the interior circumferential surface of the upper cover 11, the confluence chamber communicates with the outside through the output duct 1105, of which the interior duct opening is near to the 8/16 bottom portion of the confluence chamber and far bellow the top portion of the vertical edge of the "L". Referring to Fig.11, 12, 13, the bar confluence rod 1103 is provided 5 with an abutment surface 1103a corresponding to the interior crow surface of the cover body 1101, two first arcuate surfaces 1103b opposed invagination are symmetrically arranged to the two side surfaces vertical to the abutment surface 1103a, two second arcuate surfaces 1103c opposed invagination are arranged to the 10 top surface parallel to the abutment surface 1103a, between the first and second arcuate surfaces 1103b, 1103c forms an overflow prevention edge 1103d, the bar confluence rod 1103 near to the top end thereof is provided with a first slope 1103f and a second slope 1103e, the angle made with the first and second slopes is an obtuse 15 angle, the second slope 1103e slopes from the top end of the bar confluence rod 1103 to the abutment surface 1103a, the first slope 1103f slopes from the top end of the bar confluence rod 1103 to the abutment surface 1103a, the second arcuate surface 1103c extends to the second slope 1103e, the first arcuate surface 1103b extends 20 to the first slope 1103f. Two third slopes 1103g are symmetrically arranged to the end near to the first slope 1103f of the bar confluence rod 1103, the third slope 1103g make the end near to the first slope 1103f of the bar confluence rod 1103 form a sharp corner, a plurality of bar confluence rods are arranged in a circle 25 around the sharp corner by such arrangement that the third slopes 1103g close up by face to face. Hot water steam in the boiler 1 condenses into water drop when hot steam touches the cool first or second arcuate surfaces 1103b, 30 1103c of the bar confluence rod 1103, water drop slides into the confluence chamber along the first or second arcuate surfaces 1103b, 1103c of the bar confluence rod 1103 under the cooperation between the overflow prevention edge 1103d and the surface tension of the water drop, water dorp collects together in the 9/16 confluence chamber and then goes out of the confluence chamber through the output duct 1105. Referring to Fig.19, 20, the sampling unit 14 includes a base body 5 1401, a movable rod 1403, a limiting pin 1402, a sampling chamber 1405, a sampling bore 1404, a shaft sleeve 1407 and an isobaric chamber 1409. Referring to Fig.21, the sampling chamber 1405 goes through the 10 upper and lower surfaces of the base body 1401, the isobaric chamber 1409 is a cylindrical blind bore, the isobaric chamber 1409 passes through the front surface of the base body 1401 and through the front and back surfaces of the sampling chamber 1405; the shaft sleeve 1407 has an interference fitting with the isobaric chamber 15 1409, the movable rod 1403 hermetically slidingly mates with the axle sleeve 1407, the limiting pin 1402 is fastened on the movable rod 1403, the limiting pin 1402 mates with the front surface or the back surface of the sampling chamber 1405. The movable rod 1403 is at the withdraw position and the sampling bore (1404) is outside 20 the base body 1401 when the limiting pin 1402 mates with the front surface of the sampling chamber 1405; the movable rod 1403 is at the insertion position and the sampling bore 1404 is inside the sampling chamber 1405 when the limiting pin 1402 mates with the back surface of the sampling chamber 1405, referring to Fig.14, 15. 25 Referring to Fig.20, inside the base body 1401 is provided with a gas permeable passage 1408, of which the head and tail ends respectively communicate with the isobaric chamber 1409 and the outside atmosphere, the air permeable passage 1408 extends to the 30 front surface of the base body 1401, air pressure of the isobaric chamber 1409 is equal to the outside air pressure with the help of the gas permeable passage 1408 so as to make the movable rod 1403 moves more flexibly. The movable rod 1403 near to the front surface of the base body 1401 is provided with a handle 1406, of 10/16 which the diameter is greater than the interior diamenter of the shaft sleeve 1407, the sampling chamber 1405 of the sampling unit 14 is inside the boiler 1, the handle 1406 is outside the boiler 1. 5 Referring to Fig.24, 25, the accumulator unit 2 is provided with a barrel 21, a first duct 210, a second duct 29, a third duct 29, a fourth duct 27, a first water valve 25, a second water valve 24, a third water valve 23, a fourth water valve 22 and a water level tagging pipe 26. The barrle 21 communicates with the outside through the 10 first, second, third, and fourth ducts 210,29,28,27; the first, second, third, and fourth ducts 210,29,28,27 are switched between opening and closing respectively through the first, second, third and fourth water valves 25,24,23,22; the first duct 210 is located near to the bottom portion of the barrel 21, the interiror duct opening of the 15 second duct 29 is near to the bottom portion of the barrel (21) and above the interior duct opening of the first duct 210, the interior duct openings of the third and fourth ducts 28, 27 are near to the top portion of hte barrel 21. 20 Referring to Fig.26, the second and third water valves have synchronous action, which is achieved by such arrangement, the connecting block 20 with a contour corresponding to the handle of the water valve is respectively provided with a through hole at each end of the head and tail ends, the handle of the water valve at the 25 respective position is provided with a through hole, the connecting block 20 is configured between the two handles of the water valve, two circular shafts 201 connects the handle with the connecting block 20, and then circular shaft 201,the handle and the connecting block 20 are fastened together by retaining ring. 30 The concentrates preparation device of the invention includes a depressurization precess, a condensing process, an accumulating process, a draining process and a sampling process. 11/16 Depressurization precess Switching off the first, fourth and sixth water valves 25, 22, switching on the second, third, fifth, seventh, eighth and nineth water valves 24,23. Turning on the depressurization unit 4 to make 5 the air in the boiler 1 pass sequentially through the fifth, second, third and nineth water valves 24, 23 into the depressurization unit 4 so as to decrease the air pressure in the boiler 1. Condensing process 10 Turning on the cooling unit 3 to make cooling liquid thereof loop sequentially through the seventh water valve, the top ring, the bottom ring and the eighth water valve, the temperature of the top cover of the boiler 1 is less than the temperature of the hot steam in the boiler 1 with the help of cooling liquid. Turning on the heating 15 unit 12 in the boiler 1 to increse the temperature of the mixture therein, the boiling point of water in the mixture is decreased under the low air pressure in the boiler 1, so the water can be boiling at low temperature to produce hot steam, which condenses into water drop when touches the bar confluence rod 1103 with lower 20 temperature with respect to hot steam, water drop slides into the confluence chamber along the first or second arcuate surface 1103b, 1103c with the help of cooperation between overflow prevention edge 1103d and surface tension of water drop, the confluence chamber collects water drop slided from the bar confluence rod 25 1103. Through the condensing process, water in the mixture in the boiler 1 is continuously ransfered into the confluence chamber so as to decrease the content of water in the mixture. Accumulating process 30 When water level in the confluence chamber is over the interior duct opening of the output duct 1105, through which water is drained into the accumulator unit 2, water passes sequentially through the fifth water valve and the second water valve 24, there are three conditions in the accumulating process: 1. the air pressure 12/16 in the boiler 1 is greater than the air pressure in the accumulator unit 2; 2. the air pressure in the boiler 1 is equal to the air pressure in the accumulator unit 2; 3. the air pressure in the boiler 1 is less than the air pressure in the accumulator 2. 5 When in condition 1, water in the confluence chamber is compressed into the accumulator unit 2 by the air pressure. When in condition 2, water in the confluence chamber flows into the accumulator unit 2 under the gravity force, because the confluence chamber is above the accumulator unit. 10 When in condition 3, turning on the depressurization unit 4 to decrease the air pressure in the accumulator unit 2 and make it less than the air pressure in the boiler 1, such that the condition 3 is changed to condition 1 or condition 2; the accumulator unit 2 accumulates water through above three methods. 15 Because the heating unit 12 in the boiler 1 is heating, the temperature in the bolier is greater than the temperature in the accumulator unit 2, so the air pressure in the boiler 1 is greater than the air pressure in the accumulator unit 2, thus, heating is good for 20 condition 1. When the density of the concentrates reaches the predetermined-density, the heating unit 12 stops heating, the air pressure in the boiler 1 is decreased, turning on the 25 depressurization unit 4 for avoiding that the air pressure in the boiler 1 is less than the air pressure in the accumulator unit 2, the air pressure in the accumulator unit 2 is less than the air pressure in the boiler 1 through the depressurization unit 4 so as to compress the water condensed from the rest steam into accumulator unit 2. 30 Because the water level in the confluence chamber is above the interior duct opening of the output duct 1105, steam in the boiler 1 do not directly pass into the accumulator unit 2 so as to reduce bad impact from steam to the depressurization unit 4; because the interior duct opening of the second duct 29 is near to the bottom 13/16 portion of the barrel 21, and the interior duct opening of the third duct 28 is near to the top portion of the barrel 21, it is avoided that water compressed out of the second duct 29 is immediately inlaled into the third duct 28 when the depressurization unit 4 works, such 5 arrangement that the interior duct opening is arrange to the top portion of the barrel 21 can reduce the risk of water in the barrel 21 passing into the third duct 28. Draining process 10 When the water level tagging pipe 26 displays that the water level reaches the predetermined-density, the second water valve 24 is turn25 off firstly, because the second water valve 24 acts together with the third water valve 23 through the connecting block 24, the third water valve 23 is turned off when the second water valve 24 is 15 turned off, from then on, water condensed in the boiler 1 is stored by the conluence chamber provisionally, the accumulator unit 2 is isolated from the bioler after the second and thrid water valves 24,23 are swithed off; the fourth water valve 22 is switched on for an equal air pressure between the accumulatro unit 2 and the 20 outsiade atmosphere, the first water valve 25 is switched after achieving the equal air pressure, water in the accumulator unit 2 flows out from the first water valve 25 due to the gravity force, the draining of accumulator unit 2 is achieved. After water in the accumulator unit 2 is drained off, the first and 25 fourth water valves 25,22 are turned off firstly so as to isolate the accumulator unit 2 from the outsaide atmosphere, after that, the second water valve 24 is turned on secondly, meanwhile the third water valve 23 is turned on too, because the air pressure in the accumulator unit 2 is greater than the air pressure in the boiler 1, 30 water in the duct is compressed back to the boiler 1 at the time when the second and third water valves 24,23 are turned on, water is kept in the confluence chamber rather than flows into the concentrates due to the buffering effect of the confluence chamber. When the air pressure in the accumulator unit 2 is equal to the air 14/16 pressure in the boiler 1, condensed water provisionally stored in the confluence chamber flows to the accumulator unit 2 under the gravity force. The depressurization unit 4 is switched on for keeping a low air pressure condition in the boiler 1 so as to keep the air 5 pressure at a predetermined-density, switching on the depressurization unit 4 speeds up water in the confluence chamber flowing to the accumulator unit 2. Sampling process 10 Referring to Fig.21,22,23, the sampling unit 14 takes samples of concentrates in the boiler 1 during the concentrating process to make concentration detection of samples. Inorde to incarese detecting accuracy, the handle 1406 is rotated fastly before sampling, referring to Fig.18, the remnant sample left in the 15 sampling bore 1404 is thrown out due to the centrifugal force, then the sampling bore is upwards with the help of the direction mark on the handle 1406; the movable rod 1403 is drawn out to the withdraw position after the sampling bore 1404 is full of concentrates, detector detects the concentration of the sample in 20 the sampling bore, or detector detects the concentration of the sample in the glassware dropped from the sampling bore due to the gravity force after rotating the movable rod 1403 to make the sampling bore 1404 be downwards. The movable rod 1403 is pushed back to the insertion position after sampling process. 25 After the density of concentrates in the boiler 1 reaches the predetermined-density, the heating uit 12, the depressurization unit 14, the fifth water valve and the sixth water valve are switched off firstly, air from the outside goes into the boiler 1 through the gas 30 inlet duct 1108 so as to make the air presurre in the boiler 1 be equal to the outside air pressure, the present air pressure in the boiler 1 is grearter than the beginning of concentrating, the boiling point of water in the concentrates increases accordingly. Because the heating unit 12 is turned off, water in the concentrates do not 15/16 boil any long, the concentrates do not loose water any more, that is the present density is kept. Opening the discharging opening 16 to let the prepared concentrates flow out from the discharging opening 16, which is 5 switched off after the concentrates is drained off. Oping the feeding opening 15 to let the mixture unconcentrated flow into the boiler 1, the feeding opening 15 is swithed off when the mixture reaches the predetermined-density to isolate air in the boiler 1 from the outside atmosphere for the next concentrating process. 10 Obviously, water in the accumulator unit 2 can be drained out during the boiler 1 is inputing mixture or outputing contrates. The detailed process is that, the nineth water valve is switched off firstly, then the sixth water valve is swithed on, due to the pressure 15 difference water in the confluence chamber is compressed to the accumulator unit 2; after the air pressure in the accumulator unit 2 and in the boiler 1 is equal to the air pressure of the outsaide atmosphere, water in the confluence chamber flows to the accumulator unit 2 under the gravity force, meanwhile the first 20 water valve 25 is swithed on for draining of water from the accumulator unit 2. The depressurization unit 4 is an air extracting pump or a vacuum pump. The cooling unit 3 is provided with an input end and an 25 output end, the cooling liquid flows into the cooling unit 3 from the input end, the cooling fin of the cooling unit 3 takes heat away from the cooling liquid into the atmosphere so as to make the temperature of the cooling liquid from the output end of the cooling unit 3 cooler than the temperature of the cooling liquid 30 from the intput end of the cooling unit 3. 16/16