Patent Publication Number: US-9429484-B2

Title: Determining the heat flow emanating from a heat transporting fluid

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/CH2011/000248 filed Oct. 19, 2011, claiming priority based on Swiss Patent Application No. 1935/10 filed Nov. 18, 2010, the contents of all of which are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to the technology of measuring thermal quantities. It refers to a method for determining the heat flow emanating from a heat transporting fluid according to the preamble of claim  1 . 
     PRIOR ART 
     Binary mixtures of two fluids are often used in terminal systems, especially related to heating, cooling or air conditioning etc. A well-known binary fluid is a water/antifreeze fluid mixture, especially in form of a water/glycol mixture. When such a mixture or binary fluid transports heat energy and delivers this energy at a point of the circulating liquid system, it is necessary to know the actual mixing ratio of the heat transporting fluid, when the energy delivered shall be calculated from certain measurements at the system. 
     Unfortunately, the mixing ratio of such a binary liquid or other fluid mixtures changes with time, as, for example, water can evaporate from the system, or water is refilled, thereby changing the mixing ratio. 
     Document DE 102005043699 discloses a sensing unit for a vehicle, which determines the content of an anticorrosion medium in the fluid system of the vehicle. To determine the mixing ratio, the speed of sound is measured within the fluid. 
     Document DE 19533927 combines the capacity measurement and a measurement of the speed of sound to determine and control the concentration of a washing detergent within a cleaning fluid. 
     Document DE 3741577 discloses a method and system for measuring the mixing ratio of a binary fluid by leading a microwave signal through said liquid. 
     The cited documents are silent with respect to the determination of the heat flow emanating from a heat transporting fluid, which is a mixture of different fluids. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method for determining the heat flow emanating from a heat transporting fluid, which is a mixture of different fluids. 
     It is a further object of the invention to provide a heat flow measuring arrangement for carrying out said method. 
     These and other objects are obtained by a method according to claim  1  and heat flow measuring arrangement according to claim  13 . 
     The method according to the invention comprises the steps of:
         a) measuring the differential temperature between said first temperature and said second temperature;   b) measuring the speed of sound within said heat transporting fluid at a predetermined location of said flow space in the vicinity of said first and/or second position;   c) measuring the absolute temperature of the heat transporting fluid at said predetermined location;   d) measuring the volume flow at said predetermined location;   e) determining from said measured absolute temperature and said measured speed of sound the mixing ratio of said heat transporting fluid;   f) determining from said measured absolute temperature and said determined mixing ratio of said heat transporting fluid the density and the specific heat of said heat transporting fluid; and   g) determining from said measured differential temperature, said measured volume flow, said determined density and said determined specific heat the heat flow emanating from said heat transporting fluid.       

     According to an embodiment of the inventive method said heat transporting fluid is a binary mixture of two fluids. 
     Especially, said heat transporting fluid is a mixture of water and an antifreeze fluid. 
     More specifically, said heat transporting fluid is a water/glycol mixture. 
     According to another embodiment of the inventive method the mixing ratio of said heat transporting fluid is determined from said measured absolute temperature and said measured speed of sound by means of a data table for the relation between speed of sound, absolute temperature and mixing ratio of the specific heat transporting fluid. 
     Alternatively, the mixing ratio of said heat transporting fluid may be determined from said measured absolute temperature and said measured speed of sound by means of a mathematical relation between speed of sound, absolute temperature and mixing ratio of the specific heat transporting fluid. 
     According to another embodiment of the invention the speed of sound within said heat transporting fluid is measured by means of an ultrasonic measuring arrangement. 
     More specifically, the ultrasonic measuring arrangement comprises a first ultrasonic transducer placed at a first side of said flow space and a second ultrasonic transducer placed at a second side of said flow space, such that an ultrasonic signal travelling between said first and second ultrasonic transducers passes the fluid within said flow space. 
     Especially, the first and second ultrasonic transducers are arranged with respect to the fluid flow within said flow space, such that an ultrasonic signal travelling between said first and second ultrasonic transducers has a velocity component in the direction of said fluid flow, the speed of sound is measured in opposite directions between said first and second ultrasonic transducers, and the volume flow is derived from the measured different speeds of sound in said opposite directions. 
     When a special arrangement for the measurement of the sound of speed is used, the flow velocity of the fluid may be determined from two different measurements of the speed of sound, namely in the flow direction and opposite to the flow direction. The volume flow can then be calculated from the flow velocity and the cross-sectional area of the flow space or tube. However, according to another embodiment of the invention said volume flow is measured by means of a separate flow meter. 
     According to another embodiment the measurement of the speed of sound is based on measuring the transit time of an ultrasonic pulse travelling between said first and second ultrasonic transducers. 
     More specifically, the measurement of the speed of sound is done according to the sing-around method. 
     The heat flow measuring arrangement according to the invention comprises:
         a) first means for measuring the differential temperature between said first temperature and said second temperature;   b) second means for measuring the speed of sound within said heat transporting fluid at a predetermined location of said flow space in the vicinity of said first and/or second position;   c) third means for measuring the absolute temperature of the heat transporting fluid at said predetermined location;   d) fourth means for measuring the volume flow at said predetermined location;
 
whereby said first, second, third and fourth means are connected to an evaluation unit for determining said heat flow based on the data it receives from said first, second, third and fourth means.
       

     According to an embodiment of the inventive heat flow measuring arrangement said first means for measuring the differential temperature comprises a first temperature probe placed at said first position, and a second temperature probe placed at said second position downstream of said first position. 
     More specifically, said second means for measuring the speed of sound within said heat transporting fluid at a predetermined location of said flow space comprises an ultrasonic measuring arrangement, which is connected to an ultrasonic control unit. 
     Basically, the absolute temperature can be determined from the measured first and second temperatures of the first and second temperature probes. However, according to another embodiment of the invention said third means for measuring the absolute temperature of the heat transporting fluid at said predetermined location comprises a third temperature probe, which is placed between said first and second temperature probes in the flow direction. 
     According to another embodiment said fourth means for measuring the volume flow at said predetermined location comprises a separate flow meter. 
     According to just another embodiment a data table is provided for the relation between speed of sound, absolute temperature and mixing ratio of the specific heat transporting fluid, and the evaluation unit has access to said data table. 
     According to another embodiment of the invention said ultrasonic measuring arrangement comprises at least two ultrasonic transducers, which are arranged, such that an ultrasonic signal travelling between said at least two ultrasonic transducers passes through said heat transporting fluid. 
     More specifically, said at least two ultrasonic transducers are arranged with respect to the flow direction of said heat transporting fluid, such that the measuring track between said at least two ultrasonic transducers intersects said flow direction under an oblique angle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is now to be explained in more detail by means of different embodiments and with reference to the attached drawings. 
         FIG. 1  shows a heat flow measuring arrangement according to an embodiment of the invention; and 
         FIG. 2  shows a set of curves characteristic for the dependence of the speed of sound on temperature for a binary water/glycol mixture having a fraction of glycol of 0, 20, 40 and 60%, which can be used to determine the mixing ratio, when the speed of sound and the absolute temperature are known. 
     
    
    
     DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS OF THE INVENTION 
       FIG. 1  shows a heat flow measuring arrangement  10  according to an embodiment of the present invention. Central part of the arrangement is a flow space  11 , e.g. a tube. A fluid  12 , especially in form of a binary fluid, more specifically a water/antifreeze fluid mixture, or even more specifically a water/glycol mixture, flows through said flow space  11  with a flow direction, which is defined by the sets of arrows in  FIG. 1 . 
     At the left side of the flow space  11  the fluid  12  has a first temperature T 1 , at the right side of the flow space  11  a second temperature T 2 , which is lower than T 1 . The temperature difference or differential temperature ΔT=T 1 −T 2  is the result of a heat flow dQ/dt, which emanates from the fluid  12  and leaves the flow space  11  (see broad arrow in  FIG. 1 ). The heat flow dQ/dt may be caused by a heating radiator or a heat exchanger, or the like. 
     According to basic physical principles (see for example document U.S. Pat. No. 4,440,507) the heat flow dQ/dt can be determined using the following equation: 
                       ⅆ   Q     /     ⅆ   t       =       Q   .     =       ρ   ·   C   ·       V   .     ⁡     (       T   ⁢           ⁢   1     -     T   ⁢           ⁢   2       )         =       ρ   ·   C     ⁢           ⁢       ⅆ   V       ⅆ   t       ⁢   Δ   ⁢           ⁢   T                 (   1   )               
ρ being the density of the fluid, {dot over (V)}=dV/dt being the volume flow of the fluid, and C being its heat capacity. The differential temperature ΔT can be easily measured by measuring the temperatures T 1  and T 2  at the locations given above. The volume flow dV/dt can be easily determined from the flow velocity of the fluid  12  and the cross-sectional area of the flow space  11 . However, the situation is different for the density ρ and the heat capacity C. When the fluid  12  is a mixture of at least two different fluids, especially a mixture of water and an antifreeze fluid like glycol, which is often the case in the heating and air conditioning area, both factors depend not only on the absolute temperature, but also on the mixing ratio of the fluid  12 . If the kind of antifreeze fluid and the mixing ratio are known, it is quite simple to put the correct (T-dependent) factors ρ and C into the equation (1), above.
 
     However, it is often the case, that the mixing ratio of the fluid  12  changes in time, e.g. by evaporation of water from or adding water to the system of circulating fluid, so that the factors ρ and C change their value and the results of the determination of the heat flow dQ/dt by means of equation (1) become wrong. Accordingly, the mixing ratio must be determined at least from time to time to make sure, that the results of a heat flow calculation are correct. 
     Now, it is known in the prior art (see for example document DE 10 2005 043 699, especially section [0024]), that the mixing ratio of a mixture of fluids can be determined from the speed of sound, which is measured within said mixture. When the speed of sound has been measured in such a fluid, calibration measurements or mathematical relationships between the parameters (algorithms) can be used to determine the actual mixing ratio.  FIG. 2  shows a set of curves, which characterise the dependence of the speed of sound v s  on temperature T for a binary water/glycol mixture having a fraction of glycol of 0, 20, 40 and 60%. Although only four exemplary curves are shown, it is clear, that for a precise determination of the mixing ratio much more curves with a very narrow distance between adjacent curves are needed. 
     Taking the diagram of  FIG. 2 , the mixing ratio can be determined by finding the point of intersection in said diagram for a given absolute temperature T and a given speed of sound v s . This point of intersection lies on one of those curves, which gives the respective mixing ratio corresponding to said curve. It is clear, that such a diagram can be transformed into a data table containing discrete values of the parameters involved. Such a data table can be easily accessed by a computer to find the correct value of the mixing ratio, when the corresponding values of absolute temperature T and speed of sound v s  are known. 
     The heat flow measuring arrangement  10  of  FIG. 1  comprises an ultrasonic measuring arrangement  13 , which can be used not only to measure the speed of sound v s  within the fluid  12  of the flow space  11 , but also to measure or determine the volume flow dV/dt of the fluid  12  flowing through the flow space  11 . The ultrasonic measuring arrangement  13  comprises a first ultrasonic transducer  14  and a second ultrasonic transducer  15 . Both transducers  14 ,  15  define a measuring track within the flow space  11 , which lies with its whole length in the fluid  12 . The measuring track intersects the flow direction of the fluid  12  under an oblique angle. 
     The ultrasonic transducers  14  and  15  are able to transmit and receive ultrasonic pulses, which travel along the measuring track. When the first transducer  14  emits an ultrasonic pulse, which is received by the second transducer  15 , this pulse travels between those transducers with a downstream time t 1 , which can be expressed as: 
                       t   ⁢           ⁢   1     =     L       v   s     +     a   ·     v   f             ,           (   2   )               
where L is the length of the measuring track and (a v f ) is the component of the flow velocity v f  of the fluid  12  parallel to the direction of the measuring track.
 
     When the second transducer  15  emits an ultrasonic pulse, which is received by the first transducer  14 , this pulse travels between those transducers with an upstream time t 2 , which can be expressed as: 
     
       
         
           
             
               
                 
                   
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     By subtractive combination of equations (2) and (3), the speed of sound v s  can be eliminated, so that the flow velocity v f  of the fluid  12  is: 
                       v   f     =     k   ⁢           ⁢     L   2     ⁢     (       1     t   ⁢           ⁢   1       -     1     t   ⁢           ⁢   2         )         ,           (   4   )               
where the experimentally determined calibration factor k contains not only the factor a, above, but also effects connected with the non-ideal measuring situation (flow profile, side effects etc.).
 
     From the flow velocity v f  and the known cross-sectional area A of the flow space or tube  11 , the volume flow dV/dt can be determined as follows: 
     
       
         
           
             
               
                 
                   
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     By additive combination of equations (2) and (3), the flow velocity v f  can be eliminated to give the speed of sound v s : 
                     v   s     =         k   ′     ·     L   2       ⁢     (       1     t   ⁢           ⁢   1       +     1     t   ⁢           ⁢   2         )               (   6   )               
with another calibration factor k′ of the kind described before.
 
     The precision of the determination of the speed of sound can be improved when using the so-called “sing-around” method (see for example JP 2003302270). In the heat flow measuring device  10  of  FIG. 1  a sing-around loop is established by sending an ultrasonic pulse from transducer  14  to transducer  15 . The pulse is received and fed back into ultrasonic control unit  19 , which then excites a new ultrasonic pulse starting from transducer  14 . This loop is maintained several times, and the ultrasonic control unit  19  measures the total time it takes to complete these several sing-around loops. The time it takes for the pulse to travel along the measuring track for one-time is then determined by dividing the total time by the number of loops having been run through. 
     Thus, the ultrasonic measuring arrangement  13  with its transducers  14  and  15  and an ultrasonic control unit  19  for controlling the transducers  14  and  15  is able to measure and to determine the speed of sound v s  as well as the volume flow dV/dt within the fluid  12  and the flow space  11 . However, it is also possible, to measure the volume flow dV/dt directly by means of a separate flow meter  24 , which may be of a kind well-known in the art. The results of these measurements and determinations are sent to a central evaluation unit  20 , which contains the computer power necessary to calculate and/or determine the actual mixing ratio of the fluid  12 . 
     When a data table  21  is used, which is a numerical equivalent of a diagram like that shown in  FIG. 2 , the evaluation unit  20  takes the actual values of the speed of sound v s  and the absolute temperature T and reads out from the data table  21  the corresponding value of the mixing ratio. The absolute temperature T is measured by means of a temperature probe  17  located in the vicinity of the measuring track of the ultrasonic measuring arrangement  13 . Instead of using a data table  21 , the mixing ratio can be evaluated by using an appropriate algorithm. Alternatively, the absolute temperature T may be determined as a mean of temperatures T 1  and T 2 . 
     The mixing ratio so determined can be used in different ways. First of all, a signal can be sent out by means of an optical or acoustical signalling unit  22 , which is connected to and driven by the evaluation unit  20 , when the mixing ratio crosses preset limit. In case, where a minimum content of antifreeze fluid is necessary to avoid freezing of the system, e.g. on cold winter days, the signal may be sent out, when the antifreeze fluid content becomes smaller than a preset lower limit. 
     Furthermore, the determined or estimated mixing ratio can be used together with the measured absolute temperature T and the knowledge of the kind and parameters of the antifreeze fluid involved to determine the actual density ρ and heat capacity C of the binary fluid  12 . Using equation (1) and the measurement of the differential temperature ΔT (with temperature probes  16  and  18 ), the actual heat flow dQ/dt can then be evaluated. 
     This evaluated heat flow dQ/dt can on the one hand be integrated over time to specify the amount of thermal energy delivered from the circulating fluid system for heating cost billing purposes. On the other hand, the evaluated heat flow dQ/dt can be used to control the circulating fluid system and the delivery of thermal energy by means of a heat flow control  23 , which is connected to the evaluation unit  20 . 
     LIST OF REFERENCE NUMERALS 
     
         
           10  heat flow measuring arrangement 
           11  flow space (e.g. tube) 
           12  fluid (especially binary) 
           13  ultrasonic measuring arrangement 
           14 , 15  transducer (ultrasonic) 
           16 , 17 , 18  temperature probe 
           19  ultrasonic control unit 
           20  evaluation unit 
           21  data table 
           22  signalling unit 
           23  heat flow control 
           24  flow meter 
         T,T 1 ,T 2  temperature 
         ΔT differential temperature 
         dQ/dt heat flow 
         dV/dt volume flow 
         v f  flow velocity 
         v s  speed of sound