Patent Publication Number: US-7581515-B2

Title: Control scheme for an evaporator operating at conditions approaching thermodynamic limits

Description:
BACKGROUND OF THE INVENTION 
   This invention generally relates to a method of controlling an evaporative heat exchanger. More particularly, this invention relates to a control scheme for operating an evaporative heat exchanger that exhausts to space vacuum. 
   Evaporative heat exchangers are utilized in applications where a conventional radiator cannot be utilized. An evaporative heat exchanger includes a cooling medium that accepts heat from another system and exhausts that heat to an ambient environment. Water is a very efficient cooling medium with a latent heat of 1000 BTU/lb (2326.000 J/kg). The favorable latent heat to weight ratio makes water a suitable choice for use in vehicles operating in extreme conditions with restrictive space and weight requirements. 
   The conditions in which evaporative heat exchangers are utilized in a space vacuum are at the extreme thermodynamic conditions for water. Slight changes in pressure and temperature can result in freezing of water within the evaporator. For this reason great care must be taken to maintain operation of the evaporative heat exchanger within desired performance ranges. 
   Accordingly, it is desirable to design and develop a method and device for adapting evaporative heat exchanger operation to current operating conditions to maintain desired performance. 
   SUMMARY OF THE INVENTION 
   The example heat exchanger assembly includes a plurality of evaporative heat exchangers that are selectively fed evaporant to tailor operation to current heat load in order to maintain operation in thermodynamically extreme conditions. 
   An example evaporative heat exchange assembly includes three evaporative heat exchangers into which is fed a heat transfer medium that carries heat from a heat generating system to an inlet. Heat rejected from the heat transfer medium is accepted by an evaporant feed separately to each of the evaporative heat exchangers. The evaporant enters each of the heat exchangers in a liquid form and vaporizes upon encountering heat given off by the heat transfer medium and is exhausted into an ambient environment. 
   The example heat exchanger assembly operates in the vacuum of space. The operating environment in the vacuum of space is at or near the triple point of water. At the temperatures expected during operation, water will freeze at pressures below 0.089 psia (613.6 Pa). Therefore, pressures within each of the heat exchangers must be kept above such a pressure to prevent freezing. 
   The temperature or heat load into the heat exchanger assembly varies during operation. Incoming heat transfer fluid at lower temperatures will not vaporize evaporant at levels encountered with higher temperatures. The resulting reduction in vaporized evaporant reduces pressure within each of the heat exchangers The example system accommodates such temperature fluctuations by tailoring heat load capacity such that pressure within each of the heat exchangers remains above the triple point pressure. 
   Accordingly, the example disclosed system tailors operation to provide reliable vaporization of liquid evaporant near thermodynamic limits. 
   These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of an example evaporative heat exchange assembly. 
       FIG. 2  is a schematic view of another example evaporative heat exchange assembly. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , an example evaporative heat exchange assembly  10  includes three evaporative heat exchangers  12 ,  14 , and  16  into which is fed a heat transfer medium  44  that carries heat from a heat generating system  56  to an inlet  30  of the assembly  10 . The heat transfer medium  44  flows into the inlet  30  and rejects heat to emerge from an outlet  32  at a lower temperature. The heat rejected from the heat transfer medium  44  is accepted by an evaporant  46  fed separately to each of the evaporative heat exchangers  12 ,  14  and  16 . The evaporant  46  enters each of the heat exchangers  12 ,  14  and  16  in a liquid form and vaporizes upon encountering heat given off by the heat transfer medium  44 . The vaporized evaporant  46  is exhausted into an ambient environment  36 . 
   The example assembly  10  operates where the ambient environment  36  is at or near the vacuum of space. The example evaporant  46  is water as it is a weight efficient evaporant with a latent heat of 1000 BTU/lb (2326.000 J/kg). In vehicles and devices that operate in such extreme environments, weight and space must be allocated in the most efficient manner. Therefore the favorable latent heat to weight properties of water provides the desired efficiencies. However, the operating environment is at or near the triple point of water with temperatures at the relatively low temperature of around 32-36 F.° (0-2 C.°), with pressures approaching zero. At the example operating temperatures water will freeze at pressures below 0.089 psia. For this reason, pressures within each of the heat exchangers  12 ,  14  and  16  must be kept above such a pressure to prevent freezing. 
   Liquid water evaporant  46  entering each of the heat exchangers  12 ,  14 , and  16  is vaporized by heat from the heat transfer medium  44 . Each of the heat exchangers  12 ,  14 ,  16  provides for expansion of the vaporized evaporant to maintain a desired pressure above the triple point pressure. The vapor is then exhausted through exhaust ports  50  as water vapor  34 . The increase in pressure caused by the vaporization of the water evaporant is utilized to maintain pressures above the triple point pressure that causes water to freeze. 
   As appreciated, the temperature or heat load into the heat exchanger assembly  10  varies during operation. Incoming heat transfer fluid  44  at lower temperatures will not vaporize evaporant  46  at levels encountered with higher heat transfer medium temperatures. The resulting reduction in vaporized evaporant additionally reduces pressure within each of the heat exchangers  12 ,  14 ,  16 . In the environment in which the example system operates, such a reduction in pressure can result in freezing of liquid evaporant within the heat exchangers  12 ,  14 , and  16 . 
   The example system accommodates such temperature fluctuations by tailoring heat load capacity such that pressure within each of the heat exchangers remains above the triple point pressure. Heat load capacity is controlled by adjusting the flow of water evaporant  46  separately to each of the heat exchangers  12 ,  14 ,  16  such that the vaporization of the water evaporant produces the desired pressures at each of the outlets  50 . 
   The assembly  10  includes valves  20 ,  22 , and  24  selectively actuated by a controller  48  to control water evaporant  46  flow to each corresponding heat exchanger  12 ,  14 ,  16 . An inlet temperature sensor  52  communicates temperature information indicative of the temperature of incoming heat transfer medium  44 . An outlet temperature sensor  54  communicates information indicative of outlet temperature of the heat transfer medium. The valves  20 ,  22 , and  24  feed evaporant through a variable control valve  26 . 
   The heat exchangers  12 ,  14 , and  16  are orientated to receive the heat transfer medium in series. Heat transfer medium from the first heat exchanger  12  enters the second heat exchanger  14 , which in turn enters the third heat exchanger  16 . Combining the heat exchangers  12 ,  14 ,  16  in series results in an overall increase in turndown capacity. In the example heat exchanger assembly, each of the evaporative heat exchangers  12 ,  14 ,  16  operate at a turndown range of 1.5:1. Combining the three provides a turndown range of 3.38:1. ((1.5*1.5*1.5) =3.38:1). When less turndown range is required due to lower temperatures of the heat transfer medium  44 , one or a combination of the heat exchangers  12 ,  14 ,  16  is deactivated by closing the corresponding one of the control valves  20 ,  22 ,  24 . Further, each of the heat exchangers  12 ,  14  and  16  can provide different turndown ranges that when operated together, or in various combinations tailor heat turndown to current conditions. 
   Before one of the heat exchangers  12 ,  14 , and  16  are deactivated, the variable control valve  26  reduces flow to the currently active heat exchangers  12 ,  14 ,  16 . When the reduction in evaporant flow is not sufficient to tailor operation of the heat exchanger assembly  10  to the current temperature of the incoming heat transfer medium  44 , one or a combination of the heat exchangers  12 ,  14 , and  16  are deactivated. In the disclosed example, the third heat exchanger  16  is deactivated by closing the control valve  24 . Closing the control valve  24  stops the flow of evaporant  46  to the third heat exchanger  16 . Accordingly, the turndown capacity is reduced. Heat transfer medium  44  still flows through the third heat exchanger  16 , but no heat transfer takes place. 
   Operation continues at the reduced heat turndown capacity that vaporizes evaporant at levels corresponding to the reduced volume of the heat exchanger assembly  10  to maintain pressure above the triple point pressures. Further reductions in heat transfer medium temperatures are accommodated by deactivating the second heat exchanger  14  by closing off the control valve  22 . The resulting reductions in heat turndown range tailors operation to maintain pressure within each of the evaporative heat exchangers  12 ,  14 ,  16  above a pressure that would cause freezing of the water evaporant. 
   The heat exchangers  12 ,  14 , and  16  can be activated and deactivated in any combination to tailor the heat turndown range to current conditions. The first heat exchanger  12  and the second heat exchanger can be operated together with the third heat exchanger  16  turned off. Because each of the heat exchangers  12 ,  14 , and  16  are independently controlled by the corresponding control valve  20 ,  22 , and  24 , many combinations of heat exchanger operation can be implanted depending on current operating conditions. Other combinations of the heat exchangers can be operated by closing off one of the corresponding control valves  20 ,  22 , and  24 . 
   Referring to  FIG. 2 , another example heat exchange assembly  15  includes a fourth evaporative heat exchanger  18  that receives evaporant through a second variable control valve  28 . In operation, the first, second and third evaporative heat exchangers  12 ,  14 , and  16  are selectively feed liquid water evaporant  46  based on the inlet temperature of the heat transfer medium. 
   The fourth heat exchanger  18  provides a final turndown or temperature reduction. The fourth heat exchanger  18  reduces heat transport fluid outlet temperature to a fixed lower value. Because, the fourth heat exchanger  18  encounters a substantially constant heat load there is little temperature variation and the potential of freeze-up is mitigated. Selectively deactivating one of the first, second and third heat exchangers  12 ,  14 ,  16  provides an output of heat transport fluid  44  at a substantially constant temperature regardless of the temperature at the inlet  30 . Therefore, the fourth heat exchanger  18  is not exposed to the range of temperatures that the first three heat exchangers  12 ,  14 ,  16  encounters. The second variable control valve  28  provides a sufficient range of evaporant flow to control any small fluctuation in temperature that may occur. 
   In the disclosed example, the heat transfer medium is also water as water is an efficient heat transfer medium relative to weight. However, other heat transfer mediums may be utilized as are dictated and desired by application specific requirements. Further, the example evaporant is water. The example system is specifically designed to take advantage of the favorable latent heat to weight properties of water. The example ambient conditions expose water to the thermodynamic extremes where small changes can result in liquid water vaporizing or freezing. Accordingly, the example disclosed system tailors operation to provide reliable vaporization of liquid water near triple point pressures. 
   Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.