CA2491950A1

CA2491950A1 - Heat transfer pipe with control

Info

Publication number: CA2491950A1
Application number: CA002491950A
Authority: CA
Inventors: Maurice J. Tuchelt
Original assignee: SHEC Labs Solar Hydrogen Energy Corp
Current assignee: SHEC Labs Solar Hydrogen Energy Corp
Priority date: 2005-01-11
Filing date: 2005-01-11
Publication date: 2006-07-11
Also published as: EP1836450A1; WO2006074539A1; US20080142198A1; EP1836450A4

Abstract

A method of providing controlled heat transfer between two bodies of dissimilar temperature incorporates a pressure regulated heat pipe to control the vapor phase heat transfer characteristics of the working fluid.

Description

2 BACKGROUND
Heat pipes are employed in numerous applications that require heating or cooling. United States Patent Number 4,240,405 teaches a system of transferring heat from a thermal solar collector to a hot water storage tank using the phase change of water as the heat transfer mechanism. Water in this example is vaporized using solar energy focused on a solar collector. The vapor from the solar collector is routed to a condenser located in an insulated hot water storage tank. Energy released during the coolant phase change from vapor to liquid in the condenser releases the latent heat of vaporization. The resulting exothermic condensation reaction releases 44 KJ /mol of heat energy to the water in the storage tank.
The hot stored water may then be effectively utilized for general heating or as a hot water supply feed.
Other applications such as semiconductor heat sinks use heat pipes for cooling where alcohol, Freon or propane are the preferred working fluid. These assemblies are typically an evacuated copper pipe permanently loaded with a small quantity of fluid.
During operation, the working fluid boils at the hot end, forcing vapor into the cold opposing end where this vapor condenses exothermically, releasing its latent heat to the sink (the cold side of the heat pipe). The condensed fluid is returned as liquid to the hot end by gravity and the cycle continues. In this example, the internal pressure is determined by the lowest temperature of the heat pipe, and the vapor pressure of the fluid at this temperature. This is a relatively low temperature but effective application where limited space exists for convective cooling of high dissipation electronic components.
In the previous examples of closed system heat pipes, the working fluid vapor pressure and thus the system pressure is determined by the lowest temperature in contact with the fluid.
No attempt other than possibly a safety pressure relief is made to control the system pressure.
Thermo-chemical processing plants require precise control of heat inputs to various processes. This is generally provided by controlling the fuel flow from a fuel supply to a burner thermally coupled to endothermic chemical processes. In most chemical processing

3 plant installations, heat is routed in a top down approach where process stream heat is distributed according to the process temperature and heat energy required.
Excess heat from one process is utilized by another process in conjunction with heat exchangers or direct feed, where the product of one reactor serves as the reactant and heat source for another. The goal is to minimize the heat input and thus fuel consumption while maximizing the plant throughput.
Solar reactors are now emerging owing to the need for renewable energy sources. Because these reactors are usually positioned at the foci of solar collectors, they are by necessity very compact and not afforded the luxury of the unlimited space of their plant size counterparts.
Heat exchangers require large surface areas with the accompanying mechanical volumes to be efficient at high mass flow rates. Another complicating factor in solar reactor designs is the singular heat source of a focused solar beam and the difficulties of driving multiple reactors from one, sometimes-inconsistent heat source while achieving mass and energy balance.
This invention serves to minimize these problems by providing a simple device to transfer heat in a precisely controlled manner.
SUN»IARY OF THE INVENTION
It is a principal object of the present invention to provide an efficient heat pipe system that overcomes the operational limitations of the prior art. This invention serves to provide an open system heat pipe in which the thermal transfer properties of the invention be controlled by modulating the system pressure and thus the condensation and or boiling temperature of the working fluid in response to thermal demand.
The present invention provides a heat pipe comprising a tubular or tubular matrix structure where one end of the tube is sealed and substantially thermally coupled to a heat source and the opposing end is sealed and substantially thermally coupled to a heat sink or heat process at a Iower temperature than the source. A means is provided to admit the working fluid at a

4 controlled pressure into the heat pipe assembly with the purpose of controlling the condensation temperature, hence the thermal transfer properties of the working fluid vapor within the heat pipe.
In one embodiment of the present invention, the heat pipe is mechanically positioned between two thermo-chemical reactors operating at different temperatures. The first reactor is supplied with concentrated flux from a solar collector. Solar energy absorbed by the first reactor drives an endothermic reaction at a prescribed mass flow rate and temperature, with the products of this reactor used as the reactant feed of the second reactor.
The second reactor operating at a substantially lower temperature than the first obtains sensible heat energy from the heated reactant feed with supplementary heat provided by the heat pipe to drive the second reactor in an endothermic process.
In this example, the first reactor operates at a temperature of 900 C while the second lower temperature reactor requires 50 kW/sec of heat energy at 300 C to obtain the mass energy balance required of the second reactor. The working fluid of the heat pipe must maintain a condensation temperature of 300 C. This requires a vapor pressure of 1250 PSI
be maintained if water is the working fluid. To achieve a 50 kW/sec thermal transfer at 44kJ/mol (the phase transition energy for water} the internal phase change rate would be approximately 0.0205 kg/sec. The heat pipe, constructed of stainless steel or other high temperature con-osion resistant materials can be any shape to match the mechanical configuration of the reactors. A tubular shape however is preferred to maintain stress symmetry due to the high operational pressures and temperatures. The wall thickness of the heat pipe must also be adequate for the pressures involved.
In a second embodiment of the current invention, a heat pipe is thermally coupled to a reactor, housing an exothermic chemical process. The sink of the heat pipe is thermally coupled to a preheater assembly through which a process gas stream is flowed at 30 C. In this example, the reactor must be maintained at a temperature no greater than 340 C
or damage to the reactor will occur. This requires excess heat from the reaction be removed and recycled to the feed stream preheat. Removal of this excess heat is accomplished by pressurizing the heat pipe with water at 2200 PSI, the vapor pressure of water at 340 C. Maintaining this pressure in the heat pipe will effect heat transfer to the preheater provided the reactor is at or above 340 C. If the reactor temperature drops below this value, thermal transfer through the heat pipe effectively stops, due to the inability of the coolant to boil at this temperature and

5 pressure. This effect can be exploited to more accurately control the removal of excess heat by providing a virtual set point determined by the control pressure. The ability to program the thermal transfer given a preset pressure is a feature of this invention. The heat pipe, in this example, constructed of stainless steel or other high temperature corrosion resistant materials can be any shape to match the mechanical configuration of the reactor and preheater. A
tubular shape however is preferred to maintain stress symmetry due to the high operational pressures and temperatures. The wall thickness of the heat pipe must also be adequate for the pressures involved.
DESCRIPTION OF THE DRAWINGS
The aforementioned objects and advantages of the present invention as well as additional objects and advantages thereof will be more fully understood herein as a result of a detailed description of preferred embodiments of the invention when taken in conjunction with the following drawings where like components in the drawings are assigned like designators and where:
FIG 1 is a schematic of a prior art closed system beat pipe;
FIG 2 depicts the prior art of FIG 1 employed in a semiconductor cooling application;
FIG 3 is an embodiment of the present invention employed in a solar reactor system;
FIG 4 is a schematic of an embodiment of the present invention employed in a solar reactor preheater application;
FIG 5 is a vapor pressure graph of water at a range of temperatures;

6 FIG 6 is a schematic of a closed Poop temperature control system utilizing the present invention;
FIG 7 is a schematic cross-sectional view of a pressure control system suitable for use with the heat pipe of the present invention.
DETAILED DESCRIPTION OF THE ILLISTRATED EIHBODYMENTS
FIG 1 illustrates a prior art heat pipe. The pipe 1 is usually constructed of copper to enhance the thermal conductivity of the device. The cutout 5 shows the simplicity of the device, which is essentially a hollow tube 1 with sealed ends 3, loaded with a small fluid charge (usually Freon or alcohol) employed as the coolant. FIG 2 is an example of the prior art of FIG 1 used in a typical electronic cooling application. Heat dissipated by transistors 6 mounted on an isothermal block l0 is conducted to the hot side 2 of the heat pipe 1. The coolant contained within the pipe 1 boils at the hot end 2 absorbing heat energy by phase transition while releasing coolant vapor 8, which fills the pipe 1. The opposing cool end 4 is maintained at a temperature below the condensation temperature of the vapor 8 by heat sink

7. Condensation of the vapor occurs when the vapor 8 encounters the cooler side 4 of the heat pipe. This coolant phase change being exothermic releases the energy absorbed during vaporization. The liquid condensate 9 returns to the hot end of the pipe 2 by gravity.
In this application, the heat pipe maintains the heat source at the temperature of the heat sink while providing flexibility in the mechanical layout of the electronic application. Such prior art heat pipes are sealed and operate in a predetermined manner to transfer a predetermined amount of heat. The pressure in the heat pipe will rise and fall according to the temperatures and properties of the coolant contained therein. Critical temperature control is not possible and not required here.

The heat pipe of the present invention can be used in other applications that require more precise control of energy flow, and the ability to effectively turn off the heat path like a switch.
F1G 3 presents a simplified embodiment of the present invention employed in a thetmo chemical solar reactor. Focused solar radiation 11 from a parabolic collector enters and heats reactor 12 to a predetermined temperature where an endothermic reaction contained within said reactor absorbs a portion of the heat by converting a reactant to product. A second endothermic reactor 13 is heated by reactor 12 using heat pipe 14. In a typical example, the operating temperature of reactor 12 is 900 degrees Celsius and reactor 13 must be maintained at 300 degrees Celsius. To effect the required heat transfer, coolant in the form of water at 1250 PSI is admitted to port 17 and communicated to the heat pipe 14 by a high pressure line 16. The coolant forced into the heat pipe boils at the hot end, thermally coupled to reactor 12, creating a vapor backpressure in equilibrium with the water pressure from feed line 16.
Coolant vapor at 1250 PSI condenses on the cooler end of the heat pipe, (which is thermally coupled to reactor 13) releasing latent heat in an exothermic condensation reaction and communicating this heat to reactor 13. As reactor 13 attains a temperature of 300 degrees Celsius, the condensation reaction stops and all liquid coolant in the heat pipe is vaporized and the excess coolant is expelled from the heat pipe. This will occur when all areas of the heat pipe 14 are above the condensation temperature at the control pressure of 1250 PSI.
Condensation and heat transfer will resume if the control pressure is increased or if the temperature of reactor 13 drops below 300 degrees Celsius. Radiation and convective heat losses to the environment from the hot components are reduced by filling the casement 15 with insulation 18. This insulation also provides a barrier to unwanted thermal coupling between the internal components.
Mass transfer data of the heat pipe is excluded from this example as throughput would dictate these parameters and no attempt to limit the invention to a particular reactor configuration or size is desired. In this example, water is used as the coolant however other fluids may be more suitable depending on the required temperatures and would be deemed within the scope of the invention.

g FIG 4 illustrates a second embodiment of the present invention employed in a thermo chemical solar reactor. Focused solar radiation 11 from a parabolic collector enters and heats reactor 19 to a predetermined temperature where an endothermic reaction contained within said reactor absorbs a portion of the heat by converting a reactant to product which exits at sine 23. A second reactor 25 contained within the enclosure 15 generates heat as reactants are converted to products in an exothermic reaction. In this example reactor 19 is operated at 700 degrees Celsius and reactor 25 at 350 degrees Celsius. Upon startup, with reactor 25 cold, an electric heater 24 preheats reactor 25 to a predefined temperature to initiate the reaction.
Considerable surplus heat is generated by this reaction which if not removed would result in an overheat condition in reactor 25. To effect removal of the surplus energy, a heat pipe 14 is coupled to reactor 25 and preheater 20. The heat pipe 14 is supplied with coolant water from line 1b at 2400 PSI from a pressure control system located externally from the solar receiver.
Pressurized coolant from line 16 floods the heat pipe 14 and boils at the hot end 27 of the heat pipe creating a vapor expansion which ejects excess liquid water from the pipe 14 through line 16 until the internal pressure of the pipe 14 and control pressure from line 16 are equalized. Vapor travels to the cool end 28 of the heat pipe 14 and condenses, releasing the latent heat of vaporization to the preheater assembly 20 heating the preheater to approximately 350 degrees Celsius. The condensed liquid coolant in the heat pipe 14 is re-circulated to the hot end 27 by gravity or by a wick 29 contained within the heat pipe.
Preheater 20 is disposed with a plurality of internal passages 26 to effect heat transfer to the reactant from feed line 21 feeding reactor 19 from line 22. The reactant on exiting preheater 20 is substantially in thermal equilibrium with the preheater, and the enthalpy of the reactant from line 21 has increased by an amount equal to the heat removed from reactor 25 by the heat pipe 14.
FIG 5 is a graph of vapor pressure data at a range of temperatures for water used as coolant in the previous examples. The indicated pressures are relative to one atmosphere where zero on the graph represents 14.6 PSI absolute. The critical temperature for water is 374 degrees Celsius at 3200 PSI, which dictates the highest condensation temperature given any pressure, or the maximum cold end temperature of the heat pipe in these examples. Above 374 degrees Celsius, water transforms to a supercritical state where vapor condensation is impassible.
Other coolants however may be employed depending on the requirements of the heat pipe.
The previous examples are for illustrative purposes only and not intended to limit the invention to a particular embodiment, coolant or range of temperatures.
FIG b is a schematic of a typical application of a closed loop temperature control system incorporating the present invention where accurate temperature control is required. Heat energy 41 is applied to a process 33, thermally coupled to heat pipe 14, which is thermally coupled on the opposing end to process 34. A controller 30 receives a temperature signal 40 from thermocouple 36 in contact with process 34, and a pressure signal 39 from pressure transducer 35 monitoring pressure on node 42 via line 37. Controller 30 calculates the temperature error as the difference between a set point temperature and the temperature of process 34 while generating a signal 38 to the pressure control system 32, which supplies coolant from reservoir 31 at a controlled pressure to line 15 feeding heat pipe 14. The pressure of the coolant from pressure control system 32 controls the condensation temperature and boiling point of the coolant contained within the heat pipe 14. The result is precise heat flow control in a closed loop system to maintain process 34 at the preset temperature.
The pressure control system 32 provides coolant at a constant pressure to the line 16 and thus the heat pipe 14. The constant pressure is varied by controller 30 via communication conduit 38 to the pressure control system 32 as required to effect the desired phase change heat transfer, or to stop phase change heat transfer altogether.
FIG 7 illustrates one embodiment of a suitable pressure control system 32 for use with the heat pipe 14 of the invention. A high pressure chamber 53 is divided into a coolant portion 53A and an oil portion 53B by a flexible diaphragm 51. The coolant portion 53 contains liquid coolant and is connected at port 50 to the line 16 connected to the heat pipe 14.

A low pressure chamber 59 is divided into an air pressure portion 59A and a vented portion 59B by a low pressure piston 57. The vented portion 59B is in communication with the atmosphere through vent 60 such that atmospheric pressure only is present in the vented portion 59B. The low pressure piston 57 is connected by a shaft 56 to a high pressure piston 5 52 in bore 62 extending between the high and low pressure chambers 53, 59.
Any force exerted on the face of the low pressure piston S? is thus exerted on the high pressure piston 52 as well. The pistons 57, 52 are sealed to walls of corresponding low pressure chamber 59 and bore 62 by seals 58. The oil portion 53B of the high pressure chamber 53 and the portion of the bore 62 between the face of the high pressure piston 52 and the oil portion 53B is filled 10 with a hydraulic liquid, typically a hydraulic oil.
A coil spring 55 exerts a bias force on the low pressure piston 57 towards the air pressure portion 59A of the low pressure chamber 59, and air under pressure is introduced into the air pressure portion 59A through port 61. Port 61 is connected to a controlled source of pressurized air such that the air pressure in the air pressure portion 59A can be varied as desired. For example in the illustrated embodiment a conventional air compressor 63 is connected to a pressure vessel 64 which in turn is connected by line G5 to the port 61. The air compressor 63 maintains the air pressure in the pressure vessel 64 at a level above the maximum that will be required in the air pressure portion 59A of the low pressure chamber 59.
To increase the pressure in the air pressure portion 59A of the low pressure chamber 59, compressor valve 66 is opened to allow pressurized air from the pressure vessel 64 to enter the air gressure portion 59A, and to decrease pressure, vent valve 67 is opened to release air from the air pressure portion 59A. The air pressure requirements will typically be in the order of 100 - 150 pounds per square inch (psi), and so readily controllable by conventional means. The valves 66, 67 can be controlled by the controller 30 of FTG 6.
The face of the low pressure piston 57 has a much larger area compared to the area of the face of the high pressure piston 52 such that the pressure on the oil in the bore 62 is a multiple of the pressure of the air in the air pressure portion 59A of the low pressure chamber 59. For example where the face of the low pressure piston 57 has an area of 40 square inches, and the face of the high pressure piston 52 has an area of 2 square inches, the pressure in the bore will be 20 times that in the air pressure portion 59A. Where the air pressure is 100 psi, the oil pressure in the bore 62 and thus in the oil portion 53B of the high pressure chamber 53 will be 2000 psi. Reducing the air pressure to 80 psi will reduce the oil pressure to 1600 psi, while increasing the air pressure to 120 psi will increase the oil pressure to 2400 pst.
The oil pressure in oil portion 53B of the high pressure chamber 53 is translated through the i0 flexible diaphragm 51 to the coolant in the coolant portion 53B of the high pressure chamber 53, and thus through the line 16 to the heat pipe 14. In this manner then, the pressure of the coolant in the heat pipe 14 is controlled by controlling the air pressure in the air portion 59A
of the low pressure chamber 59.
The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous changes and modifications will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all such suitable changes or modifications in structure or operation which may be resorted to are intended to fall within the scope of the claimed invention.