US20130180593A1

US20130180593A1 - Heat pipe with controlled fluid charge and hydrophobic coating

Info

Publication number: US20130180593A1
Application number: US13/350,859
Authority: US
Inventors: Ramesh Gupta; Walter Weissman; George P. Walchuk; Randall D. Partridge; Mohsen S. Yeganeh
Original assignee: ExxonMobil Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 2012-01-16
Filing date: 2012-01-16
Publication date: 2013-07-18

Abstract

The present invention is a heat pipe having a specified volume of working fluid determined in relation to the interior volume of the heat pipe and a target temperature T_T, to provide self-temperature regulation and pressure management. The heat pipe comprises at least a evaporator region and a condenser region having a known interior volume V and a mass of working fluid given by the relationship

M _wf =D _wfat T _T ×V

- where D_wfis the density of the saturated vapor of the fluid at temperature T_T
  plus an amount of additional working fluid sufficient to have the rate of evaporation about equal to the rate of condensation for the particular heat pipe configuration. The heat pipe has at least the condenser region having an interior coating of a hydrophobic material.

Description

FIELD OF THE INVENTION

The present invention relates generally to heat pipes. More specifically the invention pertains to a heat pipe having a fluid charge particularly specific to provide regulation of temperature and pressure and a hydrophobic coating to increase the heat-up rate of the heat pipe.

FIELD OF THE INVENTION

Heat pipes are efficient heat transport devices. Their origin likely dates back to the 1940's as evidenced by U.S. Pat. No. 2,350,348. A heat pipe generally comprises a sealed container, such as a pipe with end caps, a wick structure, and an amount of working fluid. FIG. 1 illustrates a conventional heat pipe. Heat applied to the evaporator region by an external heat source vaporizes the working fluid. The vapor pressure of the vaporized working fluid drives the vapor through the adiabatic region to the condenser region where the vapor condenses, releasing its latent heat to the intended heat transfer recipient. The condensed fluid is then transported back through the adiabatic region to the evaporator region via the wick apparatus, via capillary action. The process repeats so long as there is sufficient means to drive the condensed working fluid back to the evaporator region. The use of gravity force to aid or replace the capillary function of the wick apparatus is also well known in the art. Heat pipes that do not use a wick but depend solely on gravity for transporting the condensed fluid are sometimes called thermal siphons.
The heat pipe process described above is well known, and is further detailed in Heat Pipe Science and Technology by Amir Faghri and published by Taylor & Francis Publishers (1995).
Conventionally, the “working fluid” is selected based upon the temperature range of intended application. The art generally recognizes that the useful temperature range for a given working fluid ranges generally from the temperature at which the working fluid exhibits a saturation pressure greater than 0.1 Atm, up to about 20 Atm. This generally allows containment of the fluid and its vapor without excessive pressure in the heat pipe container. For example, water exhibits a useful working fluid temperature range from about 300° K to about 550° K. A table of conventional working fluids and their suitable temperature ranges may be found in Heat Pipe Science and Technology, Amir Faghri, Taylor & Francis (1995).
Conventional heat pipes contain an ample charge of the working fluid so that the liquid never fully vaporizes in the temperature range in which the heat pipe is intended for operation. In contrast, the heat pipe of the present invention contains a specified volume of working fluid that results in at least three advantages, self-regulation of temperature, pressure management, and enhanced safety.
The heat-up rate of the heat pipe can be accelerated by increasing the fluid charge to the heat pipe. This approach, however, may impair or defeat the controlled charge temperature self-regulation. This invention overcomes that shortcoming by providing a coating of hydrophobic material on an inside wall of the heat pipe to effectively increase the fluid inventory that is available for re-boiling, without adding to the overall fluid charge to the heat pipe.

SUMMARY OF THE INVENTION

The present invention pertains to a heat pipe having a specified mass of working fluid determined in relation to the interior volume of the heat pipe, a hydrophobic coating of at least a portion of the interior of the heat pipe, a target temperature, and the thermal design characteristics and the specific configuration of the heat pipe to provide automatic temperature regulation and pressure management.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectioned illustration of a conventional heat pipe.

FIG. 2 is a cross sectioned illustration of an embodiment of the heat pipe of the present invention.

FIG. 3 is a graph of Temperature versus Pressure for the heat pipe of the present invention.

FIG. 4 is a cross sectioned illustration of an embodiment of the heat pipe of the present invention.

FIG. 5 is a graph of Temperature versus Position for a heat pipe with a limited fluid charge.

FIG. 6 is a graph of Temperature versus Time for the heat pipe of the present invention with and without a hydrophobic coating.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be understood by reference to the heat pipe configuration illustrated in FIG. 2.
The volume of the heat pipe (interior) is determined by conventional means, and is here referred to as V. The user then identifies a target temperature T_Tthat is intended to be the self-regulating temperature of the object(s) being heated by the heat pipe, and selects a working fluid that is known to be operable in the temperature range of interest.
Referring to FIG. 2, heat pipe (20) has a known interior volume V, which includes all interior volumes for the condenser region (22), adiabatic region (23) and evaporator region (24). Heat source (21) supplies heat to the evaporator region at or above T_T. The condenser region (22) transfers heat to the intended recipient (25) at about T_Tand thereupon condenses the operating vapor back to operating liquid. A hydrophobic coating (26) lines at least a portion of interim volume (20). The term “hydrophobic” is used herein to mean a material or materials that are not readily wetted by the working fluid being used in the heat pipe. i.e., the materials that do not have an affinity for the working fluid. For example, if water is the heat pipe working fluid, then the hydrophobic coating materials may comprise of hydro- or fluoro-carbon organosilicon compounds with a carbon number between 5 and 50, but preferably between 10 and 25. Another example of a hydrophobic material is Teflon when water is the working fluid. The degree of hydrophobicity can be quantified by a contact angle measurement. The contact angle between water and the coating should be between 90 and 180 degrees, and preferably between 130 and 180 degrees.
Conventionally, the hydrophobic materials may coat the interior of the heat pipe, may comprise an integral part of the heat pipe, or may otherwise constitute the surface of the interior of the heat pipe by conventional means. The hydrophobic coating serves to reduce the amount of the working fluid resident on the interior wall(s) of the condenser region (22) and adiabatic region (23). This configuration returns the working fluid more quickly from the condenser region to the evaporator region, thereby increasing the charge fluid resident in the evaporator region for faster heating and re-heating.
The total fluid charged to the heat pipe for self regulation of temperature T_Tis determined as follows:
A. A no heat load mass is calculated as M_wf=D_wf@T_T×V where M_wfis the mass of the working fluid and D_wfis density of its saturated vapor at T_T; and
B. An additional amount of working fluid is added such that the following conditions are met:

- 1. The rate of evaporation and/or boil off of the total working fluid at T_Tis about equal to the rate of condensation of the working fluid at T_T.
- 2. The rate of supply of condensate to the evaporator region is about equal to the rate of evaporation and/or boil off of working fluid.
  Conditions 1 and 2 can be empirically determined for a given heat pipe configuration, or established by fluid dynamic flow relationship available to the skilled artisan.

An additional condition may be satisfied for determining the total working fluid mass of the present invention. At a condition of maximum heat load to the condenser, a gas bubble (as defined hereinafter) of superheated gas of the working fluid, initiates in the evaporator region.

Description of Heat Pipe Operation

In a heat pipe, heat is transferred from a heat source to the heat transfer recipient. This is achieved by evaporating and boiling the working fluid in the evaporator region using heat from the heat source. The vapors generated by boiling are driven to the condenser region where the vapors condense and transfer their latent heat to the heat recipient. The condensate flows back to the evaporator region where they boil-off again, thus providing transfer of heat from the evaporator region to the condenser region.
Referring to FIG. 2, a feature of the current invention is that the heat transfer from the heat source (21) to boil-off the fluid in evaporator (24) adjusts automatically and passively to match the heat demand by the heat recipient (25) to maintain the heat recipient (25) temperature at approximately T_Tand the temperature of the condenser region at about T_Tc. T_Tcis approximately equal to T_T, where the difference is attributable to heat transfer loss between the condenser region (22) and the heat recipient (25). This temperature regulation is achieved by limiting the fluid charge to the heat pipe such that at least a portion of the evaporator region dries out when the evaporator region exceeds T_T. Since no evaporation and boiling can take place in the dry portion of the evaporator region, the total boiling rate is also limited. Essentially, a gas or vapor bubble is formed initiating in the lowest most section of the evaporator region (24) impeding heat transfer from the heat source (21) to the heat pipe. The term “gas bubble” as used herein, means a region within the heat pipe occupied by superheated gas of the working fluid (i.e. no liquid state) initiated in the evaporator region, typically at its lowest most level. The size of the dry gas bubble, by expanding or contracting, passively adjusts itself such that the heat transfer from the heat source to the heat pipe is about equal to the heat demand of the heat recipient, thereby maintaining the condenser region at approximately T_Tcand the heat recipient at T_T.
In any specific heat pipe application of the present invention, the heat demand of the heat recipient (25) may vary over a range. For example, the heat recipient may be a hydrocarbon process flow stream which needs to be maintained at a temperature of about T_T. The heat demand to maintain its temperature at about T_Twill vary as the flow rate of this stream varies. Likewise, the temperature of the heat source may also vary in a range above about T_T.
The quantity of the fluid charge in the heat pipe of the present invention is such that the gas bubble size adjusts in response to the varying heat demand and the heat source temperature, such that a near constant temperature T_Tis achieved at the heat recipient.
For example, when the heat source temperature is just above T_Tand there is heat demand by the heat recipient (25), only a very small gas bubble forms in the lower end of the evaporator region (24). The evaporator is essentially unimpeded and can accept the necessary amount of heat from the heat source (21) and transmit this heat to the working fluid to meet the heat demand of the heat recipient (25).
When the heat demand by the heat recipient drops, the amount of condensate that is returned to the evaporator region will also drop. This will cause the gas bubble initiated in the evaporator section to expand and impede the transfer of heat to the working fluid. The less the heat demand by the heat recipient the less condensate will be returned to the evaporator and increase the size of the bubble. The heat transfer from the heat source to the working fluid will be adequately impeded by the gas bubble to equal the heat demand by the heat recipient at temperature T_T.
The same principle of impeding the heat transfer to the working fluid by a self adjusting gas bubble applies when the heat source (21) temperature fluctuates substantially above T_T. The gas bubble will expand to adequately impede heat transfer to the evaporator section so as not to overheat the heat recipient above T_T. If concurrently the heat demand by the heat recipient goes down, the condensate return will be reduced further and the bubble will further expand to further impede heat transfer to the evaporator.
The following table provides a summary of how the gas bubble adjusts in size so that the heat transfer to the working fluid approximately equals the heat demand by the heat recipient to maintain its temperature at approximately T_T.

TABLE 1

	Bubble Size and	Characteristics of the
Operating	Heat Transfer	Condensate Returning to
Conditions	Characteristics	the Evaporator

(A)	High heat	Near zero bubble	High rate of condensate
	demand, low	size. Heat transfer to	return. Liquid and vapor
	heat source	working fluid from	coexist in full length
	temperature	heat source	of the evaporator.
		essentially unimpeded.
(B)	Low heat	Gas bubble expands	Low heat demand leads
	demand, low	relative to (A) to	to low rate of condensate
	heat source	adequately impede	return. Condensate
	temperature	heat transfer to	available only in the
		working fluid to match	condenser section where
		heat demand.	bubble is not present.
(C)	High heat	Gas bubble expands	High heat demand leads to
	demand, high	relative to (A) to	high rate of condensate
	heat source	impede heat transfer	return. The returning
	temperature	from the heat source	condensate fully evaporates
		to the working fluid.	before reaching the far end
			of the evaporator due to the
			high heat source
			temperature.
(D)	Low heat	Bubble expands	Low heat demand and low
	demand, high	further beyond case	rate of condensate return.
	heat source	(C), increasing	Large bubble provides
	temperature.	impedance to heat	significant impedance to
		transfer from the very	heat transfer from the very
		hot heat source.	hot heat source to the
			working fluid.

In addition to self-regulation of temperature, the heat pipe of the present invention provides effective pressure management. In a conventional heat pipe with an ample fluid charge, the vapor and liquid are essentially at boiling equilibrium and the pressure is essentially the vapor pressure of the fluid. Since the vapor pressure generally increases almost exponentially with temperature, the pressure inside the conventional heat pipe increases super linearly when heat is supplied from the heat source to increase the heat pipe temperature. In contrast, pressure in the present invention increases exponentially or super linearly only till the temperature T_Tis achieved in the evaporator region of the heat pipe. Above T_T, there is substantially no liquid and the vapor in the evaporator region is below its vapor pressure. Since the pressure of super heated vapor bubble will generally increase about linearly with the absolute temperature and not increase exponentially, the pressure rise in the heat pipe can be more easily managed. Essentially, the pressure buildup is moderated above temperature T_Tbecause liquid is not available to generate additional vapors. This temperature pressure relationship is graphically illustrated in FIG. 3 for a heat pipe using a controlled carbon dioxide charge to regulate the temperature at −3.2° C.
The problem with the limited working fluid charge heat pipe is the potential for unavailability of working fluid in the evaporator section of the heat pipe when heat transfer is actually needed. This is due to the time lag of transporting the condensed working fluid from the condenser region back to the evaporator region. The hydrophobic coating of the interior of the heat pipe substantially reduces the delay in transporting the working fluid back to the evaporator region.
The following exemplifies specific embodiments of the present invention.

Example 1

A design analysis was done for a heat pipe configured as illustrated in FIG. 1, having a diameter of 2 cm and a length of 50 cm and an internal volume of 157.08 cc. In this example, the heat pipe is used to transport heat from a variable temperature heat source to a target body at T_T=−3.2° C. Carbon dioxide is used as a working fluid. The mass of the working fluid was determined as discussed previously. The fluid mass consisted of first calculating the carbon dioxide mass required to fill the heat pipe volume with saturated carbon dioxide vapors at T_T=−3.2° C. and then adding additional carbon dioxide to account for the liquid condensate that would exist in the adiabatic and condenser regions of the heat pipe. The density of saturated CO₂vapor at −3.2° C. is 0.0885 g/cc. Accordingly, 13.9 grams of CO₂is charged to the heat pipe to fill it with saturated vapors at −3.2 degree C. and the additional carbon dioxide required to account for liquid carbon dioxide in the adiabatic and condenser sections was empirically determined as previously described. The heat pipe so configured is operated with heat source (21) whose temperature may vary in a temperature range above −3.2° C. The heat pipe self-regulates at T_Tequal to about −3.2° C.
FIG. 3 is an illustration of the temperature and pressure inside the above heat pipe designed to regulate the temperature at −3.2° C. Below −3.2° C., carbon dioxide is present as a combination of liquid and vapor. As the temperature of the heat pipe is raised to about −3.2° C. by using heat from the heat source, the liquid carbon dioxide in the evaporator section vaporizes. The heat pipe will regulate its temperature at about −3.2° C. and its temperature will not generally increase above about −3.2° C. Below −3.2° C., the vapor and liquid are essentially in boiling equilibrium and the pressure inside the heat pipe is essentially the saturation vapor pressure of carbon dioxide at the heat pipe temperature. This saturation vapor pressure increases super linearly as is shown in FIG. 3. Had the heat pipe been designed with excessive carbon dioxide, the pressure would have kept on increasing super linearly.
However, with the controlled charge, there is no liquid carbon dioxide inside the evaporator region of the heat pipe at temperature exceeding about −3.2° C. Even if the heat pipe heats up above −3.2° C. due to some extraneous heat, the pressure will increase only modestly, and only linearly with absolute temperature. This modest increase in pressure due to hypothetical extraneous heat is shown by the broken line in FIG. 3. Thus, the pressure in the heat pipe is also managed and regulated.

Example 2

In a second example of the present invention, heat was extracted from a hot gas stream whose temperature varied over a large range from 300 to 600° C. It was desired to heat a hydrocarbon stream using this heat. In this case the temperature of the hydrocarbon stream was regulated at about 240° C.
A heat pipe with an internal volume of 251 cc was built. A finned heat exchanger was used to transfer heat from a hot gas stream to the evaporator section of the heat pipe. Similarly, a condenser was built to transfer heat to a hydrocarbon stream with the aim of heating the hydrocarbon stream to a regulated temperature of about 240° C. Water was selected as the working fluid for this heat pipe application. The heat pipe was evacuated of all gases and then first charged with about 4.1 grams of water. This 4.1 grams of water was calculated by multiplying the heat pipe volume (=251 cc) by the density of saturated water vapor (steam) at 240° C. (=0.016 g/cc). An additional amount of water was added to compensate for the shuttling-water between the evaporator and the condenser, and the water retained inside the wick as described earlier. This heat pipe, using a controlled-fluid charge was then found to regulate the temperature of the hydrocarbon stream at about 240° C. when the temperature of the hot gas stream varied over the large range mentioned above. Thus, by using a prescribed quantity of fluid charge a substantially constant temperature of the hydrocarbon stream was obtained.

Example 3

FIG. 4 illustrates a heat pipe fabricated to further demonstrate the invention. The heat pipe tube (41) has an internal volume of 193 cc. The finned heat exchanger (42) with an effective area of 0.32 m2, was used to transfer heat from a hot gas stream (44) supplied at the evaporator end of the heat pipe by means of the ducted enclosure (43). A condenser heat exchanger (46) was used to transfer heat to a gasoline fuel stream (47) with the heated fuel exiting as stream (48). Temperatures were measured by means of thermocouples in the entering and exiting fluid steams (44, 45, 47, 48) and along the length of the heat pipe (49 a-e).
The evacuated heat pipe tube described was charged with 3 g of distilled water and 0.05 g of Argon. This amount of water was chosen to maintain operating temperatures of <200° C. with variable heat inputs and fuel loads. The corresponding steam density at 2000° C. is 0.0076 g/cc.
The heat pipe was mounted at a variety of tube angles from 5 to 45 degrees for testing, with the heated evaporator section at the low position. The heat pipe tube and fuel heat exchanger were insulated.
The heat pipe described by FIG. 4 was evaluated at an angle of 10 degrees to heat gasoline fuel flowing at 0.5 to 2 g/s and 450 kPa pressure. One test was conducted with no fuel flowing.
Hot nitrogen flowing at about 7.8 g/s and 375° C. was used as the heat source to heat the gasoline to a nearly constant temperature of approximately 160° C., with a variation of less than 10° C. with the fuel flow varying from 0.5 to 2 g/s.
Temperatures along the length of the heat pipe tube were measured at positions noted in FIG. 4. Temperatures at the condensate end of the heat pipe remained nearly constant with varied fuel load. Temperatures measured at the evaporator were higher indicating the presence of superheated gas bubble.
With no fuel load, temperatures increased slightly, but remained less than 200° C. with the minimal load resulting from heat losses along the length of the heat pipe. The result indicates that fuel rates lower than 0.5 g/s can be used.
The temperature profile of the device is illustrated for four fuel rates which correlate to four heat demand rates, illustrating the temperature regulating characteristics of the heat pipe with a specified mass of working fluid.

Example 4

The heat pipe illustrated in FIG. 4 was then modified to include a hydrophobic coating of the interior volume of the condensation region, here identified as coating (50).
Boiling and evaporation in the evaporator section (4), and convection of the generated vapor to the condenser section (51) is the dominant mechanism of heat transport in heat pipes. A shortcoming of the limited working fluid charge heat pipe is slowed temperature response. The slow response of the heat pipe, when using a limited fluid charge, can result from an inadequate fluid being available for boiling in the evaporator section of the heat pipe. This potential problem is exacerbated as the temperature rises, when more and more of the fluid gets boiled off, and a lesser amount of the working fluid remains available for boiling . . . thereby reducing the boiling rate further. Additionally, the condensate produced in the condenser clings to the heat pipe wall as droplets or liquid films which may travel down slowly to the evaporator. If the fluid inventory on the heat pipe wall could be instantly made available for reboiling more rapidly, the rate of heat up will increase. The hydrophobic coating of the present invention reduces the liquid clinging to the wall and reduces the transit time of the working fluid to the evaporator section of the heat pipe. This reduction in the liquid inventory on the wall increases the liquid inventory in the evaporator section. This in turn increases the boil-off rate and accelerates the heat pipe response.
The faster heat pipe response with a hydrophobic coating is depicted in FIG. 6. FIG. 6 illustrates the operation of a heat pipe which undergoes periodic shutdown after several hours of operation. Before this heat pipe is restarted, it cools down to near ambient temperature. This heat pipe has a very minimal charge of the working fluid to accomplish self regulation of temperature. To show the benefits of a hydrophobic coating, this heat pipe was operated both with and without a hydrophobic coating.
Without a hydrophobic coating, the working fluid was found to condense on the wall during cool down to the ambient temperature. This condensed fluid formed small droplets which adhered to the wall and failed to flow down to the evaporator section. Since there was no working fluid available in the evaporator during for boil-off, this heat pipe without a hydrophobic coating took a relatively long time to start. It took about 11.5 minutes for the heat to conduct from the lower section of the evaporator to the droplets on the wall before these droplets could boil-off and the heat pipe could start.
In contrast, with the hydrophobic coating, the droplets failed to adhere to the wall during the heat pipe cool down to the ambient temperatures. This resulted in the availability of a liquid working fluid in the evaporator section. Upon restart, the heat pipe responded in a relatively shorter period of only 4.0 minutes.

Claims

What is claimed is:

1. A heat pipe for providing heat transfer to a heat recipient at a temperature of about T_Tcomprising at least a evaporator region and a condenser region having a known interior volume V and a determined mass of working fluid at least equal to M_wf, whereby the mass of working fluid is determined by the relationship

M _wf =D _wfat T×V

where D_wfis the density of the saturated vapor of the fluid at temperature T_T

and at least a portion of the interior volume has vessel walls coated with hydrophobic material.

2. The heat pipe of claim 1 wherein the working fluid comprises M_wfplus an additional amount of fluid that, at the temperature of about T_T,

a. evaporation from the evaporator region is about equal to condensation, and

b. rate of supply of condensate to the evaporation region is about equal to rate of evaporation from the evaporation region.

3. The heat pipe of claim 2 wherein the mass of working fluid is further characterized as an amount sufficient, at about T_T, for initiating a gas bubble in the evaporation region when heat is transferred to the heat recipient.

4. The heat pipe of claim 3 wherein the mass of the working fluid ranges from about M_wfto about 200% M_wf.

5. The heat pipe of claim 4 wherein the mass of the working fluid ranges from about M_wfto about 150% M_wf.

6. The heat pipe of claim 1 wherein the hydrophibic material comprises: hydrocarbon chains, fluorocarbons and/or non-polar siloxane, Teflon and the like.

7. A method for controlling temperature T_Tof heat transfer in a heat pipe having at least an evaporation region and a condenser region, from a heat source to a heat recipient, comprising:

a. determining interior volume of the heat pipe,

b. selecting a working fluid for the heat pipe having a known density D_wfof its saturated vapor at T_T

c. determining a mass of working fluid according to the relationship

M _wf =D _wfat T _T ×V,

d. supplying at least the mass of working fluid of step c to the heat pipe.

8. The method of claim 7 wherein a sufficient amount of working fluid is added to initiate a gas bubble in the evaporation region whenever the temperature of the evaporation zone is above about T_T.

9. The method of claim 8 wherein the mass of the working fluid ranges from about M_wfto about 200% M_wf.

10. The method of claim 9 wherein the mass of the working fluid ranges from about M_wfto about 150% M_wf.