Self regulating heat pipe

A structure for more accurately automatically controlling the heat transfer characteristics of a heat pipe with a non-condensible gas. The gas, intermixed with the heat transfer vapor, is largely contained by an expanding and contracting bladder. This permits the vapor pressure of the heat transfer fluid to control the position of the non-condensible gas to vapor front with less back pressure from the gas which is being compressed. The bladder is contained within a structure which is itself enclosed within the interior of the heat pipe evaporator so that the non-condensible gas is held at a constant temperature.

SUMMARY OF THE INVENTION 
This invention deals generally with heat pipes and more specifically with 
the temperature control of heat pipes by the use of a non-condensible gas 
reservoir. 
The use of non-condensible gas as a means of regulating the heat transfer 
characteristics of a heat pipe is well established. In most such 
arrangements the gas is accessible to the vapor space of the heat pipe 
from a separate reservoir and its pressure or volume is controlled by some 
simple means such as changing its temperature or changing the volume of 
the reservoir, such as by a bellows. In U.S. Pat. No. 3,517,730 by T. 
Wyatt it was also shown that the bellows action could be controlled by an 
independent mechanical thermocouple device so that a feedback system was 
created which automatically controlled the heat pipe temperature. Such 
mechanical devices add complexity and size to the installation and can 
adversely affect reliability. 
Another problem in the use of the non-condensible gas is that there is 
always a significant amount of working fluid vapor mixed with the 
non-condensible gas. This can lead to problems of condensation of the 
vapor within the non-condensible gas reservoir if the temperature of the 
reservoir is low enough and this causes erratic temperature control. Wyatt 
attacks this problem by adding an electrical heater and an insulated 
container around the non-condensible gas reservoir, again adding 
complexity and size to the configuration. 
The present invention presents a self-regulating heat pipe which uses a 
non-condensible gas within a novel structure. It uses an expandable 
reservoir which is located within the evaporator region of the heat pipe 
itself but is connected with and affected by the condenser region through 
a pipe or tubing which extends from the reservoir back to the condenser 
region. 
The result is that the expandable gas reservoir is operated at a virtually 
constant temperature, that of the heat pipe evaporator, which is always 
too high to permit condensation of the working fluid vapor. Moreover, the 
resistance to the expansion of the reservoir is essentially constant 
because the gas in the secondary reservoir which resists the expansion is 
also held at the same constant temperature so that its pressure 
essentially does not increase. 
The preferred embodiment of the invention uses an expandable reservoir in 
the form of a balloon or bladder with very low resistance to expansion. 
The bladder is constructed of aluminized mylar, so that it is usable in a 
relatively low temperature heat pipe using water as a working fluid. In 
such an arrangement, the gas pressure to which the secondary reservoir is 
filled is the only resistance to expansion of the primary reservoir, and 
the primary non-condensible gas reservoir will increase or decrease its 
volume from only the action of the pressure of the working fluid vapor. 
Thus, no outside thermostatic control is required, and the result is a 
highly stable self regulating, temperature controlled heat pipe.

DETAILED DESCRIPTION OF THE INVENTION 
The FIGURE is a simplified cross section view along the axis of the 
preferred embodiment of the invention in which heat pipe 10 encloses 
non-condensible gas primary reservoir 12 and secondary reservoir 14. 
Heat pipe 10 is conventionally constructed of sealed casing 16 with 
capillary wick 18 lining the inner walls of casing 16. In operation, one 
end of heat pipe 10 is the evaporator region 20 to which heat is applied 
and the other end is the condenser region 22 from which heat is removed. 
If heat pipe 10 were evacuated and only vaporizable working fluid were 
loaded into it at fill tube 24, it would operate as a conventional heat 
pipe. 
However, when a non-condensible gas such as nitrogen is also loaded into 
heat pipe 10, it operates somewhat differently. As is well understood in 
the art, the non-condensible gas will be swept to condenser region 22 of 
the heat pipe 10 by the movement of the working fluid vapor and the gas 
will collect there, preventing that part of the heat pipe which it 
occupies from operating as a heat pipe. In fact, a boundary 26 will form 
between the volume of the heat pipe which contains non-condensible gas and 
that volume which does not. 
The present invention adds to this conventional configuration in order to 
attain self regulating temperature control for the heat pipe. 
The additional structure is essentially three items. Secondary reservoir 
14, which has a non-expandable structure is located in evaporator region 
20. It encloses primary reservoir 12 the opening of which is attached to 
conduit 28 and held in place by clamp 30. The end of conduit 28 which is 
remote from primary reservoir 14 opens into the interior of heat pipe 10 
near the end of condenser region 22 which is most remote from evaporator 
region 20. The open end of conduit 28 is located well into the region of 
the heat pipe which contains the non-condensible gas. 
During normal operation the non-condensible gas will, therefore, fill 
conduit 28 and partially inflate expandable primary reservoir 12. This 
expansion will be resisted and limited by the pressure of the 
non-condensible gas which has been loaded into secondary reservoir 14 
through its fill tube 32. 
The pressure of the gas in secondary reservoir 14 determines the heat 
pipe's temperature control point, and that pressure is one of the design 
parameters. The pressure of the gas in secondary reservoir 14 should be 
the same as the vapor pressure of the heat transfer fluid in the heat pipe 
at the nominal operating temperature. 
With the pressure of the gas in secondary reservoir 14 determined, pressure 
equilibrium will be established between secondary reservoir 14 and the gas 
and vapor mixture in expandable primary reservoir 12, and boundary 26 will 
locate where it forces the working fluid vapor pressure and the pressure 
of the mixture of vapor and non-condensible gas to also be equal. 
The automatic control phenomenon will then function as follows. 
If conditions attempt to raise the temperature of evaporator region 20, the 
vapor pressure of the heat transfer fluid will attempt to rise. This will 
push boundary 26 farther away from evaporator region 20 and thereby 
activate more surface of heat pipe 10 within condenser region 22 to afford 
more cooling to limit the temperature rise at evaporator 20. 
The movement of boundary 26 meets only slight resistance because it is 
accommodated to by the expansion of primary reservoir 12, which is, in 
effect, at the opposite end of the combined gas vapor zone from boundary 
26. The expansion of primary reservoir 12 itself meets with little 
resistance because its movement is resisted only by the gas pressure in 
secondary reservoir 14, which is,as mentioned, nominally the same as the 
vapor pressure of the heat transfer fluid. The increased volume of primary 
reservoir 12 therefore limits the temperature increase of evaporator 
region 20, and a decrease in volume of primary reservoir 12 will also 
occur to limit a decrease in temperature of evaporator region 20. 
This feedback system is aided by the fact that the non-condensible gases in 
secondary reservoir 14 and in primary reservoir 12 are essentially at the 
temperature of evaporator region 20 and are therefore at a constant 
temperature, thus eliminating any temperature change effects on pressure. 
Moreover, since the temperature of the gases is approximately that of the 
highest temperature in the system, no condensation of vapor will occur in 
expandable primary reservoir 12. 
The present invention has been tested in a heat pipe constructed of copper, 
with water as the working fluid, and having an expandable primary 
reservoir constructed of aluminized mylar. This embodiment showed superior 
self regulating properties in that, with a change in heat sink temperature 
over the range from negative 0.23 degrees C. to positive 29.4 degrees C., 
the heat pipe evaporator temperature varied only 1.15 degrees C. from the 
set point temperature of 36.1 degrees C. On the other hand a more 
conventional heat pipe with a fixed wall non-condensible gas reservoir 
could be expected to have a variation in evaporator temperature 
approximately four times as great. 
It is to be understood that the form of this invention as shown is merely a 
preferred embodiment. Various changes may be made in the function and 
arrangement of parts; equivalent means may be substituted for those 
illustrated and described; and certain features may be used independently 
from others without departing from the spirit and scope of the invention 
as defined in the following claims. 
For example, expandable primary reservoir 12 could be constructed as a 
bellows or a piston rather than as a balloon or bladder. Moreover, another 
means of resisting the expansion of the primary reservoir could be used. A 
spring could, for instance, be used in conjunction with a piston to permit 
the expandable primary reservoir to react to increased vapor pressure.