Heat-pipe thermostats of high precision

In a gas-controlled heat-pipe thermostat of high precision having a temperature-controlled chamber arranged at least partly within the evaporation and condensation cycle and a gas reservoir connected to said heat-pipe, the improvement comprising in that in the heat-pipe a cooling surface is arranged for the production of condensate which, for the scavenging of the surface of said temperature-controlled chamber in a directed manner, is connected to the outer wall of said chamber by liquid conducting capillary structures.

The invention concerns gas-controlled heat chambers, also known as 
heat-pipe thermostats, as described in the Luxemburg Pat. No. 70419. In 
particular, the invention concerns heat chambers in which the desired 
temperature can be set and maintained with a high degree of precision. 
As is known, in gas-controlled heat pipes there is a relation, in the ideal 
case, between the pressure of the control gas and the temperature of the 
heat chamber which is determined by the vapor pressure curve of the 
working fluid used. 
In practice, however, there are deviations from the ideal behavior which 
are very troublesome in heat-pipe thermostats of high accuracy. 
The research work forming the basis of the present invention has shown that 
deviations are often due to the presence of endogenic impurities in the 
working fluid on the walls of the temperature-controlled chamber. 
The impurities can be substances, for instance, which are detached from the 
walls of the chamber by the working fluid. Impurities of this kind in the 
working fluid reduce its vapor pressure at the temperature set. At a 
pre-determined control gas pressure, the detached impurities therefore 
cause a rise in the saturation temperature of the vapor and thus in the 
temperature of the heat chamber and this effect is in proportion to the 
magnitude of the concentration of detached impurities at the fluid/vapor 
phase boundary. 
It should be mentioned that this effect in practice is only caused by 
impurities of low volatility since highly volatile impurities are diffused 
into the vapor phase at the fluid/vapor phase boundary so that their 
concentration at the surface of the fluid may be regarded as practically 
zero. 
Since the contamination effect described is dependent on the constructional 
details of the heat chamber and its condition and can scarcely be defined 
or anticipated according to natural laws, the resultant reduction in 
temperature must be regarded in general as an uncertain factor in the 
absolute temperature level of the chamber and its maintenance over a 
period of time. 
The invention is concerned with the problem of how this temperature error 
factor, i.e., the presence of impurities of low volatility in the fluid on 
the walls of the chamber, can be kept small. 
The invention is based on the following considerations: high concentrations 
of impurities occur especially when 
fluid stagnates on the walls of the chamber since then there is much time 
available for the diffusion of impurities from the walls of the chamber 
into the fluid and thus the concentration of impurities in the fluid can 
rise to an equilibrium level or when 
there are zones of convergent streams of fluid on the walls of the chamber 
(i.e., vaporizing zones into which fluid streams from all sides) since 
impurities of low volatility entrained by the streams of fluid accumulate 
in such zones and in this manner their concentration can rise to the 
solubility limit. Such vaporizing zones on the walls of the chamber can 
occur when heat is passed to the walls of the chamber, either through 
exothermic processes in the interior of the chamber or from outside, e.g., 
through superheated steam striking the wall of the chamber or through the 
radiation of heat from very hot surfaces, e.g. from the actual heating 
zone of the heat pipes. 
The invention is also based on the consideration that condensate freshly 
formed in the cooling zone is particularly clean since 
impurities of low volatility are hardly present in vapor and therefore not 
in the condensate resulting from this vapor either 
time is necessary for impurities to be detached or dissolved out of the 
walls. 
In general, fresh condensate contains only volatile impurities, especially 
control gas, which are of no importance for the effect discussed here of 
the fall in vapor pressure through dissolved impurities as mentioned 
above. 
On the basis of these considerations, the invention concerns a 
gas-controlled heat-pipe thermostat of high precision with a 
temperature-controlled chamber arranged at least partially within the 
vaporization and condensation cycles, characterized in that, in the heat 
pipe, a cooling surface for the production of condensate is arranged 
which, for the scavenging of the surface in a directed manner, is 
connected to the outer wall of the temperature-controlled chamber in such 
a way that liquid can be conducted to it. 
Thus by this arrangement a high concentration of impurities on the surface 
of the fluid on the wall of the chamber is prevented since the wall of the 
chamber is constantly scavenged by a directed stream of fresh condensate. 
Through this stream the time for which each individual volume of fluid 
remains on the wall of the chamber is kept short and thus the 
concentration of impurities dissolved out of the wall in this time remains 
small. In the vaporizing areas, this stream is superimposed on the 
convergent stream with the result that fluid no longer flows into the 
vaporizing zone from all directions all the time but that now fluid flows 
in from one side and flows out the other (somewhat less, according to 
evaporation) and in this way the accumulation of impurities of low 
volatility is avoided. 
There are many possibilities for the practical execution of the invention. 
Fumdamentally, the following is necessary: 
a cooled surface which in the extreme case can also be a part of the 
chamber itself on which the condensate is produced 
a suitable `connection` which enables the condensate to pass from the 
condenser to the surface of the chamber (insofar that the surface of the 
chamber is not itself the condenser) 
a means to distribute the condensate over the surface of the chamber and 
finally 
a suitable `connection` enabling the condensate to pass from the wall of 
the chamber to an evaporator.

As shown in FIG. 1, heat is passed to the heat pipe at H to evaporate the 
working fluid in the evaporation zone (5). 
Part of the vapor (D') escapes to the cooling zone (K') which is connected 
to a gas pressure regulating system (G) via a gas buffer (7). 
Other vapor (D) passes to the cooling zone (K) which can be connected via a 
cut-off valve (8) to a low-pressure chamber not shown here. Thus fresh 
condensate is formed at (1) and passes via the capillary structures (2) 
formed from several layers of fine-meshed wire netting at the top to the 
wall of the heat-chamber (6). The condensate is distributed there via a 
similar capillary and is ultimately passed back, at the bottom, via 
another capillary structure (4) to the evaporation zone (5). 
The form of execution of the invention described is in no way restrictive. 
As is clear to those skilled in the art, the return of condensate to the 
wall of the chamber can be arranged instead of via capillary structures 
also by a distributing channel, for example, or by simply allowing it to 
drip down.