Method for carrying out maintenance work on a refrigerator, device and refrigerator for carrying out the method

The invention relates to a method for carrying out maintenance work on structural parts located within the housing of a refrigerator, the refrigerator being divided into two parts separable from each other so that the interior chamber of the refrigerator becomes accessible. The method is carried out by opening the housing, carrying out the maintenance work, closing the housing and thereafter sweeping the housing with working gas. In order to avoid condensation in the cold areas of the interior chamber of the refrigerator after the housing is opened, the method provides that the areas of the refrigerator housing endangered by condensations be heated locally to a temperature, which is so selected, that up to and including the sweeping process following the maintenance work, the temperature does not fall below the dew point of condensable gases.

BACKGROUND OF INVENTION 
The invention relates to a method for carrying out maintenance work on 
structural parts within a refrigerator housing having two parts separable 
from each other in such a manner that the interior space of the 
refrigerator housing becomes accessible, whereby the housing is opened, 
maintenance work is carried out and the housing is closed and finally 
swept with a working gas. In addition, the invention relates to a device 
and a refrigerator for carrying out the method of the invention. 
Refrigerators are low temperature cold machines, wherein thermodynamic 
circulation processes occur. A one-stage refrigerator comprises 
essentially a working space with a displacer. The working space is 
alternatingly connected in a specific way to a high pressure gas source 
and to a low pressure gas source, so that during the forced reciprocating 
motion of the displacer the thermodynamic circulation processes occur. 
Here, the working gas is led into a closed circulation system, the 
consequence of which is that from a given region of the working chamber 
heat is withdrawn. Employing two-stage refrigerators of this type with 
helium as the working gas, temperatures less than 10.degree. K. can be 
achieved. 
Generally, repair and maintenance work on structural parts located within 
the refrigerator housing is carried out by switching the refrigerator off 
and waiting for the cold regions of the refrigerator to reach room 
temperature. This waiting period was necessary in the absence of heating, 
to avoid the collection of condensable gases on the cold surfaces upon 
opening the refrigerator housing the condensation of which would endanger 
the operation of the refrigerator upon restarting. Since the temperatures 
of the cold surfaces of a refrigerator operated with helium are only a few 
degrees Kelvin (60.degree. to 80.degree. K. in a one-stage refrigerator or 
at the first cold stage of a two-stage refrigerator) nearly all gases 
present in the atmosphere must be considered condensable gases and, 
therefore, undesirable. Alternatively, the housing is sufficient 
preheated, prior to opening. The housing is then opened and the requisite 
work is carried out. The housing is then closed and thoroughly swept with 
the working gas before the refrigerator is again returned to service. 
Maintenance work carried out in this way takes a long time, considering in 
total the actual repair work time, the housing preheat time and the 
restart time. Furthermore, operation of the equipment or instrument, which 
is cooled by the refrigerator, is interrupted during this time. With 
instruments having a high fill level of liquid helium (100:1 and more) 
heating the refrigerator for the sole purpose of maintenance is, for 
economic reasons alone, out of the question because it presupposes removal 
of the liquid helium. If, for example, a refrigerator cools the magnets of 
a nuclear spin tomograph, considering the instrument being inoperable for 
two to three weeks and the additional cost of helium, extraordinary 
expenses totaling tens of thousands of dollars would have to be acceptable 
if the maintenance work is to be carried out in the conventional manner. 
In order to avoid long operating interruptions, it is known to undertake 
the maintenance work within a glove box. Using the glove box permits 
carrying out this work in a protective gas atmosphere. In this way, for 
example, condensable gases which would penetrate into the working or 
cylinder space of the displacer and condense on the cold surfaces are 
avoided. Using the glove box, therefore, has the advantage that 
maintenance work can be performed at still low temperatures without the 
requirement of preheating the cold areas of the refrigerator and the 
connected equipment or instrument. 
Using the relatively expensive glove box is possible however, only where 
sufficient space is available. Moreover, the length of time for performing 
the maintenance work is still somewhat excessive (at least 2 hours). One 
reason for this is the need for several sweeping processes with 
concomittant relatively high protective gas consumption to create a 
sufficiently pure protective gas atmosphere within the glove box. Another 
reason can be found in the high degree of difficulty inherent in handling 
tools with the gloves of the glove box. Moreover, the danger exists that, 
for example, a displacer to be exchanged touches the box sides or the 
gloves in the glove box with its cold side upon being pulled out. The 
consequence of this is the destruction of parts customarily consisting of 
synthetic foil and with that a contamination of the protective gas 
atmosphere. In addition, temperature increases occurring during the 
maintenance work, which, due to using the glove box for a relatively long 
time, are not neglible, so that the time spent on returning to operating 
temperature service is significant. Finally, the costs incurred in 
connection with the high protective gas consumption are not 
inconsiderable, particularly if helium must be used. 
Another displacer exchange method has been suggested, in which a protective 
gas stream is maintained from the moment the housing is opened until 
closing to prevent undesirable gases from penetrating into the cylinder 
chamber of the displacer. However, in this exchange method, the 
possibility exists that condensable gases will reach the displacer 
chamber, particularly, at the moment when the displacer to be exchanged is 
removed from the refrigerator housing. If this is done too rapidly, then 
not only protective gas but air also enters the displacer chamber and 
condenses immediately on the cold interior walls of the housing. The 
condensation hinders the installation of the new displacer and can only be 
removed at great technical expense. 
OBJECT AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved method for 
carrying out maintenance work on structural parts located in a 
refrigerator, which can be performed rapidly while avoiding gas 
condensation with certainty. It is another object of the present invention 
to provide a device an refrigerator for carrying out the method of the 
invention. 
According to the method of the invention the cold surfaces of the 
refrigerator housing endangered by gas condensation are locally heated to 
a temperature selected so that prior to the sweeping process and including 
the maintenance work, the surface temperature does not fall below the dew 
point of the condensable gases. The particular advantage of this method 
lies in the fact that it ensures with certainty that the surface 
temperatures do not fall below the dew point, which means that maintenance 
work can be carried out rapidly and unimpeded by undesirable 
condensations. 
Local heating of housing surfaces endangered by condensation is conducted 
after opening the housing and removing the displacement system. 
Thereafter, a hot gas can be blown into the refrigerator housing until the 
surfaces endangered by condensation have reached such a temperature, that 
subsequent to turning off the hot gas stream, the maintenance, the closing 
of the housing and the concluding sweeping process can be conducted 
without the surface temperature falling below the dew point of condensable 
gases. A particularly useful temperature to which to heat the surfaces is 
approximately 50.degree. to 60.degree. C. A final surface temperature of 
this order of magnitude ensures that in the time in which the refrigerator 
housing, after discontinuing the hot gas stream, is still open, that the 
surface temperature does not fall to values below the dew point of 
condensable gases. As a result, immediately following the sweeping 
process, the serviced refrigerator is again available for operation. 
In another embodiment of the invention, surfaces endangered by condensation 
are locally heated with heating devices to maintain the local surface 
temperature above the dew point of condensable gases. It is advisable to 
use electric heaters for this purpose, which are arranged in the area of 
the cold stage(s). Heating devices of this nature can, for intended 
maintenance, be turned on before the housing is opened and can be operated 
for a length of time which with certainty prevents condensation in the 
displacer chamber after the refrigerator housing is opened. The particular 
advantage of this method lies in that displacers frozen from contamination 
can be freed and made functioning again or can be simply exchanged. 
Further advantages and details of the invention are described below in 
conjunction with embodiments represented in the drawings.

Referring to the drawings and first to FIG. 1 there is shown a refrigerator 
1 comprised of a housing, having housing parts 2 and 3. In housing part 2, 
cylindrical work chambers 4 and 5 are provided for displacer stages 6 and 
7. 
The upper displacer stage 6 is equipped with a drive piston 8, and an 
associated cylinder 9 which is kept in a guide bush 10. The guide bush 10 
is provided with bores 11, 12, and 13. Bore 11 opens into the working 
chamber 4 and serves to supply chamber 4 with a working gas. The central 
bore 13 ends in a transverse bore 14, which is connected with a ring 
groove 15 in the outer wall of the guide bush 10. Two further bores 12 are 
indicated by dash-dot lines and serve to pneumatically drive the system 
consisting of the displacers 6 and 7. The different bores lie in planes 
differing from the plane of projection so that they do not cross each 
other, which is indicated hatched or with dash-dot lines respectively. 
In the housing part 3 a control motor 16 is located, which actuates a 
control valve 18 through shaft 17. This control valve 18 serves in a 
manner known per se to supply the various bores with high and low pressure 
working gas, preferentially helium. 
The connections for the high pressure and the low pressure working gas are 
labeled 19 and 20, respectively. A separating plane 21 between the housing 
parts 2 and 3 lies at the level of the control valve 18. Upon removal of 
the upper housing part 3 with motor 16 and valve 18 and upon further 
removal of the guide bush 10, displacers 6 and 7 become accessible and can 
be exchanged within the framework of maintenance. 
During operation of the refrigerator 1, the working gas is under high 
pressure and flows through connection 19 into the refrigerator 1. Control 
valve 18 supplies the different bores 11 and 12 with the high pressure 
working gas. After expansion in the refrigerator stages, the working gas 
penetrates into bores 13, 14 and flows through to the ring groove 15 and 
out the low pressure connection 20. The working gas pressure at the high 
pressure connection 19 is customarily 22 bar, while the working gas 
pressure at the low pressure connection 20 is approximately 7 bar. 
The structural parts within the interior of the refrigerator housing most 
subject to wear and tear are, in particular, the motor 16, the valve 18, 
and the displacement system 6 and 7 (only partially shown). Maintenance 
work on these parts can only be performed after the housing part 3 is 
removed so that even in the case where the guide bush 10 and the 
displacement system 6 and 7 can remain in the housing part 2, condensation 
in the other cold stages cannot be prevented. 
The manner of carrying out maintenance work according to the invention is 
intended to be explained in conjunction with FIG. 2. FIG. 2 represents 
schematically the lower housing part 2 of the refrigerator, which is 
connected to low temperature equipment, such as a superconducting magnet 
(not shown). To this end, the housing 2 itself is equipped with a flange 
22, which, in turn is connected to the housing of the low temperature 
equipment. To each cold stage of the refrigerator 1 a flange 23 or 24 is 
heat conductively fastened. These flanges, in turn, are connected over 
cold bridges indicated by dash-dot lines with one temperature label of the 
desired temperature (first stage flange 23: approximately 60.degree. to 
80.degree. K.; second stage flange 24: approximately 10.degree. K.). 
In order to replace the displacement system 6, 7 located within the 
interior 4, 5 of refrigerator housing 2 during operation of the 
refrigerator, the housing part 3 with motor 16 and valve 18 is lifted off 
and the guide bush 10 is removed. Subsequently, the displacement system 6, 
7 becomes accessible and can be pulled out of the lower housing part 2. 
Immediately following the process of taking out the displacement system, 
air will stream into the interior housing chamber 4,5 and condense on the 
cold surfaces. In order to remove these condensations and to heat the cold 
areas to temperatures at which condensations no longer occur, a hot gas 
blower 27, the shape of which is adapted to the interior space 4,5 of the 
housing part 2, is introduced into the housing part 2. In the embodiment 
shown in FIG. 2, air is used as hot gas. The hot gas blower 27 comprises a 
housing 28, in which a ventilator 29 as well as a preferentially 
adjustable filament winding 31 is located. The longitudinal axis of the 
system is labeled 30. The tubular sections 32 and 33 of the blower project 
into the interior chamber 4, 5 of the refrigerator, respectively. The 
diameters of the tubular sections 32 and 33 are adapted to the diameters 
of the refrigerator stages in such a way, that in each instance an annular 
space surrounding the pipe sections 32 and 33 remains. In a single-stage 
refrigerator, tubular section 33 is omitted. To heat the areas 
contaminated by condensation, the hot air blower 27 is placed on the 
housing part 2 with a collar 34 and operated. The ventilator 29 draws air 
through inlet openings 35 and across the filament winding 31 in the 
direction of the pipe sections 32 and 33. The heated air flows from the 
pipe sections 32, 33 and through the particular outer annular spaces back 
to the collar 34, which is equipped with air outlet openings 36 (cf. arrow 
37). The air outlet openings 38 and 39 of the pipe sections 32 and 33 lie 
in the region of the cold ends of the particular refrigerator stages, 
where the greatest amount of heat conduction is required. 
In the embodiment represented in FIG. 2, the outlet opening 38 of pipe 
section 32 is built as an adjustable ring nozzle. To this end, the lower 
part 41 of the pipe section 32 enlarges conically in the direction of 
stream of the hot gases. Into this conically enlarged opening a likewise 
conical shaped part 42 is set, which is supported displacably (double 
arrow 43) in the direction of the longitudinal axis 30 of the hot gas 
blower 27. By sliding the shaped part 42 in the axial direction, the width 
of conical gap 44 changes so that the quantity of air streaming out can be 
adjusted. This adjustment permits selecting the ratio of gas streaming out 
of the pipe sections 32 and 33 in such a way to ensure that approximately 
the same temperatures are obtained in both of the refrigerator stages. 
Shaped part 42 also serves to simultaneously support the pipe section 33 
projecting into the partial interior space 5 of the refrigerator housing 
2. 
After heating to sufficiently high temperatures (advisably approximately 
50.degree. to 60.degree. C.) the blower 27 can be removed from the 
refrigerator housing 2. Thereafter, the temperatures will drop again until 
they approach dew point temperatures. However, sufficient time remains to 
install the new displacement system 6, 7, assemble the housing part 3 and 
sweep the interior chamber 4, 5 with helium. Immediately following the 
helium sweep, the refrigerator 1 is again available and ready to be 
operated. 
If repairs need to be carried out only on the motor 16 or valve 18, the 
displacement system 6, 7 must likewise be removed from the housing part 2, 
in order to insert the hot gas blower 27. After sufficient heating and 
removal of condensation in the interior of the housing part 2 as well as 
on the displacement system 6, 7 and after completion of the repair, the 
refrigerator can be put back together and the concluding sweep process 
carried out. 
FIG. 3 shows an alternate embodiment, in which the air as a hot gas, is 
replaced by an inert or working gas in a compressed gas bottle 45, 
preferably helium. The compressed gas bottle 45 is connected to the blower 
27 with a hose line. Since the gas enters the housing 28 under pressure a 
ventilator 29 is not required. The remainder of the maintenance work, 
which frequently includes exchanging the displacement system 6, 7, is 
carried out in the manner described in conjunction with FIG. 2. This 
embodiment is then of particular advantage, when due to the presence of 
magnetic fields the danger of disturbing a ventilator 29 exists. 
In the embodiment according to FIG. 4, the refrigerator 1 itself is 
equipped with heating devices 51, 52. These are arranged in the region of 
the cold ends of the refrigerator stages and can, depending on the 
geometry of the available space, be built as a ring, plate or pot heater. 
If, for example, the displacement system is intended to be exchanged, the 
electric heaters 51, 52 are put into operation even before the housing 
parts 2 and 3 are opened in order to heat the cold areas to temperatures 
above the dew point. It is advisable to keep the heating devices operating 
even during the displacer exchange, so that it will not be required to 
heat to excessive temperatures in order to compensate decreases of 
temperatures during the maintenance work. It is, for example, sufficient 
if the temperatures in the areas of the two cold stages are heated until 
completion of the maintenance work to room temperature or somewhat above 
it (up to 20.degree. C.) and maintained there. This method offers the 
additional advantage, that when doing repair work on the motor 16 or the 
valve 18 it is not absolutely necessary to take the displacement system 6, 
7 out of the housing part 2, since the areas of the refrigerator open 
during operation can be heated to sufficiently high temperatures already 
before the housing is opened. 
To maintain the mentioned temperatures, it is advisable to provide control 
or regulating devices. In the represented embodiment temperature sensors 
53 and 54 are provided on both cold stages. A plug 55 connects the 
electric wires leading to heaters 51, 52 as well as the control lines 
leading to the sensors 53, 54 to regulating and power supply equipment 
(not shown).