Helium refrigerator with compressor drive control

The recompression line for the gas in the cold container (7) comprises at least one compressor (C) with which at least a first means (N.sub.1) for controlling the speed of rotation of the compressor as a function of parameters (inter alia, flow rate (D), pressure (P)) of the fluid in the line (8) is associated. In order to ensure an adequate fluid flow rate at the inlet of the compressor, the installation comprises a line (9) comprising a pilot-operated valve (V.sub.1) and by-passing the compressor, and a line (10) comprising a pilot-operated valve (V.sub.2) connecting the inlet line (4) to the compression line (8). Used, inter alia, in installations for refrigerating superconductive elements.

This invention relates to a refrigerating installation comprising a 
container containing a biphasic fluid at low pressure and low temperature, 
especially helium, supplied by a supply line, and a compression line for 
the gas connected to the container and comprising at least one compressor. 
An installation of this kind is described in the document FR-A-2 679 635 in 
the name of the Applicant. 
A refrigerating installation of this kind is used, inter alia, for 
refrigerating superconductive elements in particle accelerators, in which 
the pressure of the fluid must be reduced to a very low value of less than 
20 hPa in order to obtain a temperature of less than 4.2K in the 
container. In order to reintroduce the gaseous fluid at this very low 
pressure into the installation, one, typically several compressors 
connected in series must be used in the compression line, the operation 
thereof being difficult to control as a result of instability which may 
appear in the compression line, particularly in the starting and stopping 
phases of the installation. 
The aim of this invention is to propose an installation with a simple and 
efficient design for optimising the operation of the compression stages 
and adjusting the flow rates in the compression line in the different 
phases of operation of the installation. 
To this end, according to one feature of the invention, the installation 
comprises, associated with the compressor, at least a first means for 
controlling the speed of rotation of the compressor as a function of 
parameters of the gaseous fluid upstream of the compressor, typically as a 
function of at least the flow rate of the fluid upstream of the 
compressor. 
According to other features of the invention: 
the installation comprises a first means for controlling the flow rate of 
the fluid in the compression line, i.e. intended for the refrigerator 
downstream of this line, typically a second means for controlling this 
fluid flow rate; 
the installation comprises, associated with the compressor, a second means 
for controlling the speed of rotation of the compressor as a function of 
the pressure upstream of the compressor, the second control means 
typically being associated with the downstream compressor of the 
compression line when it comprises at least two compressors connected in 
series, with each of which one of said first speed control means is 
associated.

FIG. 1 shows a helium refrigerating installation of the type described in 
the abovementioned document FR-A-2 679 635 and essentially comprising a 
refrigerator 1 delivering after expansion at 2 liquid helium at a first 
low pressure into an intermediate container 3, from where the liquid is 
advanced via a line 4 traversing an exchanger 5 and a final expansion 
element 6 to a second supercold container 7 containing liquid and gaseous 
helium at a second lower pressure, e.g. of approximately 20 hPa and at a 
temperature of approximately 2K. The gaseous atmosphere in the container 7 
is recompressed in a compression line 8 traversing the exchanger 5 in 
order to be recycled towards the refrigerator 1. 
In the embodiment of FIG. 1, the compression line 8 comprises, downstream 
of the exchanger 5, a compressor C which can be re-cycled by a re-cycle 
line 9 provided with a pilot-operated valve V.sub.1. The line 8 comprises, 
between the container 7 and the exchanger 5, a shut-off valve V.sub.3 
downstream of which a line 10 extending from the line 4 upstream of the 
expansion element 6 opens. The line 10 comprises a pilot-operated valve 
V.sub.2. The line 8 comprises, between the container 7 and the opening of 
the line 10, a shut-off valve V.sub.3. A first control loop N.sub.1 is 
associated with the compressor C, providing at the output a control signal 
V for the speed of rotation of the compressor C and receiving moreover a 
signal V.sub.R representing the speed of rotation of the compressor, a set 
point N.sub.1C produced by a calculating means MC as a function of a 
calculation using the characteristic of the compressor and which works out 
a theoretical speed of rotation of the compressor as a function of the 
temperature T, the pressure P and the flow rate D" at the inlet of the 
compressor, measured by respective sensors 11, 12 and 13 in the line 8. A 
second control loop N.sub.2 is also associated with the compressor C, 
providing at the output a control signal for the speed of rotation of the 
compressor C as a function of a set point P.sub.C, which is the nominal 
suction pressure of the compressor, and of a signal representing the 
pressure P measured at the inlet of the compressor. 
The valve V.sub.1 for limiting the flow rate of the gas taken from the 
container 7 is controlled by a control loop D.sub.1 in response to a set 
point D.sub.C representing the desired fluid flow rate in the compression 
line for recycling to the refrigerator 1, a flow rate signal D 
representing, inter alia, the flow rate measured in the line 8 at the 
outlet of the exchanger 5 and a converted signal of the set point 
N.sub.1C. A sensor for the flow rate signal D is provided along line 8 at 
14. The valve V.sub.2 which allows the expansion element 6 to be by-passed 
is controlled by a control loop D.sub.2 as a function of a set point 
D'.sub.c representing the desired flow rate in the line 8 upstream of the 
exchanger 5 and a signal D' representing the fluid flow rate measured in 
the line 8 upstream of the exchanger 5. 
The installation operates as follows: 
1. Starting procedure: 
1.1. Starting with the container 7, without limitation of flow rate: 
The loop N.sub.1 keeps the compressor C within the permitted operating 
zone. If the gas flow rate is insufficient, the speed of rotation 
increases, as does the compression rate, and the suction pressure in the 
container 7 is reduced, thereby freeing the desired additional helium 
flow. Under these conditions, the flow rate required for the correct 
operation of the compressor is provided dynamically by the container 7. If 
the speed of the compressor does not increase rapidly enough, the flow 
rate is too low. On the other hand, if the speed of the compressor 
increases too rapidly, the flow rate is too high. In both cases, if the 
flow rate is not adapted to the speed, the compressor can fall out of 
step. The control loop N.sub.1 allows the speed of the pressure reduction 
in the container 7 to be adapted automatically as a function of the size 
of the latter, the quantity of liquid helium it contains and the flow 
emitted at constant pressure by the container as a result of heat losses. 
1.2. Starting with the container, with a controlled flow rate: 
The flow rate that can be tolerated by the refrigerating installation is 
limited. It is therefore necessary to ensure correct operation of the 
compressor C by providing it with a complementary flow by recycling. This 
is the role of the duct 9 and the loop D.sub.1. If the flow rate required 
by the compressor exceeds the set point value D.sub.C of the loop D.sub.1, 
the valve V.sub.1 ensures the complementary flow by recycling. It will be 
noted that the set point value D'.sub.c of the control loop D.sub.2 for 
the valve V.sub.2 corresponds to the flow rate that can be tolerated by 
the refrigerating installation. 
1.3. Starting without the container: 
The container 7 is shut off from the compression line 8 by the valve 
V.sub.3, e.g. following recent stoppage of the compression line. Before 
the line 8 can be connected to the container 7, a pressure equal to that 
prevailing in the container 7 must be reached in the line 8. Under these 
conditions, starting is ensured as follows. The set point value D'.sub.C 
corresponds to the permitted flow rate in the container 7 and the speed of 
the compressor evolves according to a law fixed in relation to time. When 
the suction pressure of the compressor is equal to that prevailing in the 
container 7, the loop N.sub.1 is brought into operation, the valve V.sub.3 
is opened and the valve V.sub.2 is gradually closed. 
2. Normal conditions: 
When the nominal pressure at the inlet of the compressor C has been reached 
at the end of the starting phase, the valve V.sub.3 remaining open, the 
loop N.sub.1 is deactivated and the loop N.sub.2 is brought into 
operation. When this nominal suction pressure is reached, the operation of 
the system is no longer dynamic as the container 7 can only provide part 
of the flow at constant pressure corresponding to the static gates. The 
complementary flow is thus provided by the valve V.sub.2, the value of the 
set point D'.sub.C of the loop D.sub.2 corresponding to the minimum 
permitted flow rate upstream of the line 8 corresponding to the suction 
pressure. 
3. Stoppage: 
The stopping phase is preceded by cancellation of the dynamic power 
resulting from the exchange with the articles cooled by the bath in the 
container 7, and therefore a reduction in the flow emitted at constant 
pressure by the container 7. The loop D.sub.2 therefore opens the valve 
V.sub.2 and the stopping procedure is as follows. The loop D.sub.1 is 
activated, the valve V.sub.3 is gradually closed and, once the latter is 
closed, the speed of rotation of the compressor decreases according to a 
law defined in relation to time, until the final stoppage thereof. 
FIG. 2 shows an embodiment comprising, as is often necessary, several 
compressors C.sub.i connected in series. As can be seen in FIG. 2, each 
compressor C.sub.i is provided with its own control loop N.sub.1, the 
re-cycle line 9 re-cycling all of the compressors and the control loop 
N.sub.2 only affecting the downstream compressor (C.sub.4), the input 
signal being the pressure P.sub.1 upstream of the first compressor 
(C.sub.1). The procedures are the same as those described hereinabove, 
although the evolution of the speeds of rotation as a function of time in 
the starting phases without the container or stopping phases relates only 
to the last compressor (C.sub.4) provided with the control loop N.sub.2. 
Although the invention has been described with respect to particular 
embodiments, it is not limited thereto, but, on the contrary, is subject 
to modifications and variants obvious to the person skilled in the art.