Device for liquefying a gas

In order to prevent icing of the cryogen, a device for liquefying a gas includes a heater provided in a cooling chamber at the output of a Sterling cycle engine. The heater is controlled by a gas pressure sensor in the cryostat in communication with the chamber and matches the cooling load to the cooling capacity of the Sterling cycle engine.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device for liquefying a gas. 
2. Prior art of the present invention 
A conventional device for liquefying a gas, as shown in FIG. 1, includes a 
vessel 17 in which an amount of liquid cryogen 18 such as Neon, Argon or 
Nitrogen is contained. In the liquid cryogen 18, there is immersed an 
object (not shown) to be cooled. Due to heat generation from the object, 
liquid cryogen 18 is evaporated and the resulting gas cryogen enters into 
a chamber 13 via a conduit 15 as shown in dotted-line in FIG. 1. Since an 
interior portion in the chamber 13 is cooled at or below a temparature by 
a refrigerator 10 which is driven by a motor 27, as cryogen is condensed 
back to a liquid and resulting liquid cryogen returns into the vessel 17 
via the conduit 15 as shown in solid-line in FIG. 1. 
However, the conventional device for liquefying a gas has the following 
drawbacks. That is to say, since gas cryogen and liquid cryogen flow in 
opposite directions in common conduit 15, both gas and liquid cryogen are 
mixed, to some extent, with each other, resulting in that smooth movements 
thereof are prevented. Furthermore, in the case that capacity for 
liquefying a gas in the chamber 13 is greater than heat quantity radiated 
from the object, cryogen to be returned to the vessel 17 as a liquid is 
frozen into ice. 
SUMMARY OF THE INVENTION 
It is, therefore, a principal object of the present invention to provide a 
device for liquefying a gas without the aforementioned drawbacks. 
It is another object of the present invention to provide a device for 
liquefying a gas comprising, (a) a vessel containing therein an amount of 
liquid cryogen and having a space above the surface of said liquid 
cryogen; (b) a chamber; (c) a first conduit for connecting an upper side 
of an interior portion in said chamber and said space in said vessel; (d) 
a second conduit for connecting a lower side of said interior portion in 
said chamber and an inside portion of said liquid cryogen; (e) a sensor 
for detecting the pressure in said vessel; (f) cooling means provided in 
said chamber for liquefying a gas; (g) heater means provided around said 
cooling means in said chamber; and (h) a control device for controlling 
the quantity of the electric charge to said heater means according to 
signal from said sensor. 
According to the present invention, gas cryogen, as a result of evaporation 
of the liquid cryogen due to heat-radiation from the object immersed 
therein, enters into the chamber via the first conduit and is condensed 
back to liquid cryogen by cooling in the chamber. Resulting liquid cryogen 
is, then, returned into the vessel via the second conduit. Since no 
counter-flows exist between the chamber and the vessel, liquid cryogen may 
be returned smoothly into the vessel. Furthermore, since the quantity of 
cryogen to be liquefied may be equalized to the quantity of evaporated 
cryogen by the actuation of the control device, liquefied cryogen may not 
be frozen into ice.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a Stirling cycle refrigerator 10 includes a pair 
of expansion cylinders 11 and 12. As well-known, when the Stirling cycle 
refrigerator 10 is brought into operation by a motor 27, top end portions 
of the cylinders 11 and 12 are cooled into a frozen condition. The 
cylinders 11 and 12 are extended into a chamber 13. The chamber 13 is 
covered with a vacuum case 14 so as to be heat-insulated from the 
atmosphere. 
The chamber 13 is connected to an interior portion of a vessel or cryostat 
17 by a first heat-insulated conduit 15 and a second heat-insulated 
conduit 16. In the interior portion of vessel or cryostat 17, there is 
contained an amount of liquid cryogen 18. In the liquid cryogen 18, an 
object 19 such as a semi-conductor element or a vivo is immersed to be 
cooled. Above the surface of the liquid cryogen 18, there is defined a 
space 20. A lower opening of the first conduit 15 is exposed into the 
space 20 and is in opposition to the surface of the liquid cryogen 18 at a 
distance. An upper opening of the first conduit 15 is exposed into an 
upper side of the interior portion of the chamber 13. Thus, gas cryogen as 
a result of evaporation of the liquid cryogen 18 due to heat-generation 
from the object enters into the interior portion in the chamber 13. 
In the chamber 13, a pair of heat exchangers 21 and 22 are mounted to the 
cylinders 11 and 12, respectively. Gas cryogen is cooled by the cylinders 
11 and 12 via the heat exchangers 21 and 22 so that gas cryogen is 
condensed back to liquid cryogen. Resulting liquid cryogen is, then, 
retuned into the cryostat 17 via the second conduit 16. An upper opening 
and lower opening of the second conduit 16 are, respectively, positioned 
at a lower side of the interior portion in the chamber 13 and in the 
liquid cryogen 18 in the cryostat 17. 
In the cryostat 17, there is installed a pressure sensor 26 from which a 
signal corresponding to the pressure in the cryostat 17 is continually 
transmitted to a control device 23 in the form of micro-processor. In the 
control device 23, a signal from the sensor 26 is compared with a 
reference or a set value. In accordance with difference as a result of the 
afore-mentioned comparison, the control device 23 adjusts the quantity of 
electric power to a pair of heaters 24 and 25 mounted on the heat 
exchangers 21 and 22, respectively. Due to the variation in electric power 
to the heaters 24 and 25, heat-exchange ratio therein is varied with the 
result that capacity for liquefying a gas may be controlled. 
Assuming that the object 19 with high heat content is replaced with one 
with low heat content, the amount of evaporation of the liquid cryogen 18 
is decreased thereby increasing pressure in the cryostat 17. According to 
the decrease in pressure in the cryostat 17, the control device 23 
increases the electric power to the heaters 24 and 25. Thus, capacity for 
liquefying a gas in the chamber 13 is dropped and resulting capacity is 
reduced only to liquefy the gas entered into the chamber 13. There is no 
fear that surplus capacity for liquefying a gas freezes liquid cryogen in 
the returning movement to the cryostat 17.