U.S. patent number 3,914,950 [Application Number 05/478,803] was granted by the patent office on 1975-10-28 for helium refrigerator.
This patent grant is currently assigned to The United States of America as represented by the United States Code GP. Invention is credited to James C. Administrator of the National Aeronautics and Space Fletcher, N/A, Ervin R. Wiebe.
United States Patent |
3,914,950 |
Fletcher , et al. |
October 28, 1975 |
Helium refrigerator
Abstract
An improved helium refrigerator of the type which includes a
plurality of counter-flow heat exchangers and has a maximum reserve
cooling capacity when the final or low-temperature heat exchanger
is substantially filled with liquid helium, and a minimum reserve
cooling capacity when the low-temperature heat exchanger is
substantially filled with gas. An electrical bridge circuit
including a pair of temperature-responsive resistors is mounted on
the low-temperature heat exchanger for providing a continuous
indication of the reserve cooling capacity of the refrigerator.
Inventors: |
Fletcher; James C. Administrator of
the National Aeronautics and Space (N/A), N/A (Sunland,
CA), Wiebe; Ervin R. |
Assignee: |
The United States of America as
represented by the United States Code GP (Washington,
DC)
|
Family
ID: |
23901415 |
Appl.
No.: |
05/478,803 |
Filed: |
June 12, 1974 |
Current U.S.
Class: |
62/49.1; 73/295;
62/608; 62/129 |
Current CPC
Class: |
F25B
9/10 (20130101); F17C 13/026 (20130101) |
Current International
Class: |
F25B
9/10 (20060101); F17C 13/00 (20060101); F17C
13/02 (20060101); F17C 013/02 () |
Field of
Search: |
;62/49,125,129,130
;73/295 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Rovinski, A. E.; A LHe Level Indicator; Cryogenics, Dec. 1961, p.
115..
|
Primary Examiner: Dority, Jr.; Carroll B.
Assistant Examiner: Capossela; Ronald C.
Attorney, Agent or Firm: Mott; Monte F. Grifka; Wilfred
Manning; John R.
Government Interests
ORIGIN OF THE INVENTION
The invention described herein was made in the performance of work
under a NASA contract and is subject to the provisions of Section
305 of the National Aeronautics and Space Act of 1958, Public Law
85-568 (72 Stat. 435; 42 U.S.C. 2457).
Claims
What is claimed is:
1. In combination with a helium refrigerator of the type including
a low-temperature counter-flow heat exchanger, adapted to be
substantially filled with helium in a gaseous state as well as in a
liquid state, encased in a relatively thin external jacket formed
of thermally conductive material and having a maximum reserve
cooling capacity when the heat exchanger is substantially filled
with liquid helium and a minimum reserve cooling capacity when the
heat exchanger is substantially filled with helium in a gaseous
state, means for providing a continuous indication of the reserve
cooling capacity of said refrigerator, including:
A. a pair of mutually spaced electrical resistors mounted in spaced
relation on the external surface of said jacket, near the uppermost
end thereof, each being characterized by a logarithmic coefficient
of resistance inversely proportional to temperature; and
B. circuit means for continuously providing intelligence indicative
of temperature induced changes in the differential in the
electrical conductivity of said pair of resistors including, means
connecting said pair of resistors in electrical parallelism within
a common electrical bridge circuit, means for simultaneously
applying a common voltage across said resistors, and means
including a voltmeter connected between said resistors for
comparing temperature induced changes in voltage drops across the
resistors of said pair.
2. The improvement of claim 1 wherein each of said electrical
resistance means comprises a carbon resistor characterized by a
maximum resistance at 4.4.degree. K.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention generally relates to cryogenic systems and more
particularly to an improved helium refrigerator of the type
including a series of counter-flow heat exchangers and having a
temperature-sensing bridge circuit thermally coupled with the
low-temperature counter-flow heat exchanger for detecting changes
in the quantity of liquid helium present therewithin for
continuously providing intelligence indicative of the reserve
cooling capacity of the refrigerator.
2. Description of the Prior Art
Maser systems employed in tracking stations utilized in the
tracking of bodies beyond the earth's atmosphere frequently include
cryogenic devices capable of cooling maser systems to approximately
4.4.degree. K. Often, a helium refrigerator is employed for this
purpose. Helium refrigerators employed in the cooling of masers
found in tracking stations normally are of a type characterized by
multi-stage, counter-flow heat exchangers and are known to have
reserve cooling capacities directly proportional to the level of
the liquid helium found in the low-temperature heat exchanger of
the final stage.
The reserve cooling capacity of helium refrigerators heretofore has
been determined through techniques which require that known
quantities of electrical energy be applied to the refrigerators,
through heating elements mounted on the low-temperature heat
exchangers thereof for periods sufficient to boil away the liquid
helium reserves. Of course, such operations can be performed only
during those periods when the tracking stations are quiescent. As a
consequence, a great deal of difficulty is encountered in
acquiring, in real-time, intelligence indicative of the reserve
cooling capacity for an on-line or operational refrigerator.
It is therefore the general purpose of the instant invention to
provide means for continuously providing intelligence indicative of
the reserve cooling capacity of an on-line helium refrigerator.
OBJECTS AND SUMMARY OF THE INVENTION
Is is an object of the instant invention to provide an improved
helium refrigerator.
It is another object to provide in a helium refrigerator means for
continuously providing a signal indicative of the cooling capacity
of the refrigerator.
It is another object to provide in a helium refrigerator an
electrical circuit connected with the low-temperature heat
exchanger for providing intelligence indicative of changes of
temperature in the heat exchanger, whereby the reserve cooling
capacity of the refrigerator readily can be determined during
periods when the refrigerator is in an operational mode.
These and other objects and advantages are achieved through the use
of a temperature sensing bridge circuit thermally coupled with the
low-temperature heat exchanger of selected helium refrigerators for
continuously providing an electrical signal indicative of the
change in temperatures within the heat exchanger, as will become
more readily apparent by reference to the following description and
claims in light of the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view illustrating a helium refrigerator
including temperature sensing means for providing a continuous
indication of the reserve cooling capacity of the refrigerator.
FIG. 2 is an elevational view of a low-temperature heat exchanger
employed in the refrigerator shown in FIG. 1, illustrating a pair
of electrical resistors thermally coupled with the heat
exchanger.
FIG. 3 is a schematic view illustrating a bridge circuit within
which the resistors shown in FIG. 2 are connected for providing
intelligence indicative of temperature changes within the heat
exchanger.
FIG. 4 is an elevational view illustrating alternate positions of
levels of liquid helium with a heat exchanger during periods of
operation.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings wherein like reference characters
designate like or corresponding parts throughout the several views
there is shown in FIG. 1 a helium refrigerator, generally
designated 10, which embodies the principles of the instant
invention.
The helium refrigerator 10, as shown, is of a type frequently
employed in cooling a two-stage coaxial expansion engine, generally
designated 12, provided within a maser system not shown. Since the
maser system forms no part of the instant invention a detailed
description thereof is omitted in the interest of brevity. However,
it is to be understood that the helium refrigerator has utility
separate and apart from the cooling of masers.
As shown, the helium refrigerator 10 includes a Joule-Thomson
circuit, generally designated 14. The circuit 14 includes a first
stage counter-flow heat exchanger 16, a second stage counter-flow
heat exchanger 18, and a final, low-temperature counter-flow heat
exchanger 20. These heat exchangers are of a known design and form
no specific part of the instant invention.
The heat exchanger 16 is connected with a compressor circuit 22
through suitable tubing 24. The heat exchanger 16 reduces the
temperature of the refrigerant helium delivered thereto and
conducted therethrough from ambient temperatures to 60.degree. K.
(Kelvin). The thus cooled gas is then passed through a coil,
designated 26, circumscribing a first stage of the engine 12 where
a cooling function is performed. The gas is then delivered to the
second stage heat exchanger 18 wherein the temperature of the gas
is reduced from 60.degree. K. to 15.degree. K. and thereafter
delivered through a coil 28 circumscribing the second stage of the
engine 12 where a further cooling function is performed. From the
coil 28, the gas is delivered to the low-temperature heat exchanger
20 at which the temperature of the gas is reduced from 15.degree.
K. to 4.4.degree. K.
While the structure of heat exchangers is well known and a detailed
description of the heat exchangers 16, 18 and 20 is omitted in the
interest of brevity, it is to be understood that these heat
exchangers include a pair of adjacent tubes which serve to conduct
refrigerant in opposite directions therethrough. The heat
exchangers are encased in a thermally conductive jacket 32.
As is well understood by those familiar with multistage helium
refrigerators, the maximum reserve cooling capacity for a given
refrigerator exists when the low-temperature heat exchanger is
substantially filled with helium in its liquid stage. Conversely,
the reserve cooling capacity for the refrigerator is minimized once
the liquid has been boiled out of the low-temperature heat
exchanger.
It has been discovered that it is possible to monitor, both
continuously and accurately, the reserve cooling capacity of a
refrigerator, even during those periods in which the refrigerator
is on-line. This is achieved simply by mounting a pair of
temperature sensitive resistors, designated 34 and 36, on the
jacket 32 of the low-temperature heat exchanger 20 and connecting
the resistors 34 and 36 in a first pair of arms of a temperature
sensing bridge circuit, generally designated 40, FIG. 3. Within the
other pair of arms of the bridge circuit 40 there is connected a
resistor 38, having a variable resistance, to be employed in
balancing the circuit 40 for purposes which will hereinafter be
more fully explained, and a resistor 39, having a fixed resistance
for purpose well understood by those familiar with the design and
operation of bridge circuits.
The resistors 34 and 36 have a coefficient of thermal resistance
such that as the resistance thereof varies in a manner which is
inversely proportional to changes in temperature occurring within
the heat exchanger 20. As a practical matter, the resistors 34 and
36 are 1/8 watt, 50 ohm. carbon resistors which are commercially
available and have a logarithmic temperature coefficient of
resistance, such that small changes in temperature can readily be
detected.
As illustrated in the drawings, the resistors 34 and 36 are mounted
near the high temperature end of the heat exchanger 20, in close
spatial relationship. Thus, the greatest temperature differential
which can be detected by the bridge circuit 40 exists when the
liquid helium within the heat exchanger rises to a level,
designated Level B, FIG. 4, directly opposite the lowermost
resistor 36, so that this resistor is caused to be cooled to its
lowest possible temperature. Similarly, the minimum temperature
differential which can be detected exists when the level of the
liquid helium drops to its lowest possible level, designated Level
A, within the heat exchanger 20 so that the temperatures of the
resistors 34 and 36 are substantially the same. Hence, it is
possible to continuously monitor the reserve cooling capacity of
the refrigerator simply by detecting differences in the voltage
drops occurring across the resistors 34 and 36 of the temperature
sensing circuit 40. In practice, the resistance of resistor 38 is
varied to achieve a balancing of the bridge circuit 40, prior to a
placing of the refrigerator 10 on-line.
As a practical matter, a suitable D.C. power source, generally
designated 42, is connected across the bridge circuit and serves to
establish flow of electrical current therethrough in a manner well
understood by those familiar with such circuits. Additionally, a
volt meter 44, preferably a five-place digital volt meter, is
connected across the bridge and employed for detecting differences
in the voltage drops as they occur across the resistors 34 and 36,
also in a manner well understood by those familiar with bridge
circuits. The detected differences in the voltage drops are
converted to electrical signals, by any suitable means, not shown,
and transmitted through a suitable transmission circuit, also not
shown, to a monitoring station remotely related to the refrigerator
10.
OPERATION
It is believed that in view of the foregoing description, the
operation of the device will readily be understood and it will be
briefly reviewed at this point.
Calibration of the bridge circuit is performed preparatory to
placing the refrigerator 10 on-line. To achieve this result, the
liquid helium within the low-temperature heat exchanger 20 is
substantially boiled off so that the level of the liquid is at a
level corresponding to the level indicated Level A in FIG. 4. When
the level of the liquid is at Level A, the temperature of the
resistors 34 and 36 is substantially the same, due to their remote
relationship with the level of the liquid helium. The variable
resistor 38 is now adjusted so that the voltage dropped across the
resistors 34 and 36 is equalized so that the reading taken at the
volt meter 44 indicates a balanced condition for the bridge and a
corresponding zero reserve cooling capacity for the refrigerator
10.
The refrigerator 10 is then placed on-line and caused to cool
sufficiently for causing an accumulation of liquid helium to occur
in the low-temperature heat exchanger 20. As an accumulation of
liquid helium occurs, the level of the liquid helium rises toward
the lowermost resistor 36 causing the electrical resistance of that
resistor to increase. As the liquid approaches a level opposite the
resistor 36, designated Level B, FIG. 4, the temperature of the
resistor is dropped to approximately 4.4.degree. K., while the
temperature of the resistor 34 remains at substantially 15.degree.
K. due to the warming effect of the gases being introduced into the
heat exchanger 20 as it passes from the coil 28. At this instant,
the detected differences in the voltage drop occurring across the
resistances of the resistors 34 and 36 is maximized indicating that
the liquid helium reserve is maximized. Consequently, the maximized
differences in voltage drops detected by the volt meter 44 serves
to indicate that a maximum reserve cooling capacity for the
refrigerator now exists. The reading taken at the volt meter 44
preferably is converted to electrical intelligence and transmitted
through suitable electrical circuits to remote monitoring stations,
whereby a real-time monitoring of the reserve cooling capacity of
the refrigerator 10 is facilitated.
In view of the foregoing, it should readily be apparent that the
refrigerator 10 which embodies the principles of the instant
invention provides a practical solution to the perplexing problem
of facilitating a continuous monitoring of the reserve cooling
capacity of a given cryogenic refrigerator, while the refrigerator
remains operational, all without introducing significant thermal
energy into the system.
Although the invention has been shown and described in what is
conceived to be the most practical and preferred embodiment, it is
recognized that departures may be made therefrom within the scope
of the invention, which is not to be limited to the illustrative
details disclosed.
* * * * *