U.S. patent number 3,885,939 [Application Number 05/464,078] was granted by the patent office on 1975-05-27 for cryostat control.
This patent grant is currently assigned to General Dynamics Corporation. Invention is credited to Arvel Dean Markum.
United States Patent |
3,885,939 |
Markum |
May 27, 1975 |
Cryostat control
Abstract
Flow control for a cryostat in which the refrigerant flow rate
is controlled by adding a contaminant to the refrigerant.
Inventors: |
Markum; Arvel Dean (San Juan
Capistrano, CA) |
Assignee: |
General Dynamics Corporation
(Pomona, CA)
|
Family
ID: |
23842466 |
Appl.
No.: |
05/464,078 |
Filed: |
April 25, 1974 |
Current U.S.
Class: |
62/474; 62/51.2;
62/502; 137/13 |
Current CPC
Class: |
F25J
1/0276 (20130101); F25B 9/02 (20130101); F25B
2400/12 (20130101); F25J 2280/40 (20130101); F25J
2205/20 (20130101); Y10T 137/0391 (20150401) |
Current International
Class: |
F25B
9/02 (20060101); F25J 1/00 (20060101); F25b
043/00 () |
Field of
Search: |
;137/13 ;165/40
;62/85,114,195,474,475,502,511,514 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: O'Dea; William F.
Assistant Examiner: Capossela; Ronald C.
Attorney, Agent or Firm: Miller; Albert J. Johnson; Edward
B.
Claims
What I claim is:
1. In combination:
a cryostat including a coiled tube heat exchanger;
a high pressure refrigerant gas supply to provide a refrigerant to
said cryostat, said refrigerant comprising a mixture of 16%
Freon-14 and 84% Freon-23; and
means to introduce a contaminant into the refrigerant for said
cryostat, said contaminant comprising 10 parts per million by
weight water vapor, said contaminant having a solidification point
above that of the refrigerant to alternately freeze and melt in the
coiled tube heat exchanger of said cryostat to reduce the flow of
refrigerant through said cryostat.
Description
BACKGROUND OF THE INVENTION
Joule-Thomson effect cooling devices, commonly referred to as
cryostats, as well known in the art to produce cryogenic
temperature levels. The cryostats may be employed to maintain
radiation sensing devices at the extremely low temperatures
required. Examples of conventional Joule-Thomson effect cryostats
may be found in U.S. Pat. Nos. 2,991,633, 3,095,711, 3,353,371,
3,415,078 and 3,431,750.
In order to achieve a rapid initial cool-down, large coolant or
refrigerant flows are required in conventional cryostats. Only a
fraction of this cool-down flow is, however, needed for steady
state operation of the cryostat. Thus, a cryostat designed to meet
the initial cool-down flow requirements would be inherently
inefficient during steady state operation, while a more efficient
steady state flow design would have an excessively long cool-down
period.
Since in many cryostat applications the coolant or refrigerant flow
is limited by the available supply, techniques have been developed
to provide sufficient cool-down flow without providing excessive
steady state flow. While certain self-regulating flow control
mechanisms have been developed for cryostats, these mechanisms,
which have been either thermal-mechanical, electro-mechanical, or
chemical in nature, have been rather complicated, overly complex
and often prone to operational difficulties. All rely upon external
forces, thus consuming energy such as electrical power and all
include at least some moving parts. In some cases the basic cooling
characteristics of the cryostat have been altered by the flow
regulating mechanism.
SUMMARY OF THE INVENTION
The invention is directed to a cryostat flow control in which the
refrigerant flow rate is controlled by the addition of a
contaminant or foreign fluid to the refrigerant. After initial
cool-down, the contaminant, having a higher solidification point
than the refrigerant, will solidify in the cryostat and cause a
partial or complete refrigerant flow stoppage. When the refrigerant
flow is thus reduced or stopped, refrigeration slows or ceases with
a resultant rise in cryostat temperature which in turn then melts
the solidified contaminant. The refrigerant flow will then resume
until the temperature is again reduced to freeze up or solidify the
refrigerant contaminant.
The alternate freeze-up and melting cycle achieves a greatly
reduced average steady state refrigerant flow rate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a cryostat utilizing the
control of the present invention.
FIG. 2 is an enlarged section view of a portion of the heat
exchanger tube of the cryostat of FIG. 1.
FIG. 3 is a graphical representation of the operational cycle of a
cryostat having the flow control of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The cryostat control of the present invention is applicable to any
type of cryostat (counterflow, regenerative, Joule-Thomson
expansion, etc.). For purposes of illustration, a Joule-Thomson
expansion cryostat 10 having a coiled tubing heat exchanger 12 and
liquid refrigerant reservoir 14 is illustrated in FIG. 1. A high
pressure refrigerant gas supply 16 provides refrigerant to the heat
exchanger 12 through a control valve 18. The refrigerant cooled in
the inlet side of the heat exchanger 12 is expanded through an
expansion valve 20, or alternately through a nozzle or orifice, and
collected in the liquid refrigerant reservoir 14. The liquid
refrigerant is then discharged from the cryostat 10 through a
refrigerant exhaust 22 after passing through the other side (outlet
side) or heat exchanger 12.
Initially, the refrigerant gas is at the same temperature as its
surroundings. When admitted to the cryostat 10 it passes through
the inlet side of the heat exchanger 12 and out from the heat
exchanger 12 through the expansion valve or nozzle 20. As the
refrigerant expands through the expansion valve 20, it drops in
temperature because of the Joule-Thomson effect. This lower
temperature refrigerant is then forced through the outlet side of
the heat exchanger 12 and thereby decreases the temperature of the
incoming refrigerant. This incoming refrigerant then expands
through the expansion valve 20 and drops to an even lower
temperature than the preceding increment of refrigerant. This
process continues until such time that the refrigerant becomes
liquefied at the expansion nozzle 20. The system then remains
stabilized at the boiling temperature of the refrigerant.
In order to effect control of the cryostat 10 in accordance with
the present invention, a gaseous contaminant or foreign fluid is
introduced into the refrigerant from a contaminant supply 24. A
mixing chamber 26 may be provided to uniformly distribute or
disperse the contaminant vapor throughout the refrigerant supplied
to the cryostat 10. Alternately other methods of agitation,
stirring, or heating may be utilized for this purpose.
As illustrated most clearly in FIG. 2, once cool-down has been
achieved, the contaminant 30, having a solidification temperature
higher than that of the refrigerant, will precipitate out of
solution from the refrigerant and freeze-up. This will reduce and
eventually block the flow of refrigerant through the heat exchanger
tube 28. As the refrigerant flow is reduced, refrigeration slows or
ceases until the cryostat temperature rises and melts the
solidified contaminant. Refrigerant flow then resumes and decreases
the cryostat temperature until the contaminant blockage occurs
again. The cycle of alternate freeze-up and melting occurs
indefinitely until the refrigerant supply is stopped. The operation
of the cryostat is graphically illustrated in FIG. 3.
The type of contaminant, ratio of contaminant weight to refrigerant
weight and the type of refrigerant can be varied to accommodate any
desired cooling cycle and cryostat configuration. The maximum
temperature reached during cycling, and the frequency of the
cycling is dependent upon the percentage by weight of contaminant
in the refrigerant gas supply.
In a 0.118 inch diameter, 11/2 inch long, finned tube cryostat,
having a gas flow rate of 1.1 standard liters per minute of 16%
Freon-14 and 84% Freon-23 at a supply pressure of 500 pounds per
square inch, 10 parts per million by weight of water vapor as a
contaminant in the refrigerant will cycle the refrigerated tip of
the cryostat from 250.degree. Kelvin to 170.degree. Kelvin at about
10 second intervals. While the exact location of the refrigerant
flow blockage was not determined, it is believed to occur near or
at the expansion nozzle.
Any desired coolant cycle can be tailored by proper selection of
the refrigerant and contaminant in the proper proportions. A list
of possible cooling cycles is provided below.
______________________________________ Temperature Range
Refrigerant Contaminant ______________________________________
195.degree. - 275.degree.K Freon - 23 Water Vapor 145.degree. -
275.degree.K Freon - 14 Water Vapor 145.degree. - 165.degree.K
Freon - 14 Xenon 112.degree. - 165.degree.K Methane Xenon
88.degree. - 120.degree.K Argon Krypton 78.degree. - 120.degree.K
Nitrogen Krypton 78.degree. - Nitrogenee.K Methane
______________________________________
This flow regulation control utilizes the cooling capacity of the
refrigerant to solidify the introduced contaminant in the
refrigerant within the cryostat flow passages. There are no moving
parts or external forces required for flow control and the basic
cooling characteristics of the refrigerant are not altered.
In this manner the full refrigerant flow is available for the
initial cryostat cool-down which occurs well above the
solidification point of the contaminant. Once, however, the
cryostat operating temperature is achieved, the cyclical freeze-up
will significantly reduce the flow of refrigerant flow through the
cryostat.
While specific embodiments of the invention have been illustrated
and described, it is to be understood that these embodiments are
provided by way of example only and that the invention is not to be
construed as being limited thereto, but only by the proper scope of
the following claims.
* * * * *