Laboratory freezer appliance

A laboratory freezer appliance providing a usable storage space on the order of 5 to 20 cubic feet capable of storage temperatures of -160.degree. C. and lower including an insulated freezer chamber, heat transfer tubes in proximity to the freezer chamber carrying liquid argon at ultra-low temperatures which absorbs heat from the freezer chamber thereby vaporizing the argon in the heat transfer tubes; a closed cycle, hermetically-sealed free piston Stirling cycle heat pump providing a cold end above a vertical displacer driven by a linearly reciprocating piston at a delta T to the freezer chamber of about -13.degree. C.; a condensing chamber surrounding the cold end of the heat pump for condensing argon vapor to the argon liquid; and a distributor for distributing liquid argon from the condensing chamber to the heat transfer tubes and returning argon vapor to the condensing chamber, all without mechanical pumping of the argon, in a continuous, closed cycle refrigeration system.

BACKGROUND OF THE INVENTION 
This invention relates to a cryogenic temperature storage chamber and, more 
particularly, to a laboratory freezer appliance that provides a freezer 
storage space, e.g., on the order of 5 to 20 cubic feet, capable of 
storage temperatures of -160.degree. C. and lower. 
In both research and diagnostic laboratory applications, low temperature 
refrigeration of living biological systems and biomaterials is required to 
produce satisfactory preservations. That is, the biochemical and physical 
processes by which biomaterials sustain life are affected to varying 
degrees by temperature. Thus, in applications where ultralow temperatures 
are successful in arresting these processes, lower storage temperatures 
are desired to achieve more satisfactory results, particularly for long 
term storage of biological specimens. The need therefore exists for a 
reliable laboratory freezer appliance which provides a usable freezer 
storage space at a consistent and uniform ultralow temperature, e.g., 
-160.degree. C. and lower, for essentially unattended, extended storage 
periods. 
There are two types of equipment which currently attempt to address, in 
part, this need. One is by stored refrigeration in the form of vacuum 
insulated liquid nitrogen dewars designed with a storage space in the 
vapor above the liquid nitrogen. There are a number of limitations to 
liquid nitrogen dewars. First, the only practical insulation is in the 
form of vacuum insulation. Due to strength requirements, the configuration 
of the storage chamber must necessarily be either cylindrical or spherical 
which is not an efficient use of space in traditional rectangular 
buildings and rooms. Second, nitrogen is a liquid at -196.degree. C. at 
atmospheric pressure, which is an acceptable storage temperature. However, 
in liquid nitrogen dewars, the temperature may vary greatly, for example, 
up to 100.degree. C. from top to bottom depending on the design of the 
vessel and the quality of the insulation, with a significant portion of 
the chamber maintaining temperatures much warmer than the desired 
-160.degree. C. temperature. Thus, the physical placement of specimens 
within the vapor dictates their long term storage temperature, and 
uniformity and repeatability of storage conditions, particularly over long 
storage times, is as a practical matter impossible. Third, the source of 
cooling in a liquid nitrogen dewar is the phase change of the nitrogen 
from liquid to vapor. Thus, it is necessary for the dewar to regularly 
receive a fresh supply of liquid nitrogen to replace the boiled-off 
quantity. Although liquid nitrogen is not particularly expensive, 
availability and handling do cause problems, and liquid nitrogen cannot be 
stored indefinitely at ambient temperatures of typically 20.degree. C. 
The other type of equipment attempting to provide ultralow refrigeration 
temperatures is a mechanical system using a mixture of refrigerant 
components which are compressed by one or more refrigeration compressors. 
Such refrigeration systems can include a single standard commercial 
air-conditioning compressor which serves as a pump to move the 
refrigerant, which is a mixture of fluorocarbon refrigerants, through the 
system, an air- or water-cooled condenser which cools the compressor and 
removes heat from the refrigerant by partially changing the mixture from 
vapor to liquid, a liquid/vapor separator which separates liquid 
refrigerant from vapor and returns lubricating oil to the compressor, 
multiple heat exchangers to effect the cooling process, and an evaporator 
coil through which the refrigerant flows at ultralow temperatures to 
absorb heat from the freezer interior and deliver it to the condenser for 
removal. Again, there are a number of problems with this refrigeration 
system. First, these systems currently operate at temperatures of 
-135.degree. C. to -150.degree. C. which fall short of the desired 
temperature of -160.degree. C. Second, the development of the 
refrigeration circuit for this product is highly intuitive because the 
properties of the mixed refrigerants are difficult to predict with any 
accuracy as are the heat transfer and flow characteristics of the 
mixtures. Third, the mixed refrigerants are fluorocarbon refrigerants 
which may have to be replaced in the future for environmental reasons. 
BRIEF DESCRIPTION OF THE INVENTION 
It is among the principal objectives of this invention to provide a 
laboratory freezer appliance capable of providing consistent and 
repeatable storage conditions at temperatures of -160.degree. C. and lower 
for extended and unattended storage periods. The freezer has a usable 
storage space of, e.g., 5 to 20 cubic feet. In accordance with the 
principal objectives of this invention and inherent in the term 
"appliance," the freezer operates on normally available 220 volt AC 50/60 
Hz single phase electricity; the installation and start up consists of 
little more than unpacking and leveling the unit, plugging it into the 
power source, turning the unit on and waiting for cool down from room 
temperature to operating temperature, e.g., -160.degree. C., which takes 
only about one-half day; the unit will operate continuously with only 
occasional unskilled maintenance for the first five years of continuous 
operation; the unit is configured to use space efficiently; and the 
aesthetic appearance of the freezer is pleasing and the noise and 
vibration levels are relatively low. Thus, the unit looks and sounds 
substantially like a typical household freezer. 
In its general aspects, the cryogenic freezer appliance of the present 
invention includes an insulated freezer chamber; heat transfer tubes in 
proximity to the freezer chamber carrying a refrigerant at ultralow 
temperatures which absorbs heat from the freezer chamber thereby cooling 
the storage chamber and vaporizing the refrigerant in the heat transfer 
tubes; a condensing chamber surrounding the cold end of a closed cycle, 
hermetically sealed, free piston, Stirling cycle heat pump, which includes 
a linearly reciprocating piston and a displacer in the cold end of the 
heat pump removed from the piston and driven in reciprocation by the 
alternate expansion and compression of a working gas within a working 
space above the piston, for condensing the vaporized refrigerant to a 
liquid; a distributor for distributing the liquid refrigerant back to the 
heat transfer tubes; and a rejector for removing heat from the heat pump 
in a continuous, closed cycle refrigeration system. 
In a presently preferred form of the invention, a heat transfer fluid such 
as argon enters a condensing chamber surrounding the cold end of a 
Stirling cycle heat pump in the form of argon gas. The argon gas is 
condensed therein to a liquid at an ultralow temperature and flows by 
gravity to an argon distributor having a centrally located liquid argon 
reservoir and a number of tubes extending around the periphery of the 
reservoir which substantially evenly deliver the liquid argon refrigerant 
to heat transfer tubes which extend along either side of the freezer 
storage chamber in heat transfer communication therewith. The heat 
transfer tubes slope downwardly from the argon distributor such that the 
liquid argon flows along the tubes by gravity. The heat transfer tubes are 
externally finned to provide a large heat transfer surface. The liquid 
argon in the heat transfer tubes absorbs heat from the interior of the 
freezer storage chamber causing convective flow of ultralow temperature 
air through the storage chamber in turn lowering of the chamber 
temperature to desired operating temperature which may be on the order of 
-160.degree. C. or below. The liquid argon is vaporized by the absorbed 
heat, and the vapor returns to a head space above the reservoir of the 
argon distributor in the same heat transfer tubes that carry the liquid 
argon from the distributor. The argon gas then flows from the argon 
distributor to the condensing chamber surrounding the cold end of the 
Stirling cycle heat pump where the argon is again condensed to a liquid 
and returned to the argon distributor in a continuously operating closed 
cycle refrigeration process. The operating pressure of the system is on 
the order of 5 bar. No external pumping means is provided to move the 
refrigerant through the system thereby eliminating any moving parts and 
the need for any lubricants which would freeze up at the ultralow 
temperatures involved in the cryogenic freezer. 
The Stirling cycle heat pump includes a compressor having an electrically 
linearly driven reciprocating piston and a displacer removed from the 
compressor driven in reciprocation by the alternate expansion and 
compression of a working gas, preferably helium, in the working volume 
above the piston. The piston of the compressor is driven by a linear 
electric motor at a frequency of 44 Hz to provide a generally sinusoidal 
pressure variation in the helium gas. The compressing movement of the 
piston causes pressure of the helium gas to rise from a minimum pressure 
of about 285 psig to a maximum pressure of about 375 psig. The pressure 
rise of the helium gas causes the displacer in the cold end of the heat 
pump, which is free to move in the cold end, at a point in the cycle to 
move rapidly downward. With the downward movement of the displacer, high 
pressure working gas at about ambient temperature is forced through a 
regenerator and into the cold space above the displacer. The regenerator 
absorbs heat from the flowing pressurized gas and thus reduces the 
temperature of the gas. As the compressor piston reverses direction in the 
sinusoidal drive pattern provided by the linear drive motor and begins to 
expand the volume of gas in the working space above the piston, the high 
pressure helium above the displacer is cooled even further. This cooling 
in the cold space provides refrigeration for maintaining an average 
temperature gradient of over 200.degree. Kelvin over the length of the 
regenerator and an input of about 200 watts of heat from the argon to the 
expansion space helium at the low temperatures of -160.degree. C. or less. 
At a point in the expanding movement of the piston, the pressure in the 
working volume of helium gas drops sufficiently so that the momentum of 
the displacer is overcome by a retarding force on the displacer provided 
by an internal gas spring which is stabilized at a pressure between the 
minimum and maximum pressures of the helium gas. The displacer is then 
driven to its starting position. The displacer thus cycles with the piston 
but out of phase therewith. 
The argon gas refrigerant to be cooled circulates in the condensing chamber 
which is in the form of a pressure vessel external of but surrounding a 
cold end cup of the heat pump. The cold end cup is formed of a highly heat 
conductive material that does not become brittle at temperatures of 
-160.degree. C. or lower, such as a stainless steel, and the outer surface 
of the cold end cup is provided with a heat conductive sleeve having a 
series of fins having an Adamak fin profile to increase heat transfer to 
the argon gas in the condensing chamber. 
A temperature differential of about 13.degree. C. is maintained between the 
desired storage chamber temperature and the helium in the cold end of the 
heat pump and about 10.degree. C. between the storage chamber temperature 
and the temperature of the liquid argon. Thus, for a -160.degree. C. 
storage chamber temperature, the liquid argon is at -170.degree. C. and 
the helium is at -173.degree. C. The 200 watts of heat input into the 
helium in condensing the argon gas in the pressure vessel surrounding the 
cold end cup plus the input work to drive the heat pump compression and 
expansion cycles are rejected to the environment external of the heat pump 
by a heat rejector/condenser assembly. 
As stated, the cryogenic freezer provides a suitable storage space, e.g., 
from 5 to 20 cubic feet, capable of providing a consistent and uniform 
freezer temperature of -160.degree. C. or lower. There are no external 
pumping means to pump the refrigerant through the distribution chamber and 
heat transfer tubes. Further, there is no traditional petroleum-based 
lubrication in the system which otherwise would be subject to freezing by 
virtue of the ultralow temperatures of the system and to contamination of 
the refrigerant and the working gas in the heat pump. Rather, the moving 
parts of the heat pump are spun during operation to provide hydrodynamic 
non-contact gas bearings which, since no contact between rotating and 
stationary parts is allowed, eliminates the need for traditional 
lubricants.

DETAILED DESCRIPTION OF THE INVENTION 
Freezer Cabinet 
Referring now to FIG. 1, in its general aspect, the cryogenic freezer 10 
includes a cabinet 11 which houses a freezer storage chamber 12 interiorly 
of the cabinet 11, a lid 14 to seal closed the freezer storage chamber 12 
and to provide access thereto, and a side car cabinet 16. An access panel 
18 provides access to the Stirling cycle heat pump 20 (FIG. 2), which will 
be described in detail below. A condenser 22 and blower 24 are housed in 
the side car cabinet 16. The blower 24 draws air through a grill 26 (FIG. 
1) in an end wall 27 of the cabinet 16 and over the condenser 22 to 
condense a refrigerant for removing heat from the heat pump 20, as also 
will be described in detail below. 
The freezer 10 is generally rectangular in configuration making it suitable 
for efficient use in a laboratory. The cabinet 11, lid 14, and side car 16 
are formed of 18-gauge cold-rolled steel which is painted for protection 
and aesthetics. Typical physical dimensions of the freezer 10 are an 
overall external dimension of 91" long by 46.5" high by 28.5" front to 
back. Typical interior chamber dimensions are 43.5" long by 32" high by 
161/4" deep, which provides two 42.5" long by 27" high by 61/4" deep 
usable storage volumes, or about 8 cubic feet of usable storage volume. 
Caster wheels 28 are provided to permit convenient movement of the freezer 
10 in the laboratory or other facility. The freezer operates on normally 
available 220 volt (180 to 250 VAC 50/60 Hz single phase) electricity; and 
its installation and start up consists of essentially unpacking and 
leveling the unit, plugging the unit into the power source, turning on a 
switch and waiting for the unit to cool down to its operational 
temperature of -160.degree. C., which takes approximately one-half day. 
The freezer is designed to operate continuously with only occasional 
unskilled maintenance for its first five years. 
Referring now specifically to FIGS. 2 and 3, the freezer storage chamber 12 
is formed of 20-gauge type 304 stainless steel for good thermal 
conductivity and corrosion protection. The storage chamber 12 is 
surrounded by an insulative material 30 such as a foamed-in-place urethane 
having a density on the order of 3 pounds per cubic feet. That is, the 
outer shell 11 and freezer storage chamber 12 are placed in a fixture 
which holds the inner and outer walls in place. Thereafter, the urethane 
is added as a liquid with a foaming agent and foamed in place against the 
facing walls of the shell 11 and storage chamber 12. The insulation is 
generally 6" thick at the side walls and 7" thick at the bottom between 
the bottom of the storage chamber 12 and the outer shell 11. As shown in 
FIG. 2, the insulation 30 surrounds the bottom, side, and end walls of 
storage chamber 12 and extends around the Stirling cycle heat pump 20 to 
isolate the the freezer storage chamber 12 and the cold end (shown 
generally at 32) of the heat pump 20 from the warm heat rejector 34 and 
compressor assembly 36 of the heat pump 20. 
As shown best in FIG. 3, a hard, low thermal conductivity plastic panel 38, 
such as a vinyl ester resin/fiberglass mat reinforced plastic, extends 
around the top between the freezer chamber 12 and the outer wall of the 
cabinet 11 and is adhesively joined to the steel cabinet and freezer 
chamber walls. The freezer lid 14 likewise has a foamed-in-place urethane 
insulative core 40, conveniently 5" thick, and a plastic mat lid liner 42 
joined to the lid 14. A snap-in plastic extrusion (not shown) joins the 
lid 14 and lid liner 42 for foaming of the insulative core 40 in place. 
This extrusion also retains a bulb gasket 43 in the lid 14 surrounding 
chamber 12. These fiberglass mat reinforced panels 38, 42 have decreased 
thermal expansion while maintaining flexibility. A plastic foam sublid 44 
rests above the top of the freezer chamber 12. The lid liner 42 is formed 
to receive a second gasket 46, which may conveniently be a combination of 
several feather gaskets, adhered in a groove in the plastic lid liner 42. 
A stainless steel, e.g., type 304, rack 48 is supported interiorly of the 
freezer storage chamber 12 which in turn supports standard storage boxes 
or items 50 contained in the freezer chamber 12. The rack 48 is spaced 
inwardly from the side and bottom walls of the chamber 12 and below the 
sublid 44, and the storage boxes 50 are in rows spaced from each other 
down the center of the chamber 12 (FIGS. 3 and 4). This results in an open 
space 52 at the bottom of the chamber 12, spaces 54, 56 along the sides, a 
space 57 between the rows of storage boxes 50, and a space 58 above the 
storage boxes 50 and below the sublid 44. These spaces are important to 
permit circulation by convection of ultralow temperature air in the 
freezer storage chamber 12, as described below. 
Heat Transfer System 
A series of heat transfer tubes 60 extend along the length of the storage 
chamber 12 in the spaces 54, 56 between the inner wall of the chamber 12 
and the support racks 48. The tubes 60 are formed of copper for its heat 
transfer properties and its corrosion resistance, and the tubes are 
typically 0.5" in outside diameter with a 0.022" wall. Six vertically 
spaced tubes 60 are provided along the front of the chamber 12 and six 
along the rear of the chamber 12 for a total of twelve heat transfer 
tubes. The tubes 60 are spaced about 11/4 inches apart. The tubes 60 
include external flat copper fins 62 0.008" thick to increase the heat 
transfer to the surrounding air. Four fins per inch are provided on the 
upper two tubes, six fins per inch are provided on the middle two tubes, 
and eight fins per inch are provided on the lower two tubes. 
The copper heat transfer tubes 60 circulate a heat transfer fluid along the 
length of the storage chamber. A presently preferred heat transfer fluid 
is argon as a saturated liquid at -170.degree. C. when a storage chamber 
temperature of -160.degree. C. is desired. Other heat transfer fluids such 
as oxygen, nitrogen, and natural gas could be used. Oxygen and natural 
gas, however, have the disadvantage of being flammable, and nitrogen has a 
higher saturation pressure. Argon, on the other hand, is non-flammable and 
non-explosive at room temperature and atmospheric pressure, and argon has 
a saturation pressure of less than 50 psig at -170.degree. C. The liquid 
argon at ultralow temperatures flows down the heat transfer tubes 60 along 
the length of the storage chamber 12 by the force of gravity due to the 
tubes 60 being sloped downwardly from their inlet end 64 to their opposite 
end 66. Gravity flow of the argon refrigerant eliminates the need for a 
pump which would have moving parts which would require lubrication. 
The liquid argon in the heat transfer tubes 60 absorbs heat from the 
storage chamber 12 cooling the surrounding air and causing the argon to 
vaporize in the tubes 60. The argon gas in the tubes 60 forms a gas head 
above the liquid in the heat transfer tubes 60 and is transported back to 
the inlet end 64 of the tubes 60 in a counterflowing direction to the flow 
of the liquid argon. 
Placement of the heat transfer tubes 60 at the top of the storage chamber 
12, as shown in FIGS. 2 and 3, causes a natural convective flow of 
ultralow temperature air in the chamber 12 (in the direction shown by the 
arrows in FIG. 3) surrounding the storage boxes 50. That is, the ultralow 
temperature air circulates downwardly along the side walls in spaces 54, 
56, across the bottom space 52, and upwardly in the space 57 between the 
storage boxes 50, and across the space 58 at the top of the chamber 12 
below the sublid 44 and back to the heat transfer tubes 60. 
An argon distributor 70, whose location is shown generally in FIGS. 2 and 4 
and whose details are shown in FIGS. 5 and 6, is located at the top of the 
freezer 10 outside of the storage chamber 12 between the cold end 32 of 
the heat pump 20 and the inlet end 64 of the heat transfer tubes 60. The 
argon distributor 70 consists of a domed chamber 72 formed of type 304 
stainless steel, which is welded at its base to a reservoir 74, also 
formed of type 304 stainless steel, having a circular basin 76 therein 
which is fed with liquid argon through a tube 78 communicating at its 
other end with the cold end 32 of the Stirling cycle heat pump 20. Twelve 
liquid argon distribution tubes 80 communicate with the liquid argon 
reservoir 74 about its circumference. That is, the liquid argon 
distribution tubes 80 open into the bottom of the basin 76 and are spaced 
about its circumference to achieve a substantially uniform distribution of 
the liquid argon which flows into and fills the basin 76 to each of the 
tubes 80. The distribution tubes 80 are joined at their opposite ends to 
the inlets ends 64 of the twelve heat transfer tubes 60. The argon 
distributor 70 is conveniently formed with a 3.5" diameter basin 76, a 
0.375" diameter feed tube 78, and twelve 0.5" distribution tubes 80. A 
leveling surface 82 aids in leveling of the reservoir 74 to aid in 
achieving uniform distribution of liquid argon to each of the tubes 80. In 
a presently preferred form of the invention, the tubes 80 are spaced about 
the circumference of the reservoir 74 from a 0.degree. reference line 
shown in FIG. 6 at the angles indicated for each of the twelve tubes 80. 
This is done to match the liquid argon flow to the heat capacity of the 
individual tubes 60. 
Argon gas is returned to the argon distributor 70 by flowing through the 
heat transfer tubes 60, the argon distribution tubes 80, and into a gas 
head space 84 above the liquid reservoir 74. A second tube 86, e.g., 1" in 
diameter, connects the head space 84 to a condensing chamber 90 (FIG. 7) 
at the cold end 32 of the Stirling cycle heat pump 20 where the argon gas 
is condensed to a liquid, and flows back through feed tube 78 to the 
reservoir 74 of the argon distributor 70, whereby a continuous cycle of 
argon condensation, distribution, vaporization, and condensation occurs. 
That is, the argon gas from the head space 84 in the argon distributor 70 
flows through tube 86 to the condensing chamber 90 at the cold end 32 of 
the heat pump 20 where it is condensed to a liquid at -170.degree.C. or 
lower. The liquid argon flows back through tube 78, which is slanted 
downwardly from the condenser 90 toward the distributor 70, into the 
reservoir 74 of the distributor 70, into the argon basin 76, and then out 
through the distribution tubes 80 by the force of gravity and into and 
along the heat transfer tubes 60 along the length of the freezer storage 
chamber 12. The liquid argon in the tubes 60 absorbs heat in the storage 
chamber 12 causing the liquid argon to vaporize with the argon gas then 
returning to the head space 84 in the argon distributor 70 above the 
liquid reservoir 74 in a counterflowing relation to the liquid argon and n 
a continuous sequence of argon gas condensation and vaporization. 
Stirling Cycle Heat Pump 
The source of refrigeration is the closed cycle, hermetically sealed, 
free-piston Stirling cycle heat pump 20, which is shown in detail in FIGS. 
7 and 8. The heat pump 20 is vertically disposed and includes the cold end 
32 and associated condensing chamber 90 at the top, a heat rejector 
subassembly 34 therebelow, and the compressor assembly 36 below it. As 
shown most clearly in FIG. 8, the cold end 32 and heat rejector 
subassembly 34 are supported on a main support plate 94, which in turn is 
bolted by means of bolts 95 to a main support plate 97 of the compressor 
subassembly 36. 
Cold End 
The cold end 32 of the heat pump 20 includes a cold end cup 96 made of 
12-gauge stainless steel conforming to ASTM-A-240 grade 304 and having a 
minimum wall thickness of 0.089 inch. The cold end cup 96 is surrounded by 
a similar 13-gauge stainless steel cap 98 of minimum wall thickness of 
0.078 inch to form the argon condensing chamber 90 therebetween. The cap 
98 and cold end cup 96 are formed in the shape of domes to accommodate the 
argon gas pressure which is on the order of five bars. The cold end cup 96 
and condenser chamber cap 98 are joined to an annular stainless steel 
(type 304 or 304L) flange 100 with the cold end cup 96 being welded to the 
cold end flange 100 at 101 and the chamber cap 98 being welded at annular 
groove 102 to the flange 100. (The flange is shown diagrammatically in 
FIG. 7. In practice, the flange is formed of two concentric rings which 
engage to permit assembly and disassembly for testing. The cold end cup 96 
and chamber cap 98 are welded to the inner ring. Upon completion of 
successful testing an annular seal weld seals the joint between the two 
rings to seal the cold end to prevent escape of helium.) 
The liquid argon feed tube 78 and argon vapor tube 86 are welded to the 
wall of the chamber cap 98 and communicate with openings 103 and 104, 
respectively, in the wall of the cap 98 permitting flow of argon vapor 
through opening 104 into the space 105 between the cup 96 and cap 98 where 
the vapor is condensed to a liquid which then flows out by gravity through 
opening 103 and into the liquid argon supply tube 78 for return to the 
distributor 70. 
In a presently preferred form of the invention, the cold end cup 96 has an 
inner diameter of 4.550 inches and a minimum wall thickness of 0.089 inch. 
Referring in addition to FIG. 10, the cold end cup 96 is provided with an 
aluminum sleeve 107 having on its outer surface a plurality of contoured 
fins 106 to increase the heat transfer from the wall of the cold end cup 
96 to the argon gas in the space 105. The sleeve 107 has an inner diameter 
4.76 inch and outer diameter to the tip of the fins 106 of 5.144 inch. The 
sleeve 107 including fins 106 is 2.750 inch in length, and the fins 106 
have a fin profile made according to the equations set out in the paper by 
Adamak, T., "Bestimmung der Kondensationgrossen auf feingewellten 
Oberflachen zur Auslegung optimaler Wandprofile," 
Warme-und-Stoffunbertragunh, vol. 15, 1981, pp. 255-270. An example of a 
suitable fin cutter profile is given in FIG. 11. There are 122 fins on the 
outer surface of sleeve 107 spaced on 2.937.degree. intervals equaling a 
total of 358.3.degree.. (One pair of fins is spaced 1.700). The sleeve 107 
and fins 106 are formed of aluminum to maximize heat transfer. At room 
temperature, the diameter of the sleeve 107 is such that it fits loosely 
over the O.D. of the cup 96. As the temperature of the cold end 32 lowers, 
the aluminum sleeve 107 by virtue of its relatively higher coefficient of 
thermal expansion shrinks about the cup 96 to form in effect a shrink fit 
between the two parts. The intimate metal-to-metal contact further aids in 
maximizing heat transfer to the argon gas. 
Heat Rejector 
The heat rejector assembly 34 includes an outer cylinder 108 mounted at its 
base 110 in the main support plate 94 and at its top in a groove 112 in 
the cold end flange 100, and an inner cylinder 114 mounted at its base 116 
to the support plate 94. (Again flange 100 is shown diagrammatically. In 
practice, the cylinder 10 is welded to the outer ring which in turn is 
seal welded to the inner ring.) The outer 108 and inner 114 cylinders are 
formed of Schedule 40 type 304L seamless stainless steel pipe. The heat 
rejector assembly 34 further includes a type 304L stainless steel upper 
flange 118 which is joined to the insulation support plate or pan 119 
(FIG. 2). There are 180 type 304L stainless steel tubes 120 of 0.125" 
outside diameter by 0.020" thick wall by 4.240" long mounted in the space 
122 between the inner 114 and outer 108 rejector cylinders. The tubes 120 
are mounted at their bases in openings 124 in the support plate 94 and at 
their tops in openings 126 in an upper rejector tube support sheet 128 
also formed of type 304L stainless steel. The tubes 120 are brazed in 
place as is tube support sheet 128. The tubes 120 are circumferentially 
spaced about the unit in three concentric rings. 
Upper and lower rejector assembly stubs 130 and 132, respectively, extend 
through and are welded in the wall of the outer cylinder 108 of the 
rejector assembly 34 and communicate with the space 122 surrounding the 
rejector tubes 120. As will be described below, a refrigerant is 
circulated in the space 122 to remove heat from the gas passing through 
the tubes 120. The stubs 130, 132 are 1/2" in outside diameter .times. 
0.035" in wall thickness.times.1.5" in length and are formed of type 304 
stainless steel. Four circumferentially spaced stubs 130 are provided at 
the top and two stubs 132 at the bottom. 
The tubes 120 open at their top ends into a regenerator 134 located between 
the rejector assembly 34 and the cold end cap flange 100. The regenerator 
134 is of standard construction and is a matrix formed of 22 micron 
diameter stainless steel wire having 80% porosity. Filters 136 are located 
at the top and bottom of the regenerator 134 to prevent particles of the 
stainless steel wire from becoming dislodged and being caught in the gas 
flowing through the regenerator. The filters 136 are preferably formed of 
a spunbonded sheet of continuous polyester fibers that are randomly 
arranged, highly dispersed, and bonded at the filament junctions. The 
filtration efficiency is greater than 90% for particles larger than 5 
microns and the pressure drop for air flow is 0.5 inches of water gauge at 
180 ft/min velocity. A suitable material is Reemay 2295 sold by Snow 
Filtration Co. of Cincinnati, Ohio. 
Displacer Assembly 
A displacer support plate 138 which includes a central hub 140 having a 
internally threaded recess 142 is bolted to the assembly by means of bolts 
144 passing upwardly therethrough. A gasket 146 seals the periphery 
between the displacer support plate 138 and main support plate 94. Set 
screws 148 are provided in the displacer support plate 138, as hereinafter 
described. As best seen in FIG. 9, the displacer support plate 138 has 
three openings 150 surrounding the hub 140 permitting passage of the 
helium working gas between the compressor 36 and cold end assembly 32. 
A displacer support rod 152 is screwed into the recess 142 and then welded 
to the central hub 140. 
A displacer assembly 154 includes a cylindrical displacer tube 156, a 
cylindrical displacer sleeve 157, a shell cap 158, a displacer rod guide 
160, which surrounds the displacer rod 152 and to which the displacer 
sleeve 157 is threaded at its base, a support ring 162, and an insulator 
164. 
The displacer support plate 138, support rod 152, displacer sleeve 157, and 
rod guide 160 are made of type 6061-T651 aluminum. The displacer tube 156, 
shell cap 158, and support ring 162 are all formed of phenolic grade XXX. 
The displacer tube 156 is adhered to the displacer sleeve 157 at its base, 
and the displacer sleeve 157 is adhered to an annular support flange 166 
of rod guide 160 by an adhesive sold by Hysol Aerospace Products, Dexter 
Adhesives & Structural Materials Div. of Pittsburg, CA, under the 
designation 9434. The cap 158 and ring 162 are adhered to the displacer 
tube 156 by the same adhesive. Eight circumferentially spaced 1/4" 
openings 167, which intersect the outer circumference of flange 166, are 
provided in support flange 166. 
The displacer assembly 154 is surrounded by a cold end heat exchanger 168 
formed of phenolic grade XXX which is threaded to a type 6061-T6 aluminum 
cylindrical stuffer 169. The cold end heat exchanger 168 at its end 
surrounded by the cold end cup 96 contains 30 passages 198 on its outer 
surface 0.375" wide.times.0.045" deep through which the helium gas passes 
into a gas expansion space 170, as best seen in FIG. 10. Since the gas in 
space 170 is at a temperature on the order of -173.degree. C., the 
displacer assembly 154 is made of low thermal conductivity material, such 
as phenolic grade XXX, to minimize thermal conduction losses. Likewise, 
the insulator 164 provided below the end cap 158 is made of a material 
suitable for a temperature differential between the cold end and warm end 
of the displacer on the order of 215.degree. K. A suitable insulating 
material is Solimide type TA-301 sold by IMI-Tech Corp. of Elk Grove 
Village, Ill. A Reemay 2295 filter 172 is placed between the insulator 164 
and support ring 162 to prevent any particles of insulation shedding off 
the insulator 164 from entering the working gas where they could interfere 
with clearance seals. 
The end of the stuffer 169 opposite the cold end heat exchanger 168 
receives the three bolts 144 securing the displacer support plate 138 in 
place. 
The end cap 158 may be provided with a threaded recess 174 to permit 
insertion and removal of the displacer assembly 154 in place on the 
displacer support rod 152. 
A gas spring cap 176 formed of type 6061 T6 aluminum is threaded to the top 
of the displacer rod guide 160 forming a generally closed space 178 
therein extending down through the center of the displacer support rod 
152, which is filled with helium at an average pressure between the 
maximum and minimum pressure of the working gas in the heat pump to form a 
gas spring for the displacer assembly 154. That is, the displacer assembly 
154, including displacer tube 156, end cap 158, rod guide 160, and cap 
176, reciprocate lineraly in the cold end heat exchanger 168 on the 
displacer support rod 152. Space 178 filled with helium functions as a gas 
spring in cooperation with the resonant movement of the displacer at the 
operating frequency of the compressor in accordance with the teachings of 
U.S. Pat. No. 4,183,214, which disclosure is incorporated herein by 
reference. 
A clearance seal of 0.0015" maximum exists between the inner diameter of 
the displacer rod guide 160 and the outer diameter of the displacer 
support rod 152. A clearance seal of 0.0040" maximum exists between the 
outer diameter of displacer sleeve 157 and the inner diameter of the 
stuffer 169. In both cases, the inner diameter of the external part (rod 
guide 160 and stuffer 169) are hard anodized and finished to a 4 micron 
finish, and the outer diameter of the internal part (support rod 152 and 
sleeve 157) are coated with Xylan 1014 (sold by Whitford Corp. of West 
Chester, Pa.), one mil thick, to provide a hard-on-soft bearing pair in 
case of minor contact. 
Further, the displacer assembly 154 is supon about its longitudinal axis to 
provide non-contact hydrodynamic gas bearings between support rod 152 and 
rod guide 160 in accordance with the teachings of U.S. Pat. No. 4,330,993 
and 4,412,418, which disclosures are incorporated by reference. That is, 
when the piston in the compressor section 36 of the heat pump 20 is moving 
downwardly, the working gas, in this case helium, is caused to flow down 
through the regenerator 134, through the rejector tubes 120, radially 
inwardly in a space 180 above support plate 138 and downwardly through 
openings 150 in support plate 138. In doing so, the gas under pressure 
impacts on a series of circumferentially disposed turbine fins 182 (FIGS. 
7 and 9) affixed to the base of the displacer rod guide 160. There are 36 
fins in all 0.232" in length and separated by 0.068". As the gas passes 
therethrough, the impact of the gas on the fins 182 causes the displacer 
assembly 154 to spin on its longitudinal axis between the inner diameter 
of the rod guide 160 and the outer diameter of the displacer support rod 
152 forming the non-contacting hydrodynamic gas bearing. 
In the position of the displacer assembly 154 as shown in FIG. 7, the 
turbine fins 182 are elevated out of space 180 However, when the helium 
gas is flowing downwardly toward the compressor and radially inward in 
space 180, the displacer is in a lowered position whereby the helium gas 
impacts on the turbine fins 182. When the gas is flowing in the opposite 
direction, i.e., away from the compressor toward the cold end 32, as the 
compressor piston cycles, the turbine fins are in their raised position 
shown in FIG. 7 out of space 180 whereby spinning movement is maintained 
in one direction only. When the heat pump is turned off, the displacer 154 
is at rest in a lowered position. An annular recess 184 is provided to 
accommodate the turbine fins, and an annular elastomeric bumper 186 
surrounding hub 140 prevents metal-to-metal contact between the displacer 
rod guide 160 and hub 140. 
Because gas clearance seals allow the leakage of small amounts of gas, the 
components could drift off center. To minimize this problem, four equally 
spaced small diameter ports 190 of 0.040" in diameter are provided in the 
wall of the displacer rod guide 160. A circumferential groove 188 0.039" 
wide.times. 0.059" deep intercepts four 1/8" diameter holes 192 in the 
wall of support rod 152 which are open to the helium gas in inner space 
178. When ports 190 and groove 188 come into registry, they permit 
movement of small gas quantities to equalize the pressure between the gas 
spring space 178, the surrounding interior space 194, and across the 
clearance seal therebetween. Likewise, a 0.020" port 196 is provided in 
the rod guide 160 to permit gas flow between the interior space 194 and a 
compression space 216. These centering ports 188 and 190 and 196 serve to 
maintain the proper positioning of the fixed and rotating parts in 
accordance with the teachings of U.S. Pat. No. 4,404,802, which disclosure 
is incorporated herein by reference. 
Heat Rejection System 
Referring again to FIGS. 2 and 7, heat is rejected from the rejector 
assembly 34 by the circulation of a refrigerant such as 
chlorodifluoromethane in the space 122 surrounding the rejector tubes 120. 
That is, the liquid refrigerant enters the space 122 through the pair of 
opposed lower stubs 132 (only one shown in FIGS. 2 and 7) and absorbs heat 
from the helium gas passing through the rejector tubes 120 by heat 
conduction through the tube walls. The absorption of heat from the gas 
causes the refrigerant to vaporize, and the vapor leaves the rejector 
through the four circumferentially spaced upper stubs 130 (only one shown 
in FIGS. 2 and 7). Stubs 130 connect with tubing 204 through which the 
vapor flows to the reflux condenser 22 mounted at the end wall 27 of the 
side car cabinet 16. The condenser is formed of four rows of vertical 
copper tubes 206 with sixteen tubes per row. The tubes 206 have a 3/8" 
outside diameter a wall thickness of 0.016" and communicate at their base 
with 5/8" outside diameter cross tubes. The tubes 206 are provided with 
flat copper fins 0.006" thick with 14 fins per inch to increase the heat 
transfer area. Vapor and liquid headers to which the 5/8" OD cross tubes 
connect at opposite ends of the rows are 11/8" OD copper tubing with a 
0.050" wall. The blower 24, which may include a 1/3 hp, three speed motor, 
operating at 230 volts 50/60 Hz, is located in the bottom of the side car 
cabinet 16 and draws air through the grill 26 through a standard air 
filter 207 and over the condenser tubes 206 to condense the refrigerant 
therein. The liquid refrigerant then flows by gravity from the condenser 
22 through tubing 208 connected to stubs 132 into the bottom of the heat 
rejector space 122. The system preferably also includes a vibration 
arrestor for the fan 24 and a 400 psig relief valve. 
Compressor 
Referring to FIG. 8, the Stirling cycle heat pump is driven by the 
compressor assembly 36 which includes a linear drive motor 210, a spin 
motor 212, and a piston assembly 214 which is driven by the linear motor 
210 in a reciprocating pattern to alternately expand and contract a 
working volume of helium in a compression space 216 above the piston 
assembly 214 and which is spun on its longitudinal axis by the spin motor 
212 to cause hydrodynamic support of the piston assembly 214. The volume 
216 communicates through openings 150 with space 180. 
The linear motor 210, spin motor 212, and piston assembly 214 are contained 
in a pressure vessel for containing the helium under pressure on the order 
of 330 psig average pressure. The pressure vessel includes annular main 
support plate 97, an annular bottom plate 218, and a seamless cylindrical 
side wall 220 extending therebetween, all formed of carbon steel and 
welded together. The bottom plate 218 is in turn mounted to a lower 
support plate 222 by means of bolts 224 passing upwardly through the lower 
support plate 222 and threaded into bottom plate 218. Annular upper 226 
and lower 228 flanges are welded respectively to the bottom plate 218 and 
the lower support plate 222. Once the unit is assembled and successfully 
tested, the flanges 226, 228 are welded about their circumference at 230 
to seal the unit to provide in effect a pressure vessel for containing the 
helium working gas. 
The linear drive motor 210 includes outer laminations 232 and inner 
laminations 234. The outer laminations 232 are supported between an upper 
236 and lower 238 lamination support assembly which includes upper and 
lower lamination retaining rings 240. The inner motor laminations 234 are 
likewise supported by upper 242 and lower 244 inner lamination support 
assemblies. The lamination supports 236, 238, 240, 242, 244 are formed of 
a phenolic Ryertex grade X. 
The upper outer lamination support assembly 236 is bolted to a flange on 
the piston cylinder 246 through an intermediate aluminum ring 248 which is 
epoxied to the phenolic support using Hysol 9434. The inner assembly 242, 
244 is bolted to the bottom edge of the piston cylinder 246. Bolts 250 
pass upwardly through the outer support assemblies 238, 236 and are 
threaded into main plate 97. 
The linear motor coil 252 consists of 251 turns of #9 AWG round copper 
wire. The linear motor outer laminations 232 consist of 2650 laminations 
per unit of AISI M-15 silicon steel, 30-gauge 0.014" thick with C-3 
finish. The inner laminations 234 are of like material and comprise 935 
laminations per unit. 
The spin motor 212 is supported by a spin motor support plate 252 formed of 
6061-T651 aluminum which is secured by means of bolts to the phenolic 
lower outer lamination support 238 with an aluminum ring insert 258 
interposed therebetween. The spin motor includes inner spin motor 
laminations 260 supported by an inner spin motor end frame 262, outer spin 
motor laminations 264 supported by an outer spin motor end frame 266. The 
end frame 262, 266 are formed of 6061-T651 aluminum. The inner 260 and 
outer 264 spin motor laminations are formed of 147 laminations of AISI 
M-15 silicon steel, 26-gauge 0.018" thick with a C-3 finish. 
The inner spin motor laminations are wound with two sets of windings at 
90.degree. of one another forming a 2-phase induction motor known as a 
drag-cup rotor motor. The windings comprise 94 turns for each phase of #19 
AWG copper wire. 
The piston assembly 214 includes the fixed piston cylinder 246, a piston 
sleeve 270 reciprocal in the piston cylinder 246, a piston plug 272, which 
is screwed into the I.D. of top end of the piston sleeve 270 and welded in 
place, an annular piston flange 274 which is screwed onto 0.D. of the 
bottom end of the piston sleeve 270 and welded in place, and a magnet 
paddle assembly 276 including upper and lower annular phenolic (Ryertex 
grade "XXX") support members with a magnetic ring 280 therebetween bolted 
to the outboard side of the piston flange 274. The magnet paddle assembly 
276 is located between the inner 234 and outer 232 laminations of the 
linear drive motor 210. 
The magnetic ring is made up of 13 equal sections of iron boron neodymium 
magnets having an energy product equalling 26,000,000 megagauss oersted 
with an HCI greater than 8,000 oersteds and BR greater than or equal to 
9,500 gauss. Each segment has an ID of 2.759", an OD of 3.023", a width of 
1.437", and spans an arc of 27.degree.30", The individual magnets are 
glued along their vertical mating edges by Hysol 9434. Three layers of 
Kevlar (DuPont) cloth (28.times.24 weave, 1.3 oz/sq. yd) 3.5 mils thick 
are laid up on the exterior of the paddle using a Hysol epoxy 9436. The 
upper and lower phenolic annuli are glued respectively to the upper and 
lower ends of the magnetic ring 280. The phenolic is a grade XXX Ryertex. 
The magnetization direction is radially outward. 
A cylindrical rotor 282 is fixed to the piston 270. The rotor 282 is caused 
to spin on its longitudinal axis by the spin motor 212 in turn causing 
spinning of the piston 270 in the sleeve 246 to form hydrodynamic of gas 
bearings therebetween. The rotor 282 is formed of 6061-T6 aluminum, has a 
0.040" wall thickness, and has 12 0.031" wide by 3.976" long vertical 
slits equally spaced circumferentially and extending through the rotor 
tube wall. The upper end of the rotor 282 is welded to a rotor tube 
adaptor 281 also formed of 6061-T6 aluminum which in turn is fixed to an 
annular sleeve 283 in turn fixed to the piston sleeve 246 by four upper 
and two lower 1/8" diameter pins, the six pins being spaced 60.degree. 
about the sleeve 283 circumference. 
The linear motor 210 causes vertical linear reciprocation of the magnetic 
panel assembly 276 which in turn causes vertical linear reciprocation of 
the piston 270 within the piston cylinder 246 in a nearly sinusoidal 
pattern at a frequency of 44 Hz for optimum thermodynamic performance. At 
the same time, the spin motor 212 causes spinning of the rotor 282 which 
causes spinning of the piston 270 within the piston cylinder 246. The 
piston cylinder 246 and piston sleeve 270 are made of type 6061-T651 
aluminum, hard anodized, with the facing surfaces honded to a 4 micron 
finish. The outer surface of the sleeve 270 is coated with Xylan 1014 
minimum 1 mil thick. The diametral clearance between the piston sleeve 270 
and piston cylinder 246 is 0.025" maximum. As stated, spinning of the 
piston 270 creates a hydrodynamic gas bearing with the helium in the 
pressure vessel to lubricate the piston as it reciprocates in the piston 
cylinder. 
In general, all components should have minimal outgassing or coatings which 
would foul heat exchange surfaces or build up on clearance seals. 
The piston sleeve 270 has eight 0.059" openings 284 which intersect a 
0.061" wide .times.0.059" deep groove about the outer perimeter of the 
sleeve 270. This groove aligns at the center of the piston stroke with 
eight like openings 286 in the piston cylinder 246 to provide when aligned 
pressure equalization across the gas bearing to maintain centering of the 
piston sleeve 270 in the cylinder 246 in the same manner that the 
displacer gas bearings are equalized. The pressure vessel is filled with 
helium under an average pressure of 330 psig, and openings provide 
communication of helium gas throughout the interior of the pressure 
vessel. 
In addition, a 0.020" centering port 287 (FIG. 7) in displacer support 
plate 138 communicates between space 180 and the volume in the compressor 
36 outside the cylinder 246 so that that volume has the same average 
pressure as the space 180. 
Referring back to FIG. 2, the compressor assembly 36 has a heat pump 
support plate 290 mounted to it. Springs 292 and plate 288 form a tuned 
vibration absorber. The spring constant of springs 292 and the mass of 
plate 288 are tuned to the driving frequency of the vibration force while 
the mass and driving force of the heat pump are used to size spring and 
absorber mass with acceptable amplitude in accordance with well-known 
formulas such as that appearing at pages 136-138 of Thomson, William T., 
"Theory of Vibration with Applications," 3d Ed., Prentice Hall, 1988; and 
Mark, "Standard Handbook for Mechanical Engineers," 8th Ed., McGraw/Hill, 
1978, p. 5-72. 
The vibration absorber is in turn mounted to the base of the cabinet by 
adjustable mounting screw 294 adjustable from outside of the cabinet 11 
(FIG. 2). 
Referring back to FIG. 7, set screws 148 prevent further compression of 
gasket 146 by the force of piston cylinder 246 which could otherwise move 
displacer support plate 138 and cold end heat exchanger 168 upwardly which 
is unacceptable. 
OPERATION OF HEAT PUMP 
The operation of the Stirling cycle is well known and is described herein 
in terms of the particular construction of the free piston, closed cycle 
Stirling heat pump 20 described above. At a point in the Stirling cycle, 
the piston 270 in the compressor assembly 36 is driven by the linear motor 
210 upwardly to compress the helium gas in the working space 216 above the 
piston plug 272. This compressing movement of the compressor piston 270 
causes the pressure in the working volume of helium to rise from an 
average pressure of about 330 psig to a maximum pressure on the order of 
375 psig which warms the working volume of gas. The helium gas in space 
180 being in communication with the volume 216 is likewise compressed. At 
a point in the cycle, the increasing pressure creates a sufficient 
pressure on the displacer piston 154 to cause it to move rapidly downward. 
With this movement of the displacer 154, high pressure helium at about 
ambient temperature is forced through the regenerator 134 into the cold 
space 170 above the displacer 154 and inside the cold end cup 96. As the 
helium passes upwardly through the regenerator 134 the regenerator absorbs 
heat from the flowing pressurized gas and thereby reduces the temperature 
of the gas. 
With the sinusoidal drive from the linear motor at 44 Hz, the compressor 
piston 270 then begins to move downwardly expanding the working volume in 
the space 216 above the piston head. With this expansion, the pressure 
drops to about 285 psig, and the helium in the cold space 170 is cooled 
even further. It is this cooling in the cold space which provides the 
refrigeration for maintaining a temperature gradient of over 200.degree. 
Kelvin over the length of the regenerator 134 and an input of about 200 
watts of heat from the argon to the expansion space helium at the low 
temperatures of -160.degree. C. or less. 
At some point in the expanding movement of the piston 270, the pressure in 
the working volume 216 drops sufficiently so that the momentum of the 
displacer 154 is overcome by the retarding force of the internal gas 
spring in which helium gas in space 178 is stabilized at an average 
pressure of about 330 psig. The displacer 154 is then driven upwardly thus 
driving the cool gas in the cold space 170 back through the regenerator 
134 and the cycle is repeated. The displacer 154 thus cycles at a resonant 
frequency with the piston although out of phase therewith. 
As described above, the flow of the pressurized helium through the turbine 
vanes 182 causes the spinning of the displacer rod guide 160 to maintain 
its hydrodynamic support throughout the cycle. Likewise the spin motor 
assembly 212 causes spinning of the piston assembly 214 to maintain its 
hydrodynamic support throughout the cycle. 
A temperature differential of about 13.degree. C. is maintained between the 
desired storage chamber 12 temperature and the helium in the cold end 32 
of the heat pump and about 10.degree. C. between the storage chamber 
temperature and the temperature of the liquid argon. Thus, for a 
-160.degree. C. storage chamber temperature, the liquid argon is at 
-170.degree. C. and the helium is at -173.degree. C. At temperatures of 
-160.degree. C. or less about 200 watts of heat are input into the helium 
in condensing the argon gas in the pressure vessel surrounding the cold 
end cup. The heat input to the helium plus the input work to drive the 
heat pump compression and expansion cycles and frictional losses are 
removed from the heat pump by circulating the refrigerant in the interior 
space 122 surrounding the helium flowing through the stainless steel tubes 
120. The refrigerant is vaporized by the heat absorbed from the helium 
thereby removing the heat from the system. The refrigerant vapor flows 
through tube 204 to the reflux condenser 22 of the vertical tube gravity 
flow type. A forward curved direct drive blower 24 pulls air in through 
the grill 26 in the end wall 27, through an air filter 207, and over the 
condenser tubes 206 to condense the refrigerant which then flows by 
gravity through a return tube 208 and into the bottom of the rejector 
subassembly 34 where it again is vaporized thereby removing heat from the 
gas. 
An expansion tank 296 can be located in the side car 16 for containing the 
argon gas refrigerant at room temperature when the unit is not in use. 
This argon system operates at a saturation pressure of about 50 psig, and 
the tank 296 contains the super-heated argon at a pressure of about 200 
psig at room temperature. 
If desired tubing may be provided to the bottom of the freezer chamber 12 
and a fitting provided on the back wall of the cabinet 11 to connect to a 
back up source of liquid nitrogen external of the freezer 10 in the event 
of an unintended shutdown of the freezer to maintain low storage 
temperatures in the chamber 12 until the freezer is restarted. 
Still further, instrumentation may be provided to monitor the condition of 
the heat pump including, for example, loss of helium, loss of displacer or 
piston spin, over-stroking of the displacer or piston, temperature 
setpoint, and elevated warm end temperature. For example, optical position 
sensors are desirably used to monitor piston and displacer position and 
spin. 
Although the present invention is directed principally to a laboratory 
freezer appliance capable of providing storage temperatures of 
-160.degree. C. and lower, it will be recognized that it could be operated 
at higher temperatures while still achieving the benefits of the invention 
.