Dual pneumatic volume for cryogenic cooler

A dual pneumatic volume means in the ambient end of a free displacer cyroic cooler to provide strong pneumatic braking and dwell times of the displacer movement at both the top dead center and bottom dead center of the displacer waveform. A second pneumatic piston is positioned between the end of the cooler housing and extends into the first pneumatic piston attached to the displacer to form a pneumatic spring volume within the first piston to accomplish the strong pneumatic braking.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention is in the field of remotely driven free displacer cyrogenic 
coolers based upon the balancing force at a frictional seal between an 
ambient temperature volume and a working fluid volume. 
2. Description of the Prior Art 
Generally, the ambient temperature volume of a cyrogenic cooler is 
comprised of one pneumatic volume which influences the movement of the 
free displacer according to the compressed state of the gas in the 
pneumatic volume, which acts against the sinusoidal pressure waves in the 
working fluid. The free displacer therefore follows a sinusoidal 
oscillatory wave pattern in which the pneumatic volume essentially does 
not alter the displacer waveform but only acts to restore the displacer 
back to its position with some slight phase shift while still supporting 
the sinusoidal movement thereof. The present dual pneumatic volume means 
alters the displacer movement by causing dwell times at both the top dead 
center (TDC) and bottom dead center (BDC) of the displacer movement 
waveform. 
SUMMARY OF THE INVENTION 
The dual pneumatic volume means of the present invention is located in the 
ambient temperature end of an enclosed cooler housing of a cooler, such as 
a pneumatically driven split cycle cryogenic cooler, to cushion impact and 
provide dwell times at the TDC and BDC positions of the displacer 
movement, and to restore a positive return of the displacer from these 
positions. The dual pneumatic volume means is preferably comprised of a 
first pneumatic piston which is rigidly attached by way of a self aligning 
joint to a free displacer forming an integral part thereof and which 
extends through a passageway of said cooler housing into a pneumatic 
bounce volume at the ambient temperature end of the cooler and a second 
pneumatic piston extending between the end of the pneumatic bounce volume 
and moveably positioned within the first pneumatic piston to form a 
pneumatic spring volume therein. The pneumatic spring volume provides a 
second active force in limiting the motion of the first pneumatic piston, 
and therefore the integrally connected displacer attached thereto. A 
mechanical spring means, such as a coil spring, may also be included in 
the pneumatic spring volume to further limit movement of the displacer 
especially at the TDC. The mechanical spring means K factor will however 
be designed for only one charge pressure. The first and second pneumatic 
pistons and/or the first pneumatic piston and the cooler housing 
passageway may have mating slots and keys or rounded grooves and spring 
loaded ball bearings on opposite sides thereof to provide some twisting 
motion to accommodate for thermal shock of the regenerator-displacer 
caused by the huge changes in temperatures from the cold end to the 
ambient temperature side. The mating surfaces are however preferably 
between the first and second pneumatic pistons positioned in an angular 
spiral with the first pneumatic piston having labyrinth seals around its 
outer portion at the passageway to provide a metal-to-metal calibrated 
leak seal which allows some slight leakage of the working fluid for most 
efficient operation. The labyrinth seals provide an effective frictional 
seal upon which the balanced force between the working fluid volume and 
the dual pneumatic volume means act. The working fluid may be helium or 
hydrogen.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Before detailed reference to FIG. 1, it should be noted that the displacer 
waveforms as shown by FIG. 2 are representatively not concentric, i.e. it 
takes longer for the compression stroke, represent by numeral 8, to reach 
the flattened TDC than for the expansion stroke, represented by numeral 9, 
to reach the flattened BDC. Efficiency is greatly improved however in the 
present dual pneumatic volume means. 
Refer to FIG. 1 in which a cooler 10 is shown comprised of a cold end 13 
and an ambient temperature end 11 with a passageway therebetween in an 
enclosed cooler housing. A first pneumatic piston 12 is rigidly connected 
to regenerator-displacer by a self aligning joint 40 which allows some 
lateral movement therebetween wherein twisting or spiral motion of said 
first pneumatic piston 12 accommodates the thermal stresses set up in the 
regenerator-displacer 20 at the wide temperature differences from cold end 
13 to the ambient temperature end. Piston 12 and displacer however 
reciprocate as one unit when a working fluid enters displacer 20 by way of 
a plurality of inlet ports 36 from a remote compressor and feed lines (not 
shown). The working fluid easily passes through an end plug 30, preferably 
made of sintered metal balls having about 40% porosity and maybe a screen 
mesh at the end, and the regenerator matrix 22, which may be a plurality 
of separate metal balls. Matrix 22 is contiguous with end plug 30 and may 
be enclosed at cold end 13 by another similar end plug. Displacer 20 moves 
within leap seal 16 in the conventional manner according to working fluid 
pressures developed across the displacer. Leap seal 16 may be made of 
fluorocarbon, carbon graphite, Rulon or Rulon J4 as examples. Labyrinth 
seals around piston 12 are represented by numeral 32. 
Piston 12 extends part way into pneumatic bounce volume 24. A second 
pneumatic piston 14 is moveably positioned within first pneumatic piston 
12 and extends to the end of volume 24. First and second pistons 12 and 14 
may move relative to each other by mating means such as shown by FIGS. 3A 
and 3B representing section 3--3 from FIG. 1. FIG. 3A shows a slot 41 
within piston 14 which is mated with a key 42 extending out from piston 
12. FIG. 3B shows another mating means which is comprised of rounded 
grooves 43 in piston 14 mated to spring loaded ball bearings 44 set in 
piston 12. Both pistons 12 and 14 are preferably made of Rulon. The slot 
or groove are preferably twisted as shown in FIG. 1 representing slot 41 
at an angular spiral .alpha. to provide relative spiraling between the two 
pistons. A pneumatic spring volume 26 is formed within piston 12 enclosed 
by the end of piston 14 in which volume 26 is simultaneously compressed or 
expanded with that of volume 24. A mechanical spring means 28 in volume 26 
provides further pneumatic braking for displacer 20. 
In operation, the pneumatic spring volume 26 provides a positive 
counteraction to motion of the displacer as the displacer approaches TDC 
and BDC. The size of volume 26 should be large enough in which a near 
vacuum created therein as the displacer approaches BDC holds both 
pressures across labyrinth seals 32 in equilibrium to flatten the BDC of 
the displacer movement and to flatten the TDC of displacer movement by a 
strong internal compressive pressure against piston 12, and especially 
with the aid of the mechanical spring means. 
While only one preferred embodiment has been described it is to be 
understood that other variations may be made while remaining within the 
spirit and scope of the invention which is limited only by the following 
claims.