Refrigeration – Refrigeration producer – Heat exchange between diverse function elements
Reexamination Certificate
2001-11-09
2003-11-11
Tapolcal, William E. (Department: 3744)
Refrigeration
Refrigeration producer
Heat exchange between diverse function elements
C062S612000
Reexamination Certificate
active
06644067
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to refrigeration systems and more particularly to a closed circuit refrigeration system utilizing coalescent/depth filters.
The cryogenic mixed gas refrigeration system is a familiar system and has been described in numerous prior art documents such as U.S. Pat. Nos. 2,041,725, 4,535,597, 4,597,267, 4,689,964 and 5,161,382 and ASHRAE Refrigeration handbook, 1998, section 39.2. The limited application to which this technology has been put to is in part due to a number of sufficient shortcomings in the present systems. Prior art “auto-cascade” refrigeration systems have shown mixed gas systems to be an effective method of extending the normal temperature range of a refrigeration system with a single compressive step. Such systems are capable of cryogenic temperatures as low as −160° C. By using a mixture of gases with differing thermodynamic properties, the components of which under compression may be preferentially separated on the basis of phase, provide intermediate cooling of the discharge gas system.
A mixed gas refrigeration system can be described as an extended multi-zone economizer where high pressure gas discharged from the compressor is cooled by low pressure returning gas via a heat exchanger into which condensed liquid refrigerant is evaporated through a metering device such as a capillary line or a thermal expansion valve. The phrase change from liquid to gas serves to cool the discharge gas stream further.
Prior art cryogenic systems contain a mixture of gases, which are sequentially condensed and extracted into the return gas stream to cool the discharge gases. Currently disclosed systems use continuous tube-in-tube heat exchangers and tangential/vortex type phase separators placed at suitable points along the length of the heat exchanger.
The most prevalent application of the prior art technology has been its use in “cryogenic water vapor pumps”. These are a type of vacuum pump commonly associated with the industrial applications of vacuum, e.g., coating of plastics/paper and the manufacture of semiconductors. Such systems are used to preferentially pump water vapor from high vacuum systems through the trapping of water onto a copper or stainless steel Meissner coil placed inside the vacuum chamber. The advantage of such systems has been their very fast water vapor pumping speeds. Advances in design have allowed the fast cycling of such systems through the common refrigeration practice of direct injection of hot gases into a Meissner coil. Common applications of cryogenic water vapor pumps are thin-film coatings and the processing of semiconductor devices.
During the cool-down of an “auto-cascade” system from ambient, almost all of the refrigerant charge exists in the gaseous phase, and consequently the gas flow rates are high. As the system cools to its equilibrium cryogenic temperature, certain components condense and are separated and returned through capillary lines to affect cooling of the oncoming gas stream. At equilibrium, flow rates are greatly reduced. The reduction in gas flow increases with distance from the compressor. Thus, to be efficient, any device designed to separate the two-phase components (gas/liquid) must be capable of operating effectively in two differing temperature/pressure/flow regimes. Prior art systems have employed either impingement or centrifugal (vortex) separation processes. Both separation methods operate at around 80% separation efficiency under optimal conditions. Often the two separation methods are combined which increases the operational range of the hybrid device but maximal efficiency is always compromised.
Two of the greatest technical challenges which face the engineer of these systems are the efficient separation of condensed components from the gas stream and prevention of contamination of the cryogenic parts of the system with compressor oil or less volatile components of the gas mixture. To be able to achieve the separation of condensed from non-condensed refrigerants has proved to be the limiting factor in the widespread commercial application of this technology.
The whole system must be capable of operating over a very wide range of temperatures, gas flows and pressures which exist in the system between start and achieving a cryogenic equilibrium. At start-up, the gas mixture can be deemed to be homogenous throughout the system and at high temperatures and pressure. Since all of the components are in the gas phase, the velocity of the gas is high. High gas velocities are ideal conditions for the impingement type of phase separator.
Once the system cools, the less volatile components are removed and returned to the compressor by being evaporated into the suction line further cooling the discharged gas and ultimately causing the condensation of further component. At equilibrium, each separation point corresponds to the corresponding temperature of a component gas, which is subsequently colder than the previous point. At this point in the cycle, the system is at low temperature and pressures and the gas velocity has dropped as a result of most of the gas charge being liquefied. A further consequence is that the composition of the discharge line changes with distance from the compressor.
The vortex separator (cyclonic) has been favored as it provides a lower pressure drop than mesh or sieve impingement types. The vortex type of separator separates droplets on the basis of centrifugal force. It therefore favors larger droplets moving within a high velocity gas stream. This is ideal just after start-up and at points closer to the compressor where gas velocities and mass flows are higher. However, their efficiency is greater compromised as the system cools and becomes cryogenic. The impingement filter has some similarities to a coalescent filter. However, the mean free path is small and the effective pore size large. Impingement phase separation works at low gas velocities where the droplets may have an increased residence time. Because of the opposing properties of a vortex and impingement phase separation, it is common to have both principals within the same separator.
Another principle difficulty encountered with a cryogenic auto-cascade system lies with the fact that to achieve the low temperatures gases with low boiling points such as methane or nobles gases must be used. Such gases are well above their critical temperature at normal temperatures. They thus follow the Boyles Law behaving as ideal gases where PV=NRT.
Since an auto-cascade refrigeration system as described is a closed system, the volume of system V, quantity of gas N and by definition the gas constant R do not change.
The change in state, which an ideal gas undergoes during compression, may be described by
P1
T1
=
P2
T2
Here pressure P and temperature T are expressed in absolute units (pa and ° K)
Typical refrigeration compressor operating compression ratios are between 10:1 and 20:1. In such a system compressing an ideal gas (i.e. one above its critical temperature) would cause the temperature of gas discharged from the compressor to increase by several hundred degrees Kelvin. This is far in excess of the capabilities of commercial compressors.
A solution is to use an agent to quench the discharge gas temperature. The basis of the effect lies in the fact that its boiling point is sufficiently high that it only changes from liquid to gas at discharge temperatures and pressures encountered within a typical refrigeration system. The change in state from liquid to gas absorbs a large amount of energy suppressing any adiabatic temperature increase caused by the compression of and ideal gases to achieve low compressor discharge temperatures.
Prior art has shown the use of refrigerant R123 to be effective in controlling gas discharge temperatures in large conventional refrigeration systems. The use of R123 as a chloro-carbon has been shown to cause damage to tropospheric ozone layer.
Another difficulty with auto-cascade systems is the fact that they rely upon a large compressor displacement
Ali Mohammad M.
Gray Cary Ware & Freidenrich LLP
Tapolcal William E.
Telmark Cryogenics Limited
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