Cooler

Details Cooler Cooler

1997-09-22

1999-08-17

Capossela, Ronald

Refrigeration

Cryogenic treatment of gas or gas mixture

Liquefaction

62216, F25B 1902

Patent

active

059376570

DESCRIPTION:

BRIEF SUMMARY
This invention relates to Joule Thomson coolers.
Joule Thomson coolers are devices that are used to provide point cooling to very low temperatures. Applications for these coolers include the cooling of superconducting materials. and the cooling of detector materials sensitive to infra-red radiation such as used in thermal imaging cameras and heat seeking missiles.
A Joule Thomson cooler, as shown in FIG. 1. comprises an expansion orifice 1 and a heat exchanger 2. High pressure gas, typically air. nitrogen or argon. is applied to the cooler inlet 3. When this gas passes through the expansion orifice, it expands to ambient pressure, which causes the temperature of the gas to drop. This gas, now at a temperature lower than that of the inlet gas, passes over the heat exchanger 2 before exiting the cooler at 4. The inlet as is therefore cooled in the heat exchanger by the expanded exiting gas. before reaching the expansion orifice 1, where further cooling occurs. A cumulative cooling effect takes place until the temperature drops to the point that the gas becomes liquid following expansion. The cooler is mounted within a vacuum encapsulation knows as a "dewar" 5, and a small pool of liquid 6 forms at the bottom of this dewar. Heat is extracted from the surrounding area as this liquid evaporates. The element 7 that requires cooling is also mounted within the dewar and is therefore cooled down to the boiling point of the working fluid
The time taken for the formation of liquid gas in the cooler from the commencement of gas flow is referred to as "cooldown time", and is governed by the thermal mass of the equipment, the pressure of the inlet gas, the area of the expansion orifice (and hence, the mass flow rate of gas through the orifice), the thermodynamic properties of the gas, and the efficiency of the cooler.
Once liquid gas has been formed and cooldown is achieved, the cooler begins to operate in the steady state phase. At this time, the cooler only needs to consume sufficient gas to remove the steady state heat load from the cooled element and the surroundings. If the cooled element is an infra-red detector, the steady state heat load is made up of the electrical power dissipated in the detector element, the heat conducted across the wires to the detector element, the heat that radiates from the outer dewar wall onto the detector and inner dewar wall, and the heat that conducts down the inner dewar wall and cooler core from the "warm" end to the "cold" end.
The orifice area required to provide sufficient gas flow to absorb these heat loads is usually substantially less than that required to give a satisfactory cooldown time. The gas mass flow required to maintain the cooled element at the desired temperature is dependent upon the inlet pressure and ambient temperature, as shown in FIG. 2.
The simplest cooler construction would involve the use of a fixed expansion orifice, and the size of this orifice would be governed by the need to provide the required cooldown time, and to maintain cooldown at worst case conditions of maximum ambient temperature and minimum available inlet pressure. However, a fixed orifice gives a flow characteristic such that the flow rate increases during the cooldown phase as temperature falls, as shown in FIG. 4. The steady state flow rate is governed by the orifice area and the inlet gas pressure. A fixed orifice cooler will therefore be operating inefficiently for most of the time, because at any pressure and temperature other than the design point. the cooler will consume more gas than is required by the laws of thermodynamics to maintain cooldown.
The implications of inefficient operation depend on the source of high pressure gas for the cooler. If a rechargeable gas bottle is used. then the duration between bottle changing and recharging is much reduced. and if a compressor is supplying gas directly, then a larger flow capacity is required and any maintenance activities based on usage occur more frequently.
One known method of improving efficiency is to provide a means of changing

REFERENCES:
patent: 3823575 (1974-07-01), Parel
patent: 4080802 (1978-03-01), Annable
patent: 4126017 (1978-11-01), Bytniewski et al.
patent: 4750338 (1988-06-01), Hingst
patent: 4779428 (1988-10-01), Chan et al.
patent: 5119637 (1992-06-01), Bard et al.

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