Patent Description:
The basic principal of operation of a GM cycle refrigerator is described in <CIT> A GM cycle refrigerator consists of a compressor that supplies gas at a discharge pressure to an inlet valve which admits gas to an expansion space through a regenerator, expands the gas adiabatically within a cold end heat exchanger where it receives heat from an object being cooled, then returns the gas at low pressure to the compressor through the regenerator and an outlet valve. The GM cycle has become the dominant means of producing cryogenic temperatures in small commercial refrigerators primarily because it can utilize mass produced oil-lubricated air-conditioning compressors to build reliable, long life, refrigerators at minimal cost. GM cycle refrigerators operate well at pressures and power inputs within the design limits of air-conditioning compressors, even though helium is substituted for the design refrigerants. Typically, GM refrigerators operate at a high pressure of about <NUM> MPa, and a low pressure of about <NUM> MPa.

A system that operates on the Brayton cycle to produce refrigeration consists of a compressor that supplies gas at a discharge pressure to a heat exchanger, from which gas is admitted to an expansion space through an inlet valve, expands the gas adiabatically, exhausts the expanded gas (which is colder) through in outlet valve, circulates the cold gas through a load being cooled, then returns it to the compressor at a low pressure through the heat exchanger. Brayton cycle refrigerators operating at cryogenic temperatures can also be designed to operate with the same compressors that are used for GM cycle refrigerators.

The cold expander in a GM refrigerator is typically separated from the compressor by <NUM> to <NUM> long gas lines. The expanders and compressors are usually mounted indoors and the compressor is usually cooled by water, most frequently water that is circulated by a water chiller unit at a temperature that is typically in the midrange of <NUM> to <NUM> for which the compressor is designed. Air cooled compressors that are mounted indoors are typically cooled by air conditioned air where the temperature is in the range of <NUM> to <NUM>.

Disadvantageously, compressors designed for air-conditioning service require additional cooling when compressing helium because monatomic gases including helium get a lot hotter when compressed than standard refrigerants. <CIT> describes a means of adapting a scroll compressor manufactured by Copeland Corp. by injecting oil along with helium into the scroll such that about <NUM>% of the displacement is used to pump oil. Approximately <NUM>% of the heat of compression leaves the compressor in the hot oil and the balance in the hot helium. The Copeland compressor is oriented horizontally and requires an external bulk oil separator to remove most of the oil from the helium.

Another scroll compressor that is widely used for compressing helium is manufactured by Hitachi Inc. The Hitachi compressor is oriented vertically and brings the helium and oil directly into the scroll through separate ports at the top of the compressor and discharges it inside the shell of the compressor. Most of the oil separates from the helium inside the shell and flows out of the shell near the bottom while the helium flows out near the top.

Helium compressor systems that use the Copeland and Hitachi scroll compressors have separate channels in one or more after-coolers for the helium and oil. Heat is transferred from the oil and helium to either air or water. The cooled oil is returned to the compressor and the cooled helium passes through a second oil separator and an adsorber before flowing to the expander. <CIT> shows after-cooler <NUM> as being a single heat exchanger cooled by water. This is a typical arrangement for helium compressor systems that operate indoors where chilled water is available. Air cooled compressors have been designed for operation either indoors or outdoors. Figures 3A and 3B in <CIT> shows an arrangement with a Hitachi scroll compressor that has two air cooled oil coolers, one indoors and one outdoors while all the other components are indoors with the helium always cooled by air. As explained in the '<NUM> patent, keeping all of the components that have helium in them indoors in an air condition environment, where the temperature is in the range of <NUM> to <NUM>, minimizes the contaminants that evolve from hot oil and increases the life of the final adsorber. Rejecting some or all of the heat outdoors in the summer reduces the load on an air conditioning system while rejecting heat to the indoor air in the winter reduces the load on the heating system. Two compressors, one air cooled operating either indoors or outdoors and the other water cooled operating indoors, can provide redundancy if one fails and can extend the time between services if each is operated for a significant part of the year. Air cooled oil lubricated helium compressors that are used outdoors are typically designed to operate in the temperature range of -<NUM> to <NUM>. The power input to these compressors is typically in the range of <NUM> to <NUM> kW.

<CIT> shows a cryogenic system including two compressor units to supply refrigerant gas to a refrigerator and a control unit configured to configured to perform operation control of the compressor units (see <FIG>). Each of the compressor units of the system is provided with pressure sensors to detect pressure of gas in low pressure pipe and in high pressure pipe, and the measured pressure values, in particular the differences of the measured pressure values of each compressor unit, are used to control the operating frequency of the compressor motor in each compressor unit.

The objective of this invention is to provide redundancy in the helium compressor system operating with a GM cycle expander to produce refrigeration at cryogenic temperatures. An important application is the cooling of superconducting MRI magnets which operate at temperatures near <NUM> and require very reliable operation. Most MRI systems are located in hospitals and have chilled water available, so the primary helium compressor is water cooled. In the event of a failure in the water cooling system or the water cooled compressor this invention provides a backup air cooled helium compressor connected to a common manifold in such a way that the cross-over from one compressor to the other does not affect the operation of the expander. An oil-lubricated helium compressor sytem and a method thereof according to the present invention are defined in claims <NUM> and <NUM>, respectively.

Parts that are the same or similar in the drawings have the same numbers and descriptions which are not repeated. <FIG> is a schematic diagram showing how air cooled oil lubricated helium compressor <NUM> can be manifolded with water cooled oil lubricated helium compressor <NUM> to supply gas to a GM expander. Gas returning from the expander enters low pressure mainifold <NUM> through coupling <NUM> and is split to flow to air cooled compressor <NUM> through check valve <NUM> or to water cooled compressor <NUM> through check valve <NUM>. Both compressors are connected to high pressure manifold <NUM> and the GM expander the expander enters low pressure mainifold <NUM> through coupling <NUM> and is split to flow to air cooled compressor <NUM> through check valve <NUM> or to water cooled compressor <NUM> through check valve <NUM>. Both compressors are connected to high pressure manifold <NUM> and the GM expander through coupling <NUM>. Check valves <NUM> and <NUM> prevent gas from flowing into the return gas manifold <NUM> when the compressors are turned off. Having both compressors connected directly to high pressure manifold <NUM> results in the compressor that is off being at high pressure and also prevents oil from migrating out of the compressor that is "off" to the one that is "on". When a GM refrigerator with a single oil lubricated compressor shuts down the equilibrium pressure will be closer to the high pressure than the low pressure because there is typically more volume at high pressure, e.g. in the oil separator and adsorber, than low pressure. When two compressors are connected in parallel and only one is running while the other has high pressure in it requires that the equilibrium pressure when they are both off be higher than when they are connected separately to an expander.

<FIG> is a schematic diagram of oil-lubricated helium compressor system <NUM> which has an air cooled after-cooler and <FIG> is a schematic diagram of oil-lubricated helium compressor system <NUM> which has a water cooled after-cooler. The standard compressor systems that are presently being manufactured by the assignee of this invention are essentially the same as shown in these figures. These figures show the vertical Hitachi scroll compressors but the schematics for the horizontal Copeland compressors are similar.

Compressor system components that are common to both of the figures are: compressor shell <NUM>, high pressure volume <NUM> in the shell, compressor scroll <NUM>, drive shaft <NUM>, motor <NUM>, oil pump <NUM>, oil in the bottom of the compressor <NUM>, oil return line <NUM>, helium return line <NUM>, helium/oil mixture discharge from the scroll <NUM>, oil separator <NUM>, adsorber <NUM>, main oil flow control orifice <NUM>, orifice <NUM> which controls the flow rate of oil from the oil separator, gas line <NUM> from oil separator <NUM> to adsorber <NUM> internal relief valve <NUM> and pressure equalization solenoid valve <NUM>, gas line <NUM> from internal relief valve <NUM> and pressure equalization solenoid valve <NUM> to helium return line <NUM>, adsorber inlet gas coupling <NUM>, adsorber outlet gas coupling <NUM> which supplies high pressure helium to the expander, and coupling <NUM> which receives low pressure helium from the expander.

Air cooled compressor system <NUM> in <FIG> shows high pressure helium flowing from compressor <NUM> through line <NUM> which extends through air cooled after-cooler <NUM> to oil separator <NUM>. High pressure oil flows from compressor <NUM> through line <NUM> which extends through air cooled after-cooler <NUM> to main oil control orifice <NUM>. Fan <NUM> drives air through after-cooler <NUM> in a counter-flow heat transfer relation with the helium and oil.

Water cooled compressor system <NUM> in <FIG> shows high pressure helium flowing from compressor <NUM> through line <NUM> which extends through water cooled after-cooler <NUM> to oil separator <NUM>. High pressure oil flows from compressor <NUM> through line <NUM> which extends through water cooled after-cooler <NUM> to main oil control orifice <NUM>. Cooling water <NUM> flows through after-cooler <NUM> in a counter-flow heat transfer relation with the helium and oil.

A primary concern in using oil lubricated compressors that are designed for air conditioning refrigerants is the management of oil. First a lot more oil is compressed along with the gas in order to cool the helium and secondly the cryogenic expanders cannot tolerate any oil thus requiring an extensive oil removal system. There is also a concern for oil migration during start up and shut down. Pressure equalization solenoid valve <NUM> opens when the compressor turns off in order to avoid having high pressure gas in compressor <NUM> blow oil back through return line <NUM> where it can migrate to the expander.

The preference for having the water cooled after-cooler as the primary cooler is typical but there may be circumstances when the air cooled after-cooler is the primary cooler and the water cooled after-cooler is used as a backup. Some MRI magnets are kept cold during transport by running the refrigerator using the air cooled compressor because electrical power is available but not cooling water. It is also possible that the air cooled after-cooler is used in the winter to help heat the building and the water cooled after-cooler is used in the summer to minimize the load on the air conditioner.

The most likely causes of failures in a water cooled after-cooler are fouling of the heat exchanger, low cooling water flow rate, and high inlet water temperature. For an air cooled after-cooler the most likely causes are blockage of the air flow, failure of the fan, and high air temperature. Temperature and pressure sensors are used to monitor the operation of the refrigeration system. Temperature sensors that are critical to detect a failure are located on one or more of the following lines: oil out of water cooled after-cooler <NUM>, oil out of air cooled after-cooler <NUM>, helium discharge temperature in line <NUM>, oil temperature leaving the compressor in line <NUM>, water line <NUM> in and out of water cooled after-cooler <NUM>, and indoor and outdoor air temperatures. Other fault sensors such as a cooling water flow rate sensor might be used.

The system that is being cooled, such as an MRI magnet, generally has the control system that determines which of the two compressors is running. The designer of the control system determines which sensors in each of the compressors provide critical signals that can be used to determine when to switch from one compressor to the other. Switching can be done with the operating compressor turned off before the other is turned on, but it is preferable for the one that is off to be turned on before the other is turned off. Having both compressors on at the same time results in gas by-passing through internal relief valves <NUM>. The control system keeps the expander operating if at least one compressor is turned on.

While this invention has been described in most detail for GM cycle refrigerators cooling MRI magnets at <NUM> it is also applicable to Brayton cycle refrigerators and applications such as cooling cryopumping panels at <NUM>. It will also be understood that it is capable of further modification, uses and/or adaptations, following in general the principal of the invention, and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains, and as may be applied to the essential features herein before set forth, as fall within the scope of the invention or the limits of the appended claims. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.

Claim 1:
An oil-lubricated helium compressor system configured to supply gas to an expander operating at cryogenic temperatures, the compressor system comprising:
an air cooled compressor (<NUM>) having a first supply side (<NUM>) and a first return side (<NUM>);
a water cooled compressor (<NUM>) having a second supply side (<NUM>) and a second return side (<NUM>);
a gas supply manifold (<NUM>) connected to the respective supply side (<NUM>) of each compressor (<NUM>, <NUM>) and a high pressure side of the expander;
a gas return manifold (<NUM>) connected to the respective return side (<NUM>) of each compressor (<NUM>, <NUM>) and the low pressure side of the expander;
check valves (<NUM>, <NUM>) configured to prevent gas from flowing from either compressor (<NUM>, <NUM>) into the gas return manifold (<NUM>);
a controller configured to control the cross-over from one compressor to the other; and
a plurality of sensors configured to detect critical operating parameters connected to the controller and configured to provide critical signals to the controller that are used to determine when to switch from the one compressor to the other such that the operation of the expander is not affected, wherein the sensors include temperature sensors for measuring one or more of the following temperatures: temperature of oil out of a water cooled after-cooler (<NUM>), of oil out of an air cooled after-cooler (<NUM>), of helium discharged out of the compressors (<NUM>), of oil leaving the compressor (<NUM>), of water (<NUM>) in and out of a water cooled after-cooler (<NUM>), and indoor and outdoor air temperatures.