Miniature Long Life Stirling Cryocoolers
Honeywell Hymatic
Redditch, Worcestershire, United Kingdom B98 9HJ
C R Aubon Technical Manager
SUMMARY
This paper describes the life testing that has been conducted on Stirling cryocoolers that have been designed and built by Honeywell Hymatic over the last 12 years. The information includes both in house and customer evaluation data.
INTRODUCTION
One of the main issues for the designer of an infrared imaging instrument or system has been the choice of cryogenic cooling, particularly in the case of high performance systems. Whilst liquid nitrogen has proved suitable for static systems such as laboratory instruments and Joule Thomson cryostats have been successful in both ground based and air carried military equipments, both bring a logistical burden which is inconvenient for the operator. Infrared sensing has been applied to industrial process control and this market is expanding as a result of the drive for price and quality competitiveness as well as the tightening regime of environmental legislation.
Over the last decade a range of important devices based on High Temperature Superconductors has emerged from the research opening up new applications in sensing and telecommunications. Depending on the selected material the operating temperature will be in the range from 60K to 110K.
Sensors for other parts of the electromagnetic spectrum have also require cooling to cryogenic temperature.
A mechanical refrigerator, such as those using the Gifford-McMahon and Stirling cycles provides an attractive alternative to bulk liquid cryogen and Joule Thomson (JT) systems, requiring only an electrical supply. The attraction is offset by the size and cost of acquisition in the case of G-M machines and, previously by the short life and poor reliability of Stirling cryocoolers. Over the decade since 1990 Stirling coolers from a range of manufacturers have come to maturity and both life and reliability extended to levels acceptable for many military applications, typically 4000 to 10000 hours of operation. However for applications in process control and communications an even longer life is necessary with an expectation of 5 years minimum of installed and continuous operation. This brings the industrial cryocooler up to the life and reliability standards hitherto only achieved in "space" coolers.
The range of cryocoolers which are the subject of this paper provide an optimum solution for the system designer and user by offering very long life and high reliability within a small package and at a modest cost of acquisition. Their design emanates from spaceflight coolers designed and flown by the Rutherford Appleton Laboratory at The University of Oxford. This work was taken forward by the Department of Engineering Science at the University.
Background
Honeywell Hymatic has been one of the major developers and suppliers of JT cryostats and their support systems for over 40 years and has an international reputation in that field.
The University of Oxford has a long background in research and manufacture of long life Stirling coolers particularly for space applications. They also have an international reputation in the Stirling coolers, particularly associated with linear, long life designs.
The Honeywell Hymatic cryocoolers result from a long term technology license agreement and close collaboration over 15 years between The University of Oxford and Honeywell Hymatic. All use linear, non contact, compressors suspended on spiral springs. This is a feature in common with the Oxford spaceflight coolers.
DESIGN FEATURES FOR LONG LIFE
The Honeywell Hymatic design addresses each of the failure mechanisms that limit the life of Stirling cryocoolers.
Wear The compressor achieves a dynamic pressure seal by the maintenance of very small, non contacting clearances between the piston and cylinder. Light contact can occur between the displacer and cold finger at the warm end in the region of the flange. The rubbing pair in this case are a loaded, high temperature plastic and stainless steel. The absence of contact between the cylinder and compressor eliminates this prime cause of degradation which can result from rubbing contact.
Fatigue The only components in the design that are subject to fatigue are the flat spiral springs that suspend both the moving cylinder of the compressor and the warm end of the displacer. Careful design ensures that these springs are subjected to a stress much lower than their high cycle fatigue limit thus giving unlimited life. Qualification testing and life testing has proved the design on many sizes of springs. Lot testing at high frequency ensures that the requirements of the component design are maintained on a batch to batch basis.
Charge gas leakage Honeywell Hymatic cryocoolers have three static pressure seals which are hermetically joined using electron beam welding. The final seal, which is made after the cryocooler has been charged, is produced using a metal to metal (cold weld) seal. This sealing technology has been well proven by Honeywell Hymatic on long life bottle systems at pressures up to 600Bar (10,000psi) which now have demonstrated lives of more than 20 years. In practice it is possible to measure external leakage at the level that is required by using modern leak measurement technology.
Contamination The design principles and processes developed by Honeywell Hymatic to achieve high reliability in their JT product range have been used as the basis the cryocooler development. The cryocooler is less sensitive to contamination than a JT expansion orifice. Careful selection of materials which are then processed by chemical cleaning and vacuum bake out at the highest practical temperature is the key. This is coupled with stringent care of components once they have been cleaned. The same principles are applied to the completed cryocooler prior to charging.
AVAILABLE RELIABILITY EVIDENCE
The ‘Oxford’ Type of Cryocooler
The primary technical differentiation of the Honeywell Hymatic cryocooler is that of life. The available evidence gives confidence in an operating life in excess of 5 years. The pedigree of the design begins with the ‘space’ cryocoolers designed at Oxford University and various licensees. An analysis by JPL of these devices showed that a cumulative total of 500,000 running hours had been completed by 21 units, with 3 of these having been terminated due to a degradation in performance assigned to contamination. Some of these life tests are ongoing.
Experimental prototype
The first Stirling cooler built by Honeywell Hymatic was an Oxford designed 200K, 500mW air filled model. The aim was to establish that a simplified design and assembly process evolved by Oxford did provide the basis for translating the technology used in long life space cryocoolers to the needs of a volume market. A batch of seven coolers were built with varied assembly tolerances in order to establish the key features of the design. The work was undertaken in 1989/90.
This design, as a development prototype was not hermetically sealed, instead using bolted joints with silastomer seals. The working fluid is air and leakage is surprisingly low.
Two of these sample coolers were set running, in the laboratory, at the end of 1990 and for the first two years the performance was logged. This showed no sign of degradation. Since then the pair have been left running, with occasional switch off, and each accumulated approximately 100,000 hours of running before the test was stopped at which time the coolers were running satisfactorily. Being not hermetically sealed, it has been necessary to occasionally top up their charge gas (air). This has been roughly once a year.
Development Cryocooler
The second stage of development was to design and build helium filled cryocoolers with a nominal cooling power of 500mW at 80K. The aim was to establish the design features and processes for a hermetically sealed, helium filled cryocooler. The design is a monobloc with a 6 mm diameter coldfinger. The external sealing is electron beam welded stainless steel.
This work was undertaken in 1993 and a batch of five were built. Two cryocoolers from this batch were assigned to long term testing in laboratory conditions. They had been fully assembled with the three external joints electron beam welded. For the first 6,400/5,400 hours the charge connection was not finally sealed in case some contamination occurred. As no problem was found the cryocoolers were finally charged and sealed in 1994 and have, at the 24 April 2001, had run for respectively 52463 and 53702 hours with full data logging. No performance degradation is discernable. The running has included 52 power up routines one of the pair has been subjected to typical cyclic yearly temperatures of –27ºC to +40ºC.
200mW Monobloc Production Cryocooler
This design delivers more than 200 mW of cooling at room temperature and more than 100 mW at 70ºC, both measured with a cold tip temperature of 80K. The cryocooler is suited to integrated dewar assemblies (IDCA). Construction is similar to the 500mW development model.
Honeywell Hymatic's demonstration cryocooler, which is portable and used for customer presentations, has been logged. Because of its use the duty has been varied subject to both long and short term periods of running both at Honeywell Hymatic and around the world. At November 2003 it has run for more than 54000 hours with 263 power up cycles. Cold end temperature is monitored and has remained steady at 75.7/75.9K throughout the period but input power has not been measured. No Joule heat load has be introduced but the cooling load is increased by the low cold end temperature.
1 Watt Split Production Cryocooler
This new product line draws on the design heritage of the 200mW monobloc design by utilizing the compressor and coldfinger configurations but arranged to ease manufacture. Twin opposed compressors are mounted on a single centre plate that acts as the mounting and provides connection to the coldfinger.
Every effort has been made to facilitate high volume production whilst at the same time retaining the features, demonstrated on previous designs, that ensure very long life.
Further details on the interface and performance of this cryocooler can be viewed on the cryocooler datasheet.
SUMMARY
The above information supports 20000 operating hours life warranty given to Honeywell Hymatic Cryocooler products and also supports the expectation of operating lives well beyond 5 years.
The ultimate failure mechanism is not known and the intention is to continue life testing, to failure and increase the number of samples and forms of test. It is reasonable to expect that an operating life of 10 or more years is achievable.
Product is now being shipped to programmes where the cooler is likely to run continuously and an operating life of 5 years is the minimum that will be acceptable. An affordable cryocooler with an operating life beyond 5 years is now a reality.