Improving Temperature Uniformity at –86 °C in a ... - Biomedical

T E C H N I C A L A R T I C L EFigure 1 The SANYO cascade refrigeration system employs twoindependent refrigeration circuits indirectly connected by an interstageheat exchanger. The company’s proprietary integrated lubricating oilcoolingsystem (patent pending) automatically apportions the workloadbetween compressors and permits both compressors to operate well withinthe expanded performance envelope. 1) Freezer cabinet with evaporator:The evaporator coil is strategically wrapped around the interior chamberand concealed within the composite wall of vacuum insulation panelsand conventional foamed-in-place urethane insulation. 2) Low-stageheat exchanger: Energy is absorbed by the refrigerant gas and transferredto the low-stage heat exchanger to cool discharge gas. 3) Low-stagecompressor: The compressor pumps refrigerant through the low-stagecircuit. 4) Low-stage oil reservoir: High-stage refrigerant passes throughthe low-stage oil sump to cool lubricating oil, resulting in high-stagecompressor energy being used to minimize the workload on the low-stagecompressor. 5) Interstage heat exchanger: Energy is transferred to thehigh-stage circuit. 6) Low-stage capillary tube: Liquid refrigerant underpressure is passed through the capillary tube where it flash-evaporates inthe low-stage evaporator to absorb energy (heat) from the product storedin the freezer. 7) High-stage compressor: The compressor pumps refrigerantthrough the high-stage circuit. 8) High-stage oil reservoir: High-stagerefrigerant passes through the high-stage sump to cool lubricating oil enroute to the low-stage compressor through the air-cooled precondenser.9) Air-cooled precondenser: Removes energy (heat) from the high-stagerefrigerant en route to the low-stage compressor. 10) Main condenserand motor/fan assembly: The motor/fan assembly blows ambient airacross condenser coils to move energy (heat) from the high-stage refrigerantto the ambient environment. 11) High-stage capillary tube: Liquidrefrigerant under pressure is passed through the capillary tube where itflash-evaporates in the interstage heat exchanger to absorb energy (heat)from the low-stage refrigerant circuit. 12) Air-cooled precondenser:Removes energy (heat) from the high-stage refrigerant en route to thehigh-stage oil reservoir. 13) High-stage refrigerant: Commonly availableworldwide. A combination of R134a and R410a (Puron ® ) selected foroptimum cooling performance in compliance with international environmentalprotection laws. 14) Low-stage refrigerant: Commonly availableworldwide, R508. 15) Instrumentation (not shown): Temperature andpressure sensors throughout the high- and low-stage circuits transmitinformation to the Status 3 central controller for operation, monitoring,and interpretation.new, environmentally safe refrigerants. Collaboratingwith industry leaders in refrigeration chemistry,lubrication pathways, natural and synthetic oils, andcabinet insulation technologies, SANYO engineershave created a highly reliable ultralow temperaturefreezer based on the performance of compressorsdoing the work they were specificallydesigned to do.Cabinet designprerequisitesConcurrent with refrigeration system research,SANYO pioneered the development of a compositecabinet wall based on a combination ofconventional, high-density foamed-in-placeinsulation and new, state-of-the-art vacuuminsulation panels (SANYO VIP ® , U.S. patentno. 6,260,377) permitting a thinner wall profileand increased interior volume. This design optimizesuse of available laboratory space by allowingmore storage in the same footprint.Since the original VIP design was introduced,company researchers developed proprietaryimprovements in the open cell panel technique,creating better matrices to support a sustainablevacuum, and giving manufacturing engineersmore latitude in composite orientation withconventional foam. As a result, evaporator coilswithin the thin-wall cabinet are arranged foroptimal interior uniformity and best heat removal(energy transfer), further reducing the burden onthe cascade refrigeration system. A byproductof the design permits a hot gas bypass to circulatearound the peripheral edges of the cabinetdoor gasket, warming the gasket to mitigate thebuildup of moisture to prevent ice formation ormold on external services (see Table 1).Vertical componentintegrationThe concept of vertical component integrationis central to the company’s entire product line.SANYO manufactures its own oil separators,circuit boards, and vacuum insulation panels,and designs its own compressors. This integratedsupply chain ensures component quality fromsource to application, and permits the companyto evaluate and improve its own componentswithout third-party involvement.Together with other refrigeration system componentssuch as filter dryers, heat exchanges,condenser and evaporator tubing, meteringdevices, motor windings, and sophisticatedmicroprocessor-based electronic controls, thecompany has created a holistic solution to biologicalpreservation.Key compressor componentsEach component of the SANYO compressorhas been electronically designed and modeledin stereolithographic beta form to exceedactual operating conditions. Pistons, connectingarms, valve plates, and wrist pins aredesigned to handle high load capacities. Alltesting is performed in a +35 °C ambient.*Proper delivery and return of lubricants is a key factorin extending component life. Compressor motor sizingis predicated on refrigerant flow as well as energyrequired for initial pull-down, and then sustainedultralow temperature with reserve capacity. Motorwindings are configured to accommodate fluctuatingelectrical supplies in many institutional settings. Atthe same time, SANYO’s commitment to “greenproducts” is expressed through more efficient motoroperation with reduced energy consumption.A better compressor yieldsbetter uniformity andimproved reliabilityHeat from multiple sources contributes to compressorwear. Heat is generated by compressors working tocompress low-density refrigerants required in the lowstage of the cascade loop. Additional heat is absorbedby room-temperature product placed into the freezer,as well as migration from the ambient environment.In the SANYO research and development laboratory,prototype compressors were tested under harshenvironmental conditions to exceed actual freezeruse in typical laboratories. Because the companycontrols its own compressor design, all amendmentsand reengineering options were explored as needed,with new prototypes brought into test quickly. Lifetesting and tear-downs delivered critical data toSANYO engineers, permitting beta test results to besynthesized into the complete design program in supportof the global ultralow freezer program.Design mandate: reducedischarge temperatureThe ultralow temperature compressor employs anorientation of conventional components to reducedischarge temperatures and compressor heat whileusing commercially available refrigerants and lubricants.Heat reduction results range from as low as 25°C below previous SANYO compressors and morethan 40 °C below leading-brand compressors used bynumerous competitors.At the heart of the solution is a compressor oilcoolingloop that reapportions the working heatbyproduct between the low-stage compressor and thehigh-stage compressor. Due to low molecular weightsin low-stage refrigerant formulations, low-stage compressorsmust work harder to achieve cooling targets.The SANYO technique uses existing lubricating oilto cool the low-stage compressor, passing the resultingheat load to the high-stage compressor, which,by design, is already doing less work.By shifting a portion of the burden from the lowstage to the high stage, the load on both compressorsis balanced while reducing operating pressuresand keeping heat loads and discharge temperatureswell within tolerances required to prevent chemicalbreakdown of oils and refrigerants. As a result,refrigeration capacity is expanded and structuralengineers have more latitude in strategic applicationof evaporator coils around the interior chamber, akey to temperature uniformity and, ultimately, tocell viability.Applying the benefitWith lower compressor discharge temperatures and pressures,newer refrigerants can be more effective. Combinedwith VIP insulation, the migration of ambient*Testing and evaluation: Consumer Advisory Performance testing and published data on SANYO ultralow temperature freezers is based on extensive testing in a +35 °C ambient to simulate worstcaseconditions. When comparing performance of competitive ultralow freezers, it is important to establish and verify conditions under which tests are performed. Tests performed at temperaturesbelow +35 °C may exhibit lower compressor discharge temperatures; these discharge temperatures are not typical of real-world conditions, however, and should be used with caution when evaluatingnew or replacement freezer selection. For summary information on test conditions contact SANYO (Table 2).00 / SEPTEMBER 2007 • AMERICAN BIOTECHNOLOGY LABORATORY