Charge minimization in hydrocarbon systems
This paper was presented at the IIR Gustav Lorentzen Conference in Glasgow (Sep. 2004). It presents the results of the project to minimize charge in a R290 system that has 1-1.5 kW refrigeration capacity. Currently, there are many guidelines within industry to limit the charge of flammable refrigerants to less than 150g. This charge limitation is suitable for small applications such as household refrigerators, but for larger applications the maximum charge constraint is an obstacle. Therefore, in order to make flammable or toxic refrigerants useful for systems larger than a refrigerator, it is necessary to reduce their charge below the levels necessary to warrant a toxic or flammable situation in the case of a system leak. Another constraint introduced with the reduction of charge in a system, is the maintaining of a baseline thermal performance of the system including capacity, compressor power and COP. As tubes and components are made smaller, the capacity and COP can suffer due to a higher pressure drop even though the charge is reduced. The only way to make a system smaller and know if it has maintained a set of performance characteristics is to develop a baseline system with which new systems can be compared. A low charge hydrocarbon refrigeration system (less than 130g of propane and producing between 1 and 2 kW of cooling capacity) was built by using microchannel heat exchangers. An experimental analysis of the system charge was also completed showing the charge distribution among the various components in the system for a variety of operating conditions. The largest single component charge amount was found to be within the compressor, then the condenser and then the evaporator. The use of microchannel tubes reduced the total system charge from approximately 200g in conventional systems to less than 130g. Further analysis including a comparison between various microchannel HX options and compressor designs is underway. The analysis and construction of new prototypes indicate that in the next step it could be possible to go below 50g/kW at 1kW basis, as the ability to design lower charge heat exchangers as well as to use compressors with less oil and higher void fractions makes it possible to reduce the charge from the experimental baseline of 130g to less than 60g without drastically redesigning any components, but simply by using the internal volumes. Source: 6th IIR Gustav Lorentzen Natural Working Fluids Conference, Conference papers from Day 1 Sessions 1&2, 2/C/16.30. Authors: M. Hoehne, P. Hrnjak. You can order the proceeding CD-ROM of the Glasgow conference by clicking here.