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272 R. M. Lazzarin Figure 12. Comparison between a microchannel heat exchanger and a conventional tube. This effort has continued recently with microchannel heat exchange surfaces (Fig. 12) where the refrigerant charge can be reduced (by more than 40%) and perfor- mance improved keeping the same volume of heat transfer surface. Other efforts were devoted to the CFC substitutes both toward fluids such as R410A and natural refrigerants, particularly propane, ammonia and above all carbon dioxide. Perhaps the most outstanding innovations were related to compression and expan- sion processes from condenser and evaporator (and vice versa). As far as compres- sion is concerned, a two stage process was realised, particularly for fluid characterised by high pressure lift such as carbon dioxide. A possibility was the injection of liquid refrigerant during the compression. This is possible both in screw and centrifugal compressors (Fig. 13). For operation at partial loads a system less expensive than inverter speed modula- tion was conceived. It operates on scroll compressors under the commercial name of digital scroll. A simple lifting of 1 mm of the fixed scroll stops the gas supply that is bypassed between inlet and outlet of the compressor (Fig. 14). The lifting allows modulating alternating period of full compressor load with periods of by pass by 20 seconds cycles. The full compressor load can extend from 20 seconds (maximum capacity) to 10 seconds (half capacity) till 2 seconds (20% capacity) thus modulating in a simple and reliable way. Downloaded from https://academic.oup.com/ijlct/article/2/3/262/758006 by guest on 21 June 2022 International Journal of Low Carbon Technologies 2/3PDF Image | Technological innovations in heat pump systems
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