Flash gas bypass for improving the performance of transcritical R744 systems that use microchannel evaporators

Stefan Elbel, Pega Hrnjak

Research output: Contribution to journalArticlepeer-review


The performance of transcritical R744 systems with direct expansion (DX) can be significantly improved by implementing a Flash Gas Bypass (FGB). The idea behind the concept is to bypass refrigerant vapor, created during the isenthalpic expansion process, around the evaporator. By feeding the evaporator with liquid refrigerant, pressure drop is reduced and refrigerant distribution is improved. With R744 as the working fluid, increased refrigerant-side heat transfer coefficients are expected as well. In addition, the FGB concept proves to be beneficial in terms of system design, in particular for combined air-conditioning and heat pumping applications. An experimental comparison to a conventional DX-system reveals that FGB increases the cooling capacity and COP at the same time by up to 9 and 7%, respectively. Even larger improvements are possible in case a variable speed compressor is utilized to match the performance of the conventional DX-system. A simulation model helps to separate the individual improvement mechanisms. It was found that the reduction of refrigerant-side pressure drop is the dominant improvement mechanism of FGB.

Original languageEnglish (US)
Pages (from-to)724-735
Number of pages12
JournalInternational Journal of Refrigeration
Issue number7 SPEC. ISS.
StatePublished - Nov 2004


  • Carbon dioxide
  • Design
  • Expansion
  • R744
  • Refrigerating circuit
  • Transcritical cycle

ASJC Scopus subject areas

  • Building and Construction
  • Mechanical Engineering


Dive into the research topics of 'Flash gas bypass for improving the performance of transcritical R744 systems that use microchannel evaporators'. Together they form a unique fingerprint.

Cite this