A　rotary　booster　driven　free-cooling　unit　using　ambient　cold　was　proposed　for　small　and　medium　sized　data　center　free　cooling.　The　influences　of　heat　exchanging　area,　refrigerant　charging　quality　and　condenser　fan　frequency　on　cooling　capacity,　rotary　booster　power　and　energy　efficiency　ratio　(EER)　were　studied　experimentally　under　different　working　conditions.　Results　indicated　both　the　cooling　capacity　and　EER　increase　with　the　refrigerant　charging　quality.　With　the　increase　of　indoor　and　outdoor　temperature　difference　?t,　the　rotary　booster　power　reduced　to　about　0.6　kW　while　the　temperature　difference　was　25?.　And　the　EER　was　improved　by　appropriately　reducing　the　frequency　of　the　condenser　fan.　When　the　refrigerant　charging　quality　was　5　kg　and　condenser　fan　frequency　was　30Hz,　the　cooling　capacity　of　free-cooling　unit　with　dual　heat　exchangers　was　20.2　kW　and　the　EER　was　18.4　when　the　indoor　and　outdoor　temperature　difference　was　25?.　©　2019　International　Institute　of　Refrigeration.　All　rights　reserved.