Air-water chiller for external installation with a twin-screw inverter compressor, suitable for large users of air conditioning in buildings for residential, commercial and industrial use. NSMI is a single or dual circuit chiller loaded with R134a gas, axial fans, one or two twin-screw compressors, one of which has an inverter, a shell and tube exchanger, electronic valve and "V block" microchannel coils. The units are designed to provide high yield at full load, and very high efficiency at partial loads, with the increasingly optimised compression yield of the inverter compressor. It is available in the high efficiency and high efficiency silenced version; some available options include a desuperheater, for the recovery of part of the resulting heat during cooling, inverter fans, single or twin pumping units, and any special treatments or operations of external coils for use in aggressive environments.