A review of the literature shows no data for horizontal two-phase one-component flow with evaporation; therefore, visual studies have been made of the flow patterns of evaporating R-22 (commercially pure dichloro-difluoro-methane) inside horizontal tube sections under pressure, temperature, flow, and heat load conditions simulating actual operation of a small air-conditioning coil. Description of the patterns observed are given along with data to use in checking with volume flow fraction, Froude number flow regime maps and with maps using Baker’s [1] parameters. A discussion on the qualitative relation between coefficients, the type of flow, effect of return bends, and flow instabilities is included.