Experimental work on single tubes, twin tubes and a bundle of 18 tubes is reported. With the results for single tubes, bundle-factors can be formed which allow an estimate of bundle heat transfer coefficients from single tube experiments. For twin tubes the effect of increased on flow velocity to the upper tube is shown. For the tube bundle the effects of tube pitch, ratio, tube arrangement and number of heated rows upon the bundle factor are presented. An equation for the bundle factor is suggested. For theoretical predictions a procedure is presented which is based on data from literature only. The agreement with experimental data is within ±25 %.