A wall-jet of a different fluid (or the same fluid at a different thermodynamic state) is blown along the inner surface of a pipe to prevent a contact between the main central flow and the pipe wall. Although motivated by problems encountered in measuring radioactivity, the analogy between heat and mass transfer was utilised for the experiments and concurrent numerical computations which investigated transport of heat from the central air flow towards cool pipe wall, across the protective cool air wall-jet. Similar protective cooling of walls may be actually itself an important application. Another one may be segregation required due to gas chemical aggressiveness. This first part discusses the details of the experiment and the character of the temperature and flow fields evaluated from measured profiles of time-mean velocity and temperature. Analysis of the data, using mutually supporting information from the experiment as well as the computations, is handled in the next, second part. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier BM. All rights reserved.