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Show 3. DEPOSITION TUBE BANK 3.1. Deposition Probes The first tube bank we selected is shown in Fig.5. The bank has to operate as a deposition probe when inserted into the flue gas. It consists essentially of 12 staggered tubes of austenitic steel (Cr 18%, Ni 8%, Mo 3%) 32 mm outer diameter into which a few chromel-alumel thermocouples are fitted to measure the skin temperature at the lead~ng and trailing edge of the tube. The surface temperature, and hence the heat flux, is controlled by cooling air such that it does not exceed 700°C, which represents the upper temperature experienced by superheaters in the zone directly above the boiler nose. Due to the supplying compressor which cuts in and out and to the many tubes, it was diffult to keep surface temperature and heat flux under control in order to obtain ash deposits whose deposition history could be referred to a precise gas and surface temperature. Therefore, we designed the more instrumented deposi tion probe, 't:ith less exposed surface, shown in Fig.6. It consists of two tubes, each fitted with 8 chromel-alumel thermocouples within the wall thickness of the tube four by four at different heights 50 mm apart at 90° distance. At the end of a single run the deposits left in-situ are collected and analyzed together with the ash dislodged during operation collected in the ash hopper beneath the tubes. 3.2 Heat Transfer Measurements and Video Camera Recording The heat flux due to the combined radiation/ convection to the tubes is measured and adjusted during the test. We could chose to operate at constant heat flux or at constant skin temperature. In fact it is not possible to have both at a constant value because the build-up |
