For Optimised Flow
Background: The Velocity Distribution Issue - Design vs Reality
The thermal and hydraulic design of the heat exchanger is typically developed with the assumption of a uniform velocity distribution of the process stream at the inlet faces. In practice, however, the velocity distribution at the inlet to the heat exchanger is not uniform, and is dictated mostly by such factors as the pumping fan’s characteristics, its distance from the heat exchanger and the ductwork’s shape and dimensions between these two.
Due to the non-uniform velocity distribution of the stream to the heat exchanger (in some cases the local velocity can be more than twice higher than the mean design velocity), the heat transfer surface temperature is affected, being either sub-cooled below the design value, or super-heated over the design value. Typically, this issue leads for example to corrosion problems in the “cold-end” of the Air Preheaters due to sub-cooling of the heat transfer surface below the Acid Dew Point of the flue gas.
Figure 1 presents a comparison of the heat transfer surface temperature distribution at the flue gas outlet face of the Air Preheater. The picture on the left presents the ideal design case, where the combustion air velocity distribution to the Air Preheater is uniform. The picture on the right presents the
simulation of the actual field case, where the combustion air velocity distribution to the Air Preheater is non-uniform. The comparison shows that in the field case the heat transfer surface minimum temperature is ~30% lower than the design minimum temperature, which results in condensation of the acid on the heat transfer surface and corrosion.
Figure 1. Heat transfer surface temperature profiles at the flue gas outlet face of the Air Preheater; Design case (left), CFD simulation of the actual field case (right).
Extensive Laboratory Studies
Engineers at APEX-Research conducted extensive laboratory studies of the Velocity Distribution Issue employing cold flow modelling and laser optical measurement techniques (Laser Doppler Anemometry, Particle Image Velocimetry). The results of the optical measurements were used to define the correct boundary conditions for the CFD (Computational Fluid Dynamics) simulations.
Figure 2 presents a comparison of the CFD simulation (on the left) with the optical measurement (on the right) taken on the scaled model of a Gas-Gas Heat Exchanger with charging duct and forced draft fan. Please notice the qualitative similarity of the results in both approaches.
Figure 2. Comparison of results from CFD (on the left) and optical measurements (on the right).
Uniform Velocity Distribution with APEX-delfino® Flow Conditioner
All these efforts led us to developing the APEX-delfino® Flow-Conditioner – proprietary device used to ensure the uniform velocity distribution of any process stream to the downstream equipment, for example combustion air to the Air Preheater, flue gas to the SCR Catalyst or to the ESP or bag filters.
Comparison Using CFD (Computational Fluid Dynamics)
Figure 3 presents a comparison between CFD simulation of the field actual case (on the left) and the CFD results of the APEX-delfino® Technology employed by APEX-Research engineers (on the right). In the field case, due to the non-uniform velocity distribution of the combustion air stream to the Air Preheater
(maximum local velocity 70% higher than the mean design velocity) the heat transfer elements have been locally sub-cooled and suffered corrosion due to condensation of the flue gas acid. By incorporating the APEX-delfino® Flow-Conditioner and optimizing the shape and size of the charging duct, the stream
velocity distribution has been improved to satisfactory level ensuring no sub-cooling of the heat transfer surface across the complete inlet face of the Air Preheater (maximum jet velocity achieved 10% higher than the mean design velocity).