CRITICAL IMPACT ANGLE FOR MITIGATING DEPOSITION OF SKIM MILK POWDER ON TUBES IN CROSS-FLOW

Particulate fouling on the gas side of heat recovery equipment is a common industrial problem and its occurrence drastically degrades the thermal and hydraulic recuperator performance. The aim of this study is to characterize the deposition of Skim Milk Powder (SMP) on a single bare tube in cross-flow. Tubes tested include: round, elliptical, and turned square tube. A custom built rig is applied to simulate exhaust air conditions that is experienced in an exhaust exchanger. Results show that deposition mostly occurs on the front faces of the tubes with the rate of deposition decreasing through the duration of the test until equilibrium is reached. For a constant air flow rate, increasing particle stickiness resulted in greater deposition coverage around the front of the round and elliptical tubes whereas the turned square tube tended to be either clear or covered. On the round and elliptical tubes, the frontal deposit formed an apex shape. At equilibrium, the amount of deposition coverage on the tube varied depending on the tube shape, particle stickiness, and air velocity. Where deposition coverage stopped, the normal angle to the surface was calculated and has been shown to be well correlated to stickiness and less dependent of tube shape. INTRODUCTION Heat exchanger fouling is costly and affects a wide range of industries (Bott, 1995). Numerous studies have looked at the thermal and hydraulic impacts of fouling in many industrial settings, such as boilers, power stations, Heating, Ventilation and Cooling systems, to name only a few. In many situations, fouling can significantly degrade the thermal and hydraulic performance of heat exchangers. As fouling initiates and grows, it provides additional resistance to fluid flow increasing the system pressure drop and placing added electrical load on fans or pumps to maintain a constant flow rate. A fouling layer is also a barrier to heat transfer degrading the overall heat transfer film coefficient and exchanger duty. A simple technique to reduce fouling has been to increase the fluid velocity resulting in higher fluid shear stresses at exchanger walls. For example, Stehlík (2011) reports that inserts have been placed between tubes in a tube bank as an attempt to increase fluid velocity and move the separation point of the fluid from the tube closer to the rear of the tube. Abd-Elhady et al. (2004) developed the concept of a dimensionless rolling moment, which is defined as the hydrodynamic rolling moment divided by the adhesion resting moment, to calculate a limiting fouling velocity that theoretically predicts the onset of fouling. Estimates of the limiting fouling velocity for copper particles have been shown to be in close agreement with lab-scale experimental results (Jegla et al., 2010). However increasing velocity as a way to reduce fouling can dramatically increase the fluid pressure drop significantly increasing electrical load on fans and pumps. Various heat exchanger geometries have been used to reduce fouling without adversely affecting the ratio of heat duty to pressure drop. For example, Zhang et al. (1992) investigated the application of a front row of “spoiler” tubes as a way to reduce deposition without significantly affecting heat exchanger pressure drop. A spoiler tube row is a nonheat transferring row of tubes placed in front of a tube bank, specifically designed to increase air turbulence and cause localized jetting. Recently, Abd-Elhady et al. (2011) demonstrated the effectiveness of using cone-shaped (a round tube with an apex on the front) tubes for reducing fouling yet increasing the heat transfer to pressure drop ratio. Paz et al. (2012) focused on fouling in diesel exhaust gases and showed the close relationship between wall shear stress and deposition. Using this link between wall shear stress and fouling, Walmsley et al. (2012b) developed Computational Fluid Dynamics (CFD)models to examine a wide range of tubes for their fouling, heat transfer and pressure drop characteristics. Deposition of particles may result from a combination of several discrete transport processes and mechanisms (Guha, 2008). These processes are largely dependent on the dimensionless particle relaxation time (t), which is a function of the particle’s density and radius and the fluid’s density, viscosity, and wall shear stress. The particle relaxation time represents the time scale with which the particles respond to changes in the slip velocity. Typically, three general transport regimes are identified: (1) turbulent diffusion (t < 0.1), (2) turbulent diffusion-eddy impaction (0.1 < t < 10) and (3) particle inertia moderated (t > 10). In the turbulent diffusion regime, deposition is a mass transport problem described by Fick’s law. The second regime is a transitional regime whereas particles that fall in the particle inertia regime respond slowly to changes in velocity flow field changes as indicated by the large t. Unpublished work by the authors has indicated that for milk powder deposition in heat exchangers, the dominant Proceedings of International Conference on Heat Exchanger Fouling and Cleaning 2013 (Peer-reviewed) June 09 14, 2013, Budapest, Hungary Editors: M.R. Malayeri, H. Müller-Steinhagen and A.P. Watkinson