Heat exchangers particle deposition

The conical coal injector was replaced with a blunt cyclin-der with a single axial jet so the flame could be stabilized at lower swirl numbers, thereby reducing the centrifugal deposition on the furnace walls. The radiation shield between the combustor and heat exchanger was removed to reduce particle losses further. The increased radiative transfer decreased the wall temperature substantially. The later experiments were also carried out at lower fuel-air equivalence ratios, i.e., (J> = 0.57. The combination of increased heat losses and increased dilution with excess air reduced the maximum wall temperature to 990°C for the experiments reported below. 

X-Ray powder diffraction patterns are catalogued in the JCPDS data file,7 and can be used to identify crystalline solids, either as pure phases or as mixtures. Again, both the positions and the relative intensities of the features are important in interpretation of powder diffraction patterns, although it should be borne in mind that diffraction peak heights in the readout from the photon counter are somewhat dependent on particle size. For example, a solid deposit accumulating in a heat exchanger can be quickly identified from its X-ray powder diffraction pattern, and its source or mechanism of formation may be deduced—for instance, is it a corrosion product (if so, what is it, and where does it come from) or a contaminant introduced with the feedwater ... 

Thermal deposition of particles inside boilers or heat exchangers can lead to reduced efficiency of the units (Fuchs, 1964) whereas... 

The alkali silicates and sulphates so produced have melting points as low as 700 C, and tend to deposit on the reactor walls or on the heat exchange surfaces, when conventional grate-fired systems are used whereas, in the case of fluidised bed reactors, they significantly contribute to bed sintering and defluidisation of the inert material through the development of a sticky deposit on the surface of the bed particles. 

The deposition of particles can occur in heat exchangers both in liquid streams and in processes involving the flow of gases. The origin of the particles is extremely diverse. They may already be in the fluid being processed, unintentionally produced as a result of the process itself, or corrosion products from upstream. 

Example In certain types of heat exchangers, a gas flows normal to a bank of tubes carrying fluid at a different temperature, and heat transfer occurs at the interface. Fouling of the outside surface of the tubes by particles depositing from the flow reduces the heat transfer rate. If the tubes are I in. (outside diameter) and the gas velocity is 10 ft/sec, estimate the diameter of the largest particle [pp — 2 g/cm ) that can be permitted in the gas stream without deposition by impaction on the... 

Fig. 7.S is based on the data of Fig. 7.4 and plots the mass transfer coefficient for deposition at different velocities for 10 particles these data neglect Brownian motion. The conditions represented on Fig. 7.5 are idealised conditions that could apply to an air blown heat exchanger. The figure shows that there is a rapid increase in mass transfer coefficient for deposition as velocity increases up to around 3 m/s. Above this velocity there is a reduced but steady, rate of increase of mass transfer coefficient with velocity.

Where fine particles are likely to be present in a fluid passing through heat exchange equipment subject to a high temperature difference, it is necessary to be aware that the diffusional and inertial deposition of particles is likely to be augmented by thermophoresis effects. 

The temperature and flow conditions within the heat exchanger will determine the location at which these various stages occur. For instance the supersaturation and crystallite formation may occur in the bulk fluid with the growing crystals moving towards the wall to form the deposit. The movement of foulant will under these circumstances, follow the processes described in Chapter 7 for particulate deposition. It is possible that due to the level of turbulence within the system, that some (or possibly many) of the crystallites formed are swept into re ons where the solution is not supersaturated. Under these conditions the particles will redissolve. On the other hand crystallisation may occur near or at the heat transfer surface. The presence of nucleation sites on a solid surface may encourage the formation of scale on the surface. Under these circumstances the process is largely governed by the mechanics of the crystallisation process. 

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