Heat exchanger surface condition effect

J. D. Bernal (London) In my opinion many, if not all, so-called oxide covered surfaces are, at ordinary and even under high temperature conditions, effectively hydroxyl-covered. I would like to recall in connection an observation many years ago by Farkas. He was filling an evacuated and baked out silica apparatus with pure Da gas and noted after pumping out that the gas was contaminated with Ha. On repeating the same process with pure R2 he found a smaller Da contamination. The simplest explanation was that the surface of silica is normally covered by OH groups from which the hydrogen is not driven even at red heat, but which can exchange easily with Da to form OD and HD. Other x-ray evidence on finely divided quartz shows that the hydroxide layer may be thicker than monomolecular. 

Figure 3.9. Steam heaters, (a) Flow of steam is controlled off the PF outlet temperature, and condensate is removed with a steam trap or under liquid level control. Subject to difficulties when condensation pressure is below atmospheric, (b) Temperature control on the condensate removal has the effect of varying the amount of flooding of the heat transfer surface and hence the rate of condensation. Because the flow of condensate through the valve is relatively slow, this mode of control is sluggish compared with (a). However, the liquid valve is cheaper than the vapor one. (c) Bypass of process fluid around the exchanger. The condensing pressure is maintained above atmospheric so that the trap can discharge freely, (d) Cascade control. The steam pressure responds quickly to upsets in steam supply conditions. The more sluggish PF temperature is used to adjust the pressure so as to maintain the proper rate of heat transfer.

Attack by organisms other than SRB. Ammonia and amines are produced by microbial decomposition of organic matter under both aerobic and anaerobic conditions (ammo-nification). (Stott)5 These compounds are oxidized to nitrite by aerobic bacteria such as Nitrosomonas or Nitrobacter species. Nitrobacter is very efficient at destroying the corrosion-inhibition properties, of nitrate-based corrosion inhibitors by oxidation, unless a biocidal agent is included in the formulation. The release of ammonia at the surfaces of heat-exchanger tubes has a detrimental effect. (Stott)5... 

Additional parameters specified in the numerical model include the electrode exchange current densities and several gap electrical contact resistances. These quantities were determined empirically by comparing FLUENT predictions with stack performance data. The FLUENT model uses the electrode exchange current densities to quantify the magnitude of the activation overpotentials via a Butler-Volmer equation [1], A radiation heat transfer boundary condition was applied around the periphery of the model to simulate the thermal conditions of our experimental stack, situated in a high-temperature electrically heated radiant furnace. The edges ofthe numerical model are treated as a small surface in a large enclosure with an effective emissivity of 1.0, subjected to a radiant temperature of 1 103 K, equal to the gas-inlet temperatures. 

The thermal environment is sometimes very complex. Convection, radiation and conduction are the common means of heat exchange and they vary independently over time and location. The final effects on the surface heat exchange of the human body are important factors for heat balance and for perception of the thermal conditions. Assessment of the thermal environment in a modern office or a car can create difficulties due to the complex interaction of the ventilation system with the situation close to the person and the external, environmental factors (e.g. radiation, air temperature and air movements). Furthermore, measurements in reality, as well as in the laboratory, contain various methodological problems. In this chapter some important aspects of dynamic water vapour and heat transport through fabrics are discussed. 

---