Tube Wall Temperatures

Catalytic methanation processes include (/) fixed or fluidized catalyst-bed reactors where temperature rise is controlled by heat exchange or by direct cooling using product gas recycle (2) through wall-cooled reactor where temperature is controlled by heat removal through the walls of catalyst-filled tubes (J) tube-wall reactors where a nickel—aluminum alloy is flame-sprayed and treated to form a Raney-nickel catalyst bonded to the reactor tube heat-exchange surface and (4) slurry or Hquid-phase (oil) methanation. 

Elevated temperatures at the boiler tube wall or deposits can result in some precipitation of phosphate. This effect, termed phosphate hideout, usually occurs when loads increase. When the load is reduced, phosphate reappears. 

A cylindrical tube in a chemical plant is subjected to an excess internal pressure of 6 MN m , which leads to a circumferential stress in the tube wall. The tube wall is required to withstand this stress at a temperature of 510°C for 9 years. A designer has specified tubes of 40 mm bore and 2 mm wall thickness made from a stainless alloy of iron with 15% by weight of chromium. The manufacturer s specification for this alloy gives the following information ... 

Fig 13 5 Temperature distribution across the water-tube wall. 

Good heat transfer on the outside of the reactor tube is essential but not sufficient because the heat transfer is limited at low flow rates at the inside film coefficient in the reacting stream. The same holds between catalyst particles and the streaming fluid, as in the case between the fluid and inside tube wall. This is why these reactors frequently exhibit ignition-extinction phenomena and non-reproducibility of results. Laboratory research workers untrained in the field of reactor thermal stability usually observe that the rate is not a continuous function of the temperature, as the Arrhenius relationship predicts, but that a definite minimum temperature is required to start the reaction. This is not a property of the reaction but a characteristic of the given system consisting of a reaction and a particular reactor. 

Figure 9.7.2 Plug-flow reactor simulation. Inside temperature vs. Tube length at various tube wall temperatures, in K ...

Yi = Composition of the condensible component in equilibrium with liquid at tube wall temperature... 

The calculations are made as follows. The exchanger is divided into small increments to allow numerical integrations. A tube wall temperature is first calculated and then QAV. The gas temperature and composition from an increment can then be calculated. If the gas composition is above saturation for the temperature, any excess condensation can occur as a fog. This allows the degree of fogging tendency to be quantified. Whenever possible, experimental data should be used to determine the ratio of heat transfer to m.ass transfer coefficients. This can be done with a simple wet and dry bulb temperature measurement using the components involved. 

A temperature profile of vapor condensing in the presence of a noncondensable gas on a tube wall, as shown in Figure 16 indicates the resistance to heat flow. Heat is transferred in two ways from the vapor to the interface. The sensible heat is removed in cooling the vapor from t to t, at the convection gas cooling rate. The latent heat is removed only after the condensable vapor has been able to diffuse through the noncondensable part to reach the tube wall. This means the latent heat transfer is governed by mass transfer laws. 

Figure 16. Temperature profile showing effect of vapor condensation on a tube wall in the presence of a noncondensable gas.

The bulk fluid temperature at which the fluid properties are obtained should be the average temperature between the fluid inlet and outlet temperatures. The viscosity at the tube wall should be the fluid viscosity at the arithmetic average temperature between the inside fluid bulk temper-... 

The furnace was fitted with interlocks that should have isolated the fuel supply if the tube wall temperature or the pressure of the heat transfer oil got too high. Neither interlock worked, and neither had been tested or maintained. The set-point of the high tube wall interlock had been raised far above its original set-point, from 433°C to 870°C, a simple way of putting it out of action [15]. Changing the set-point of an interlock is a modification and should be allowed only when the equipment is capable of withstanding the new conditions (see Chapter 2). 

Figure 10-28. Tube wall conditions affecting overall heat transfer and associated temperature profile.

Refer to Figure 10-28. The temperature of the outside of the tube wall is based on hot fluid being on the outside of the tubes ... 

The outside tube wall temperature for hot fluid on the inside of the tubes is... 

Often this may be assumed based upon the temperature of the fluids flowing on each side of the tube wall. For a more accurate estimate, and one that requires a trial-and-error solution, neglecting the drop-through tube metal wall (usually small) ... 

Edmister and Marchello present a tube wall temperature equation ... 

Ganapathy presents a shortcut technique for estimating heat exchanger tube wall temperature, which so often is needed in establishing the fluid film temperature at the tube wall ... 

For example, a hot flue gas flows outside a tube and shell exchanger at 900°F (C) while a hot liquid is flowing into the tubes at 325°F (k). The film coefficients have been estimated to be hj = 225°F and h = 16 Btu/(hr) (fti) (°F). Estimate the tube wall temperature using hj as hj corrected to the outside surface for the inside coefficient ... 

This calculation neglects the temperature drop across the metal tube wall and considers the entire tube to be at the temperature of the outside surface of the wall, k. Kem for the same equation suggests using the caloric temperature for k and h. 

This relationship can be used for estimating surface temperature and for back-checking estimating assumptions. For many situations involving liquids and their tube walls, the temperature difference (tg — tg) across the wall is small and equals tg — tg for practical purposes. 

Dp = equivalent tube diameter, ft dp = equivalent tube diameter, in. a, = flow area across the tube bundle, fF B = baffle spacing, in. c = specific heat of fluid, Btu/lb (°F) p. = viscosity at the caloric temperature, Ib/ft (hr) py, = viscosity at the tube wall temperature, Ib/ft (hr)... 

U = temperature of vapor, °F = temperature of tube wall, °F X,. = distance from top (effective) of tube, ft... 

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