Tube length

Since the investment per unit area of heat transfer surface is less for long exchangers with relatively small shell diameter, minimum restrictions on length should be observed. 

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The bonding of the HT cable to the tube reduces production costs and tube length. 

Fig. 18. (a) Differential-temperature thermal flow meter, (b) Tube length-temperature profiles for (—) 2ero flow and ( ) flow. AT is the temperature... 

Reynolds Number. The Reynolds number, Ke, is named after Osborne Reynolds, who studied the flow of fluids, and in particular the transition from laminar to turbulent flow conditions. This transition was found to depend on flow velocity, viscosity, density, tube diameter, and tube length. Using a nondimensional group, defined as p NDJp, the transition from laminar to turbulent flow for any internal flow takes place at a value of approximately 2100. Hence, the dimensionless Reynolds number is commonly used to describe whether a flow is laminar or turbulent. Thus... 

If U varies along the tube length or the stream temperature profile is not a smooth curve, then divide the entire tube length into a number of small heat-exchange elements, apply steps (2) through (8) to each element, and sum up the resulting area requitements as follows ... 

Translate the heat-transfer area deterrnined in steps (8) or (9) into corresponding tube bundle dimensions (ie, number of tubes, diameter, and tube length). 

Translate the heat-transfer area deterrnined above into corresponding tube bundle dimensions. If different from those assumed in step (2), repeat steps (2) through (8) until satisfactory agreement is reached. The method caimot be appHed to cases in which U varies along the tube length or the stream... 

The highly exothermic nature of the butane-to-maleic anhydride reaction and the principal by-product reactions require substantial heat removal from the reactor. Thus the reaction is carried out in what is effectively a large multitubular heat exchanger which circulates a mixture of 53% potassium nitrate [7757-79-1/, KNO 40% sodium nitrite [7632-00-0], NaN02 and 7% sodium nitrate [7631-99-4], NaNO. Reaction tube diameters are kept at a minimum 25—30 mm in outside diameter to faciUtate heat removal. Reactor tube lengths are between 3 and 6 meters. The exothermic heat of reaction is removed from the salt mixture by the production of steam in an external salt cooler. Reactor temperatures are in the range of 390 to 430°C. Despite the rapid circulation of salt on the shell side of the reactor, catalyst temperatures can be 40 to 60°C higher than the salt temperature. The butane to maleic anhydride reaction typically reaches its maximum efficiency (maximum yield) at about 85% butane conversion. Reported molar yields are typically 50 to 60%. 

Fig. 4. Schematic diagram of a TOF spectrometer where is the extraction field, E the acceleration field, and E tube length (1).

The location of exchangers is the key to maintenance. Usually the back head is kept at a distance of about three meters from the piperack support columns. Access equipment must be able to get in and remove the sheU cover and flange head. Access area must also be provided to handle and remove the sheU cover usually located under the piperack. The tube-pulling or rodding-out area must be kept clear to allow access to the channel end. This space should be at least equal to the tube length and about two meters from the tube sheet location. Tube removal space should be allowed for but is not mandatory if grade-mounted heat exchangers are used and mobile maintenance equipment employed to pick up the entire unit and transfer it to the repair shop. 

Eig. 4. Falling-film crystallizer, semibatch. Tube length is 12.2 m tube diameter is 7.6 cm. 

It is assumed that process conditions and physical properties are known and the following are known or specified tube outside diameter D, tube geometrical arrangement (unit cell), shell inside diameter D shell outer tube limit baffle cut 4, baffle spacing and number of sealing strips N,. The effective tube length between tube sheets L may be either specified or calculated after the heat-transfer coefficient has been determined. If additional specific information (e.g., tube-baffle clearance) is available, the exact values (instead of estimates) of certain parameters may be used in the calculation with some improvement in accuracy. To complete the rating, it is necessary to know also the tube material and wall thickness or inside diameter. 

Si e. Sizes of shells (and tube Lundies) shall he designated hy niimhers describing shell (and tiihe-hiindle) diameters and tube lengths as follows ... 

Tubes Standard heat-exchanger tubing is V4, Yh, V2., Yh, %, 1, IV4, and IV2. in in outside diameter (in X 25.4 = mm). Wall thickness is measured in Birmingham wire gauge (BWG) units. (A comprehensive list of tubing characteristics and sizes is given in section 9, table D-7 of TEMA.) The most commonly used tubes in chemical plants and petroleum refineries are 19- and 25-mm (%- and 1-in) outside diameter. Standard tube lengths are 8, 10, 12, 16, and 20 ft, with 20 ft now the most common (ft x 0.3048 = m). 

Tube lengths vary and may be as great as 18.3 m (60 ft). When tube length exceeds 12.2 m (40 ft), three fans are generally installed in each bay. Frequently used tube lengths vaiy from 6.1 to 12.2 m (20 to 40 ft). 

Cathodic protection can be useful, although its ability to protect tube interiors is generally limited to the first 4 to 6 in. of tube length. Such systems, however, must be properly designed and maintained to be effective. Corrosion can be intensified if the polarity of the cathodic protection system is inadvertently reversed. 

In a small integral reactor at each step of the stepwise increasing temperature, one point on a conversion versus temperature curve is received. These are all at the same feed rate and feed composition, constant pressure, and each is at a different but practically constant temperature along the tube length within every step. Since the reactor is small the attainment of steady-state can be achieved in a short time. The steadiness of conditions can be asserted by a few repeated analyses. 

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

This result means that the reactor is insensitive if the temperature profile is concave toward the reactor length axis, and the inflection point is avoided. If the AT exceeds that permitted by the previous criterion—the limit set by RT /E— an inflection of the temperature vs., tube length will occur and thermal runaway will set in. Just before runway sets in the temperature at the hot spot can be 1.4 times higher than RT /E. 

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