Pressure drop in heat exchangers

Operating data of an exchanger are shown on the sketch. These data include 

With this method, unknown terminal temperatures are found without trial calculations. 

Evaluation of the boiling heat transfer coefficient in vertical tubes, as in thermosyphon reboilers, is based on a group of equations, (42)-(48), of Table 8.10. A suitable procedure is listed following these equations in that table. 

Although the rate of heat transfer to or from fluids is improved by increase of linear velocity, such improvements are limited by the 

Calculation of tube-side pressure drop is straightforward, even of vapor-liquid mixtures when their proportions can be estimated. Example 8.7 employs the methods of Chapter 6 for pressure drop in a thermosiphon reboiler. 

Pressure drop in heat exchangers Tube side 

Pressure drop on the tube-side of a shell and tube exchanger is made up of the friction toss in the tubes and losses due to sudden contractions and expansions and flow reversals experienced by the tube-side fluid. The friction loss may be estimated by the methods outlined in Section 3.4.3 from which the basic equation for isothermal flow is given by equation 3.18 which can be written as  

There is no entirely satisfactory method for estimating losses due to contraction at the tube inlets, expansion at the exits and flow reversals, although suggests 

A-fluid flowing through the clearance between the tube and the hole in the baffle. 

Although the rate of heat transfer to or from fluids is improved by increase of linear velocity, such improvements are limited by the economic balance between value of equipment saving and cost of pumping. A practical rule is that pressure drop in vacuum condensers be limited to 0.5-1. Opsi (25-50 Ton) or less, depending on the required upstream process pressure. In liquid service, pressure drops of 5-10 psi are employed as a minimum, and up to 15% or so of the upstream pressure. 

The shell side with a number of segmental baffles presents more of a problem. It may be treated as a series of ideal tube banks connected by window zones, but also accompanied by some bypassing of the tube bundles and leakage through the baffles. A hand calculation based on this mechanism (ascribed to K.J. Bell) is illustrated by Ganapathy (1982, pp. 292-302), but the calculation usually is made with proprietary computer programs, that of HTRI for instance. 

A simpler method due to Kem (1950, pp. 147-152) nominally considers only the drop across the tube banks, but actually takes account of the added pressure drop through baffle windows by employing a higher than normal friction factor to evaluate pressure drop across the tube banks. Example 8.8 employs this procedure. According to Taborek (HEDH, 1983, 3.3.2), the Kern predictions usually are high, and therefore considered safe, by a factor as high as 2, except in laminar flow where the results are uncertain. In the case worked out by Ganapathy (1982, pp. 292-302), however, the Bell and Kem results are essentially the same. 

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Short, B. E., Heat Transfer and Pressure Drop in Heat Exchangers, Engineering Research Series No. 37, University of Texas, Austin, TX, pub. No. 4324, pp. 1-55 (1943). 

Short, B.E. Univ. of Texas Pub. No. 4324 (1943). Heat transfer and pressure drop in heat exchangers. 

Frictional pressure drop, in heat exchanger design, 13 260-261 Friction and Wear Databank, 15 205 Friction coefficients, 10 178 11 749, 750 13 247... 

Siif RT, B..E. Univ. of Texas Pub. No. 4324 (1943). Heat transfer and pressure drop in heat exchangers. 30. DfiNotuiE, D.A.. hul. Eng. Chem. 41 (1949) 2499. Heat transfer and pressure drop in heat exchangers. 

The total pressure drop in heat exchangers is due principally to the core frictional pressure drop. There are also contributions to the pressure drop from inlet and exit losses and from flow acceleration or deceleration caused by heating or cooling. The total pressure drop can be conveniently summed by... 

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