Heat exchangers inside tubes

A double-pipe heat exchanger (inside tube diameter of 0.5 m cools engine oil from 160°C to 60°C. Water at 25°C is used as a coolant. Mass flow rates are 2 kg/sec for both fluids. Estimate the length of the exchanger if the overall heat transfer coefficient is 250 W/m °K. 

Hot water at 90°C and 4 m/sec flows in the inner tube of a double-pipe steel heat exchanger (inner-tube inside diameter of 2.5 cm and wall thickness of 0.8 mm outside tubes inside diameter is 3.75 cm). Oil at 20°C flows in the annulus at 7 m/sec. If the exchanger length is 6 m calculate U. 

Figure 5.23. Schematic of a shell and tube heat exchanger. The tube fluid enters the inlet header and is distributed into tubes. Typical heat exchangers have hundreds of tubes. The shell fluid flows inside a cylindrical shell, around the outside of the tubes.

Friction Coefficient. In the design of a heat exchanger, the pumping requirement is an important consideration. For a fully developed laminar flow, the pressure drop inside a tube is inversely proportional to the fourth power of the inside tube diameter. For a turbulent flow, the pressure drop is inversely proportional to D where n Hes between 4.8 and 5. In general, the internal tube diameter, plays the most important role in the deterrnination of the pumping requirement. It can be calculated using the Darcy friction coefficient,, defined as... 

Fypass Flow Effects. There are several bypass flows, particularly on the sheUside of a heat exchanger, and these include a bypass flow between the tube bundle and the shell, bypass flow between the baffle plate and the shell, and bypass flow between the shell and the bundle outer shroud. Some high temperature nuclear heat exchangers have shrouds inside the shell to protect the shell from thermal transient effects. The effect of bypass flow is the degradation of the exchanger thermal performance. Therefore additional heat-transfer surface area must be provided to compensate for this performance degradation. 

Frictiona.1 Pressure Drop. The frictional pressure drop inside a heat exchanger results when fluid particles move at different velocities because of the presence of stmctural walls such as tubes, shell, channels, etc. It is calculated from a weU-known expression of... 

Typical Examples (A) Split-ring floating-heat exchanger with removahle channel and cover, single-pass shell, 591-mm (23V4-in) inside diameter with tubes 4.9 m (16 ft) long. SIZE 23-192 TYPE AES. 

The U-tube bundle has the advantage of providing minimum clearance between the outer tube hmit and the inside of the shell for any of the removable-tube-bundle constructions. Clearances are of the same magnitude as for fixed-tube-sheet heat exchangers. 

Evaporator is the heat exchanger where refrigerant (water) evaporates (being sprayed over the tubes) due to low pressure in the vessel. Evaporation chills water flow inside the tubes that bring heat from the external system to be cooled. 

Tube ventilation In this system cooling tubes which work as heat exchangers are welded between the core packet and the outer frame and are open only to the atmosphere. See to Figures 1.20 (a)-(c). One fan inside the stator, mounted on the rotor shaft, transfers the internal hot air through the tube walls which form the internal closed cooling circuit. A second fan mounted outside at the NDE blows out the internal hot air of the tubes to the atmosphere and replaces it with fresh cool air from the other side. This forms a separate external cooling circuit. 

The simplest type of shell-and-tube heat exchanger is shown in Eigure 3-1. The essential parts are a shell (1), equipped with two nozzles and having tube sheets (2) at both ends, which also serve as flanges for the attachment of the two channels or beads ( 3) and their respective channel covers (4). The tubes are expanded into both tube sheets and are equipped w nil transverse baffles (5) on the shell side for support. The calculation of the effective heat transfer surface is based on the distance between the inside faces of the tube sheets instead of the overall tube length. 

One challenge in the design of U-tube heat exchangers is to determine the effective length of the tubes. For example, when U-tube bundles are fabricated from 12-ft tubes, the maximum length tube in the bundle is 12 ft which is in the outside tube row. The inside tube is the shortest and is less than 12 ft long. 

The heat transfer area, A ft, in an exchanger is usually estahlished as the outside surface of all the plain or hare tubes or the total finned surface on the outside of all the finned tubes in the tube bundle. As will be illustrated later, factors that inherendy are a part of the inside of the tube (such as the inside scale, transfer film coefficient, etc.) are often corrected for convenience to equivalent outside conditions to be consistent. When not stated, transfer area in conventional shell and tube heat exchangers is considered as outside tube area. 

For heat exchangers in true counter-current (fluids flowing in opposite directions inside or outside a tube) or true co-current (fluids flowing inside and outside of a tube, parallel to each other in direction), with essentially constant heat capacities of the respective fluids and constant heat transfer coefficients, the log mean temperature difference may be appropriately applied, see Figure 10-33. ... 

Figure 10-40B. Fouling resistance for various conditions of surface fouling on heat exchanger surfaces. Thermal resistance of typical uniform deposits. Note that the abscissa reads for either the inside, r or outside, r , fouling resistance of the bulidup of the resistance layer or film on/in the tube surface. (Used by permission Standards of Tubular Exchanger Manufacturers Association, 6 Ed, p. 138, 1978. Tubular Exchanger Manufacturers Association, Inc. All rights reserved.)...

Pressure loss through the inside of the tubes during heating or cooling in heat exchangers is given for liquids and gases by ... 

Figure 10-144. Approximate relationship of the overall coefficient fouled, and the fouling factor of inside tubes for predicting the economical use of finned tubes in shell and tube units. (Used by permission Williams, R. B., and Katz, D. L. Performance of Finned Tubes and Shell and Tube Heat Exchangers, 1951. University of Michigan. Note For reference only, 1950 costs.)...

Because finned tubes of the low-fm design are standard tubes, the inside heat exchange and pressure drop performance will be the same as determined for plain or bare tubes. Use the appropriate information from earlier design sections. 

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