Heat transfer outside tubes

This design is not well adapted to free-convection heat transfer outside a tube or coil therefore, for this discussion only agitation is considered using a submerged helical coil, Oldshue and Kern . 

AO = OUTSIDE HEAT-TRANSFER AREA-TUBE, ft 2/ft (FOR FINNED... 

Toluene exit temperature = 257.4°F Styrene exit temperature = 175.9°F Tube-side tube pressure drop = 3.59 psi Tube-side nozzle pressure drop = 0.56 psi Toluene exit pressure = 85.85 psia Shell-side baffled pressure drop = 4.57 psia Shell-side nozzle pressure drop = 4.92 psia Styrene exit pressure = 40.52 psia Heat transfer area (tube outside) = 3,217 ft ... 

Heat transfer area (tube outside) = 3,663.2 ft or 1,221.1 ft in each of the three shells Heat duty = 9,970,000 Btu/hr... 

Men et al. (2014) have conducted experiments on the natural convection heat transfer for a PRHRS HEX in an in-containment refueling water storage tank. Several empirical correlations for the forced convection flow internal to the HEX tube and the natural convection heat transfer outside of the tube in the tank, for the vertical and horizontal portion of the tube, were compared with experimental data. The Dittus-Boelter forced convection correlation and the McAdams correlations for natural convection proved to give the better model of the data. Wenbin et al. (2014) have conducted experiments for the secondary loop of the Chinese Advance Pressurized Water Reactor for validation of the MIS AP20 models and code. These and other papers are in a special issue of the Science and Technology of Nuclear Installations journal published in 2014 as indicated by the cited references. 

The overall heat transfer coefficient is depended on tube inside heat transfer coefficient, themnal oonductivity of the tube and thickness of tube wall, tube outside heat transfer coefficient, tube inside fouling factor, and tube outside fouling factor. For heat transfer through tube, it is calculated by following equation. 

BMA Process. This route to HCN was developed by Degussa of Germany in 1949. According to one source, the reaction involves the direct combination of ammonia and methane (see first equation in this section). The reaction takes place on the inside of ceramic alumina tubes that are coated on the inside with a very thin (15 microns) layer of platinum alloyed with minor amounts of other metals (such as ruthenium and aluminum). The heat of reaction is provided outside the tubes by burning natural gas. To achieve adequate heat transfer the tubes are quite small, about inch in diameter and 6 feet long. 

In the forced convection heat transfer, the heat-transfer coefficient, mainly depends on the fluid velocity because the contribution from natural convection is negligibly small. The dependence of the heat-transfer coefficient, on fluid velocity, which has been observed empirically (1—3), for laminar flow inside tubes, is h for turbulent flow inside tubes, h and for flow outside tubes, h. Flow may be classified as laminar or... 

The sheU-and-tube exchanger is the workhorse of power, chemical, refining, and other industries (Fig. 8). One fluid flows on the inside of the tubes whereas the other fluid is flowing through the sheU and over the outside of the tubes. Baffles are used to ensure that the sheUside fluid flows across the tubes, thus inducing high heat transfer. 

The minimum velocity requited to maintain fully developed turbulent flow, assumed to occur at Reynolds number (R ) of 8000, is inside a 16-mm inner diameter tube. The physical property contribution to the heat-transfer coefficient inside and outside the tubes are based on the following correlations (39) ... 

Commonly used heat-transfer surfaces are internal coils and external jackets. Coils are particularly suitable for low viscosity Hquids in combination with turbine impellers, but are unsuitable with process Hquids that foul. Jackets are more effective when using close-clearance impellers for high viscosity fluids. For jacketed vessels, wall baffles should be used with turbines if the fluid viscosity is less than 5 Pa-s (50 P). For vessels equipped with cods, wall baffles should be used if the clear space between turns is at least twice the outside diameter of the cod tubing and the fluid viscosity is less than 1 Pa-s (10... 

The inside of the convection tubes rarely foul, but occasionally the Hquid unsaturates in feedstocks tend to polymerize and stick to the walls and thus reduce the heat transfer. This soft coke is normally removed by mechanical means. In limited cases, the coke can also be burnt off with air and steam. Normally, the outside surface of the convection section fouls due to dust and particles in the flue gas. Periodically (6 to 36 months), the outside surface is cleaned by steam lancing. With Hquid fuel firing, the surface may require more frequent cleaning. 

For the general case, the treatment suggested by Kern (Pmcc.s.s Heat Transfer, McGraw-Hill, New York, 1950, p. 512) is recommended. Because of the wide variations in fin-tube construction, it is convenient to convert all film coefficients to values based on the inside bare surface of the tube. Thus to convert the film coefficient based on outside area (finned side) to a value based on inside area Kern gives the following relationship ... 

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. 

Work in connection with desahnation of seawater has shown that specially modified surfaces can have a profound effect on heat-transfer coefficients in evaporators. Figure 11-26 (Alexander and Hoffman, Oak Ridge National Laboratory TM-2203) compares overall coefficients for some of these surfaces when boiling fresh water in 0.051-m (2-in) tubes 2.44-m (8-ft) long at atmospheric pressure in both upflow and downflow. The area basis used was the nominal outside area. Tube 20 was a smooth 0.0016-m- (0.062-in-) wall aluminum brass tube that had accumulated about 6 years of fouhng in seawater service and exhibited a fouling resistance of about (2.6)(10 ) (m s K)/ J [0.00015 (fF -h-°F)/Btu]. Tube 23 was a clean aluminum tube with 20 spiral corrugations of 0.0032-m (lA-in) radius on a 0.254-m (10 -in)... 

Of these special surfaces, only the double-fluted tube has seen extended services. Most of the gain in heat-transfer coefficient is due to the condensing side the flutes tend to collect the condensate and leave the lauds bare [Caruavos, Proc. First Int. Symp. Water Desalination, 2, 205 (1965)]. The coudeusiug-film coefficient (based on the actual outside area, which is 28 percent greater than the nominal area) may be approximated from the equation... 

The modified Palen-SmaU method can be employed for reboiler design using finned tubes, but the maximum flux is calculated from A, the total outside heat-transfer area including fins. The resulting value of refers to A. ... 

Functional Definitions Heat-transfer equipment can be designated by type (e.g., fixed tube sheet, outside packed head, etc.) or by... 

Common practice is to specify exchanger surface in terms of total external square feet of tubing. The effective outside heat-transfer surface is based on the length of tubes measured between the inner faces of tube sheets. In most heat exchangers there is little difference between the total and the effective surface. Significant differences are usually found in high-pressure and double-tube-sheet designs. 

Frame surface cooled (using the surrounding medium) The primary coolant is circulated in a closed circuit and dissipates heat to the secondary ccxilant. which is the surrounding medium in contact with the outside surface of the machine. The surface may be smooth or ribbed, to improve on heat transfer efficiency (as, in a TEFC or tube venulated motor (Figures 1.19 and 1.20) 4 ... 

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. 

Open Tube Sections (Water cooled) Tubes require no shell, only end headers, usually long, water sprays over surface, sheds scales on outside tubes by expansion and contraction. Can also be used in water box. Condensing, relatively low heat loads on sensible transfer. Transfer coefficient is low, takes up less space than pipe coil. 0.8-1.1... 

Common to all air cooled heat exchangers is the tube, through which the process fluid flows. To compensate for the poor heat transfer properties of air, which flows across the outside of the tube, and to reduce the overall dimensions of the heat exchanger, external fins are added to the outside of the tube. A wide variety of finned tube types are available for use in air cooled exchangers. These vary in geometry, materials, and methods of construction, which affect both air side thermal performance and air side pressure drop. In addition, particular... 

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