管壳式换热器的有效设计翻译

管壳式换热器的有效设计翻译内容摘要：

ding line sizes, especially for small lines. 10. preferred tube size. Tube size is designated as . 180。 thickness 180。 length. Some plant owners have a preferred . 180。 thickness (usually based upon inventory considerations), and the available plot area will determine the maximum tube length. Many plant owners prefer to standardize all three dimensions, again based upon 6 inventory considerations. 11. maximum shell diameter. This is based upon tubebundle removal requirements and is limited by crane capacities. Such limitations apply only to exchangers with removable tube bundles, namely Utube and floatinghead. For fixedtubesheet exchangers, the only limitation is the manufacturer’s fabrication capability and the availability of ponents such as dished ends and flanges. Thus, floatinghead heat exchangers are often limited to a shell . of – m and a tube length of 6 m or 9 m, whereas fixedtubesheet heat exchangers can have shells as large as 3 m and tubes lengths up to 12 m or more. 12. materials of construction. If the tubes and shell are made of identical materials, all ponents should be of this material. Thus, only the shell and tube materials of construction need to be specified. However, if the shell and tubes are of different metallurgy, the materials of all principal ponents should be specified to avoid any ambiguity. The principal ponents are shell (and shell cover), tubes, channel (and channel cover), tubesheets, and baffles. Tubesheets may be lined or clad. 13. special considerations. These include cycling, upset conditions, alternative operating scenarios, and whether operation is continuous or intermittent. Tubeside design Tubeside calculations are quite straightforward, since tubeside flow represents a simple case of flow through a circular conduit. Heattransfer coefficient and pressure drop both vary with tubeside velocity, the latter more strongly so. A good design will make the best use of the allowable pressure drop, as this will yield the highest heattransfer coefficient. If all the tubeside fluid were to flow through all the tubes (one tube pass), it would lead to a certain velocity. Usually, this velocity is unacceptably low and therefore has to be increased. By incorporating pass partition plates (with appropriate gasketing) in the channels, the tubeside fluid is made to flow several times through a fraction of the total number of tubes. Thus, in a heat exchanger with 200 tubes and two passes, the fluid flows through 100 tubes at a time, and the velocity will be twice what it would be if there were only one pass. The number of tube passes is usually one, two, four, six, eight, and so on. 7 Heattransfer coefficient The tubeside heattransfer coefficient is a function of the Reynolds number, the Prandtl number, and the tube diameter. These can be broken down into the following fundamental parameters: physical properties (namely viscosity, thermal conductivity, and specific heat)。 tube diameter。 and, very importantly, mass velocity. The variation in liquid viscosity is quite considerable。 so, this physical property has the most dramatic effect on heattransfer coefficient. The fundamental equation for turbulent heattransfer inside tubes is: Nu = (Re) (Pr) (1a) or (hD/k) = (DG/m) (cm/k) (1b) Rearranging: h = (DG/m)(cm/k)(k/D) (1c) Viscosity influences the heattransfer coefficient in two opposing ways — as a parameter of the Reynolds number, and as a parameter of Prandtl number. Thus, from Eq. 1c: h a (m)– (2a) h a (m)– (2b) In other words, the heattransfer coefficient is inversely proportional to viscosity to the power. Similarly, the heattransfer coefficient is directly proportional to thermal conductivity to the power. These two facts lead to some interesting generalities about heat transfer. A high thermal conductivity promotes a high heattransfer coefficient. Thus, cooling water (thermal conductivity of around kcal/h•m•176。 C) has an extremely high heattransfer coefficient of typically 6,000 kcal/h•m2•176。 C, followed by hydrocarbon liquids (thermal conductivity between and kcal/h•m•176。 C) at 250–1,300 kcal/h•m2•176。 C, and then hydrocarbon gases (thermal conductivity between and kcal/h•m•176。 C) at 50–500 kcal/h•m2•176。 C. Hydrogen is an unusual gas, because it has an exceptionally high thermal conductivity (greater than that of hydrocarbon liquids). Thus, its heattransfer coefficient is toward the upper limit of the range for hydrocarbon liquids. The range 8 of heattransfer coefficients for hydrocarbon liquids is rather large due to the large variation in their viscosity, from less than cP for ethylene and propylene to more than 1,000 cP or more for bitumen. The large variation in the heattransfer coefficients of hydrocarbon gases is attributable to the large variation in operating pressure. As operating pressure rises, gas density increases. Pressure drop is directly proportional to the square of mass velocity and inversely proportional to density. Therefore, for the same pressure drop, a higher mass velocity can be maintained when the density is higher. This larger mass velocity translates into a higher heattransfer coefficient. Pressure drop Mass velocity strongly influences the heatt。