【正文】 both the insides and outsides of the tubes. Thus, floatinghead SHTEs can be used for services where both the shellside and the tubeside fluids are dirty — making this the standard construction type used in dirty services, such as in petroleum refineries. There are various types of floatinghead construction. The two most mon are the pullthrough with backing device (TEMA S) and pullthrough (TEMA T) designs. The TEMA S design (Figure 4) is the most mon configuration in the chemical process industries (CPI). The floatinghead cover is secured against the floating tubesheet by bolting it to an ingenious split backing ring. This floatinghead closure is located beyond the end of the shell and contained by a shell cover of a larger diameter. To dismantle the heat exchanger, the shell cover is removed first, then the split backing ring, and then the floatinghead cover, after which the tube bundle can be removed from the stationary end. In the TEMA T construction (Figure5), the entire tube bundle, including the floatinghead assembly, can be removed from the stationary end, since the shell diameter is larger than the floatinghead flange. The floatinghead cover is bolted directly to the floating tubesheet so that a split backing ring is not required. The advantage of this construction is that the tube bundle may be removed from the shell without removing either the shell or the floatinghead cover, thus reducing maintenance time. This design is particularly suited to kettle reboilers having a dirty 4 heating medium where Utubes cannot be employed. Due to the enlarged shell, this construction has the highest cost of all exchanger types. There are also two types of packed floatinghead construction — outsidepacked stuffingbox (TEMA P) and outsidepacked lantern ring (TEMAW) (see Figure 1). However, since they are prone to leakage, their use is limited to services with shellside fluids that are nonhazardous and nontoxic and that have moderate pressures and temperatures (40 kg/cm2 and 300176。 ? condensing (one side condensing and the other singlephase)。 ? condensing/vaporizing (one side condensing and the other side vaporizing). The following nomenclature is usually used: Heat exchanger: both sides singlephase and process streams (that is, not a utility). Cooler: one stream a process fluid and the other cooling water or air. Heater: one stream a process fluid and the other a hot utility, such as steam or hot oil. Condenser: one stream a condensing vapor and the other cooling water or air. Chiller: one stream a process fluid being condensed at subatmospheric temperatures and the other a boiling refrigerant or process stream. Reboiler: one stream a bottoms stream from a distillation column and the other a hot utility (steam or hot oil) or a process stream. This article will focus specifically on singlephase applications. Design data Before discussing actual thermal design, let us look at the data that must be furnished by the process licensor before design can begin: 5 1. flow rates of both streams. 2. inlet and outlet temperatures of both streams. 3. operating pressure of both streams. This is required for gases, especially if the gas density is not furnished。thickness 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 unacce