Shell and tube exchangers

The Stainicaibon process is described in Figures 3—7. The synthesis section of the plant consists of the reactor, stripper, high pressure carbamate condenser, and a high pressure reactor off-gas scmbber. In order to obtain a maximum urea yield pet pass through the reactor, a pressure of 14 MPa (140 bar) and a 2.95/1 NH —CO2 molar ratio is maintained. The reactor effluent is distributed over the stripper tubes (falling-film type shell and tube exchanger) and contacted by the CO2, countercurrendy. This causes the partial NH pressure to decrease and the carbamate to decompose. 

Impervious graphite heat exchangers machined from solid blocks are also available (15,16). The solid block constmction is less susceptible to damage by mechanical shock, such as steam and water hammer, than are shell and tube exchangers. Block exchangers are limited in size and cost from 50—100% more than shell and tube units on an equivalent area basis. 

If the CO is not completely combusted to CO2 in the regenerator, a CO boiler is used to complete the combustion. The resulting heat of combustion and the sensible heat of the flue gas along with any heat from auxiUary fired fuel are recovered in the form of high pressure steam. When the regenerator is operated in total CO bum, the CO boiler is replaced with either a shell and tube exchanger or a box-type waste heat boiler (see Heat... 

Transverse fins upon tubes are used in low-pressure gas sei vices. The primary application is in air-cooled heat exchangers (as discussed under that heading), but shell-and-tube exchangers with these tubes are in sei vice. 

Impervious graphite heat-exchanger equipment is made in a variety of forms, including outside-packed-head shell-and-tube exchangers. They are fabricated with impervious graphite tubes and... 

Cost data for shell-and-tube exchangers from 15 sources were correlated and found to be consistent wrien scaled by the Marshall and Swift index [Woods et al., Can. J. Chem. Eng., 54, 469-489 (December 1976)]. 

Induced-draft units are less likely to recirculate the hot exhaust air, since the exit air velocity is several times that of the forced-draft unit. Induced-draft design more readily permits the installation of the aircooled equipment above other mechanical equipment such as pipe racks or shell-and-tube exchangers. 

Ground area and. space requirement.s. Comparisons of the overall space requirements for plants using air cooling versus water cooling are not consistent. Some air-cooled units are installed above other equipment—pipe racks, shell-and-tube exchangers, etc. Some plants avoid such inst ations because of safety considerations, as discussed later. 

When the outlet temperatures of both fluids are identical, the MTD correction fac tor for a 1 2 shell-and-tube exchanger (one pass shell... 

Overall coefficients are determined hke shell and tube exchangers that is, sum all the resistances, then invert. The resistances include the hot-side coefficient, the cold-side coefficient, the fouhng factor (usually only a total value not individual values per fluid side) and the wall resistance. 

Shell and Tube Exchanger Selection Guide (Cost Increases from Left to Right)... 

Calculation of Tubeside Pressure Drop in Shell and Tube Exchangers... 

The price of a shell and tube exchanger depends on the type of exchanger, i.e., fixed tube, U-tube, double tube sheets, and removable bundles. The tube side pressure, shell side pressure, and materials of construction also affect the price. If prices cannot be obtained from endors, correlating in-house data by plotting /fr vs. number of ft with correction factors for the variables that affect price will allow estimating with fair accuracy. If not enough in-house data is available to establish good correlations. it will be necessary to use the literature, such as References 16. 17. and 18. 

Figure 4. Common shell-and-tube exchanger configurations.

These high velocities occur at the bundle entrance and exit areas, in the baffle windows, through pass lanes and in the vicinity of tie rods, which secure the baffles in their proper position. In conjunction with this, the shell side fluid generally will take the path of least resistance and will travel at a greater velocity in the free areas or by-pass lanes, than it will through the bundle proper, where the tubes are on a closely spaced pitch. All factors considered, it appears a formidable task to accurately predict heat transfer characteristics of a shell and tube exchanger. 

The shell-and-tube exchanger is by far the most common type of heat exchanger used in production operations. It can be applied to liquid/liquid, liquid/vapor, or vapor/vapor heat transfer services. The TEMA standards dcTine the design requirements for virtually all ranges of temperature and pressure that would be encountered in an oil or gas production facility. 

Shell-and-tube exchangers contain several types of baffles to help direct the flow of both tube-side and shelbside fluids. Pass partition baffles force the fluid to flow through several groups of parallel tubes. Each of these groups of tubes is called a pass, . since it passes the fluid from one head to another. By adding pass partition baffles on each end. the tube-side fluid can be forced to take as many passe.s through the exchanger as desired. 

In. sclcciing an exchanger, one must know the advantages and disadvantages td each type. The three basic types of shell-and-tube exchangers are fixed tube sheet, floating head, and U-tube, Table 3-1 summarizes the comparison between these three exchangers. 

TEMA standards provide for two classes of shell and tube exchanger qualities. Class C is the less stringent and is typically used in onshoie applications and where the temperature is above - 20°F. Class R uoi mally used offshore and in cold temperature service. Table 3-2 shmv s the most important differences between a Class R and a Class C Tr-A1. exchanger. 

To size a shell-and-tube exchanger, first the duty is calculated. Then ii is delermined which fluid will be in the shell and which in the tube, ami i heat dansfer coefficient assumed or calculated. A choice is made of ihc number of shell and tube passes to get a reasonable LMTD correction factor (F), and a corrected LMTD as calculated from Equation 3-1. 

A double-pipe exchanger is made up of one pipe containing the tube fluid concentric with another pipe, which serves as the shell. The tube is often finned to give additional surface area. The double-pipe exchanger was developed to fit applications that are too small to economicall apply the requirements of TEMA for shell and tube exchangers. 

The procedure for calculating the number of tubes required for an aerial cooler is similar to that for a shell- and-tube exchanger. Table 3-6 shows approximate overall heat transfer coefficients. Ub should be used when the outside surface area of the bare tube (neglecting fins) is used in the heat... 

Rich/lean amine exchangers are usually shell-and-tube exchangers with the corrosive rich amine flowing through the tubes. The purpose of these exchangers is to reduce the reboiler duty by recovering some of the sensible heat from the lean amine. 

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