Page 75 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10B 5/30/2001 4:17 PM Page 53

Heat Transfer 53

From Figure 10-27B: Types of Heat Exchange Operations

Effective tube length for 288 tubes 9.6 ft. The process engineer identifies heat exchange equipment
2,142 in a process by the operation or function it serves at a particu-
Calculate new number of tubes 330
19.6210.6782 lar location in the flow cycle. For example, the bottom vapor-
Effective tube length 9.42 ft. izer on a product finishing distillation column is usually termed
2,142 “Finishing Column Reboiler E-16,” or “Reboiler E-16;” the over-
Calculate new number of tubes 336
19.422 10.6782 head vapor condenser on this column is termed “Condenser E-
Effective tube length for 336 tubes 9.4 ft. 17;” etc. The usual operations involved in developing a process
Thus, required number of tubes 336 flowsheet are described in Table 10-11, or Chapter 1, Volume 1.

The equations of Figure 10-27B correlations are as follows: Thermal Design
Engineering thermal design of heat transfer equipment is
L e Effective tube length, ft.
concerned with heat flow mechanisms of the following three
N t Number of U-tubes
types—simply or in combination: (1) conduction, (2) con-
L Nominal tube length, ft.
vection, and (3) radiation. Shell and tube exchangers are con-
cerned primarily with convection and conduction; whereas
For / 4 -in. U-tubes:
3
heaters and furnaces involve convection and radiation.
Radiation is not generally considered in conventional
L e 1L 0.52 17.4007 10 3 2 1N t 2
heat transfer equipment except for direct gas/oil-fired
2
18.5791 10 6 2 1N t 2 13.7873 10 9 2 1N t 2 3 (10-6) heaters and cracking units. These later types are not a part
of this chapter, because they are specialty items of their own
For 1-in. U-tubes: as far as design considerations are concerned.
Conduction is heat transfer through a solid nonporous bar-
L e 1L 0.52 19.2722 10 3 2 1N t 2 rier when a temperature difference exists across the barrier.
The thermal transfer capability of the specific barrier or wall
2
11.1895 5 2 1N t 2 18.4977 10 9 2 1N t 2 3 (10-7)
material, known as thermal conductivity, determines the tem-
perature gradient that will exist through the material.
k a k a
Nozzle Connections to Shell and Heads Q A1t 2 t 1 2 A t (10-8)
L c L c
Inlet and outlet liquid nozzles are sized by conventional Referring to Figure 10-28, conduction occurs through the
pressure drop evaluations or by the more common velocity tube wall and is represented by a temperature drop t 4 t 5 and
guides. For low-pressure vacuum services, velocities should through the scale of fouling by the drops t 3 t 4 and t 5 t 6 .
not be used to establish any critical connection size. (Figure Convection is heat transfer between portions of a fluid existing
10-63 is a useful guide for the usual case.) under a thermal gradient. The rate of convection heat transfer
Safety valves are often required on the shell side of is often slow for natural or free convection to rapid for forced
exchangers and sometimes on the tube side. These valves convection when artificial means are used to mix or agitate the
may require sizing based upon process reaction, overpres- fluid. The basic equation for designing heat exchangers is
sure, etc., or on external fire. For details, see Chapter 7, Vol.
I on safety-relieving devices. Q UA 1t 2 t 1 2 UA t (10-9)
Drains are necessary on the shell and on the bottom of
most heads. Sometimes several drains are necessary on the where
shell side to facilitate drainage between baffles when flush- (t 2 t 1 ) represents the temperature difference across a
ing is a part of the operation. single fluid film. Referring to Figure 10-28, convection occurs
Vents are usually placed on the shell and on the tube-side through the fluid t 1 t 3 and also t 6 t 8 .
heads to allow venting of inert gasses or other material. A
where
1 in.-6,000 lb. half or full-coupling is recommended for both
A net external surface area of tubes exposed to fluid
vent and drain, unless other sizes are indicated. heat transfer (not just the length of the individual
Couplings are handy to have on the process inlet and out- tubes), ft .
2
let nozzles on both the tube and shell sides. These may be Q heat load, Btu/hr
2
used for flushing, sampling, or thermometer wells, thermo- U overall heat-transfer coefficient, Btu/(hr-ft - °F)
couple bulbs, or pressure gages. T mean temperature difference, °F, corrected