ME421Heat Exchanger and

Steam Generator Design

Lecture Notes 7 Part 1

Shell-and-Tube Heat Exchangers

Introduction

stationary-end head shell rear-end head

Introduction• Most widely used HEX, round tubes mounted in a shell

(cylindrical)

• One fluid flows inside the tubes, other flows inside the shell

• Large area-to-volume and area-to-weight ratios

• Easy to clean

• Suitable for high-pressure applications and high pressure differences between working fluids

• Most common applications are as condensers and as steam generators

• Major components are tubes (tube bundle), shell, front- and rear-end heads, baffles, and tube sheets.

Basic ComponentsShell Types

• Front and rear head types and shell types are standardized by TEMA, identified by alphabetic characters (Fig. 8.2)

• E-shell is the most common– cheap and simple configuration

– one-shell pass and one- or multiple-tube

passes

– if one-tube pass, nominal counterflow is achieved

– most common for single-phase shell fluid applications

• F-shell used when there are two tube passes and pure counterflow is desired– longitudinal baffle results in two-shell passes

– units in series, each shell pass represents one unit

– higher pressure drop than that for E-shell

Shell Types (continued)

• J-shell has divided flow– for low pressure drop applications

– normally, single nozzle for shell-fluid at tube

center, two nozzles near tube ends

– when used for condensing the shell fluid, two inlets for shell-side vapor and one central outlet for condensate (figure)

• X-shell has cross flow– central shell-fluid entry and exit

– no baffles are used

– very low pressure drop

– used for vacuum condensers and low-pressure gases

• G-shell and H-shell are single- and double-split flow

Divided Flow

Shell Types (continued)

• G-shell and H-shell are single- and double-split flow– G-shell has a horizontal baffle with ends

removed, central shell-fluid entry and exit

– H-shell is similar, but with two baffles,

and two nozzles at the entry and exit

• K-shell is a “kettle reboiler”– tube bundle covers about 60% of shell diameter

– liquid covers the tube bundle, vapor occupies the

upper space without tubes

Figure 8.2

TEMA’s Standard Shell, Front-end and Rear-end Types

Tube Bundle Types

• Main objectives in design are to accommodate thermal expansion and allow easy cleaning (or to provide the least expensive construction)

• U-tube configuration (Fig. 8.4)– allows independent expansion of tubes and shell (unlimited thermal

expansion)

– only one tube sheet is needed (least expensive construction)

– tube-side cannot be mechanically cleaned

– even number of tube passes

– individual tubes cannot be replaced (except those in the outer row)

Tube Bundle Types (continued)

• Fixed tube sheet configuration (Fig. 8.5)– allows mechanical cleaning of inside of tubes but not outside

because shell is welded to the tube sheets

– low-cost

– limited thermal expansion

– individual tubes replaceable

• Pull-through floating head (Fig. 8.6)– allows the tube sheet to float – move with thermal expansion

– the tube bundle can be removed easily for cleaning – suitable for heavily fouling applications

Tubes and Tube Passes

• Multiple units are used in series when E-shell and F-shell are not used (only they result in pure counterflow) or when the unit cannot deliver the desired temperatures

• A large number of tube passes are used to increase fluid velocity and heat transfer coefficient, and to minimize fouling

• Tube wall thickness is standardized in terms of the Birmingham Wire Gauge (BWG) of the tube (Tables 8.1 & 8.2)

• Small tube diameters for larger area/volume ratios, but limited for in-tube cleaning

• Larger tube diameters suitable for condensers and boilers

• Fins used on the outside of tubes when low heat transfer coefficient fluid is present on the shell-side

• Longer tubes → fewer tubes, fewer holes drilled, smaller shell diameter, lower cost. However limitations due to several factors result in 1/5 – 1/15 shell-diameter-to-tube-length ratio

Tube Layout

• Angle between the tubes

• 30o results in greatest tube density, most common

• PT/do is between 1.25 and 1.50

• Maximum number of tubes that can be accommodated within a shell under specified conditions given in Table 8.3

Baffle Type and Geometry

• Baffles support the tubes for structural rigidity, thus prevent tube vibration and sagging

• They also divert the flow across the tube bundle to obtain a higher heat transfer coefficient

• Baffles can be transverse or longitudinal

• Transverse baffles are plate type or rod type

• Plate baffles– single and double segmental most common

– baffle spacing is critical (optimum between 0.4 and 0.6 of the shell diameter)

– triple and no-tubes-in-window segmental baffles for low pressure drop applications

Figure 8.8 Plate Baffle Types

Figure 8.8 Plate Baffle Types (continued)

Allocation of Streams

• Selection of which fluid flows through the tubes and which fluid flows through the shell. In general, the following are considered and a trade-off is made between choices:– More seriously fouling fluid flows through the tubes (easier to clean).

– High pressure fluid flows through the tubes (small diameter).

– Corrosive fluid flows through the tubes, otherwise both shell and tube will be corroded. Cheaper to provide special alloy tubes than shell.

– The fluid with the lower heat transfer coefficient flows through the shell, can have outside finned tubes.

– The fluid with the lower mass flow rate flows through the shell, turbulent flow is obtained at lower Re on the shell side.