Transcript ME421 Heat Exchanger and Steam Generator Design

ME421
Heat 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 Components
Shell 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
Divided Flow
– 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
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.