Heat Exchanger Design by Bell Method Part 2 Shell-and-Tube Shell-and-Tu be Heat Exchangers
Basic Design Procedure Flow rates & compositions, temperatures, pressures. Process Eng Design Eng Shell and head types, baffles, tube passes, etc. Preliminary design/analysis Use heat transfer and pressure drop correlations
Basic Design Procedure Flow rates & compositions, temperatures, pressures. Process Eng Design Eng Shell and head types, baffles, tube passes, etc. Preliminary design/analysis Use heat transfer and pressure drop correlations
Preliminary Design • Estimate heat transfer coefficients and fouling resistances. • With h, Rf ’s, Rw, and overall surface efficiencies (in case of fins on either side) estimated, evaluate the overall heat transfer coefficient
• This is the most general expression, also estimate U c. • Take F = 1.0 for counterflow HEX (single tube pass), or F = 0.9 for any even number of tube passes.
Preliminary Design (continued) • Estimate heat load • Calculate Tlm,cf • Estimate the size of the HEX
• This area is also related to tube diameter d o and number of tubes Nt • The objective is to find the number of tubes with diameter d o, and shell diameter D s to accommodate the number of tubes, with the given tube length.
Preliminary Design (continued) • Shell diameter, D s is
CL is the tube layout constant – CL = 1.0 for 90 o and 45o, CL = 0.87 for 30o and 60o
CTP is the tube count calculation constant – CTP = 0.93 for one tube pass – CTP = 0.90 for two tube passes – CTP = 0.85 for three tube passes
PR is the tube pitch ratio, P T/do
• Number of tubes, N t is
Rating of the Preliminary Design (continued) Which one is complicated b/w tube and shell side? Tube side: Shell side: more complicated
• If rating output is not acceptable, modify – HEX cannot deliver the heat required: increase h or area • To increase hi, increase um in tubes, thus number of passes • To increase ho, decrease baffle spacing or decrease baffle cut • To increase area, increase length or shell diameter, or use shells in series
– ptube > pall: decrease number of tube passes or increase tube diameter (thus decrease tube length, increase shell diameter and number of tubes) – pshell > pall: increase baffle spacing, tube pitch and baffle cut, or change type of baffles
Shell Side Analysis Kern Method (simple method) Shell Side Heat Transfer Coefficient • Baffles increase heat transfer coefficient due to increased turbulence, tube correlations are not applicable
• Without baffles, h can be based on D e, similar to double-pipe HEX, and Chapter 3 correlations can be used • On the shell side, McAdams correlation for Nu
square
triangular
Kern Method (simple method) Shell Side Heat Transfer Coefficient (continued) Gs (shell side mass velocity) can be evaluated from
where
is the bundle crossflow area at the center of the shell
Ds: shell diameter C: clearance between adjacent tubes B: baffle spacing PT: pitch size
• Gs evaluated here is a fictional value because there is actually no free-flow area on the shell side. This value is based on the bundle crossflow area at the hypothetical tube row possessing the maximum flow area corresponding to the center of the shell
Kern Method (simple method) Shell Side Pressure Drop
• Depends on the number of tubes the fluid passes through in the bundle between baffles and the length of each crossing. • The following correlation uses the product of distance across the bundle, taken as D s, and the number of times the bundle is crossed.
s = (b/w)0.14
Nb = L/B – 1 is the number of baffles (Nb + 1) is the number of times the shell fluid passes the tube bundle f takes into account entrance and exit losses
where
Kern Method (simple method) Tube Side Pressure Drop
• Total pressure drop including sudden expansions and contractions during a return (for multiple tube passes)
• Ignore second term if single tube pass
Bell-Delaware Method (complex method)
• Shell side flow is complex, combines crossflow and baffle window flow, as well as baffle-shell and bundle-shell bypass streams and other complex flow patterns • Five different streams are identified; A, B, C, E, and F • Bell-Delaware method takes into account the leakage and bypass streams, most reliable method for shell side • B-stream is the main stream, others reduce it and change shell side temperature profile, thus decrease h • A: leakage through tube/baffle clearance, C: bundle bypass
Bell-Delaware Method Shell Side Heat Transfer Coefficient
hideal is the ideal heat transfer coefficient for pure crossflow in an ideal tube bank J’s are correction factors ji is the Colburn j-factor for an ideal tube bank (Figures 8.158.17, depend on shell side Re, , tube layout, and pitch size; or correlation 8.25) As is the crossflow area at the centerline of the shell for one crossflow between baffles, A s = Ds CB/PT Note that Res is different for this method (based on d o)
Bell-Delaware Method Shell Side Heat Transfer Coefficient (continued)
• Correlation for the Colburn j-factor for an ideal tube bank
a1 – a4 from Table 8.6 in book
• Correlation for ideal friction factor
b1 – b4 from Table 8.6 in book as well
Shell side pressure drop
Tube side heat transfer coefficient
Tube side pressure drop
Bell-Delaware Method
• Number of tube rows crossed in one crossflow section, N c
• Lc is the baffle cut distance from baffle tip to inside of shell
