ME421 Heat Exchanger and Steam Generator Design Lecture Notes 6 Double-Pipe Heat Exchangers.
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Transcript of ME421 Heat Exchanger and Steam Generator Design Lecture Notes 6 Double-Pipe Heat Exchangers.
Introduction
• DP HEX: one pipe placed concentrically inside another
• One fluid flows through inner pipe, the other through the annulus
• Outer pipe is sometimes called the shell
• Inner pipe connected by U-shaped return bends enclosed in a return-bend housing to make up a hairpin, so DP HEX = hairpin HEX
• Hairpins are based on modular principles: they can be arranged in series, parallel, or series-parallel combinations to meet pressure drop and MTD requirements; add-remove as necessary
• Sensible heating / cooling, small HT areas (up to 50 m2)
• High pressure fluids, due to small tube diameters
• Suitable for gas / viscous liquid (small volume fluids)
• Suitable for severe fouling conditions (easy to clean and maintain)
• Finned tubes can be used to increase HT surface per unit length, thus reduce length and Nhp
• Outside-finned inner tubes most efficient when low h fluid (oil or gas) flows through annulus
• Multiple tubes can be used inside the shell
• Used as counterflow HEX, so they can be used as an alternative to shell-and-tube HEX
Usage Areas / Advantages
Thermal / Hydraulic DesignInner Tube
• Use correlations to find HT coefficient and friction factor
• Total pressure drop
2
uN
d
2L4fp
2m
hpi
Annulus
• Same procedure as above, but use
– Hydraulic diameter, Dh = 4Ac/Pw for Re calculation
– Equivalent diameter, De = 4Ac/Ph for Nu calculation
• For a hairpin HEX with Bare Inner Tube,
– Dh = Di - do
– De = (Di2 - do
2)/do
Study Example 6.1 (detailed analysis)
• For a hairpin HEX with Multitube Longitudinal Finned Inner Tubes
– Get Dh and De using
– Unfinned, finned, and total outside HT surface areas
ftft2o
2ic
ftftoh
ftftoiw
NNHNdD4
A
)formulaincorrectionnote(NNH2NdP
NNH2NdDP
ffofut
fftf
fotu
HN2dLtN2AAA
H2LNN2A
LNLdN2A
Thermal / Hydraulic Design (continued)
– Overall HT coefficient based on outer area of inner tubes
where
is the overall surface efficiency
Area ratios At/Ai and Af/At are needed
Rw is for bare tube wall
f is the efficiency of a rectangular continuous longitudinal fin (for other types of fins, use references)
* h affects fin efficiency; have the fluid with the poorest HT properties on the finned side
ooo
fowtfi
i
t
ii
tf
h1R
RARA
A
hA
A1
U
t
ffo A
A11
*
ff
ff k
h2m,
mH
mHtanh
Thermal / Hydraulic Design (continued)
• The heat transfer equation is (heat duty equation)
• The design problem, in general, includes determining the total outer surface area of the inner tubes from the above equation.
• If the length of hairpins is fixed, then Nhp can be calculated.
• U can also be based on the inner area of the inner tubes, Ai
• For counterflow and parallel flow arrangements, no correction is necessary for Tm. However, if hairpins are arranged in series/parallel, a correction must be made (later).
• Study Example 6.2 (detailed analysis)
Thermal / Hydraulic Design (continued)
mthp TAUNQ
mihpi
tii
TANUQ
LNd2A
• If the design indicates large Nhp, it may not be practical to connect them all in series for pure counterflow. A large quantity of fluid through pipes may result in p > pallowable
• Solution: Separate mass flow into parallel streams, then connect smaller mass flow rate side in series. This is a parallel-series arrangement.
• If such a combination is used, the temperature difference of the inner pipe fluid will be different for each hairpin.
• Thus, in each hairpin section, different amounts of heat will be transferred and true mean temperature difference, Tm will be different from the LMTD.
Parallel / Series Arrangement of Hairpins
• The true mean temperature difference in
becomes
dimensionless quantity S is
• For n hairpins, S depends on the number of hot-cold streams and their series-parallel arrangement.
• Simplest case is to either divide the cold fluid equally between n hairpins in parallel or to divide the hot fluid equally between n hairpins in parallel.
mthp TAUNQ
1c1hm TTST
1c1ht
2h1hp
TTUA
TTcmS
• For one-series hot fluid and n1-parallel cold streams,
• For one-series cold fluid and n2-parallel hot streams,
• Then, the total heat transfer rate is
1c2c1
2h1h1
1c1h
1c2h1
1
n/1
11
1
1
11
1
TTn
TTR,
TT
TTP
R1
P1
R1R
ln1R
Rn
P1S
1
1c2c
2h1h22
1c1h
2c1h2
2
n/1
22
2
2
2
TT
TTnR,
TT
TTP
RP1
R1lnR1
n
P1S
2
1c1h TTUASQ
• In the previous equations, it is assumed that U and cp of the fluids are constant, and the heat transfer rates of the two units are equal.
• Graphs are available in literature for LMTD correction factor F as well.
• If number of tube-side parallel paths is equal to the number of shell-side parallel paths, regular LMTD should be used.
• Total pressure drop includes frictional pressure drop, entrance and exit pressure drops, static-head, and the momentum-change pressure drop.
• Frictional pressure drop is
• For frictional pressure drop, use correlations from Chapter 4 or Moody diagram. Add equivalent length of the U-bend to the L in tube-side (Dh = di) pressure drop.
• You may need to account for the effect of property variations on friction factor.
Total Pressure Drop
2
uN
D
2L4fp
2m
hph
• Entrance and exit pressure drops through inlet and outlet nozzles is evaluated from
where Kc = 1.0 at the inlet and 0.5 at the outlet nozzle.
• Static head is pf = H, where H is the elevation difference between inlet and outlet nozzles.
• For fully developed conditions, momentum-change pressure drop is
• In all pressure drop calculations for design, allowable p must be considered.
• Cut-and-twist technique increases h in longitudinal finned-tube HEX. See book for p details.
Total Pressure Drop (continued)
2
uKp
2m
cn
io
2m
11Gp
• In hairpin HEX, two double pipes are joined at one end by a U-tube bend welded to the inner pipes, and a return bend housing on the shell-side. The housing has a removable cover to allow removal of inner tubes.
• Double-pipe HEX have four key design components– shell nozzles
– tube nozzles
– return-bend housing and cover plate on U-bend side
– shell-to-tube closure on other side of hairpin(s)
• The longitudinal fins made from steel are welded onto the inner pipe. Other materials can be joined by soldering.
• Multiple units can be joined by bolts and gaskets.
• For low heat duty applications, simple constructions, easy assembly, lightweight elements and minimum number of parts contribute to minimizing costs.
Design and Operational Features