Transcript ME421 Heat Exchanger and Steam Generator Design

ME421
Heat Exchanger and
Steam Generator Design
Lecture Notes 6
Double-Pipe Heat Exchangers
Introduction
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
Usage Areas / Advantages
• 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
Thermal / Hydraulic Design
Inner Tube
• Use correlations to find HT coefficient and friction factor
• Total pressure drop
2
um
2L
p  4f
Nhp
di
2
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)/d
Study Example 6.1 (detailed analysis)
o
Thermal / Hydraulic Design (continued)
• For a hairpin HEX with Multitube Longitudinal Finned
Inner Tubes
– Get Dh and De using
Pw  Di  doNt   2Hf NtNf
Ph  doNt  2Hf NtNf
Ac 

(note correction in formula)

 2
Di  do2Nt  Hf NtNf
4
– Unfinned, finned, and total outside HT surface areas
A u  2Nt doL  Nf L
A f  2NtNf L2Hf  
A t  A u  A f  2NtLdo  2Nf Hf 
Thermal / Hydraulic Design (continued)
– Overall HT coefficient based on outer area of inner tubes
Uf 
where
1
At
A
R
1
 t Rfi  A tR w  fo 
A ihi A i
ho hoho

Af 
ho  1  1  hf 
 is the overall surface efficiency
At 

Area ratios At/Ai and Af/At are needed
Rw is for bare tube wall
hf
tanhm Hf 

,
m Hf
m
2h
k f
*
hf 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
Thermal / Hydraulic Design (continued)
• The heat transfer equation is (heat duty equation)
Q  UNhp A t Tm
• 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
A i  2diLNt 
Q  UiNhp A i Tm
• 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)
Parallel / Series Arrangement of Hairpins
• 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.
• The true mean temperature difference in Q  UNhp A t Tm
becomes
Tm  STh1  Tc1
dimensionless quantity S is
S
 c p Th1  Th2 
m
UA t Th1  Tc1 
• 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.
• For one-series hot fluid and n1-parallel cold streams,
S
1  P1
1/ n1

 n1R1  R1  1  1 
1


 ln
 

R1 
 R1  1   R1  P1 

Th2  Tc1
Th1  Th2
P1 
, R1 
Th1  Tc1
n1Tc 2  Tc1 
• For one-series cold fluid and n2-parallel hot streams,
S
1  P2
1/ n 2


 n2 
 1

 ln1  R 2  
 R2 

 1  R 2  
 P2 

Th1  Tc 2
n2 Th1  Th2 
P2 
, R2 
Tc 2  Tc1 
Th1  Tc1
• Then, the total heat transfer rate is Q  UASTh1  Tc1 
• 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
• Total pressure drop includes frictional pressure drop,
entrance and exit pressure drops, static-head, and the
momentum-change pressure drop.
• Frictional pressure drop is
2
um
2L
p  4f
Nhp
Dh
2
• 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 (continued)
• Entrance and exit pressure drops through inlet and outlet
nozzles is evaluated from
2
um
pn  K c
2
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


1
1
pm  G 
 
 o i 
2
• In all pressure drop calculations for design, allowable p
must be considered.
• Cut-and-twist technique increases h in longitudinal finnedtube HEX. See book for p details.
Design and Operational Features
• 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.
IPS: inch per second (unit system)
NFA: net flow area