1. Double-Pipe Heat Exchangers Introduction Double Pipe Heat
Exchanger (DP HEX): one pipe placed concentrically inside another
where one fluid flows through inner pipe the other through the
annulusinner pipe, the other through the annulus Outer pipe is
sometimes called the shell I i t d b U h d t b d l d Inner pipe
connected by U-shaped return bends enclosed in a return-bend
housing to make up a hairpin, so DP HEX = hairpin HEX= hairpin HEX
Hairpins are based on modular principles: they can be arranged in
series parallel or series-parallel combinationsarranged in series,
parallel, or series parallel combinations to meet pressure drop and
MTD requirements; add-remove as necessary Introduction
2. Two hairpins connected in series H i i ith liHairpins with
annuli connected in series and inner pipes connected ininner pipes
connected in parallel
3. Four double pipe sections; To increase the capacity 50%, To
decrease the capacity, shut d b kpipe sections; 2 parallel x 2
series capacity 50%, merely add one bank to be six do ble pipe down
one bank; 1 parallel x 2 series double pipe sections; 3 parallel x
2 series 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 andg ( y
maintain) Finned tubes can be used to increase HT surface per unitp
length, thus reduce length and number of hairpin (Nhp)
Outside-finned inner tubes most efficient when low h fluid (oil( or
gas) flows through annulus Multiple tubes can be used inside the
shellp Used as counterflow HEX, so they can be used as an
alternative to shell-and-tube HEX
4. Double-pipe Hairpin Bare and Multi-tube Advantages: 1. Tube
bundle is removable; therefore mechanical cleaning is possible on
the shell side. 2. The U shape bundle is free for expansion and
contraction inside thep p Hairpin shell eliminating the need for
expansion joint. 3. Are capable of carrying the maximum pressure
allowable by ASME Code per given wall thickness (Up to 14600 psi
with no corrosion allowance)per given wall thickness. (Up to 14600
psi with no corrosion allowance). Higher pressure ratings are
possible using materials with higher stress values 4 For processes
that require frequent mechanical cleaning bare tube offers4. For
processes that require frequent mechanical cleaning, bare tube
offers ease of cleaning and accessibility. 5. Bare Multi-Tube and
Double-Pipe Exchangers offer the least pressure d t hdrop among
most exchangers. 6. Very often a process might be modified or
completely changed. The streams flow rate or other conditions might
also change. A Double Pipe Hairpin is designed so as to accommodate
these changes, simply by rearranging the sections. Unlimited
numbers of sections could be arranged in parallel and series to fit
new requirements of the process cong p q p Limitations: Only
chemical tube side cleaning is possible. Finned Double Pipe &
Multi-tube Ad tAdvantages: 1. The same advantages as the bare tubes
mentioned above 2 The finned hairpin usually has up to four times
more heat transfer2. The finned hairpin usually has up to four
times more heat transfer surface than bare tube hairpin. This would
especially be more advantageous when the shell side heat transfer
coefficient is low, th f i ftherefore requires more surface 3. Good
application for high shell side viscosity with low heat transfer
coefficient. Finned hairpins are particularly good application for
cooling viscous fluids. The viscosity on the fin wall is higher
than the average bulk viscosity which produces a lower film
coefficient on the fin, and reduces heat transfer and causes
excessive fouling. Limitations: Only chemical cleaning of the tubes
finned surface is possible.
5. I h i i HEX t d bl i j i d t d b 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 removablehousing on the shell
side. The housing has a removable cover to allow removal of inner
tubes. Double-pipe HEX have four key design componentsDouble 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, easyy pp , p
, y assembly, lightweight elements and minimum number of parts
contribute to minimizing costs.
6. Cross section of a longitudinal finned inner tube heat
exchangerg g Thermal / Hydraulic Designy g Inner Tube Use
correlations to find HT coefficient and friction factor Use
correlations to find HT coefficient and friction factor Total
pressure drop 2L 2 2 u N d 2L 4fp 2 m hp i AnnulusAnnulus 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 - doh i o De = (Di 2 -
do 2)/do
7. Thermal / Hydraulic Design (continued) For a hairpin HEX
with Multitube Longitudinal Finned Inner Tubes Get Dh and De using
ftftoiw NNHNdDP 2 ftftoh ftftoiw NNHNdP 2 U fi d fi d d t t l t id
HT f ftftoic NNHNdDA 22 4 Un-finned, finned, and total outside HT
surface areas fotu LNLdNA 2 fftf fotu HNdLNAAA HLNNA 22 22 ffotfut
HNdLNAAA 22 Thermal / Hydraulic Design (continued) Overall HT
coefficient based on outer area of inner tubes f RAA U 1 1 where
ooo fo wtfi i t ii t h R RAR A A hA A 1 A is the overall surface
efficiency t f fo A A 11 Area ratios At /Ai and Af / At are needed
Rw is for bare tube wall *w * 2 , tanh ff f f k h m mH mH f is the
efficiency of a rectangular continuous longitudinal fin (for other
types of fins, use references) * Note that since h affects the fin
efficiency, the fluid with the poorest HT properties should be
allocated on the finned side
