ME421Heat 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

• 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

IPS: inch per second (unit system)NFA: net flow area