Thermal Unit Operation

(ChEg3113)

Instructor: Mr. Tedla Yeshitila (M.Sc.)

Lecture 7- Double Pipe Heat Exchanger Design

Today…

• Review

• Some points about double pipe design

• Continue Example

Review

Deign steps for double pipe heat exchanger

Example

Chapter 4

Design of Double Pipe Heat Exchanger The standard sizes of Tees and return heads for double pipe heat

exchanger fittings are given in the table below.

For standard arrangements of double pipes the flow area and

equivalent diameter is given in the table below.

Outer pipe, IPS Inner pipe, IPS

2 1 ¼

2 ½ 1 ¼

3 2

4 3

Exchanger,

IPS

Flow area, in2 Annulus, in Annulus Pipe de de`

2 X1 ¼ 1.19 1.50 0.915 0.40

2 ½ X 1 ¼ 2.63 1.50 2.02 0.81

3X2 2.93 3.35 1.57 0.60

4X3 3.14 7.38 1.14 0.53

Chapter 4

Design of Double Pipe Heat Exchanger Double pipe exchangers are usually assembled in 12,15 or 20 ft

effective length.

Effective length means the distance in each leg over which heat

transfer occurs and excludes inner pipe protruding (extend)

beyond the exchanger section.

When hairpins are employed in excess of 20ft in length

corresponding to 40 effective linear feet or more of double pipe,

the inner pipe tends to sag and touch the outer pipe, which

causes poor flow distribution in the annulus.

Chapter 4

Design of Double Pipe Heat Exchanger The principal disadvantage to the use of double pipe exchangers

lies in the small amount of heat transfer surfaces contained in a

single hairpin.

When it used with distillation equipment's on industrial process

a very large number are required. These requires considerable

space, and each double pipe exchanger introduces no fewer than

14 points at which leakage might occur.

The time and expense required for dismantling and periodically

cleaning the double pipe are larger compared with other types of

equipment's.

Chapter 4

Design of Double Pipe Heat Exchanger However, the double pipe exchanger is of great use where the

total required heat transfer surface is small 100 to 200 ft2 or less.

For fluid in pipes and tubes, Sieder and Tate made a correlation

for both heating and cooling fluids, principally petroleum

fractions, in horizontal and vertical tube.

For streamline flow where Re=𝐷𝐺

𝜇 < 2,100

ℎ𝑖𝐷

𝑘= 1.86

𝐷𝐺

𝜇

𝑐𝜇

𝑘

𝐷

𝐿

1/2μ

μ𝑤

0.14

= 1.864𝑤𝑐

𝛱 𝑘𝐿

1/2 μ

μ𝑤

0.14

Where Re is Reynold number, μ𝑤

is the viscosity at the tube wall

temperature and μ is the viscosity at caloric temperature.

Chapter 4

Design of Double Pipe Heat Exchanger The above equation gave maximum mean deviation of

approximately ±12% from Re=100 to Re=21,000 except for water.

Beyond the transition range, the data may be extended to turbulent

flow where Re=𝐷𝐺

𝜇 > 10,000

ℎ𝑖𝐷

𝑘= 0.027

𝐷𝐺

𝜇

0.5𝑐𝜇

𝑘

1/3 μ

μ𝑤

0.14

The above equation gave maximum mean deviation of

approximately +15% and -10% for Re above10,000 except for

water.

The above equations can be applicable for organic liquids, aqueous

solutions, and gases. But they are not conservative for water, and

additional data for water must be given.

Chapter 4

Design of Double Pipe Heat Exchanger Even though the above equations are obtained for tubes, they will

also be used indiscriminately for pipes.

Pipes are rougher than tubes and produces more turbulence for

equal Reynold number.

Coefficients calculated from tube-data correlations are actually

lower and safer than corresponding calculation on pipe data. And

also there are no pipe correlation in the literature as tube

correlations.

Chapter 4

Design of Double Pipe Heat Exchanger

The above two equations can be also shown graphically using

single pair of coordinates. So, using the ordinate

𝑗𝐻 =ℎ0𝐷𝑒

𝑘

𝑐𝜇

𝑘

−1

3 𝜇

𝜇𝑤

−0.26 and abscissa

𝐷𝐺𝑝

𝜇

Chapter 4

Design of Double Pipe Heat Exchanger When a fluid flows in a conduit having other than a circular cross

section, such as an annulus, it is convenient to express heat

transfer coefficient and friction factors by the same types of

equations and curves used for pipes and tubes.

To permit this type of representation for annulus heat transfer, it

has been advantageous to employ an equivalent diameter De.

This equivalent diameter is four times the hydraulic radius.

The hydraulic radius is the radius of pipe equivalent to the

annulus cross section.

The hydraulic radius is obtained as the ratio of the flow area to

the wetted perimeter.

Chapter 4

Design of Double Pipe Heat Exchanger For a fluid flowing in annulus, the flow area is (𝛱/4) (D2

2-D12),

but the wetted perimeter for heat transfer and pressure drop are

different.

For heat transfer, the wetted perimeter is the outer circumference

of the inner pipe with diameter D1, so 𝛱D1, so

De=4rh=4∗𝑓𝑙𝑜𝑤 𝑎𝑟𝑒𝑎

𝑤𝑒𝑡𝑡𝑒𝑑 𝑝𝑒𝑟𝑖𝑚𝑒𝑡𝑒𝑟=

4𝛱 𝐷22−𝐷1

2

4𝛱𝐷1=

𝐷22−𝐷1

2

𝐷1

In pressure drop calculation, the friction not only results from

the resistance of the outer pipe, but is also affected by the outer

surface of the inner pipe.

Chapter 4

Design of Double Pipe Heat Exchanger The total wetted perimeter is 𝛱 𝐷2 + 𝐷1 , and the pressure

drop in annuli will be:

De`=4rh=4∗𝑓𝑙𝑜𝑤 𝑎𝑟𝑒𝑎

𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑤𝑒𝑡𝑡𝑒𝑑 𝑝𝑒𝑟𝑖𝑚𝑒𝑡𝑒𝑟=

4𝛱 𝐷22−𝐷1

2

4𝛱 𝐷2+𝐷1= 𝐷2 − 𝐷1

Therefore, Re for the same conditions, w, G, and μ are different

for heat transfer and pressure drop since De might be above

2,100 while De` is below 2,100.

Actually, both Re should be considered only approximations,

since the sharp distinction between streamline and turbulent flow

at Re of 2,100 is not completely valid.

Chapter 4

Design of Double Pipe Heat Exchanger Even tough D differ from De, ho is effective at the outside

diameter of the inner pipe.

In double pipe exchangers, it is customary to use outside surface

of the inner pipe as the reference surface in Q=UA𝛥T, and hence

hi has to be determined for Ai and not A, it must be corrected.

hi is based on the area corresponding to the inside diameter

where the surface per foot of length is 𝛱*ID. On the outside of

the pipe the surface per foot of length is 𝛱*OD.

By letting ℎ𝑖𝑜 be the value of hi referred to the outside

diameter. ℎ𝑖𝑜 = ℎ𝑖𝐴𝑖

𝐴= ℎ𝑖

𝐼𝐷

𝑂𝐷

Chapter 4

Design of Double Pipe Heat Exchanger

Uc will remain constant if the scale or dirt deposit does not alter

the mass velocity by constructing the fluid flow area.

UD and 𝛥T will obviously change as the dirt accumulates

because the temperature of the fluids will vary from time the

surface is freshly placed in service until it becomes fouled.

a) Annulus diameters and b) location of coefficients

Chapter 4

Design of Double Pipe Heat Exchanger If 𝛥T calculated from observed temperature instead of

process temperature, then Q= UD A𝛥T may be used to

determine 𝑅𝑑 for a given fouling period.

𝑅𝑑 =1

𝑈𝑐−

1

𝑈𝐷=

𝑈𝑐−𝑈𝐷

𝑈𝑐 𝑈𝐷

When a cylinder is very thin compared with its diameter, as a

layer of dirt, its resistance is nearly the same as that of

thorough a flat. Error <1%.

For thick scale, the error may be appreciable.

Chapter 4

Design of Double Pipe Heat Exchanger When 𝑅𝑑 (deposited) > 𝑅𝑑 (allowed), as after service period the

apparatus no longer delivers a quantity of heat equal to the

process requirements and must be cleaned.

𝑅𝑑 value important for two reasons:

1. To protect the heat exchanger from delivering less than

required process heat load for specific period

2. To decide or consider dismantling and cleaning interval

Numerical values of the dirt or fouling factors for a variety of

process service are provided in Kern book Appendix Table 12.

Chapter 4

Design of Double Pipe Heat Exchanger The tabulated values may differ from those encountered by

experience in particular service.

If too frequent cleaning is necessary, a greater value of

𝑅𝑑 should be kept in mind for future design.

It is expected that heat transfer equipment will transfer more

heat than the process requirements when newly placed in service

and that it will deteriorate through operation as result of dirt. In

this case use Uc instead of UD which is useful to check whether

or not clean exchanger will be able to deliver the process heat

requirement when it becomes dirty.

Chapter 4

Design of Double Pipe Heat Exchanger Example 1: Double pipe Benzene - Toluene Exchanger

It is desired to heat 9,820 lb/hr of cold benzene from 80 to

120OF using hot toluene which is cooled from 160 to 100OF.

The specific gravities at 68 OF are 0.88 and 0.87, respectively.

The other fluid properties will be found from Appendix.

A fouling factor of 0.001 should be provided for each stream,

and the allowable pressure drop on each stream is 10.0psi.

A number of 20 ft hairpins of 2 by 1 ¼ in. IPS pipe are available.

How many hairpins are required?

At the end of this class:

• You will be able to design double pipe heat exchanger

– Thermal design

End of lecture -7