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TKS 3249

Perpindahan Panas 

Kuliah 7

Design of Double-Pipe Heat Exchangers (Part 2) 

Semester Genap 2015-2016

Kelas A –

 Program Sarjana Teknik KimiaUniversitas Riau

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Finned-Pipe Exchangers

• The inner pipe of a double-pipe exchanger can

be equipped with rectangular fins.

•

Fins can be attached to both the internal andexternal pipe surfaces, external fins are most

frequently used.

• The fin material need not be the same as thepipe material.

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Finned-Pipe Exchangers 

FIGURE 4.3 (a) Rectangular fins on heat-exchanger pipes,

(b) Cross-section of a finned-pipe exchanger

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Finned-Pipe Exchangers 

Standard pipe pressure with schedule 40 pipe,and for high-pressure service using schedule 80

pipe.

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Finned-Pipe Exchangers 

The fin thickness is the same in both single and

multi-tube units.

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Fin Efficiency

• The efficiency of a rectangular fin,

where:

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Fin Efficiency 

• The weighted efficiency of the entire finned

surface is given by:

where,

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Overall Heat-Transfer Coefficient

• The overall coefficient is based on the total

external surface area, ATot , of the inner pipe.

•The overall resistance to heat transfer is thus1/UATot , and is the sum of the inner and outer

convective resistances, the conductive

resistance of the pipe wall, and the resistancesof the fouling layers (if present).

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Overall Heat-Transfer Coefficient 

• The outer convective resistance in a finned

annulus is:

• Therefore, viewing RDo as a reciprocal heat-

transfer coefficient, we have:

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Overall Heat-Transfer Coefficient 

• With Ai  = πDi L,

• The equation for the clean overall coefficient

is obtained by dropping the fouling terms:

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Flow Area and Equivalent Diameter

• The flow area and wetted perimeter are:

• The equivalent diameter is,

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Flow Area and Equivalent Diameter 

where:

D1 = OD of inner pipe.

D2 = ID of outer pipe.

nt  = number of inner pipes.

n f  = number of rectangular fins.

b = height of fin.

  = thickness of fin.

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Heat-Transfer Coefficients and

Friction Factors for Finned Annuli

• Heat-transfer coefficients and friction factors

were,

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Heat-Transfer Coefficients and

Friction Factors for Finned Annuli 

Heat-transfer coefficients for finned annuli

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Heat-Transfer Coefficients and

Friction Factors for Finned Annuli 

Friction factors for finned annuli

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Wall Temperature for Finned Pipes

• From energy balance and the weighted

average temperature gives,

• To reiterate, T  p is used  i  to find fi and T wtd  is

used for  o.

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All design criteria are met, with the exception of ∆Pi = 20.6 psi, which is

slightly above the specified maximum pressure drop of 20 psi. This discrepancyis not large enough to be a concern in most situations. If necessary, however, it

could be eliminated by using hairpins of length 24 ft rather than 25 ft.

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• The high over-surface is simply a reflection of the large effect

that internal fouling has on the design overall heat-transfer

coefficient.• The effect is much greater in finned exchangers because the ratio

of external to internal surface area is so high (5.93 in this case

versus 1.18 for an un-finned 1.5-in. schedule 40 pipe).

• With the fouling factors specified in this example, the two

fouling resistances account for 30% of the total thermalresistance.

• Clearly, in the design of finned exchangers, care must be

exercised to ensure that an appropriate value is selected for the

inner fouling factor.• To summarize, for the problem as given, the final design consists

of four hairpins connected in series on both sides with water in

the inner pipe and oil in the annulus

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class exercise

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Coursework due to 11/04/16

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Coursework due to 11/04/16

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Coursework due to 11/04/16

• Coursework merupakan tugas midtest.

• Jawaban harus diketik dengan MS Word dan

dicetak pada kertas HVS 70 gram berwarna

putih.

• Sebelum mengumpulkan coursework

mahasiswa wajib mendiskusikan pekerjaannya

dengan dosen/asissten dosen paling sedikit 3

(tiga) kali.

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Coursework due to 11/04/16 

• Baik hardcopy maupun softcopy jawaban akan

dikumpulkan, untuk itu dilarang keras

memberi jawaban atau file jawaban kepada

peserta kuliah lain.• Selain perhitungan disain, jawaban juga harus

dilengkapi dengan spesification sheet untuk

HE yang saudara disain.• Beberapa ketentuan lainnya akan disampaikan

ketika berdiskusi.