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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.