RESPONSI MATA KULIAH: DASAR KETEKNIKAN PENGOLAHAN

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1 RESPONSI MATA KULIAH: DASAR KETEKNIKAN PENGOLAHAN Gedung Pascasarjana THP, JUNI 2014

2 Fluid flow 1

3 Fluid flow 2

4 Fluid flow 3

5 Fluid flow Item Pipa lurus Fitting Contraction Enlargment f EF Laminar/ Turbulen(pipa halus/kasar) K f (V 1 ) 2 /α [1-(A 1 /A 2 ) 2 ](V 1 ) 2 /α ΔP 2f(v 2 )Lρ/D 2f(v 2 )L ρ/d EF x ρ EF x ρ L /D lihat Tabel 6.3 4

6 Contoh soal Sebuah sample diuji menggunakan viskometer rotasional. Pada skala pembacaan penuh, percobaan tersebut menghasilkan konstanta pegas = 6374 dyne-cm. Tentukan parameter reologinya jika OD = 2 cm, ID= 1.5 cm, serta L= 6cm. Hasil percobaan menunjukkan hasil sbb : N (RPM) Torsi (skala penuh) Solution 5

7 Contoh soal Hitung tekanan yang harus diberikan pompa untuk mengalirkan 400 L/menit fluida yang mempunyai densitas g/cm 3 dan viskositas 100 cp. Fluida tersebut mengalir melalui sanitary pipe (1.50-in nominal) dengan panjang 60 m. Pipa lurus dan mempunyai ketinggian yng sama, dan bagian pengeluaran terbuka (tekanan atmosfir). Diketahui bahwa sanitary pipe yang digunakan merupakan pipa kasar terbuat dari commercial steel. 6

8 Solution: Q = 600 L menit 3 1m x 1000L x 1menit 60 det 3 m 0.01 det 6 3 g 10 cm kg = 1.52 x x cm m 10 g kg m 3 Diameter (1.50 in nominal, sanitary pipe)< : D = in = m g kg = 100 cp 1poise Pa.s cm.s m.s L = 60 Q area 3 m x det (0.0356/2) = 2 2 m m / det 7

9 Solution: D Re 1520x10.052x f ( ) L P D 2x0.0092x(10.052) Pa 2 x60x1520 (turbulen) f melihat diagram Moody, ɛ = mm = ɛ/d = / = f =

10 Contoh soal Sebuah fluida dengan 1.05 g/cm 3 dan viskositas 200 cp dialirkan melalui sanitary pipe (3-in nominal) sepanjang 20 m yang disambung dengan sanitary pipe (2-in nominal) sepanjang 20 m dan memiliki bagian exhaust pipa yang terbuka. Pada pipa yang lebih besar dipasang 90 elbow std. Berpakah power (dalam KW) yang harus dihasilkan pompa agar memenuhi laju alir fluida sebesar 12 L/detik? Solution 9

11 Heat Transfer Konduksi melalui Dinding datar Konduksi Melalui Silinder Konveksi q = T 1 T 2 x/ka q = (T 1 T 2 ) (r 2 r 1 )/(ka lm ) = (T 1 T 2 ) R A lm = A 2 A 1 ln(a 2 /A 1 ) R=ln(r 2 /r 1 )/(2 kl) q = ha (T w T f ) 10

12 Heat Transfer Kombinasi melalui Dinding datar q = U A T overall, U = 1/ (1/h i +k A / x A + 1/h o ) Silinder q = T 1 -T 4 1/h i A i + (r o -r i )/k A A Alm + 1/h o A o q = U i A i (T 1 -T 4 ) = U o A o (T 1 -T 4 ) U i = 1 1/h i +(r o -r i )A i /k A A Alm + A i /A o h o U o = 1 A o /A i h i +(r o -r i )A o /k A A Alm + 1/h o 11

13 Contoh soal 1. What is the rate of saturated steam(kg/h) with kpa pressure is required to heat 100 kg/h of juice from 58 C to 95 C? Assume that the heat capacity of the juice is 4 kj/kg C. solution: Q = m.cp.δt = m. ΔH use steam table Q = 100 kg/h x 4 kj/kg C x (95-58) C = kj/h ΔH = = kj/kg m = Q/ΔH = kg/h 12

14 Latihan soal a. For quick preparation of a cup of hot chocolate in a cafeteria, cocoa powder and sugar are added in a cup of water and the solution is heated by direct steam injection. If the initial temperature of all the ingredients is 15 C, the final temperature is 95 C, the mass of the solution is 150 g initially, and the heat capacity of the solution is 3.8 kj/kg C, calculate how much saturated steam at 110 C will be used. State your assumptions. 12

15 Contoh soal Contoh: 2. Hitung penyerapan panas (secara konduksi) per m 2 luas permukaan dari es balok dengan ketebalan 15 cm, dengan suhu di dalam 270 K dan suhu luar 273 K. Konduktivitas panas es 2.25 W/m.K Penyelesaian: Q/A = k.δt/ Δx = 2.25 ( )/0.15 = 45 W/m 2 12

16 Contoh soal Contoh: 3. Hitung panas yang diserap secara konveksi per m 2 luas permukaan dari es balok dengan ketebalan 15 cm yang diletakkan pada bak penyimpanan (bagian ujung-unjung dan bawah terisolasi) dan bagian atas terkena udara. Suhu di dalam es 270 K dan suhu luar es 273 K. Konduktivitas panas es 2.25 W/m.K, koefisien transfer panas udara(h),6 W/m 2.K asumsikan suhu udara 298 K. Penyelesaian: (Q/A) 2 =h(t udara T es )=6 ( ) = 150 W/m 2 12

17 Contoh soal Contoh: 4. Hitung panas total yang diserap per m 2 luas permukaan dari es balok dengan ketebalan 15 cm yang diletakkan pada bak penyimpanan (bagian ujung-unjung dan bawah terisolasi) dan bagian atas terkena udara. Suhu di dalam es 270 K dan suhu luar es 273 K. Konduktivitas panas es 2.25 W/m.K, koefisien transfer panas udara(h),6 W/m 2.K asumsikan suhu udara 298 K. Penyelesaian: (Q/A) 1 = U T overall = (1/6+2.25/0.15)x ( ) = W/m 2 12

18 Contoh soal 5. A stainless steel pipe (k= 17 W/m C) is being used to convey heated oil. The inside surface temperature is 130 C. The pipe is 2 cm thick with an inside diameter of 8 cm. The pipe is insulated with 0.04 m thick insulation (k= W/m C). The outer insulation temperature is 25 C. Calculate the temperature of interface between steel and isulation. Assume steady-state conditions. Pipa silinder seri 13

19 6. Find the heat transfer per unit area through the composite wall. Assume one-dimensional heat flow. Given: k A = 150 W/m C k B = 30 W/m C k C = 50 W/m C k D = 70 W/m C q A B C A A = A C = 0.1 m 2 A B = A D D T = 370 C T = 66 C 2.5 cm 7.5 cm 5.0 cm plate seri, peralel 14

20 Latihan soal b. A certain material has a thickness of 30 cm and a thermal conductivity of 0.04 W/m C. At a particular instant in time the temperature distribution with x, the distance from the left face, is T = 150x 2-30x, where x is in meters. Calculate the heat flow rates at x = 0 and x = 30 cm. Is the solid heating up or cooling down? c. A certain material 2.5 cm thick, with a cross-sectional area of 0.1 m 2, has one side maintained at 35 C and the other at 95 C. The temperature at the center plane of the material is 62 C, and the heat flow through the material is 1 kw. Obtain an expression for the thermal conductivity of the material as a function of temperature. 16

21 d. One side of a copper block 5 cm thick is maintained at 260 C. The other side is covered with a layer of fiber glass 2.5 cm thick. The outside of the fiber glass is maintained at 38 C, and the total heat flow through the copper-fiber-glass combination is 44 kw. What is the area of the slab? 15

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