Examen la disciplina FENEOMENE DE TRANSFER 1. ntr-o instalaie de evaporare din industria zahrului, care funcioneaz n regim continuu,

staionar, se supune concentrrii o soluie avnd iniial 10% masice zahr n ap, pn la concentraia final de 40% masice. Se cere:

1.1. S se calculeze debitul de ap evaporat (kg/h), tiind c instalaia este alimentat cu 100 m3/h soluie diluat, avnd temperatura de 20C; (1 punct) 1.2. Ce debit volumic (L/min) de soluie cu 40% masice zahr iese din instalaie, tiind c temperatura de ieire a soluiei este de 60C; (1 punct) 1.3. Soluia concentrat (100 m3/h; 40% masice zahr; 60C) este evacuat din instalaie printr-o conduct circular avnd lungimea de 45 m i diametrul interior de 50 mm. Calculai numrul Re al soluiei i determinai regimul de curgere al acesteia. (1 punct)

2. Soluia concentrat de zahr (40% masice zahr i 60C) se supune rcirii n vederea cristalizrii, pn la temperatura de 40C, ntr-un schimbtor de cldur cu fascicul tubular (SCFT) avnd urmtoarele caracteristici: lungimea evilor, L = 2 m; diametrul interior al evilor, di = 50 mm; grosimea peretelui evilor, = 2 mm; diametrul interior al mantalei, Di = 40 cm; material de construcie: OL ( = 46,5 W.m-1.K-1).

A

A

Sectiunea A - AApa

sol.zahar

tevi

manta

Se cere: 2.1. S se calculeze necesarul de ap de rcire (n m3/h i kg/s) , tiind c debitul de soluie de zahr supus rcirii este de 4 t/h, apa de rcire intr n schimbtor cu 10C i iese cu 30C; (1,5 puncte) 2.2. S se reprezinte grafic variaia temperaturii celor dou fluide de-a lungul SCFT i s calculeze diferena medie de temperatur ntre apa de rcire i soluia de zahr la circulaia n echicurent, respectiv n contracurent; (1,5 puncte) 2.3. Dac apa de rcire circul printre evi (adic prin manta) cu o vitez medie de 0,5 m/s, i se nclzete de la 10 la 30C, s se calculeze coeficientul individual de transfer de cldur al apei. (3 puncte)

Proprietile termofizice ale soluiei de zahr n funcie de concentraie i temperatur: Concentraie 10 % masice 40 % masice Temperatura 20C 40C 60C 20C 40C 60C Densitate (g/cm3) 1,040 1,030 1,020 1,180 1,140 1,100 Viscozitate (mPa.s) 1,945 1,184 0,810 6,167 3,241 1,970 Capacitate termic (kJ.kg-1.K-1) 3,9386 3,9805 3,9805 3,2263 3,3101 3,3520 Conductivitate termic (W.m-1.K-1) 0,5664 0,5943 0,6176 0,4698 0,5024 0,5117

Proprietile termofizice ale apei n funcie de temperatur:

Temperatura 283 K 293 K 303 K Densitate (kg/L) 1,000 0,998 0,996 Viscozitate 106 (N.s.m-2) 1310 1000 804 Capacitate termic (J.kg-1.K-1) 4190 4190 4190 Conductivitate termic (W.m-1.K-1) 0,575 0,599 0,618

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