BKF2422 HEAT TRANSFER

ASSIGNMENT 2: CHAPTER 4 & 5

Due Date: 1/4/2011

One assignment report per group

Heat Exchanger

1. 5.795 kg/s of oil flows through the shell side of a two-shell pass, four tube-pass oil cooler. The oil enters at 181°C and leaves at 38°C. Water flows in the tubes, entering at 32°C and leaving at 49°C. In addition, cpoil= 2282 J/kg·K and U = 416 W/m2K. Find how much area the heat exchanger must have. (R = Z and P = Y). Hint: F(P,R) = F(PR, 1/R) (3.15). Thus, if R is greater than unity, one need only evaluate F using PR in place of P and 1/R in place of R.

2. Consider the following parallel-flow heat exchanger specification:

cold flow enters at 40°C: Cc = 20, 000 W/Khot flow enters at 150°C: Ch = 10, 000 W/KU = 500 W/m2K.

(a) Determine the heat transfer and the exit temperatures when is A = 30 m2.

(b) Determine the area that would bring the hot flow out at 90 °C.

3. A heat exchanger is to be designed to cool mh = 8.7 kg/s an ethyl alcohol solution (cph

= 3840 J/kg.°C) from 75°C to 45°C with cooling water (cpc = 4180 J/kg.°C) entering the tube side at 15°C at a rate of mc = 9.6 kg/s. The overall heat transfer coefficient based on the outer tube surface is Uo = 500 W/m2.°C. Calculate the heat transfer area for each of the following flow arrangements:

(a) Parallel flow, shell and tube(b) Counter flow, shell and tube(c) One shell pass and two tube pass(d) Cross flow, both fluids unmixed

(Refer figure 4.9-5 in text book)

Radiation

4. Two very large and parallel planes each have an emissivity of 0.7. Surface 1 is at 866.5 K and surface 2 is at 588.8 K. (a) What is the net radiation loss of surface 1?(b) To reduce this loss, two additional radiation shields also having emissivity of 0.7

are placed between the original surfaces. What is the new radiation loss?

5. Two parallel plates at T1 = 800 K and T2 = 600 K have an emissivity of ε1 = 0.5 and ε2

= 0.8, respectively. A radiation shield having an emissivity ε = 0.1 on one side and an emissivity ε3,2 = 0.05 on the other side is placed between the plates. Calculate the heat transfer rate by radiation per square meter with and without the radiation shield.

6. Two adjacent rectangles are perpendicular to each other. The first rectangle is 1.52 x 2.44 m and the second 1.83 x 2.44 m, with the 2.44 m side common to both. The temperature of the first surface is 699 K and that of the second is 478 K. Both surfaces are black. Calculate the radiant heat transfer between the two surfaces.

7. A radiator may be treated as a black body with a true surface area of 6 m2 and an envelope area of 4 m2. It has a surface temperature of 40°C and is situated in a dark room at 20°C. The surface heat transfer coefficient is 4 W/m2.K. Calculate the radiated heat transfer and the convected heat transfer rate. Calculate the radiated surface heat transfer coefficient and obtain the same answers using it.(985 W and 6.32 W/m2.K4)

8. Two concentric spheres of diameter D1= 0.8 m and D2= 1.2 m are separated by an air space and have surface temperatures of T1 = 400 K and T2 = 300 K.

a. If the surfaces are black, what is the net rate of radiation exchange between the spheres?

b. What is the net rate of radiation exchange between the surfaces if they are diffuse and gray with ε1 = 0.5 and ε2 = 0.05?

c. What is the net rate of radiation exchange if D2 is increased to 20 m, with ε2 = 0.05, ε1 = 0.5, and D1= 0.8 m? What error would be introduced by assuming blackbody behaviour for the outer surface (ε2 = 1), with all other conditions remaining the same?

9. Determine the shape factor, F12, for the rectangles shown in the Figure 1 below. Use Figure 13.4 and Figure 13.6 in the reference from Incropera (5 th Edition) to assist you in the calculations. Use also reciprocity and additive rules in solving this problem. a. Perpendicular rectangles without a common edge.

Figure 1

b. Parallel rectangles of unequal areas.