ROSE-HULMAN Institute of Technology

Department of Mechanical Engineering


ME462

Thermal Design

Homework 3

Problem 1

Water enters a parallel flow, double-pipe heat exchanger at a rate of 70 kg/min and is heated from 15°C to 60°C by an oil with a specific heat of 1.9 kJ/kg-K. The oil enters at 116°C and leaves at 27°C. The overall heat transfer coefficient is 300 W/m2-K. What heat transfer area is required? (Compare to HW 2, problem 3.)

Problem 2

A double-pipe heat exchanger with the dimensions shown in the figure is used to heat water (Tin = 50°F). Steam at 250°F is condensing on the outside with convective heat transfer coefficient ho = 25000 Btu/ft2-hr-°F. The overall heat transfer coefficient is Uo = 337 B/hr-ft2-°F. If the water velocity is 3 ft/s, find the outlet temperature of the water.

  1. Find the exit temperature of the water for the heat exchanger as drawn in the figure; i.e., for counter flow.
  2. Find the exit temperature of the water for a parallel flow arrangement instead.
HW3_2.GIF

Problems 3-6

Complete LMTD-F problems handed out in class. Each is to be completed as a separate problem.

Problem 7

In a single-pass counterflow heat exchanger, 10,000 lbm/hr of water (c = 1.0 Btu/lbm-°F) enters at 60°F and cools 20,000 lbm/hr of oil (c = 0.5 Btu/lbm-°F), which enters at 200°F. For the exchanger, UA = 5,500 Btu/hr-°F. Determine the fluid outlet temperatures.

Problem 8

In a very long (A → ∞) parallel-flow heat exchanger, Cmin (hot) is 0.75Cmax (cold). If the inlet temperature on the hot side is 300°F and the inlet temperature on the cold side is 100°F, find the exit temperatures of the cold and hot fluids. What is the effectiveness?