Heat Exchangers

Abstract

A heat exchanger is a heat transfer device that exchanges heat between two or more process fluids. Heat exchangers have widespread industrial and domestic applications. Many types of heat exchangers have been developed for use in steam power plants, chemical processing plants, building heat and air conditioning systems, transportation power systems and refrigeration units.

Problems

Problem 13.1: Rework Example 13.1 using the effectiveness-NTU method of analysis.

Problem 13.2: Instead of a single tube in Example 13.1, assume the water is in a tube bank that is 15 tubes on a side with the same dimensions as the tube in Example 13.1 and spaced on 35 mm centers. The oil now flows between the tubes and an effective hydraulic diameter must be calculated. All other specifications of the problem remain the same. Estimate the length of the tube bank required.

Problem 13.3: A tube and shell recuperator for a gas turbine engine has been designed with 1.5 cm diameter tubes on 2.5 cm centers (pitch = 2.5 cm) for the cold air flow. The tube material is copper and is 2 mm thick. The hot air flows through the shell in a counter flow arrangement. The overall length of the active section is 5 m. It has a cross section of 2 m by 2m. The cold flow enters at 15 atm pressure and 600 K. The hot flow enters at 1 atm and 800 K. Both flows are 2.0 kg/sec. Estimate the hot and cold flow exit temperatures and the total heat transferred based on the log-mean-temperature method. (Note: This will require iteration)

Problem 13.4: A tube and shell recuperator for a gas turbine engine has been designed with 1.5 cm diameter tubes on 2.5 cm centers (pitch = 2.5 cm) for the cold air flow. The tube material is copper and is 2 mm thick. The hot air flows through the shell in a counter flow arrangement. The overall length of the active section is 5 m. It has a cross section of 2 m by 2 m. The cold flow enters at 15 atm pressure and 600 K. The hot flow enters at 1 atm and 800 K. Both flows are 2.0 kg/sec. Estimate the hot and cold flow exit temperatures and the total heat transferred using the effectiveness-NTU method.

Problem 13.5: Add 1 mm copper webs between the tubes, like in Example 13.2, to the recuperator in problem 4 to improve its performance and recalculate the exit temperatures and total heat transferred.

Problem 13.6: A cross flow tube and shell condenser is installed below a 10 MW steam turbine that exhausts saturated steam at 20 kPa and 5.5 kg/s. The cooling water flows through copper tubes 1.5 cm in diameter on a 2.5 cm pitch in a square array. The tubes are 2 mm thick. The tube length is 3 m and the shell cross sectional area is 2 m by 3 m. The cooling water enters at 17 °C and flows at 10 kg/s. Estimate the cooling water exit temperature and the quality of the steam exiting the condenser.