Heat Recovery From Multi-Turbine Installations
Authors: Smith A.E. , Schaefer G.T. 
Published: December 2007

In an era when energy conservation and fuel shortages are not uncommon, mechanical systems designed to improve the thermal efficiency of fuel-consuming equipment have become a necessity. This paper presents an approach to the process and mechanical design of a energy in a gas-plant process.

Introduction

Rapidly rising energy costs have resulted in intensified searches for improving gas-plant energy balances and have provided the economic incentive to develop better energy recovery systems. Exxon Co., U.S.A., recently completed two multi-turbine energy recovery systems to supply heat in natural gas processing plants. These installations have been operating successfully for several months. Recognizing that special considerations will be required for each particular application, the design concepts and objectives outline below represent the major considerations for these two installations and are intended to serve as a guide for future applications. 1. Evaluate the gas-plant process fuel requirements and determine if energy recovery from gas-fired turbines could be justified to reduce process fuel volumes. 2. Evaluate and select the optimum heat-transfer fluid to transfer the recovered energy to the process area or utilization. 3. Design and fabricate an energy recovery heater and duct-work system that will allow the turbine exhaust gases to heat the transfer fluid efficiently. 4. Evaluate and de sign special structural requirements of the system to avoid any interference with the turbine operation. 5. Design and install an automatic control system with appropriate alarms and shutdowns to ensure the protection of the equipment.

Equipment Description

Using a modular design concept, a turbine-exhaust heat recovery system can be divided into five functional elements:Gas-fired turbines supply the recoverable heat energy (see Figs. 1 and 2).Turbine ducts transport the exhaust gases from the individual turbine to a common collection duct (Figs. 1 and 2).The common collection duct accumulates the exhaust gases from the turbines and conveys them into the heater (Figs. 1 and 2).The waste-heat exchanger transfer energy from the turbine exhaust gases to an appropriate heat-transfer fluid (Figs. 1 and 2).The heat-transfer-fluid circulation system transports the recovered energy from the heater to the process equipment for utilization. This circulation system includes such equipment as pumps and surge vessel. Photographs of an actual equipment installation are presented in Figs. 3 and 4. presented in Figs. 3 and 4. Heat-Recovery Application

Process heat requirements first must be evaluated to Process heat requirements first must be evaluated to determine whether a heat recovery operation is feasible. This work should begin by reviewing the thermal efficiencies of all fuel-consuming equipment in the plant. Equipment such as old boilers and gas-fired turbines are usually thermally inefficient (20 to 50 percent) and lend themselves to energy-recovery operations. this is particularly true if the old boilers are the heat source particularly true if the old boilers are the heat source for the processing plant. Gas-fired turbines reject 70 to 80 percent of their fuel's energy to the atmosphere.

JPT

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