Method for Optimization of Heat-Exchange Units Working in Heat Recovery Systems

Abstract

The optimization of heat exchangers used in heat-recovery systems of heated media exiting process plants is considered. Such systems are widespread and used in various industries: energy, engineering, ferrous metallurgy, engineering systems of buildings. The authors propose an optimization method that uses target functions that take into account the mass and size characteristics of heat exchangers as well as the difference between the heat returned to the installation and the heat equivalent of electric energy required for pumping heat carriers through heat exchangers. Independent variables are total heat capacities of the mass flow rate of the heat carriers. Possible limitations are the maximum speeds of the heat carriers causing a large hydraulic resistance along the path. It is established that the target functions have a pronounced maximum. At certain values of the operating parameters, the target function acquires negative values and the use of heat exchangers becomes impractical. To correctly select the operating parameters of a specific heat-recovery unit, it is proposed to use thermohydraulic characteristics—graphs that represent the dependence of the target function on the heat-transfer heat capacities of the mass flow rate. Such dependences are obtained as a result of multiple thermal and hydraulic checking calculations for heat exchangers. Thermohydraulic characteristics can be built for each possible temperature mode of operation of the heat exchanger. They can be attached to the technical data sheet of devices and allow to determine the optimal flow rates of heat carriers, providing maximum economy of energy recourses. They can also be used to select the desired apparatus from the standard design list. The thermal-hydraulic characteristics calculated for the devices of the most common designs are presented: a plate heat exchanger with water–water heat carriers, a tubular finned gas-liquid heat exchanger, and a plate gas-air heat exchanger. It is shown that the maximum value of the target function when using the same heat carriers lies near the diagonal of the characteristic, i.e., at approximately equal values of the heat capacity. It is also shown that the use of thermodynamic criteria for optimizing heat exchangers of heat-recovery plants is not always advisable.