Computation of the Heat-Transfer Factor in Operating a Boil-Type Evaporator

Abstract

An empirical dependence for computing the heat-transfer factor in a boil-type evaporator obtained on the basis of data of start and make-ready work and thermochemical tests of multistage evaporating installations at various thermal plants is presented. Results of computation of the heat-transfer factor and of the capacity of an evaporating installation by known methods are compared with the results obtained with the help of the suggested dependence. A dependence for evaluating the change in the heat-transfer factor with the operating time is suggested.Keywords: multistage evaporating installation, boil-type evaporator, computation of the heat-transfer factor in evaporator.Today, thermal desalting with the help of power evaporators is used in the CIS countries at over 30 thermal power plants equipped primarily with high-pressure (9 MPa) and superhigh-pressure (13 MPa) boilers. This number includes several cogeneration plants and five condensing plants with supercritical pressure units (24 MPa). Evaporating installations are used in regions with a high salt content or a high content of organic compounds in natural water or in regions with limited possibility of waste disposal.The efficiency of heat transfer in heat exchangers is determined by the heat-transfer factor. Computation of the heat-transfer factor K e for an evaporator by the method presented in [1] is quite complicated. Evaluation of the consumption of heating steam at a thermal power plant at a specified capacity or at a capacity at specified consumption of heating steam is easier to make using dependences of type (1) given, for example, in recommendations [2] and having the form of an approximationwhere Ät e is the temperature difference in the evaporator (between the saturation temperatures of the heating steam and of the secondary steam) and t boil is the saturation temperature of the secondary steam. However, in some cases (for example at high temperature differences and long service times of the evaporators [3]), the use of these formulas can lead to substantial errors in the determination of the heat-transfer factor. In our opinion, the capacity of an evaporator and the state of the heating surface should be determined from dependences derived on the basis of results of thermal tests.Today we are lacking a simple and reliable dependence for computing the heat-transfer factor that would permit us to determine the capacity of the evaporator and of the installation as a whole reliably enough. In the present paper we made an attempt to find a dependence for computing the heat-transfer factor by processing the results of tests of several new evaporating installations working with water of different composition at a wide range of evaporator parameters and temperature differences.In order to determine the heat-transfer factor in the operation of an evaporator, we will use the equations of heat balance and heat transfer, i.e., Q e = K e F e Ät e ;(2)where Q e , Q h , and Q s represent the heat transferred through the heating section of the evaporator intr...