Experimental Study of Throttling of Solid Fuel Direct-Flow Steam Boilers

Abstract

The object of study of this work is the thermophysical processes of water boiling in narrow channels and its phase transition at variable heat flux and steam consumption. The use of traditional steam drum boilers is becoming more problematic, both in terms of safe operation and energy efficiency. This problem can be partially solved by using direct-flow steam boilers fuelled by solid fuel. The aim of the study is to determine the main operating modes of a steam boiler with a variable specific heat flux and variable steam flow over time of the required quality. The main task of direct-flow heat exchangers is to produce steam of proper quality: saturated, dry or superheated. The type of fuel used has a significant impact on this task. In the case of solid fuels, this process is complicated for several reasons: the calorific value of the fuel, its moisture content, the inertia of the combustion chamber and the loading device. A system for controlling steam generation of a certain quality was experimentally tested by dividing a straight-tube heat exchanger into two parts with a throttle washer: with superheated water at the inlet section of the heat exchanger and steam-liquid water at the boiler outlet. The control system monitored the pressure drop across the throttle washer depending on the steam flow rate, and the feedback was based on the steam temperature. The study found that with a slight change in steam flow and low frequency, it is possible to maintain the pressure at the outlet of the heat exchanger constant. Limiting external influences on the physical picture of water evaporation in a straight-tube evaporator and using a proportional-integral-differential (PID) controller made it possible to simplify the control system of a straight-tube steam boiler at different steam flow rates with a relatively stable pressure at the outlet of the heat exchanger.