The design optimization principles for thermal system
The design optimization principles of thermal system:
(1) To maximize the superheated steam production. For the use of mid-low temperature waste heat, the most important thing is to make full use it in the permissible scope of the process and plants and to make the equipment most effective which can maximize the waste heat power generation. For the low-parameter steam turbine, there are 3 main parameters affect the power generation: the flow-rate, pressure and temperature of the superheated steam. Among them, flow-rate plays a vital role to the power generation; pressure and temperature will affect the inner efficiency (heat energy transfer into mechanical energy) of the steam turbine and the enthalpy of the unit mass of the steam. However, the effect is far less than the influence made by flow-rate. Generally speaking, the SP waste heat account for over 60% of the waste heat that can be used, so, while considering about the superheated steam flow-rate in the whole production line, we will firstly consider how to maximize the superheated production from SP by coordinating AQC with SP.
(2) High temperature of superheated steam. High superheated steam temperature can not only improve the power generation by raising enthalpy value of the superheated steam and the inner efficiency of the steam turbine, the most important thing is to improve the safety of steam turbine operation. Being affected by boiler heat transfer efficiency and manufacturing costs, the temperature difference must be suitable (more than 20℃) between superheated steam and flue gas.
(3) Suitable drum working pressure. Considering that in the process of heat transfer, it’s reasonable to keep the temperature difference remain more than 20℃ without changing the temperature of steam and water mixture in evaporating heating surface. The temperature of mixture will be affected by the pressure directly, so, we will choose a reasonable pressure to create conditions for the arrangements of heating surfaces to avoid high costs of the boilers.
(4) Fully reduce the exhaust temperature. For the SP exhaust will be used to dry the raw materials, the temperature of the SP exhaust can only reduced to around 225℃ while the AQC exhaust should be cut down fully, however, if the AQC exhaust was cut too much which will cause to increase the boiler costs and meanwhile absorbing too much low-quality heat cannot improve the power generation, so the reduction of AQC exhaust will be enough if there is full drum feed water for both AQC and SP boilers. According to the calculation of the heat distribution and energy balance, AQC exhaust can meet the need in the scope of 96～98℃.
(5) Arrange the heating surfaces reasonably. While arranging the heating surfaces, the gas temperature characteristics of AQC and SP and also how to choose a suitable temperature difference to lower the boiler costs must be taken into account.