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Effective design, analysis, and integration of steam turbines can help optimize steam supply reliability and overall energy efficiency across your plant.
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Steam turbines are important components of process plant utility systems. They offer opportunities for optimizing steam supply reliability, as well as sitewide energy efficiency. Steam turbines are most common in the oil refining, ammonia and urea, methanol, ethylene, and pulp and paper industries, where they are generally sized to produce 10-60 MW of power. Good economics are also possible at smaller sizes as low as 2 MW, which are more common in the food and beverage industries, as well as in small to medium-sized plants in the chemical process industries (CPI).
Achieving favorable steam turbine economics depends on choosing the right type of turbine (e.g., backpressure vs. condensing) in the right size, as well as integrating it correctly with the heat exchanger network (HEN) in accordance with the appropriate placement principle of pinch analysis.
This article reviews the thermodynamic relationships and equations that link steam flow conditions and power output, which are useful for estimating preliminary economics of new turbines and analyzing the performance of existing units.
The basics
Any device that converts the chemical energy contained in a fuel into mechanical energy (i.e., shaftwork) via combustion is called a heat engine. Heat engines are generally classified according to the thermodynamic cycle that they follow. The most common heat engines in industrial applications are steam turbines (Rankine cycle), gas turbines (Brayton cycle), and internal combustion engines (Otto cycle).
Although gas turbines can also play an important role in the economic optimization of the combined heat and power (CHP) utilities at manufacturing plants, this article focuses exclusively on steam turbines. Steam turbines have four typical operating modes:
Backpressure steam turbines (BPSTs) produce lowpressure (LP) exhaust steam that can be used for one or more process heating duties (Figure la). The objectives are to provide the process with steam of the quantity and pressure required by the process, while generating the maximum amount of power so as to reduce the need for purchased power. Because BPSTs cogenerate two energy products (i.e., steam and power) simultaneously, they have an effective heat rate of4,500-5,500 Btu/kWh, which represents an energy efficiency two to...