Back-pressure turbines

Sinee eaeh section will have a certain isoentropic efficiency, the aetual steam rate for each will be different. The aetual steam rates can be expressed by the formulas presented below. 

From the above, the total output less generator loss is expressed by the following  

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Because of increased emphasis on maximizing cogenerated power, newer plants are trying to utilize back-pressure turbines only in applications where efficiencies above 70% can be attained. This typically means limiting the applications to the large (>1000 kW) drives, and usiag small machines only where they are necessary for the safe shutdown of the unit. Multistage turbiaes are used even on the smaller loads. 

Steam needed for process, so that a back-pressure turbine should be considered... 

One of the first questions the designer must answer concerns which type steam turbine should be used. The back pressure turbine is. selected when process steam demands are greater than the steam required for process drivers such as large compressors. This type turbine is also selected when various steam levels are required by the process. 

The guidelines are quite general, but will at least act as introduction to the types of turbines available to the process designer. Sometimes the decision on which type turbine to select is not obvious. The back pressure turbine is most frequently selected. It has lower capital cost, simple construction, is the most suitable turbine for high speeds, and is generally more reliable. 

The condensing turbine has several advantages and disadvantages over the back pressure turbine. The advantages are that it requires less change in the live steam for various turbine loads and is therefore easier to control. It also requires less steam because the enthalpy drop is larger. Finally, only one steam level is affected for a change in power requirements. 

The upper limit in exhaust pressure for back pressure multistage turbines varies between 350 psia and 500 psia. For small, single-stage, back-pressure turbines, the standard is somewhere between atmospheric and 65 p ia. 

The simple back-pressure turbine provides maximum economy with the simplest installation. An ideal backpressure turbogenerator set relies on the process steam requirements to match the power demand. However, this ideal is seldom realized in practice. In most installations the power and heat demands will fluctuate widely, with a fall in electrical demand when steam flow, for instance, rises. 

This process steam flow will dictate output generated by the turbo-alternator and excess or deficiency is made up by export or import to the supply utility, as appropriate. The alternative to the system in Figure 15.15 is to use a back-pressure turbine with bypass reducing valve and dump condenser, as shown in Figure 15.16. 

Figure 15.16 A back-pressure turbine with PRDs valve and dump condenser...

Noncondensing turbines (back-pressure turbines) are widely used by industry to fulfill any number of process requirements. As a general mle, turbine generator design efficiency is reduced by the absence of a... 

Smaller industrial cogenerators may produce electricity only as a secondary function by coupling back-pressure turbines (0.2-5MW sets) to their various process operations. 

In addition to condensing turbines, manufacturing industries widely use noncondensing (back-pressure) turbines. These back-pressure units are of various designs and are employed for electricity generation, typically within the range 600 to 30,000 KWh at 115 to 1,250 psig, and for a variety of process applications. 

The exhaust pressure of a steam turbine is fixed by the operating pressure of the downstream equipment. Figure 23.9a shows a back-pressure turbine operating between a high-pressure and low-pressure steam mains. The pressure of the low-pressure steam mains will be controlled elsewhere (see later). 

For back-pressure turbines, the turbine exhausts to a steam main. Therefore, in order to enable estimation of steam main conditions, it is important to predict the condition of the exhausted steam also. With a value of the mechanical efficiency, the enthalpy of the exhaust steam can be calculated from an energy balance9 ... 

Example 23.2 A process heating duty of 25 MW is to be supplied by the exhaust steam of a back-pressure turbine. High-pressure steam at 100 barg with a temperature of 485°C is to be expanded to 20 barg for process heating. The heating duty of the 20 barg steam can be assumed to be the sum of the superheat... 

Figure 23.20 illustrates the features of a typical steam system. It is usual to have at least three levels of steam. On larger sites, steam may also be generated at a very high pressure, which will only be used for power generation in the boiler house. Steam would then be distributed typically at three pressures around the site. Back-pressure turbines let steam down from the high-pressure mains to... 

If a steam turbine generates electricity, then the flow through the turbine can be varied within the minimum and maximum flows allowed by the machine. If a steam turbine is connected directly to a drive (e.g. a back-pressure turbine driving a large pump), then there is likely to be no flexibility to change the flowrate through the steam turbine as this is fixed by the power requirements of the process machine. 

Steam turbines are used to convert part of the energy of the steam into power and can be configured in different ways. Steam turbines can be divided into two basic classes back-pressure turbines and condensing turbines. The efficiency of the turbine and its power output depend on the flowrate of steam to the turbine. The performance characteristics can be modeled by a simple linear relationship over a reasonable range of operation. 

Background variables, 8 386-388 Back-pressure turbines, 10 146-147 steam, 10 140-141... 

A back-pressure turbine is able to convert natural gas or fuels into electric power with an efficiency of more than 80%, which makes it one of the most efficient distributed generation systems. The C02 emissions are low as well as pollution emissions. 

Back-pressure turbine control system for the generation of LP steam, provided with valve position-based optimizer. 

Tillman (1985). The generation mode is fora back-pressure turbine. The minimum heat rate is based on large systems and biomass fuel containing 15 wt % moisture. The maximum heat rate is based on small systems and biomass fuel containing 50 wt % moisture. 

Brazilian sugar mills, similar to mills throughout the world, have process steam consumption of approximately 500 kg of steam per tonne of cane processed. By generating steam at 22bar/300°C, sufiicient electric and mechanical power (using back-pressure turbines) to run the plant is achieved and nearly all bagasse produced is consumed Thus, fiiel availability, and power and thermal energy requirements are balanced. 

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