Turbines efficiency

A number of alternative pricing systems have been proposed that hinge on turbine efficiency and the relative pricing of fuel and electricity. AH... 

Is there a program to monitor turbine efficiency by checking temperatures in and out ... 

Silica. Sihca is not actually a corrodent of turbines. However, it can deposit on and cause blocking of turbine passages, thus reducing turbine capacity and efficiency. As Httie as 76 pm (3 mils) of deposit can cause measurable loss in turbine efficiency. Severe deposition can also cause imbalance of the turbine and vibration. The solubihty in steam and water is shown in Figure 15, as is a typical steam turbine expansion. Sihca is not a problem except in low pressure turbines unless the concentrations are extraordinarily high. 

Thrust-Bearing Power Loss The power consumed by various thrust bearing types is an important consideration in any system. Power losses must be accurately predicted so that turbine efficiency can be computed and the oil supply system properly designed. 

The actual steam rate is obtained by dividing the theoretical steam rate by the turbine efficiency, which includes thermodynamic and mechanical losses. Alternatively, internal efficiency can be used, and mechanical losses applied in a second step. 

Steam cost and value of turbine efficiency, so that consideration can be given to stage and valve options... 

The previous section dealt with the concepts of the various cycles. Work output and efficiency of all actual cycles are considerably less than those of the corresponding ideal cycles because of the effect of compressor, combustor, and turbine efficiencies and pressure losses in the system. 

The simple cycle is the most common type of cycle being used in gas turbines in the field today. The actual open simple cycle as shown in Figure 2-9 indicates the inefficiency of the compressor and turbine and the loss in pressure through the burner. Assuming the compressor efficiency is rjc and the turbine efficiency is t], then the actual compressor work and the actual turbine work is given by ... 

Injection of Water or Steam at the Gas Turbine Compressor Exit. Steam injection or water injection has been often used to augment the power generated from the turbine as seen in Figure 2-42. Steam can be generated from the exhaust gases of the gas turbine. The HRSG for such a unit is very elementary as the pressures are low. This technique augments power and also increases the turbine efficiency. The amount of steam is limited to about... 

Figure 3-16 shows the effeet of efficiency as a function of the load for both the compressor and turbine. Part-load turbine efficiencies are affected more than compressor efficiencies. The discrepancy results from the compressor operating at a relatively constant inlet temperature, pressure, and pressure ratio, while the turbine inlet temperature is greatly varied (Figure 3-17). 

Figure 3-16. Compressor and turbine efficiency as a function of ioad.

The boundary layer along the blade surfaces must be well energized so that no separation of the flow occurs. Figure 8-16 shows a schematic of the flow in a radial-inflow impeller. Off-design work indicates that radial-inflow turbine efficiency is not affected by changes in flow and pressure ratio to the extent of an axial-flow turbine. 

The injection of coolant air in the turbine rotor or stator causes a slight decrease in turbine efficiency however, the higher turbine inlet temperature usually makes up for the loss of the turbine component efficiency, giving an overall increase in cycle efficiency. Tests by NASA on three different types of cooled stator blades were conducted on a specially built 30-inch turbine cold-air test facility. The outer shell profile of all three blade types was the same, as seen in Figure 9-24. 

Figure 9-25. The effect of various types of cooling on turbine efficiency.

Unburnt hydrocarbon (UHC) and carbon monoxide (CO) are only produced in incomplete combustion typical of idle conditions. It appears probable that idling efficiency can be improved by detailed design to provide better atomization and higher local temperatures. CO2 production is a direct function of the fuel burnt (3.14 times the fuel burnt) it is not possible to control the production of CO2 in fossil fuel combustion, the best control is the increasing of the turbine efficiency, thus requiring less fuel to be burnt for the same power produced. 

The gas turbine efficiency drops off quickly at part load as would be expected, as the gas turbine is very dependent on turbine firing temperature and mass flow of the incoming air. The gas turbine heat rate increases rapidly at part load conditions. 

The increase in unit size and complexity together with the higher turbine inlet temperature, and higher pressure ratio has lead to an increase in overall gas turbine efficiency. The increase in efficiency of 7-10% has in many cases lead to an availability decrease of the same amount or even more as seen in Figure 21-5. A 1% reduction in plant availability could cost 500,000/yr in income on a 100 MW plant, thus in many cases offsetting gains in efficiency. 

The theoretical steam rate must then be divided by the efficiency to obtain the actual steam rate. See the section on Steam Turbines Efficiency. 

Evans provides the following graph of steam turbine efficiencies. 

Steam cost, and the value of turbine efficiency. Should it be single-stage or multistage Should it be single-valve or multivalve Is the steam an inexpensive process by-product, or is the entire cost of generating the steam chargeable to the driver ... 

Calculation of the specific work and the arbitrary overall efficiency may now be made parallel to the method used for the a/s cycle. The maximum and minimum temperatures are specified, together with compressor and turbine efficiencies. A compressor pressure ratio (r) is selected, and with the pressure loss coefficients specified, the corresponding turbine pressure ratio is obtained. With the compressor exit temperature T2 known and Tt, specified, the temperature change in combustion is also known, and the fuel-air ratio / may then be obtained. Approximate mean values of specific heats are then obtained from Fig. 3.12. Either they may be employed directly, or n and n may be obtained and used. 

Thus there are three modifications to the a/s efficiency analysis, involving (i) the specific heats ( and n ), (ii) the fuel-air ratio / and the increased turbine mass flow (I +/), and (iii) the pressure loss term S. The second of these is small for most gas turbines which have large air-fuel ratios and / is of the order of l/IOO. The third, which can be significant, can also be allowed for a modification of the a/s turbine efficiency, as given in Hawthorne and Davis [I]. (However, this is not very convenient as the isentropic efficiency tjt then varies with r and jc, leading to substantial modifications of the Hawthome-Davis chart.)... 

Fig. 5.2 shows that for the single-step cooled CBT plant at a given combustion temperature, the overall efficiency of the cooled gas turbine efficiency increases with pressure ratio initially but, compared with an uncooled cycle, reaches a maximum at a lower optimum pressure ratio. Fig. 5.3 shows that for a given pressure ratio the efficiency generally increases with the combustion temperature even though the required cooling fraction increases. 

Knowing the turbine efficiency, an approximate condition line for the expansion through the steam turbine can be drawn (to state f at pressure pbO and an estimate made of the steam enthalpy hf. If a fraction of the steam flow is bled at this point then the heat balance for a direct heater raising the water from near the condenser temperature to 7b is approximately... 

Here lies the crux of the major problem in the early development of the gas turbine. The compressor must be highly efficient-it must use the minimum power to compress the gas the turbine must also be highly efficient-it must deliver the maximum power if it is to drive the compressor and have power over. With low compressor and turbine efficiency, the plant can only just be self-sustaining-the turbine can drive the compressor but do no more than that. 

Available horsepower from a gas turbine is a function of air compressor pressure ratio, combustor temperature, air compressor and turbine efficiencies, ambient temperature, and barometric pressure. High ambient temperatures and/or low barometric pressure will reduce available horsepower while low ambient temperatures and/or high barometric pressure will increase available horsepower. All industrial turbines will have high-temperature protection, but in areas subject to very low ambient temperatures horsepower limiting may be required. 

Where hot ambient temperatures are expected, overall turbine efficiency and horsepower output can be increased by installing an evaporative cooler in the inlet. Inlet air flows through a spray of cold water. The temperature of the water and the cooling effect caused by the inlet air evaporating some of the water cools the inlet air. In desert areas where the inlet air is dry and thus able to evaporate more water before becoming saturated with water vapor, this process is particularly effective at increasing turbine efficiency. 

Figure 16-16 shows the performance characteristic of a split-shaft turbine where the only power output limitation is the maximum allowable temperature at the inlet of the turbine section. In actual practice a torque limit, increased exhaust temperature, loss of turbine efficiency, aud/or a lubrication problem on the driven equipment usually preclude operating at very low power turbine speeds. The useful characteristic of the split-shaft engine is its ability to supply a more or less constant horsepower output over a wide range of power turbine speeds. The air compressor essentially sets a power level and the output shaft attains a speed to pnivide the required torque balance. Compressors, pumps, and various mechanical tinvc systems make very good applications for split-shaft designs. 

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