Steam Turbine Experiment 286340

Y A steam turbine is a device that takes hot, high-pressure steam and extracts mechanical energy from it. This energy can then be used to do useful work to make our lives easier and better. An example of an application that uses steam turbines is the steam power plant located here on the campus of San Jose State University, which generates electricity used in everyday life. The analysis of steady-flow devices such as steam turbines is described in Thermodynamics textbook. (For example, see Chapter 6-12, p , by Cengel and Boles. Other excellent textbooks include p by Reynolds and Perkins, and p by Huang.) A schematic of a steam turbine and its thermodynamic representation on a T-s diagram is shown in the following figure:

An energy balance on the steam turbine tells us the relationship between the power output and the inlet and outlet states of the steam. A turbine is analyzed as an adiabatic (i.e., no heat transfer) device. The power output is then calculated as the mass flow rate of the steam multiplied by the difference in enthalpy between the inlet and outlet, or:

where is the power output by the turbine, is the mass flow rate of the steam, and h1 and h2 are the specific enthalpies at the inlet and outlet, respectively. The inlet and outlet states are indicated on the T-S diagram by the points 1 and 2. The point 2s on the T-S diagram is the state that is used to calculate the isentropic efficiency of the steam turbine. It is determined by the intersection of the outlet pressure and the inlet entropy. , the second law of thermodynamics dictates that the outlet entropy cannot be less than the inlet entropy. Therefore, if entropy stays the same, this indicates that maximum possible energy has been extracted from the fluid at the two given pressures. With this knowledge, the isentropic efficiency can be defined as the following: c c c



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where

is the maximum possible extracted power, and h2s is the enthalpy associated with

state 2s. The isentropic efficiency, , must be between 0 and 1 by definition. Efficiencies for steam turbines range from 0.3 to 0.9, but can vary widely depending on its design, size, and operating conditions. Y Now that we have discussed the theory, how do we use this knowledge to measure and analyze real steam turbines? What form does the output power come in, and how do we quantify it? What kinds of variables affect the power output and the isentropic efficiency? Steam turbines all find some way to convert the enthalpy of hot, high-pressure steam into rotational energy of some sort. Design of turbo machinery involves complicated mechanisms and design features that optimize this energy conversion and is a whole research field in itself. However, it isn¶t too difficult to imagine roughly how such a device could be implemented. The pressure difference between the inlet and outlet induces steam flow. The kinetic energy and pressure from the steam gets transferred to moving blades which causes a shaft to rotate (think windmills«). The variables used to quantify the output power are the RPM (i.e. the revolutions per minute of the shaft), and the torque (i.e. how much force it can generate per radius arm length). The torque is usually measured in lbf-ft since the US still does a lot of mechanical engineering in British units. The RPM and torque can be used to calculate horsepower, which is used to rate most turbomachines and engines, including your car. If you look up the power specifications on your car on the web or in your owner¶s manual, they would probably make a lot more sense to you now. Of course if you want to use the horsepower you calculated in your energy balance calculations, you need to convert it to Watts or Btu/hr so that your units are consistent. The load on the turbine is a variable that can affect the output power and the efficiency. The load is a number between 0 and 100, and it represents a braking or drag resistance to the shaft rotation. The greater the load, the greater the resistance applied. The load affects the power output because it influences the RPM and the mass flow rate of the steam. If you have a steam turbine in operation and you suddenly increase the load, your RPM is going to plummet unless you crank up the mass flow rate. It turns out that both the power output and efficiency are a function of the load and RPM because they both affect the mass flow rate, among other things. The rated power from the manufacturer is the maximum power output. The manufacturer will often set the rated power specification slightly below the maximum power output so that they have a bit of margin and can for manufacturing variations.

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A dynamometer is a machine that can measure all of these variables so that engineers can evaluate the performance of turbo machines and engines. It measures RPM and torque, and is capable of applying a load. If you use this instrument in conjunction with mass flow rate meters and temperature sensors in your inlet and outlet steam flows, you will be able to measure the quantities you need to analyze and characterize your steam turbine.

Data 1: (For Gil Bryan Arevalo) 1.Ê Load = 60% 2400 rpm 94.7 gpm 30 lbf-ft P2 = 83.2psig P3 = 0.1 psig T2 = 327.1 F T3 = 211.8 F T4 = 90.9 F T5 = 101.3 F T6 = 108 F T7 = 195.2 F

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i 1.Ê Before you get started, copy or print the schematic for this experiment. It will be handy to have a drawing with all of the temperatures and pressures labeled. 2.Ê Let's start with the steam going into the turbine inlet. You have measured its pressure (P2) and temperature (T2). Look in the steam tables in your textbook (p by Cengel and Boles), i.e. Tables A-4 through A-7 in Appendix 2. Is it superheated, saturated, or subcooled water? How can you tell? What is its specific entropy and enthalpy? If saturated, assume it is a saturated vapor. 3.Ê Now let's quantify the power output of the turbine. You have measured RPM and torque with a dynamometer. What is the power output in British (Btu/hr) and SI units (W), in addition to horsepower (hp)? Reread the introduction to this experiment if you are unclear as to how to relate RPM and torque to power. 4.Ê Draw a control volume around the steam turbine and perform an energy balance on it using the first law of thermodynamics. You can use assumptions commonly used to analyze turbines such as a) adiabatic, b) steady-flow, and c) change in KE and PE negligle. What variables are unknown so far? 5.Ê Now let's work backwards to the steam turbine from the condensors. We measured the temperature of the turbine steam exiting the condensers (T7), which is a subcooled liquid by this point. How do you find the properties of subcooled (i.e. compressed) liquids if you know its temperature? Why is it ok to use the saturated liquid properties at this temperature? What is its enthalpy? 6.Ê Take look at the water supply (T4) and return (T6) for the condensors. Assuming we have subcooled water all the way through the condensors, what are the specific enthapies of the water supply and return? What is the average density? Calculate the mass flow rate of the water supply using the volumetric flow rate (FLOW) you measured. 7.Ê Perform an energy balance on the condensors. Assume that the heat leaving the turbine fluid is equal to the heat gained by the water supply, and that there is no heat lost to the ambient air. What unknown variables do you have from this energy balance? 8.Ê You should now be at a point where you have two equations (energy balance on turbine, and energy balance on condensors) and two unknown variables (mass flow rate of steam and specific enthalpy at turbine outlet). Solve for the unknown variables. 9.Ê Is the steam exiting the turbine a superheated vapor or a saturated liquid-vapor mixture? 10.ÊYou looked up the entropy of the steam entering the turbine. What is the isentropic power output of the steam turbine? (i.e. the power output that would result if entropy is constant between the inlet and outlet pressure.) What does this power output represent with regard to the second law of thermodynamics? 11.ÊWhat is the isentropic efficiency at this RPM and load condition? 12.ÊRepeat Steps 3 through 12 above for all six cases. § 1.Ê Plot the output power of the turbine as a function of the mass flow rate of the steam. Plot both RPMs on the same graph. The power should have the units of Btu/hr. The mass flow rate should have the units of lbm/hr.

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2.Ê Plot the efficiency of the turbine as a function of turbine power. Plot both RPMs on the same graph. Efficiency is non- dimensional, so you can choose any power units you'd like. I'd recommend that you use the same power units as in your first graph. 3.Ê Pick one of your six cases. Sketch a T-s diagram, and mark the turbine inlet and outlet states in relation to the vapor dome. In addition, indicate the actual and isentropic processes for this case. Label the quality for all points inside the vapor dome. Label lines of constant pressure where appropriate. i 1.Ê This turbine is a Worthington Moore single state impulse turbine, and is rated for 30 hp. This means that the manufacturer will guarantee a power output of 30 hp near optimal operating conditions. Do the data validate this claim? 2.Ê Why does the turbine power seem to be a function of the mass flow rate of steam? (Hint: look at the specific enthalpies at the turbine inlet and outlet for all cases.) How does mass flow rate vary with load and RPM? 3.Ê What is the turbine power corresponding to the maximum efficiency? What range of power outputs is optimal for this particular turbine? 4.Ê Discuss any similarities or differences between your T-s or h-s diagram 5.Ê Discuss factors in the experiment that might contribute to measurement fluctuations. Data 2: (For Ronil Arteza) Load = 80% 2475 rpm 95 rpm 67.4 lbf-ft P2 = 778 psig P3 = 0.3 psig T2 = 323.2 F T3 = 212.3 F T4 = 91.3 F T5 = 107.6 F T6 = 133.2 F T7 = 189.8 F c c c



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Data 3: (For Reymart Famisaran) Load = 30% 2301 rpm 94.2 gpm 7.3 ft-lbf P2 = 51.2 psig P3 = 0 psig T3 = 211.6 F T4 = 94.5F T5 = 96.3 F T6 = 103.8 F T7 = 184.3 F Data 4: (For Samuel Falame) Load = 70% 2300 rpm 945 gpm 8.7 ft-lbf P2 = 51.2 psig P3 = 0 psig T3 = 211.6 F T4 = 94.5F T5 = 96.3 F T6 = 105.4 F

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T7 = 189.1 F

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