Economics of Replacing a Valve by a Turbine

| January 30, 2015

 

A cryogenic manufacturing facility handles liquid methane at 115 K and 5 MPa at a rate of 0.280 m3/s . A process requires dropping the pressure of liquid methane to 1 MPa, which is done by throttling the liquid methane by passing it through a flow resistance such as a valve. A recently hired engineer proposes to replace the throttling valve by a turbine in order to produce power while dropping the pressure to 1 MPa. Using data from Table 7–1, determine the maximum amount of power that can be produced by such a turbine. Also, determine how much this turbine will save the facility from electricity usage costs per year if the turbine operates continuously (8760 h/yr) and the facility pays $0.075/kWh for electricity.

TABLE 7–1
Properties of liquid methane
Specific
Temp., Pressure, Density, Enthalpy, Entropy, heat,
T, K P, MPa r, kg/m3 h, kJ/kg s, kJ/kg · K cp, kJ/kg · K
110 0.5 425.3 208.3 4.878 3.476
1.0 425.8 209.0 4.875 3.471
2.0 426.6 210.5 4.867 3.460
5.0 429.1 215.0 4.844 3.432
120 0.5 410.4 243.4 5.185 3.551
1.0 411.0 244.1 5.180 3.543
2.0 412.0 245.4 5.171 3.528
5.0 415.2 249.6 5.145 3.486

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FIGURE 7–30

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Category: Coursework

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