With reference to the chart, if a boiler generates saturated steam at 385.3 psig, how much heat per pound was required to change the water into steam if the feedwater temperature was initially 220°F? Illustration SG- 0004
• Use Table 2 – Thermodynamic Properties of Saturated Steam (Pressure) to find values at the boiler pressure. • Convert gauge pressure (psig) to absolute pressure (psia) before using the pressure table. • Use enthalpy of liquid at the feedwater temperature from Table 1 – Saturated Steam (Temperature) and compare it to the enthalpy of vapor at boiler pressure.
• What absolute pressure (psia) corresponds to 385.3 psig, and which row in Table 2 is closest to that value? • Once you know the enthalpy of saturated vapor at boiler pressure, how do you account for the fact that the feedwater is already at 220°F (not at 32°F or 0°F)? • How do you combine the enthalpy value for liquid water at 220°F with the enthalpy of saturated vapor at the boiler pressure to find the heat added per pound?
• Be sure you are using absolute pressure from the chart, not psig. • Confirm you are subtracting enthalpy of liquid at 220°F (from Table 1) from enthalpy of saturated vapor at the correct pressure (from Table 2). • Double‑check units: all values should be in BTU per pound (BTU/lb) before you compare to the choices.
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