Show answer Engineers worry about steam \"losing some of its heat\" while it flo
ID: 1859423 • Letter: S
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Engineers worry about steam "losing some of its heat" while it flows through long pipes even if the pipe is insulated. Therefore they are interested in determining the quality (mass fraction that is still vapor) of the steam in the pipe. One method of measuring quality involves using a throttling calorimeter, which consists of a valve, a short pipe, and a thermometer as shown below. When the valve is "cracked" (opened slightly), some of the high pressure "wet" steam (a saturated liquidvapor mixture) in the line is throttled (Joule-Thomson expansion) to atmospheric pressure thus producing superheated vapor (hopefully). If the steam in the pipeline is at 1200 kPa: What is the quality of the steam originally in the pipe if the temperature of the exit atmospheric (100 kPa) steam is 150 degree C. Estimate the increase in entropy (kJ/kg-K) when 1 kg of "wet" steam flows through the valve. If the temperature of the exit atmospheric steam is 99.62 degree C, can you still calculate the quality of the steam in the pipe? If not, why not?Explanation / Answer
For steam properties, see http://www.irc.wisc.edu/properties/
a)
At 100 kPa, 150 deg C, we get h2 = 2780 kJ/kg (superheated), s2 = 7.61 kJ/kg-K
For throttling, h1 = h2
At P1 = 1200 kPa and h1 = 2780 kJ/kg, we get x1 = 0.998 and s1 = 6.51 kJ/kg-K
b)
Increase in entropy = m(s2 - s1) = 1*(7.61 - 6.51) = 1.1 kJ/K
c)
Saturation temperature at 100 kPa is 99.62 deg C. Hence, in order to know the state of the steam after throttling, we need to know one more variable (for example, quality) in order to predict the quality upstream of valve. Therefore, we cannot predict.