An existing Rankine heat engine has been operating with an overall efficiency of 15% for some months. The efficiency today is only 13%. Which of the following process upsets is NOT a possible explanation for the decrease in efficiency?

Question

A. Operators allowing the controls to drift
B. Calculation mistakes when designing
C. Fouling (hard water minerals) buildup on the boiler
D. Loss of lubrication on the turbine bearings
E. Pump impeller wear

Answer 1

A. Incorrect. While not discussed openly in the textbook, all of these equipments require operators to control, monitor and manipulate them. An operator error can wreck a process’s efficiency (or the process itself).
B. Correct. If mistakes were made in the design, the efficiency would have been low from the beginning, rather than suddenly occurring after operation for months.
C. Incorrect. Fouling is an important cause of slow efficiency loss over time in heat exchangers, boilers and other places water contacts metal.
D. Incorrect. This is a major concern anywhere rotating shafts or sliding bars are present. Friction in the turbine leads directly to a loss of efficiency.
E. Incorrect. If, for example, the pump’s impeller did not fit in the housing fully it would require more shaftwork to pump the same amount of liquid as previously, lowering efficiency.