Thermal Energy Audit Questions and Answers

Answer: C

Solids enter with the feedwater and concentrate as steam is raised, and beyond a limit they cause carryover and scale. Discarding a fraction of the hot water keeps the concentration within bounds and takes its heat with it.

Answer: D

Water at boiler pressure carries several hundred kilojoules a kilogram of sensible heat, and discarding ten per cent of the feed throws away a corresponding share of the fuel. A flash vessel and a heat exchanger recover much of it.

Answer: A

Correct answer: improving the feedwater treatment so fewer solids enter

Blowdown exists to hold the dissolved solids in the drum below a limit, and its rate is set by how fast those solids arrive. Removing them upstream - better softening, demineralisation, more condensate returned - means less water has to be discharged to hold the same concentration. Nothing done to the pressure or the air changes the solids balance.

Answer: B

Conductivity stands proxy for dissolved solids, so a controller can hold the concentration just inside the limit instead of blowing down on a fixed and generous schedule. Manual blowdown is almost always more than is needed.

Answer: C

The water is above the saturation temperature of the lower pressure, so part of it boils immediately and the rest cools to the new saturation temperature. Capturing that steam in a flash vessel and using it at the lower pressure recovers heat that would otherwise be vented.

Answer: D

Correct answer: the difference in saturation enthalpy between the two pressures

When condensate is released to a lower pressure it is hotter than the new saturation temperature, and the surplus enthalpy boils part of it off: \[ x = \frac{h_{f1}-h_{f2}}{h_{fg2}}. \] The fraction is a property of the two pressures alone, so the mass released and the pipe size change how much flash steam there is, not what proportion flashes.

Answer: A

Condensate in a main causes water hammer and blocks heat transfer, so it must be removed, and any trap that also passes steam is throwing away fuel. Testing traps regularly is among the most profitable routine tasks in a steam plant.

Answer: B

Blowing steam represents fuel burnt for nothing, and a single failed trap on a high pressure main can waste a great deal over a year. A failed-closed trap is a production problem rather than an energy one, though it causes water hammer.

Answer: C

Correct answer: hot and almost free of dissolved solids

Condensate returns at close to boiling, so far less fuel is needed to bring it back to steam, and it is essentially distilled water, so it dilutes the drum and lets the blowdown rate fall. Those two effects compound: the returned condensate saves sensible heat directly and saves more by displacing blowdown.

Answer: D

The loss from a hot surface is the area multiplied by a heat transfer coefficient and by the rise above ambient, so a long large-diameter main at high temperature loses most. Valves and flanges left bare are a large share of many sites' losses.