Thermal Energy Audit Questions and Answers

Answer: A

Correct answer: the sum of the insulation cost and the heat loss cost is least

The two costs pull opposite ways - more insulation costs more to install and saves more fuel - so their sum has a minimum, and that thickness is the economic one. Adding material beyond it still reduces the loss, just by less than it costs, since each further layer works against the resistance already in place.

Answer: B

Lower saturation temperature means smaller losses from every metre of main and every trap, and less flashing at each let-down. The latent heat per kilogram actually rises as pressure falls, which is a further small benefit.

Answer: C

Correct answer: feedwater

An economiser sits in the flue and transfers heat from the leaving gas into the feedwater on its way to the drum, cutting the flue gas temperature and so the dry flue gas loss. Recovering the same heat into the combustion air instead is an air preheater - a different exchanger doing the analogous job on the other stream.

Answer: D

Hotter air raises the flame temperature and reduces the fuel needed, and the same heat is taken from gas that would have gone up the stack. Both devices are limited by the acid dew point below which the flue gas corrodes them.

Answer: A

Sulphur in the fuel gives sulphur trioxide, which with water vapour condenses as acid on any surface below its dew point, and the metal is attacked rapidly. It is the reason stack temperatures are held well above ambient on sulphur-bearing fuels.

Answer: B

A recuperator is a continuous heat exchanger with a dividing wall; a regenerator stores heat in a matrix that the hot and cold streams pass through in turn. The second recovers more but allows some carry-over between the streams.

Answer: C

The wheel turns slowly between the two ducts, taking heat from the warmer stream and giving it to the cooler, and a hygroscopic coating transfers moisture as well. Its penalty is a small cross-leakage between the streams and the fan power to overcome its resistance.

Answer: D

Furnace and engine exhausts at several hundred degrees will raise steam at a useful pressure, which is why they are the classic application. A low-grade stream is better used for space or water heating, and an intermittent one may not justify the plant.

Answer: A

Correct answer: a rising approach temperature for the same duty

A fouling layer adds thermal resistance, so the exchanger transfers less heat per degree available and the streams leave further apart. Pressure drop rises rather than falls, since the deposit narrows the passage, and the entering hot stream is set by the process upstream and is unaffected. Tracking the approach over time is the standard way of deciding when to clean.

Answer: B

Correct answer: the outlet of one stream and the inlet of the other

It is the gap between what a stream achieved and the best it could have achieved - the temperature of the stream it was exchanging against. A small approach means a close, effective exchange and needs a large surface; a large approach means heat was available and left unrecovered.