Energy Efficiency in HVAC Questions and Answers

Answer: A

It is the rate of heat removal that would freeze a short ton of ice in a day, and it comes to 3.517 kilowatts. It measures the cooling delivered rather than the electricity consumed, and confusing the two is a frequent error.

Answer: B

Being a ratio of two powers it is dimensionless, and a value of four means four kilowatts of cooling for each kilowatt of input. Its reciprocal expressed per ton is the kilowatts per ton figure the industry usually quotes.

Answer: C

Correct answer: lower

Kilowatts per ton is power divided by cooling delivered, so it is an inverse measure - less power for the same cooling gives a smaller number. A good centrifugal machine runs at 0.5 to 0.6 kW/TR at full load and a poor one above 1.0. It is the one efficiency figure in this subject that improves as it falls, which is why it is so easily misread.

Answer: D

Correct answer: 4.0

One ton of refrigeration is 3.517 kW of cooling, so \[ \mathrm{COP} = \frac{3.517}{0.88} = 4.0. \] The two figures say the same thing in opposite directions, and the constant 3.517 converts between them: \( \mathrm{COP} = 3.517/(\mathrm{kW/TR}) \). A COP of 4 means four units of heat moved for each unit of electricity, which is possible because the machine moves heat rather than creating cold.

Answer: A

A machine sized for the hottest afternoon runs at a third or a half of that for most of the season, and its efficiency there may be better or worse than at full load. A single full-load figure therefore misrepresents the annual consumption.

Answer: B

The same evaporator and condenser surface serving a smaller load gives smaller approach temperatures and therefore a smaller lift for the compressor. The gain is lost if the condenser water temperature is held up artificially.

Answer: C

Evaporation in the cooling tower reaches within a few degrees of the wet bulb, which on a humid day is several degrees below the dry bulb and on a dry day much more. The lower condensing temperature reduces the compressor's lift and its power.

Answer: D

Make-up water, blowdown, chemical treatment and the risk of legionella all attach to the tower, along with the tower fan and the condenser pump. Where water is scarce or dear the air-cooled machine wins despite its poorer efficiency.

Answer: A

A rising approach at constant duty says the tubes are fouled or the charge is low, and the compressor must then work against a lower evaporating temperature for the same chilled water. Trending it is the standard chiller health check.

Answer: B

Correct answer: fouled condenser tubes or non-condensable gas in the circuit

The approach is the gap between the condensing refrigerant temperature and the leaving condenser water, and it measures how well heat crosses the tube wall. Fouling adds resistance to that path, and non-condensable gas blankets the tube surface and raises the condensing pressure. Either forces the compressor to work against a higher head, so a rising approach is an early warning of increasing power for the same duty.