This process is similar to what happens when hot, humid air comes into contact with a glass of
ice water. You have all seen the condensation that forms on the outside surface of a glass of ice
water as the neighboring hot, humid air cools down. We will discuss what we mean by absolute
and relative humidity in Chapter 17.
The efficiency of a refrigeration system or an air-conditioning unit is given by the
coefficient of performance (COP) which is defined as(13.20)
You should use consistent units to calculate the coefficient of performance. The COP of most
vapor compression units is 2.9 to 4.9. In the United States, it is customary to express the
coefficient of performance of a refrigeration or an air-conditioning system using mixed SI
and U.S. units. Quite often, the coefficient of performance is called energy efficiency ratio
(EER) or the seasonal energy efficiency ratio (SEER). In such cases, the heat removal is
expressed in Btu, and the energy input to the compressor is expressed in watt-hours, and
because 1 Wh3.412 Btu, EER or SEER values of greater than 10 are obtained for the
coefficient of performance. Therefore, keep in mind that in the United States, the EER is
defined in the following manner:(13.21)
The reason for using the units of Wh for energy input to the compressor is that compres-
sors are powered by electricity, and electricity consumption is measured (even in the United
States) in kWh. Today’s air-conditioning units have SEER values that range from approxi-
mately 10 to 17. In fact, all new air-conditioning units sold in the United States must have
a SEER value of at least 10. In 1992, the United States government established the mini-
mum standard efficiencies for various appliances, including air-conditioning units and gas
furnaces.
In a gas furnace, natural gas is burned, and as a result the hot combustion products go
through the inside of a heat exchanger where thermal energy is transported to cold indoor air
that is passing over the heat exchanger. The warm air is distributed to various parts of the house
through conduits. As mentioned, in 1992 the United States government established a minimum
annual fuel utilization efficiency (AFUE) rating of 78% for furnaces installed in new homes, so
manufacturers must design their gas furnaces to adhere to this standard. Today, most high-
efficiency furnaces offer AFUE ratings in the range of 80 to 96%.Example 13.11 An air-conditioning unit has a cooling capacity of 24,000 Btu /h. If the unit has a rated energy
efficiency ratio (EER) of 10, how much electrical energy is consumed by the unit in 1 h? If a
power company charges 12 cents per kWh usage, how much would it cost to run the air con-
ditioning unit for a month (30 days), assuming the unit runs 10 hours a day? What is the
coefficient of performance (COP) for the given air-conditioning unit?
We can compute the energy consumption of the given air-conditioning unit using
Equation (13.21).EER
heat removal from the evaporator 1 Btu 2
energy input to the compressor 1 Wh 2
COP
heat removal from the evaporator
energy input to the compressor
392 Chapter 13 Energy and Power
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