Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

13.4 Watts and Horsepower 383

The U.S. Customary units of power are related to the SI unit of power, watt (W), in the follow-

ing manner:

(13.13)

(13.14)

Remember that 1(lbf#ft /s) is slightly greater in magnitude than 1 W. Also keep in mind

that 1 hp is slightly smaller than 1 kW. Another unit that is sometimes confused for the unit

of power is kilowatt hour, used in measuring the consumption of electricity by homes and

the manufacturing sector. First, kilowatt hour (kWh) is a unit of energy — not power. One

kilowatt hour represents the amount of energy consumed during 1 hour by a device that uses

one kilowatt (kW) or 1000 joules per second ( J/s). Therefore,

In heating, ventilating, and air-conditioning (HVAC) applications, Btu per hour (Btu /h)

is used to represent the heat loss from a building during cold months and the heat gained by

the building during summer months. The units of Btu /h are related to the unit of watt in the

following manner:

Another common unit used in the United States in air-conditioning and refrigeration systems

iston of refrigeration or cooling. One ton of refrigeration represents the capacity of a refrigera-

tion system to freeze 2000 lbmor 1 ton of liquid water at 32F into 32F ice in 24 hours. It is

In the case of an air-conditioning unit, one ton of cooling represents the capacity of the air-con-

ditioning system to remove 12,000 Btu thermal energy from a building in 1 hour. Clearly, the

capacity of a residential air-conditioning system depends on the size of the building, its con-

struction, shading, the orientation of its windows, and its climatic location. Residential air-

conditioning units generally have a 1- to 5-ton capacity. The sizes of home gas furnaces in the

United States are also expressed in units of Btu /h. The size of a typical single-family-home gas

furnace used in moderate winter conditions is 60,000 Btu /h.

To get a feel for the relative magnitudes that watt and horsepower physically represent,

consider the following examples.

Example 13.6 Determine the power required to move 30 people, with an average mass of 61 kg (135 lbm) per

person, between two floors of a building, a vertical distance of 5 m (16 ft) in 2 s.

The required power is determined by

power

work

time



1 30 persons2a 61

kg

person

ba9.81

m

s

2 b15 m^2

2 s

45,000 W

1 ton of refrigeration12,000 Btu/h

1 Btu/h1055 W1.055 kW

1 kWh3.6 MJ

1 kWh 1 1000 J/s 21 3600 s 2 3,600,000 J3.6 MJ

1 kW1000 W1000 J/s

1 hp745.69 W746 W

1

lbf#ft

s

1.3558 W1.36 W

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