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

There are other gases present in the atmosphere, including carbon dioxide, sulfur dioxide,

and nitrogen oxide. The atmosphere also contains water vapor. The concentration level of these

gases depends on the altitude and geographical location. At higher altitudes (10 to 50 km), the

earth’s atmosphere also contains ozone. Even though these gases make up a small percentage of

earth’s atmosphere, they play a significant role in maintaining a thermally comfortable envi-

ronment for us and other living species. For example, the ozone absorbs most of the ultravio-

let radiation arriving from the sun that could harm us. The carbon dioxide plays an important

role in sustaining plant life; however, if the atmosphere contains too much carbon dioxide, it

will not allow the earth to cool down effectively by radiation. Water vapor in the atmosphere

in the form of clouds allows for transport of water from the ocean to land in the form of rain

and snow.

Humidity There are two common ways of expressing the amount of water vapor in air:

absolute humidity or humidity ratio, and relative humidity. The absolute humidity is defined

as the ratio of mass of water vapor in a unit mass of dry air, according to

(17.1)

For humans, the level of a comfortable environment is better expressed by relative humidity,

which is defined as the ratio of the amount of water vapor or moisture in the air to the maxi-

mum amount of moisture that the air can hold at a given temperature. Therefore, relative

humidity is defined as

(17.2)

Most people feel comfortable when the relative humidity is around 30 to 50%. The higher the

temperature of air, the more water vapor the air can hold before it is fully saturated. Because of

its abundance, air is commonly used in food processing, especially in food drying processes to

make dried fruits, spaghetti, cereals, and soup mixes. Hot air is transported over the food to

absorb water vapors and thus remove them from the source.

Understanding how air behaves at given pressures and temperatures is also important when

designing cars to overcome air resistance or designing buildings to withstand wind loading.

Water

You already know that every living thing needs water to sustain life. In addition to drinking

water, we also need water for washing, laundry, grooming, cooking, and fire protection. You may

also know that two-thirds of the earth’s surface is covered with water, but most of this water can-

not be consumed directly; it contains salt and other minerals that must be removed first. Radi-

ation from the sun evaporates water; water vapors form into clouds and eventually, under

favorable conditions, water vapors turn into liquid water or snow and fall back on the land and

the ocean. On land, depending on the amount of precipitation, part of the water infiltrates the

soil, part of it may be absorbed by vegetation, and part runs as streams or rivers and collects into

natural reservoirs called lakes. Surface water refers to water in reservoirs, lakes, rivers, and

streams. Groundwater, on the other hand, refers to the water that has infiltrated the ground;

surface and groundwaters eventually return to the ocean, and the water cycle is completed. As

relative humidity

the amount of moisture in the air 1 kg 2

the maximum amount of moisture that air can hold 1 kg 2

absolute humidity

mass of water vapor 1 kg 2

mass of dry air 1 kg 2

572 Chapter 17 Engineering Materials

Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀