Chapter 14 The Energy Transmitting Medium 1
Objectives Describe the characteristics of free air. Identify the problems associated with the use of unconditioned air in a pneumatic system. Compare the composition of compressed air in a pneumatic system with the characteristics of free air. Explain the terms used to describe characteristics of compressed air. Goodheart-Willcox Co., Inc. 3 Objectives Compare the scales used to measure the pressure of ffree and compressed air. Compare the isothermal, adiabatic, and actual modes of air compression. Apply the principles of the general gas law to the compression and expansion of the air used in a pneumatic system. Goodheart-Willcox Co., Inc. 4 2
Basic Source of System Air The source of air used in pneumatic systems is the atmosphere The atmosphere blankets Earth in a layer of gas approximately 360 miles deep Goodheart-Willcox Co., Inc. 5 Basic Source of System Air The atmosphere contains several layers Goodheart-Willcox Co., Inc. 6 3
Basic Source of System Air The four layers in the atmosphere are: Troposphere Stratosphere Mesosphere Ionosphere We live in the troposphere, which is approximately 10 miles deep Goodheart-Willcox Co., Inc. 7 Basic Source of System Air The gases in atmospheric air are: Nitrogen (79%) Oxygen (20%) Other gases (1%) Goodheart-Willcox Co., Inc. 8 4
Basic Source of System Air Composition of atmospheric air Goodheart-Willcox Co., Inc. 9 Basic Source of System Air In addition to gases, the atmosphere contains water vapor and entrapped ddirt Both of these influence air compression and the final quality of the system air Goodheart-Willcox Co., Inc. 10 5
Basic Source of System Air The weight of the gases in the atmosphere exerts pressure Atmospheric pressure is 14.7 pounds per square inch at sea level 0 psig 14.7 psia Goodheart-Willcox Co., Inc. 11 Basic Source of System Air Atmospheric pressure varies by elevation Goodheart-Willcox Co., Inc. 12 6
Pneumatic System Atmospheric air is typically referred to as free air Free air must be conditioned before it can be used in a pneumatic system Certain locations require considerable preparation of free air to make it usable in a pneumatic system Goodheart-Willcox Co., Inc. 13 Pneumatic System Free air at construction sites often requires extra filtration Manufactured Housing Institute Goodheart-Willcox Co., Inc. 14 7
Pneumatic System Quarrying operations are very dusty Atlas Copco Goodheart-Willcox Co., Inc. 15 Pneumatic System The conditioning of compressed air for use in pneumatic systems involves: Removal of entrapped dirt Removal of water vapor Removal of heat Incorporation of lubricants Goodheart-Willcox Co., Inc. 16 8
Pneumatic System The amount of water vapor air can hold depends d on the temperature t of fthe air The higher the temperature, the greater the amount of water that can be retained by the air Saturation is reached when air holds the maximum amount of water for the given temperature Goodheart-Willcox Co., Inc. 17 Pneumatic System Water legs are used to collect and remove liquid water from pneumatic lines Goodheart-Willcox Co., Inc. 18 9
Pneumatic System Relative humidity expresses the percentage of water in the air compared dto the maximum amount that can be held at the specified temperature Dew point is the temperature at which water vapor in the saturated air begins to be released in liquid form Goodheart-Willcox Co., Inc. 19 Pneumatic System At the dew point, any increase in humidity is released as liquid id water, as on a fogged mirror Goodheart-Willcox Co., Inc. 20 10
Pneumatic System Dry compressed air contains water vapor, but the relative humidity is sufficiently i low to prevent the formation of liquid water at the ambient temperature of the workstation Goodheart-Willcox Co., Inc. 21 Pneumatic System A lubricant is added to dry compressed air distributed ib t d by the pneumatic system workstation This is for protection of system components Goodheart-Willcox Co., Inc. 22 11
Pneumatic System A lubricator for a pneumatic workstation ti Goodheart-Willcox Co., Inc. 23 Compression and Expansion of Air In an operating pneumatic system, the continuous interaction ti of temperature, t pressure, and volume changes make calculations complex Goodheart-Willcox Co., Inc. 24 12
Compression and Expansion of Air Two compression models are used to express air compression Isothermal compression Adiabatic compression These models are used for expansion as well Goodheart-Willcox Co., Inc. 25 Compression and Expansion of Air Isothermal compression assumes that all heat is removed, resulting in a constant t temperature t Adiabatic compression assumes all heat is retained, resulting in both increased temperature and pressure Actual compression is somewhere between isothermal and adiabatic compression Goodheart-Willcox Co., Inc. 26 13
Compression and Expansion of Air Comparison of compression models Goodheart-Willcox Co., Inc. 27 Reaction of Air to Temperature, Pressure, and Volume When air is compressed, there are changes in temperature, t pressure, and volume that t follow the relationships expressed by the general gas law (P1 V1) T1 = (P2 V2) T2 Specific system pressure, temperature, and volume changes may be difficult to verify Goodheart-Willcox Co., Inc. 28 14
Reaction of Air to Temperature, Pressure, and Volume Any change in air pressure results in temperature or volume changes Goodheart-Willcox Co., Inc. 29 Reaction of Air to Temperature, Pressure, and Volume Changes in the volume of air result in pressure or temperature changes Goodheart-Willcox Co., Inc. 30 15
Reaction of Air to Temperature, Pressure, and Volume Increases or decreases in air temperature result in pressure or volume changes Goodheart-Willcox Co., Inc. 31 Reaction of Air to Temperature, Pressure, and Volume Engineering data are available from component manufacturers and ddata handbooks that tcan be used to estimate performance from compressors and other system components Goodheart-Willcox Co., Inc. 32 16
Review Question The air we breath and use as the source of air for pneumatic systems contains several gases. Name the gases and indicate the percentage at which they exist in the atmosphere. A. Nitrogen 79%, B. oxygen 20%, and C. other gases 1% (includes argon, ozone, and carbon dioxide) Goodheart-Willcox Co., Inc. 33 Review Question The atmosphere blankets Earth with a layer of gases approximately miles deep. 360 Goodheart-Willcox Co., Inc. 34 17
Review Question refers to the amount of water vapor actually carried dby a volume of air compared to the maximum amount it can carry at the specified temperature. Relative humidity Goodheart-Willcox Co., Inc. 35 Review Question Standard atmospheric pressure is psia at sea level. l 14.7 Goodheart-Willcox Co., Inc. 36 18
Review Question Define saturation. The point where the maximum amount of water is held by air for a given temperature. Goodheart-Willcox Co., Inc. 37 Review Question The process that assumes all heat is retained in the air during compression is known as compression. adiabatic Goodheart-Willcox Co., Inc. 38 19
Review Question According to the general gas law, what happens when the pressure of air is changed? Temperature and/or volume also changes. Goodheart-Willcox Co., Inc. 39 Glossary Adiabatic compression The process in which no heat is transferred as the volume of a gas is decreased. Atmosphere 1. The mix of gases that surround Earth between its surface and space. 2. The pressure created by a column of gas extending between space and the surface of Earth at sea level. 3. The pressure 14.7 psia. Goodheart-Willcox Co., Inc. 40 20
Glossary Dew point The temperature at which the water vapors in air will begin to condense (form dew). Free air Air displaying the characteristics of the atmospheric air at a specific location. Goodheart-Willcox Co., Inc. 41 Glossary Ionosphere The uppermost layer of the atmosphere, which extends from approximately 50 miles to 360 miles above Earth s surface. Isothermal compression Compression in which any heat produced by compressing the gas is removed, producing a final pressure based only on the decrease in volume. Goodheart-Willcox Co., Inc. 42 21
Glossary Lubricant A substance used to reduce friction between moving surfaces, such as those found in bearings. Mesosphere The portion of the atmosphere that extends from the stratosphere to the ionosphere. Goodheart-Willcox Co., Inc. 43 Glossary Relative humidity The relationship between the actual amount of water in the air and the maximum amount the air could hold at a given temperature. Saturation The condition when air will not hold additional water vapor. Goodheart-Willcox Co., Inc. 44 22
Glossary Stratosphere The part of the atmosphere that extends from 10 to 30 miles above Earth s surface. Troposphere The layer of the atmosphere in which we live. This layer extends to 10 miles above Earth s surface. Goodheart-Willcox Co., Inc. 45 Glossary Water vapor Water in its gaseous form. Note: fog or mist is not water vapor, but atomized liquid water. Goodheart-Willcox Co., Inc. 46 23