chapter 14 compressed air

1

Chapter 14Chapter 14

Compressed AirCompressed Air

The Energy Transmitting MediumThe Energy Transmitting Medium

2

ObjectivesObjectives

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

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.3

free air.

Explain the terms used to describe characteristics of compressed air.

ObjectivesObjectives

Compare the scales used to measure the f f d d ipressure of free 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


the compression and expansion of the air used in a pneumatic system.

3

Basic Source of System AirBasic Source of System Air

The source of air used in pneumatic systems is th t hthe atmosphere

The atmosphere blankets Earth in a layer of gas approximately 360 miles deep



The atmosphere contains several layers


4


The four layers in the atmosphere are:– Troposphere

– Stratosphere

– Mesosphere

– Ionosphere

We live in the troposphere which is


We live in the troposphere, which is approximately 10 miles deep


The gases in atmospheric air are:– Nitrogen (79%)

– Oxygen (20%)

– Other gases (1%)


5


Composition of atmospheric air



In addition to gases, the atmosphere contains t d t d di twater vapor and entrapped dirt

Both of these influence air compression and the final quality of the system air


6


The weight of the gases in the atmosphere texerts pressure

Atmospheric pressure is 14.7 pounds per square inch at sea level– 0 psig

– 14 7 psia


14.7 psia


Atmospheric pressure varies by elevation


7

Pneumatic System Pneumatic System Compressed AirCompressed Air

Atmospheric air is typically referred to as f ifree 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


preparation of free air to make it usable in a pneumatic system


Free air at construction it ft isites often requires

extra filtration


Manufactured Housing Institute

8


Quarrying operations d tare very dusty


Atlas Copco


The conditioning of compressed air for use in ti t i lpneumatic systems involves:

– Removal of entrapped dirt

– Removal of water vapor

– Removal of heat

– Incorporation of lubricants


Incorporation of lubricants

9


The amount of water vapor air can hold d d th t t f th idepends on the temperature of the 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


Water legs are used t ll t dto collect and remove liquid water from pneumatic lines


10


Relative humidity expresses the percentage of t i th i d t th iwater in the air compared to 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


vapor in the saturated air begins to be released in liquid form


At the dew point, any increase in humidity is l d li id t f d ireleased as liquid water, as on a fogged mirror


11


Dry compressed air contains water vapor, but th l ti h idit i ffi i tl l tthe relative humidity is sufficiently low to prevent the formation of liquid water at the ambient temperature of the workstation



A lubricant is added to dry compressed air di t ib t d b th ti tdistributed by the pneumatic system workstation

This is for protection of system components


12


A lubricator for a ti k t tipneumatic workstation


Compression andCompression andExpansion of AirExpansion of Air

In an operating pneumatic system, the ti i t ti f t tcontinuous interaction of temperature, pressure,

and volume changes make calculations complex


13


Two compression models are used to express i iair compression– Isothermal compression

– Adiabatic compression

These models are used for expansion as well



Isothermal compression assumes that all heat i d lti i t t t tis removed, resulting in a constant temperature

Adiabatic compression assumes all heat is retained, resulting in both increased temperature and pressure

Actual compression is somewhere between


Actual compression is somewhere between isothermal and adiabatic compression

14


Comparison of compression models


Reaction of Air to Temperature, Reaction of Air to Temperature, Pressure, and VolumePressure, and Volume

When air is compressed, there are changes in t t d l th t f lltemperature, pressure, and volume that follow the relationships expressed by the general gas law– (P1 V1) T1 = (P2 V2) T2

– Specific system pressure, temperature, and volume


p y p , p ,changes may be difficult to verify

15


Any change in air lt ipressure results in

temperature or volume changes


Changes in the volume of air result in pressure or temperature changes


or temperature changes


16

Increases or decreases in air temperature result in pressure or volume changes


result in pressure or volume changes


Engineering data are available from component f t d d t h db k th t b


manufacturers and data handbooks that can be used to estimate performance from compressors and other system components


17

Review QuestionReview Question

The air we breath and use as the source of air f ti t t i lfor 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%,


g , yg ,and C. other gases 1% (includes argon, ozone, and carbon dioxide)


The atmosphere blankets Earth with a layer of i t l il dgases approximately _____ miles deep.

360


18


_____ refers to the amount of water vapor t ll i d b l f i dactually carried by a volume of air compared

to the maximum amount it can carry at the specified temperature.

Relative humidity


Relative humidity


Standard atmospheric pressure is _____ psia at l lsea level.

14.7


19


Define saturation.

The point where the maximum amount of water is held by air for a given temperature.


p


The process that assumes all heat is retained in th i d i i i kthe air during compression is known as _____ compression.

adiabatic


20


According to the general gas law, what happens h th f i i h d?when the pressure of air is changed?

Temperature and/or volume also changes.


GlossaryGlossary

Adiabatic compression The process in which no heat is transferred as the– 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


surface of Earth at sea level. 3. The pressure 14.7 psia.

21

GlossaryGlossary

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.


p

GlossaryGlossary

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


p y p ycompressing the gas is removed, producing a final pressure based only on the decrease in volume.

22

GlossaryGlossary

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.


p p

GlossaryGlossary

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.

23

GlossaryGlossary

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.


y

GlossaryGlossary

Water vapor– Water in its gaseous form. Note: fog or mist is not

water vapor, but atomized liquid water.


chapter 14 compressed air

Documents