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Basic Principles of Pneumatics

Topics

Absolute Pressure and TemperatureGas LawsGas FlowVacuum

Pneumatic Systems

Chapter 10

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Basic Principles of Pneumatics

Comparison HYDRAULICS PNEUMATICS

Uses a relatively incompressible liquid Uses a compressible gas

Slow, smooth motion Quick, jumpy motion

Very precise Not as precise as hydraulics

Self Lubricating Lubricant must be added

Not as clean Cleanest

Pressures of 500 to 5000 psi Pressures around 100 psi

Pneumatics – The use of gas to transmit power from one point to another.

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Absolute Pressure and Temperature

Gauge Pressure – Pressure that is above atmospheric pressure. Atmospheric pressure is due to the weight of the air above us.

Absolute Pressure – The total pressure exerted on a system, including atmospheric pressure.

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Absolute Pressure and Temperature

Temperature:

Absolute Zero -

Standard UnitsAbsolute Temperature: Rankine =

Metric UnitsAbsolute Temperature: kelvin (K)

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Gas Laws

Isothermal process – A process in which temperature remains constant.

Boyle’s Law – the absolute pressure of a confined gas is inversely proportional to its volume, as long as its temperature remains constant.

Absolute pressure must be used!

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Gas Laws

EX: A cylinder is initially filled with of air at a pressure of . The air is compressed to a volume of . What is the pressure in psig after compression?

Convert back to psig:

Calculate the new pressure:

Convert to psia:

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Gas Laws

EX: A cylinder is initially filled with of air at a pressure of . The air is compressed to a volume of . What is the pressure in kPa after compression?

Convert back to kPa gauge:

Calculate the new pressure:

Convert kPa to abs:

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Gas Laws

Gay-Lussac’s Law – The absolute pressure of a confined gas is proportional to its temperature, provided its volume remains constant.

Used when heating or cooling gases.

= absolute pressure of the gas at state 1= absolute temperature of the gas at state 1= absolute pressure of the gas at state 2= absolute temperature of the gas at state 2

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Gas Laws

EX: Air in a fixed volume container is initially at atmospheric pressure (0 psig) and . Its temperature is then raised to . What is the gauge pressure at the final temperature.

Convert to absolute temperature :

Convert to psia:

Calculate the final pressure:

Convert back to psig:

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Gas Laws

EX: Air in a fixed volume container is initially at atmospheric pressure (0 kPa gauge) and . Its temperature is then raised to . What is the gauge pressure at the final temperature.

Convert to absolute temperature :

Convert kPa abs:

Calculate the final pressure:

Convert back to kPa:

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Gas Laws

Charles law – The volume of a confined gas is proportional to its temperature, provided its pressure remains constant.

= volume of the gas at state 1= absolute temperature of the gas at state 1= volume of the gas at state 2= absolute temperature of the gas at state 2

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Gas Laws

EX: Air is held in a container with a free sliding piston. It is initially at a temperature of and a volume of . Its temperature is then raised to . What is the volume of the air after being heated?

Convert to absolute temperatures (:

Calculate the final volume:

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Gas Laws

General Gas Law – relates temperature, pressure and volume. (nothing remains constant)

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Gas Laws

EX: Air initially at and 800kPa occupies a volume of . If the air is heated to while its volume is reduced to , what is the resulting final pressure?

Convert to absolute temperature:

Convert to absolute pressure:

Calculate the final pressure:

Convert back to kPa gauge:

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Gas Laws

Adiabatic Process – none of the heat is transferred to the atmosphere, so the temperature of the gas will increase to a maximum when compressed or expanded. The temperature changes without heat being added.

𝑝1 ∙𝑉 1𝑘=𝑝2 ∙𝑉 2

𝑘

𝑝1

𝑇1

𝑘𝑘−1

=𝑝2

𝑇 2

𝑘𝑘−1

k is a constant that measures the heat absorption properties of the gas, for Air k =1.4

𝑝1 ∙𝑉 11.4=𝑝2 ∙𝑉 2

1.4

𝑝1𝑇1

3.5=𝑝2𝑇 2

3.5

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Gas Laws

EX: Air that is initially at and atmospheric pressure is compressed from to . Assuming the process is adiabatic, what is the final pressure and temperature of the gas?

Convert to absolute temperature:

Calculate the final pressure:

Calculate the final temperature:

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Gas Flow

Units – scfm (standard cubic feet per minute)

metric

Standard conditions: 14.7 psia and relative humidity 36%

Equivalent flow of atmospheric air, not necessarily pressurized air

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Vacuum

Use pressures that are below atmospheric pressure to create suction.

Used for pick and place to lift and manipulate parts.

Measured in inches of mercury (Hg), using a mercury barometer.

mmHg in metric

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VacuumMercury is used because it is a heavy liquid.

At sea level the air pressure will push mercury to a height of 30 inches.

30 in Hg represents a perfect vacuum.

At higher elevations, it will be less than 30 in Hg because atmospheric pressure is lower.

A vacuum generates less force at higher elevations.

MECH1300Vacuum

Vacuum pumps never generate a perfect vacuum because they cannot evacuate all the air.

Industry pumps operate around 20in Hg

at sea levelat sea level

As air is removed, pressure decrease

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VacuumEX: A suction cup with an inside diameter of 3 in is used in a vacuum system that pulls 20 in Hg. What is the maximum lifting force?

Convert to psig:

Calculate the area:

Calculate the Force:

The negative indicates that the object is being drawn toward the suction cup

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VacuumEX: A suction cup with an inside diameter of 40mm is used in a vacuum system that pulls 200mmHg. What is the maximum lifting force?

Convert to kPa guage:

Calculate the area:

Calculate the Force:

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Pneumatic Systems3 segments –

1. Power Supply – Power supply segment (compressor, receiver tank). Prime mover is still usually an electric motor. Compressor pumps air into the receiver tank, storing

air.System draws air from the tank as needed.

2. Control Valves – directional, pressure and flow

3. Output – actuator (motor, cylinder) and load