MMET 401 FINAL STUDY GUIDE
Pneumatic systems - Answer -Use pressurized gases (air) to transmit and control
power
-Pressures are low due to compressor design limitations (<250 psi)
-Low power applications
Why air - Answer -Cheap and readily available, environmentally friendly, inflammable
Pneumatic vs hydraulic - Answer -Weight of oil is a problem when
accelerating/decelerating actuators and suddenly opening/closing valves
Less frictional pressure and power losses
Hydraulics require special reservoirs and no leak system designs
Less expensive
Compressibility of air - Answer -Impossible to obtain precise, controlled actuator
velocities or precise positioning control with pneumatic systems
Air - Answer -Mixture of gases containing 21% oxygen, 78% nitrogen, and 1% other
gases
Contains up to 4% water vapor
Absolute temperature - Answer -All molecular motion ceases to exist and the volume
and pressure of a gas become zero
Boyle's law - Answer -If the temperature of a given amount of gas is held constant, the
volume of the gas will change inversely with the absolute pressure of the gas
Charles' law - Answer -If the pressure on a given amount of gas is held constant, the
volume of the gas will change in direct proportion to the absolute temperature
Gay-Lussac's law - Answer -If the volume of a given gas is held constant, the pressure
exerted by the gas is directly proportional to its absolute temperature
Compressor - Answer -A machine that compresses air from a low inlet pressure to a
higher desired pressure level by reducing the volume of the gas
Positive displacement units: either reciprocating piston, rotary screw or rotary vane
types
, Start, run, deliver air to the system as needed, stop, and be ready to start again without
the attention of an operator
Piston compressor - Answer -Compresses air by extending and retracting a piston
inside a cylinder
Portable and small industrial compressors are normally air-cooled, whereas larger units
must be water cooled
More than one cylinder: staging can be used to increase efficiency
Successive cylinder sizes decrease, the intercooling removes the heat of compression =
increase air density and volumetric efficiency
Air fins - Answer -Dissipate heat to increase density of air, which increases volumetric
efficiency in a compressor
Multistage compressor - Answer -Effective cooling can be implemented between
stages, which increases efficiency and reduces input power requirements
Screw compressor - Answer -Rolling trapped air into a progressively smaller volume as
the screws rotate
Rotors turn freely, with controlled clearance between both rotors and the housing,
protected by a film of oil
Metal to metal contact is eliminated = rotor wear will not occur
Air lubricates the rotors, seals the rotor clearances for high compression efficiency and
absorbs heat of compression = low discharge air temps
Vane compressor - Answer -Each rotor slot contains a rectangular vane, which slides in
and out of the slot due to centrifugal force
As rotor turns, air is trapped and compressed between the vanes and then discharged
through a port to the receiver
Sliding vanes decrease air volume as the rotor rotates
Air receiver - Answer -Supply air at constant pressure and damped pressure pulses
either coming from the compressor or the pneumatic system during valve shifting and
component operation
-Pneumatic systems demand air at a flow rate that exceeds the compressor capability,
receivers handle this
Pneumatic systems - Answer -Use pressurized gases (air) to transmit and control
power
-Pressures are low due to compressor design limitations (<250 psi)
-Low power applications
Why air - Answer -Cheap and readily available, environmentally friendly, inflammable
Pneumatic vs hydraulic - Answer -Weight of oil is a problem when
accelerating/decelerating actuators and suddenly opening/closing valves
Less frictional pressure and power losses
Hydraulics require special reservoirs and no leak system designs
Less expensive
Compressibility of air - Answer -Impossible to obtain precise, controlled actuator
velocities or precise positioning control with pneumatic systems
Air - Answer -Mixture of gases containing 21% oxygen, 78% nitrogen, and 1% other
gases
Contains up to 4% water vapor
Absolute temperature - Answer -All molecular motion ceases to exist and the volume
and pressure of a gas become zero
Boyle's law - Answer -If the temperature of a given amount of gas is held constant, the
volume of the gas will change inversely with the absolute pressure of the gas
Charles' law - Answer -If the pressure on a given amount of gas is held constant, the
volume of the gas will change in direct proportion to the absolute temperature
Gay-Lussac's law - Answer -If the volume of a given gas is held constant, the pressure
exerted by the gas is directly proportional to its absolute temperature
Compressor - Answer -A machine that compresses air from a low inlet pressure to a
higher desired pressure level by reducing the volume of the gas
Positive displacement units: either reciprocating piston, rotary screw or rotary vane
types
, Start, run, deliver air to the system as needed, stop, and be ready to start again without
the attention of an operator
Piston compressor - Answer -Compresses air by extending and retracting a piston
inside a cylinder
Portable and small industrial compressors are normally air-cooled, whereas larger units
must be water cooled
More than one cylinder: staging can be used to increase efficiency
Successive cylinder sizes decrease, the intercooling removes the heat of compression =
increase air density and volumetric efficiency
Air fins - Answer -Dissipate heat to increase density of air, which increases volumetric
efficiency in a compressor
Multistage compressor - Answer -Effective cooling can be implemented between
stages, which increases efficiency and reduces input power requirements
Screw compressor - Answer -Rolling trapped air into a progressively smaller volume as
the screws rotate
Rotors turn freely, with controlled clearance between both rotors and the housing,
protected by a film of oil
Metal to metal contact is eliminated = rotor wear will not occur
Air lubricates the rotors, seals the rotor clearances for high compression efficiency and
absorbs heat of compression = low discharge air temps
Vane compressor - Answer -Each rotor slot contains a rectangular vane, which slides in
and out of the slot due to centrifugal force
As rotor turns, air is trapped and compressed between the vanes and then discharged
through a port to the receiver
Sliding vanes decrease air volume as the rotor rotates
Air receiver - Answer -Supply air at constant pressure and damped pressure pulses
either coming from the compressor or the pneumatic system during valve shifting and
component operation
-Pneumatic systems demand air at a flow rate that exceeds the compressor capability,
receivers handle this
