HYDRAULICS & PNEUMATICS Compressed Air Source of Pneumatic Power Presented by: Dr. Abootorabi

Basic Source of System Air The source of air used in pneumatic systems is the atmosphere. The gases in atmospheric air are: Nitrogen (79%) Oxygen (20%) Other gases (1%)

Basic Source of System Air Composition of atmospheric air

Basic Source of System Air In addition to gases, the atmosphere contains water vapor and entrapped dirt. Both of these influence air compression and the final quality of the system air. The weight of the gases in the atmosphere exerts pressure. Atmospheric pressure is 14.7 pounds per square inch at sea level.

Basic Source of System Air Atmospheric pressure varies by elevation

Pneumatic System Compressed Air 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.

Pneumatic System Compressed Air 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

Pneumatic System Compressed Air The amount of water vapor air can hold depends 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

Pneumatic System Compressed Air Water legs are used to collect and remove liquid water from pneumatic lines.

Pneumatic System Compressed Air Relative humidity expresses the percentage of water 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 in liquid form.

Pneumatic System Compressed Air Dry compressed air contains water vapor, but the 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 distributed by the pneumatic system workstation. This is for protection of system components.

Pneumatic System Compressed Air A lubricator for a pneumatic workstation:

Compression and Expansion of Air In an operating pneumatic system, the continuous interaction of temperature, pressure, and volume changes make calculations complex. Engineering data are available from component manufacturers and data handbooks that can be used to estimate performance from compressors and other system components.

Reaction of Air to Temperature, Pressure, and Volume When air is compressed, there are changes in temperature, 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 changes may be difficult to verify

Compressed-Air Unit The source of compressed air for a pneumatic system is the compressed-air unit: Prime mover Compressor Other components to condition and store the pressurized air used by the system workstations Compressed air units vary in size.

Compressed-Air Unit Very small packages may produce only a fraction of a cubic foot of air per minute (cfm). 1 ft3 ≈ 0.028 m3

Compressed-Air Unit Large, industrial units may produce thousands of cfm.

Compressed-Air Unit Compressed-air units can be classified as portable units or central air supplies: Physical size is not the only factor in placing a unit in one of these classes Easy transport of a unit from one location to another is a more important factor Many portable units have a larger capacity than many stationary central air supplies

Compressed-Air Unit A portable unit may be large or small.

Compressed-Air Unit Portable units allow the compressor to be moved to the work site.

Compressed-Air Unit A compressed-air unit consists of: Prime mover Compressor Coupling Receiver Capacity-limiting system Safety valve Air filter May have a cooler and dryer

Compressed-Air Unit The prime mover in a compressed-air unit may be: Electric motor Internal combustion engine Steam or gas turbine A coupling connects the prime mover to the compressor

Compressed-Air Unit Belt coupling: DeVilbiss Air Power Company

Compressed-Air Unit Mechanical coupling: DeVilbiss Air Power Company

Basic Compressor Design A variety of designs are used for air compressors in the compressed-air unit: Reciprocating piston Rotary, sliding vane Rotary screw Dynamic

Basic Compressor Design Reciprocating-piston compressors are the most common. Rotary screw compressors are popular in new installations.

Basic Compressor Design The basic operation of any compressor includes three phases: Air intake Air compression Air discharge Component parts and physical operation varies between compressor designs.

Basic Compressor Classifications Compressors are classified as: Positive or non-positive displacement Reciprocating or rotary Positive-displacement compressors mechanically reduce the compression chamber size to achieved compression. Non-positive-displacement compressors use air velocity to increase pressure.

Basic Compressor Classifications A reciprocating compressor has a positive displacement. DeVilbiss Air Power Company

Compressor Design and Operation Reciprocating compressors use a cylinder and a reciprocating piston to achieve compression. Rotary compressors use continuously rotating vanes, screws, or lobed impellers to move and compress the air.

Compressor Design and Operation Reciprocating compressors are commonly used in pneumatic systems: Very small, single-cylinder, portable compressors for consumer use Large, industrial, stationary units may produce thousands of cubic feet of compressed air per minute

Compressor Design and Operation Large, industrial, reciprocating compressor: Atlas Copco

Compressor Design and Operation Reciprocating compressors use a single-acting or double-acting compression arrangement Single-acting compressors compress air during one direction of piston travel Double-acting compressors have two compression chambers, allowing compression on both extension and retraction of the piston

Compressor Design and Operation Double-acting compressor

Compressor Design and Operation Multiple cylinders may be arranged as: Inline Opposed V type W type Other cylinder configuration

Compressor Design and Operation Inline reciprocating compressor:

Compressor Design and Operation V-type reciprocating compressor:

Compressor Design and Operation Rotary, sliding-vane compressors use a slotted rotor containing movable vanes to compress air: Rotor is placed off center in a circular compression chamber, allowing the chamber volume to change during rotation These volume changes allow the intake, compression, and discharge of air during compressor rotation

Compressor Design and Operation Centrifugal force keeps the vanes in contact with the walls

Compressor Design and Operation Rotary screw compressors use intermeshing, helical screws to form chambers that move air from the atmosphere into the system on a continuous basis. This produces a nonpulsating flow of air at the desired pressure level.

Compressor Design and Operation Rotary screw compressors have intermeshing, helical screws:

Compressor Design and Operation Rotary screw compressors have become popular for larger industrial installations: Lower initial cost Lower maintenance cost Adaptable to sophisticated electronic control systems

Compressor Design and Operation Sliding vane and screw compressor designs often inject oil into the airstream moving through the compressors: Reduces wear on vane and screw contact surfaces Improves the seal between the surfaces Oil is removed by a separator to provide near-oilless compressed air for the pneumatic system.

Compressor Design and Operation Lobe-type compressors consist of two impellers with two or three lobes that operate in an elongated chamber in the compressor body: Spinning impellers trap air in chambers that form between the lobes As the impellers turn, this trapped air is swept from the inlet port to the outlet port to increase system pressure

Compressor Design and Operation Impellers from a lobe-type compressor.

Compressor Design and Operation Lobe-type compressors are often called blowers. They are typically used in applications requiring air pressure of only 10 to 20 psi.

Compressor Design and Operation The basic operating theory of dynamic compressors is converting the kinetic energy of high-speed air into pressure. Dynamic compressor designs are either: Centrifugal Axial

Compressor Design and Operation Centrifugal dynamic compressor: An impeller increases airspeed Prime mover energy is converted into kinetic energy as air speed rapidly increases through the impeller Kinetic energy is converted to air pressure as air movement slows in the volute collector

Compressor Design and Operation Centrifugal dynamic compressor:

Compressor Design and Operation Axial-flow dynamic compressor: Rotating rotor blades increase airspeed Fixed stator blades decrease airspeed Kinetic energy is converted to air pressure Series of rotor and stator sections are staged to form the axial-flow compressor

Compressor Design and Operation Axial-flow dynamic compressor:

Compressor Design and Operation Axial-flow dynamic compressor:

Compressor Design and Operation Pressure is created when high-speed air is slowed by the fixed stator blades.

Compressor Design and Operation Dynamic compressor designs are used to compress air and other gases for large, industrial applications: Oil refineries Chemical plants Steel mills

Compressor Design and Operation Compressor staging involves connecting a number of basic compressor units in series to raise air pressure in small increments. This method permits easier control of air temperature, which results in more-efficient compressor package operation.

Compressor Design and Operation Inline, staged, reciprocating compressor:

Compressor-Capacity Control Compressor-capacity control refers to the system that matches the compressed-air output to the system-air demand. The better the air output of the compressor matches system consumption, the more cost effective the operation of the system.

Compressor-Capacity Control Compressor-capacity control systems include: Bypass Start-stop Inlet valve unloading Speed variation Inlet size variation

Compressor-Capacity Control Bypass control uses a relief-type valve to exhaust excess air. Air is continuously delivered to the system at the compressor’s maximum flow rate. This type of control is not considered desirable as it is inefficient.

Compressor-Capacity Control Start-stop capacity control is commonly used with small, electric motor-driven compressor packages that operate pneumatic systems consuming air on an intermittent basis.

Compressor-Capacity Control Start-stop control uses a pressure-sensitive switch to start and stop the compressor to maintain a preselected pressure range.

Compressor-Capacity Control Start-stop control: compressor start

Compressor-Capacity Control Start-stop control: compressor stop

Compressor-Capacity Control Inlet valve unloading controls compressor output by holding the inlet valve open whenever maximum system pressure is achieved: Allows the prime mover to operate continuously Can be used in systems having internal combustion engines or electric motors as the prime mover

Compressor-Capacity Control Varying compressor speed can control compressor capacity: Can be used with reciprocating and rotary compressor designs Primarily used on large, industrial installations Sensors monitor pressure and send a signal to control compressor speed

Compressor-Capacity Control Varying the size of the compressor inlet can control compressor capacity: Compressor operates at a constant speed The volume of air that can enter the compressor is restricted Output varies with the size of the inlet Primarily used on dynamic compressors

Selecting a Compressor Package Establishing the level of system air consumption is a key factor when selecting a compressor This can be accomplished by identifying: Actuators used in the system Compressed-air needs of each item Percentage of time each functions

Selecting a Compressor Package Other factors must be considered during system compressor selection: Compressor and prime mover type Method of compressor-capacity control Auxiliary controls such as coolers, separators, and driers System instrumentation

The end.