Air Compressors and Pumps in Construction

Air Compressors on Construction Projects

• Compressed air is vital in construction for powering various tools and equipment.
• Applications:
  • Drills
  • Pile-driving hammers
  • Air motors
  • Hand tools
  • Pumps
• Air compressors power the tools but do not directly perform the construction task (e.g., driving a pile).

Air Compressor Definitions

• Aftercooler: A heat exchanger that cools air after it exits the compressor.
• Centrifugal Compressor: Uses a rotating vane or impeller to increase air velocity, thus creating pressure.
• Compression Ratio: The ratio of absolute discharge pressure to absolute inlet pressure.
• Compressor Efficiency: The ratio of theoretical horsepower to the actual brake horsepower required by the compressor.
• Discharge Pressure: The absolute air pressure at the compressor's outlet.
• Diversity Factor: The ratio of the actual total air quantity needed to the sum of individual air requirements.
• Free Air: Air under normal atmospheric conditions at a specific location.
• Inlet Pressure: The absolute air pressure at the compressor's inlet.
• Intercooler: A heat exchanger positioned between compression stages to remove heat.
• Load Factor: The ratio of average load to the compressor's maximum rated load over a period.
• Multistage Compressor: Achieves the desired final pressure through multiple compression stages.
• Reciprocating Compressor: Compresses air using a piston within a cylinder.
• Single-Stage Compressor: Compresses air from atmospheric pressure to the required discharge pressure in one step.
• Two-Stage Compressor: Compresses air in two steps: first to an intermediate pressure, then to the final pressure.

Air Compressor Capacity

• Capacity is measured by the amount of free air compressed to a specified pressure in one minute.
• Standard conditions are an absolute pressure of 14.7 psi at 59°F.
• The number of tools an air compressor can operate depends on their specific air requirements.

Types of Air Compressors

• Stationary Compressors:
  • Used for long-term compressed air needs at fixed locations.
  • A single large compressor is usually cheaper than multiple smaller ones for the same capacity.
  • Multiple compressors offer flexibility for varying load demands.
  • Having multiple compressors avoids complete shutdown for repairs.
• Portable Compressors:
  • Common on construction sites with changing demands.
  • Mounted on tires or skids.
  • Powered by gasoline or diesel engines.
  • Most construction compressors are rotary type.
• Rotary Compressors:
  • Use rotating elements for compression.
• Rotary-Screw Compressors:
  • Have two helical rotors: a male rotor with four lobes and a female rotor with six flutes.
  • The male rotor rotates 50% faster than the female rotor.
  • Air is compressed in the space between the lobes and flutes.
  • Inlet and outlet ports are automatically covered and uncovered by the rotor ends.
• Monoscrew Compressors:
  • Operate similarly to rotary-screw compressors.

Pumping Water in Construction

• Pumps are widely used in construction for:
  • Removing water from pits, tunnels, and excavations.
  • Dewatering cofferdams.
  • Supplying water for jetting and sluicing.
  • Lowering the water table for excavations.

Factors in Pump Selection:

• Dependability
• Availability of repair parts
• Simplicity for easy repairs
• Economical installation and operation
• Operating power requirements

Dimensional Terminology for Pumping Operations

• Discharge Head: The total discharge head is the sum of the static discharge head plus head losses in the discharge line.
• Discharge Hose: Carries liquid from the pump's discharge side.
• Self-Priming: The pump's ability to separate air from liquid to create a vacuum, drawing liquid into the pump.
• Static Discharge Head: Vertical distance from the pump impeller's centerline to the point of discharge.
• Static Suction Lift: Vertical distance from the pump impeller's centerline to the surface of the liquid being pumped.
• Strainer: A cover on the suction hose's end that restricts solids entering the pump.
• Suction Head: The sum of the static suction lift plus the suction line head losses.
• Suction Hose: Connected to the pump's suction side; made of reinforced rubber or plastic to prevent collapsing.
• Total Static Head: The sum of the suction head and the discharge head.
• Suction capability is limited by atmospheric pressure; maximum practical suction lift is 25 ft.
• Decreasing suction lift increases the volume that can be pumped.

Classification of Pumps

• Displacement pumps
• Centrifugal pumps

Reciprocating Pumps

• Operate via a piston moving within a cylinder.
• Piston movement forces water out of one end and draws it into the other.

Diaphragm Pumps

• A type of displacement pump.
• A flexible diaphragm is raised and lowered by a pump rod connected to a walking beam.
• Suitable for handling clear water or water with mud, sand, sludge, and trash.
• Effective for variable water quantities and can pump air and water mixtures.

Centrifugal Pumps

• Use a rotating impeller to impart velocity to water, enabling it to flow against pressure.
• Water possesses potential energy (height above a datum) and kinetic energy (velocity).
• Kinetic energy from the impeller causes water to rise to a certain height.