Air Compressor Types

Reference data and engineering information about air compressor types for gases and compressed air applications.

Overview

Engineering reference data for Air Compressor Types in gases and compressed air.

Key Formulas

Ideal Gas Law

$PV = nRT$

Pressure × Volume = moles × gas constant × temperature.

Boyle's Law

$P_1 V_1 = P_2 V_2$

At constant temperature.

Charles's Law

$\frac{V_1}{T_1} = \frac{V_2}{T_2}$

At constant pressure.

Variables

Symbol	Description	Unit
$P$	Pressure	Pa
$V$	Volume	m³
$T$	Temperature	K
$R$	Gas constant	8.314 J/(mol·K)

Technical Specifications

Pressure and Capacity Ranges

Compressor Type	Typical Pressure Range (psig)	Typical HP Range	Common Applications
Reciprocating (Single-Stage)	70 – 100	1 – 50	General shop air, small workshops
Reciprocating (Two-Stage)	100 – 250	1 – 50	Higher-pressure applications
Rotary Screw	Wide range	100 and above	Continuous-duty industrial use
Centrifugal	High	High-capacity	Large-scale, continuous processes

Engineering Note: A useful rule of thumb for reciprocating compressors is: $1 \text{ HP} \approx 4 \text{ CFM} @ 100 \text{ psi}$

Capacity Control Methods

Compressor Type	Primary Control Method	Description
Reciprocating	Throttling / Bypass	Suction throttling or internal/external air bypass. Speed control for engine-driven units.
Rotary Screw	Variable Speed / Variable Displacement (Slide Valve)	Adjusts motor speed or uses a slide valve to bypass compressed air internally.
Centrifugal	Inlet Guide Vanes	Adjusting the angle of vanes at the inlet to control airflow and capacity.

Cooling and Lubrication Characteristics

Rotary Screw: Primarily oil-cooled (with air or water coolers). The oil also seals internal clearances. Available in oil-flooded and oil-free designs.
Centrifugal: Inherently oil-free by design. The air compression chamber is isolated from oil-lubricated bearings and gears by shaft seals.
Reciprocating: Available in both air-cooled and water-cooled configurations, and in lubricated and non-lubricated (oil-free) models.

Reciprocating compressors are available in single-stage and two-stage configurations, each suited for different pressure applications. The relationship between power (HP) and air delivery (CFM) is an important operational consideration.

Single-stage compressors: Typically operate in the 70 psig to 100 psig pressure range.
Two-stage compressors: Typically operate in the higher 100 psig to 250 psig pressure range.
Power-to-capacity rule of thumb: For reciprocating compressors, 1 HP ≈ 4 CFM at 100 psi.
Typical horsepower ranges: Units from 1 to 50 HP are typically reciprocating. Compressors 100 HP and above are typically rotary screw or centrifugal.

Classification Criteria

Air compressors are further specified by several key characteristics beyond the basic type (reciprocating, rotary screw, centrifugal). These criteria define a compressor's construction, application, and performance.

Number of compression stages
Cooling method: Air, water, or oil
Drive method: Motor, engine, steam, or other
Lubrication:
- Oil-lubricated: Lubricating oil contacts the compressed air.
- Oil-Free: No lubricating oil contacts the compressed air.
Configuration: Packaged or custom-built

Compressor Type Operational Data

The following table summarizes key operational characteristics for each primary air compressor type.