Gas compression is the process of physically forcing a volume of gas into a smaller space, utilizing mechanical energy to overcome the repulsive forces between gas molecules. This action directly results in a substantial increase in the gas’s pressure. The ability to compress gases is foundational to modern industry, enabling high-density storage, efficient transportation through pipelines, and various applications in manufacturing and energy systems. The physics that govern this process dictate the design of the machinery used, creating two distinct families of compression equipment.
The Fundamental Physics of Gas Compression
The physical act of reducing a gas’s volume is governed by the interconnected relationship between pressure, volume, and temperature. This relationship is summarized by the gas laws, which describe how these properties change when a gas is contained. Boyle’s Law states that for a fixed amount of gas at a constant temperature, pressure and volume are inversely proportional; halving the volume will approximately double the pressure.
This inverse relationship explains the pressure increase, but the mechanics of compression introduce an unavoidable side effect: heat generation. As mechanical work is performed on the gas to push the molecules closer together, that energy is transferred into the gas itself, increasing the kinetic energy of the molecules. Charles’s Law highlights that gas volume is directly proportional to its absolute temperature at a constant pressure, meaning the resulting pressure increase is compounded by the simultaneous temperature rise. Compressing a gas therefore requires more work to overcome the increased internal pressure exerted by the hotter, faster-moving molecules, a challenge managed with specific cooling strategies.
Positive Displacement Compressors
Positive displacement compressors operate by trapping a fixed, discrete volume of gas and then mechanically squeezing that volume into a smaller space to increase its pressure. This method directly applies the principle of reducing volume to achieve a pressure surge. The resulting flow from these machines tends to be constant despite changes in discharge pressure, making them suitable for applications requiring high-pressure ratios.
The reciprocating or piston compressor is a prominent example, functioning much like an internal combustion engine. Gas is drawn into a cylinder as a piston moves away from the head, creating a low-pressure area. The piston then reverses direction, forcing the trapped gas into a much smaller space and discharging it through a valve at high pressure. Reciprocating units are generally employed when a process demands very high discharge pressures, with multi-stage designs capable of reaching extreme pressures.
Another widely used type is the rotary screw compressor, which achieves volume reduction through continuous rotational motion. This machine features two intermeshing helical rotors, often called male and female screws, rotating within a chamber. As the rotors turn, they draw gas in at the suction end, trap it between the lobes and the casing, and continuously move the pockets of gas toward the discharge end. The helical design gradually reduces the volume of the trapped air, resulting in a smooth, pulse-free flow of compressed gas. Oil is often injected into the compression chamber to seal gaps, lubricate components, and absorb the heat of compression.
Dynamic Compressors
Dynamic compressors, also known as turbomachinery, operate on a fundamentally different principle than positive displacement machines. Instead of trapping and squeezing a volume, they impart high velocity, or kinetic energy, to the flowing gas stream. The compression effect is achieved by rapidly slowing the gas down in a diffuser section, which converts the kinetic energy into static pressure. These machines are characterized by a continuous flow of gas and are typically chosen for high-volume applications.
Centrifugal compressors utilize a rapidly spinning impeller, which draws gas in near its center and slings it radially outward using centrifugal force. This action dramatically accelerates the gas. The gas then enters a stationary diffuser section with a progressively increasing cross-sectional area. As the gas rapidly decelerates within the diffuser, its kinetic energy is recovered and converted into the desired high static pressure.
Axial compressors move the gas parallel to the main shaft, hence the term “axial.” They use multiple stages, with each stage consisting of a row of rotating blades (rotors) followed by a row of stationary blades (stators). The rotors accelerate the gas and impart energy, while the stators immediately slow it down, converting velocity into a pressure increase and preparing the flow for the next rotating stage. This design allows for a very high flow rate, making them the standard choice for large-scale applications such as jet engines and high-volume industrial air separation plants.
Managing Heat and Multi-Stage Compression
The generation of heat during compression presents a significant engineering challenge, as hotter gas is less dense and harder to compress, reducing overall efficiency. When gas temperature rises, the power required for the next increment of compression increases disproportionately. Additionally, excessive heat can damage internal compressor components and poses a safety risk, especially when compressing flammable gases.
The most common solution to this problem is the use of multi-stage compression, which divides the total pressure rise into several smaller steps. Compressing the gas incrementally prevents the temperature from reaching damaging or severely inefficient levels in a single step. Between each stage of compression, the gas is routed through a heat exchanger known as an intercooler.
The intercooler’s function is to cool the gas back toward the original inlet temperature, increasing its density before it enters the next compression stage. This cooling process significantly reduces the work required for the subsequent compression, dramatically improving the overall energy efficiency of the system. After the final compression stage, an aftercooler reduces the gas temperature to a level suitable for its intended use or storage. This final cooling step often causes moisture in the gas to condense, allowing it to be separated and removed, which protects downstream equipment from corrosion.