What Is Pressure Rise? - Quincy Compressor

Pressure rise

Pressure rise is the net increase in static pressure achieved by an air compressor, calculated as the difference between the discharge (outlet) and suction (inlet) pressures. It represents the fundamental “work” the machine performs on the gas stream.

Formulaically, pressure rise is expressed as:

Pressure Rise = P_discharge− P_inlet

For a standard industrial air compressor operating at sea level, the inlet pressure is typically atmospheric (approximately 14.7 PSIA or 0 PSIG). Therefore, if the machine compresses the air to a final tank pressure of 100 PSIG, the pressure rise is effectively 100 PSI. However, in boosted applications like nitrogen boosters or secondary compression stages, the inlet pressure may already be elevated. In these cases, the pressure rise is strictly the added value, not the total final pressure.

Engineers utilize this metric to determine the mechanical stress load on compressor components. A higher pressure rise requires greater force from the piston or rotor, increasing torque requirements for the motor and thermal loads on the cooling system.

FAQs

While both terms describe the pressure change, they are used for different engineering calculations:

Pressure rise is a subtraction calculation (Output-Input). It quantifies the net force added and is used to size piping, valves and storage tanks.
Compression ratio is a division calculation (Absolute Output Pressure/Absolute Input Pressure). This ratio is critical for thermodynamics, as it determines the amount of heat generated during the process. For example, a compressor pressure increase from 14.7 PSIA to 114.7 PSIA represents a pressure rise of 100 PSI, but a compression ratio of approximately 8:1.

Altitude significantly impacts the work required to achieve a specific pressure rise. Because atmospheric pressure decreases with increasing elevation, the “starting point” (inlet pressure) is lower. To maintain a standard discharge pressure of 100 PSIG at high altitude, the compressor must achieve a greater differential pressure rise than it would at sea level. This additional workload often reduces the machine’s volumetric efficiency and may require derating the motor to prevent overheating.

In single-stage compressors, the entire pressure rise occurs in one stroke or rotation. This process generates significant heat. To manage this, multistage compressors (like Quincy’s reciprocating piston series) divide the total work into smaller steps.

Stage 1: Compresses atmospheric air to an intermediate pressure (e.g., 40 PSIG).
Intercooling: Cools the air to remove the heat of compression.
Stage 2: Compresses the intermediate air to the final discharge pressure (e.g., 175 PSIG).

By splitting the total pressure rise across multiple stages, the compressor operates at lower internal temperatures, reduces seal wear, and consumes less energy overall compared to a single-stage unit attempting the same total increase.

There is a direct physical correlation between pressure rise and temperature. According to the ideal gas law, compressing gas molecules closer together increases their kinetic energy, which manifests as heat. If the pressure rise occurs too rapidly without adequate cooling (as seen in high-ratio single-stage compression), the discharge temperature can exceed the lubricant’s breakdown point. This is why limiting the pressure rise per stage is a critical design factor for long-term equipment reliability.