Painesuhde

Painesuhde, in English often referred to as pressure ratio, is a dimensionless quantity used in engineering to express how much a fluid’s pressure changes across a component or process. It is commonly defined as the outlet pressure divided by the inlet pressure: PR = P2 / P1, where P1 and P2 are absolute pressures.

Because pressure ratios rely on absolute pressures, if gauge pressures are used, they must be converted by adding the atmospheric pressure to both sides. The concept is widely used in turbomachinery, internal combustion engines with turbochargers, HVAC systems, and hydraulic or pneumatic equipment to characterize how much a fluid is compressed or expanded.

In turbomachinery, the compressor pressure ratio describes the level of compression achieved, while in turbines it relates to the pressure drop across the expander. In engines, the pressure ratio across a compressor or turbocharger influences power, efficiency, and thermal loads. In HVAC and fluid systems, PR helps predict flow behavior, work requirements, and performance.

Calculation and thermodynamic relations: PR is dimensionless and depends on the process and gas properties. For an ideal gas undergoing isentropic compression, the temperatures relate to the pressures by T2/T1 = (P2/P1)^((gamma-1)/gamma), where gamma is the gas’s specific heat ratio. Real systems show deviations due to non-ideal effects and inefficiencies.

Example: A compressor increases pressure from 1 bar to 3 bar absolute, giving PR = 3. The resulting temperature rise depends on gamma and the isentropic efficiency of the stage.

See also: pressure drop, turbomachinery, isentropic process.