The pressure/inlet correction updates full-load and maximum power/airflow before model evaluation. Centrifugal compressor models use their discharge-pressure curve adjustment instead. For screw compressors the rated inlet pressure is used as the pressure-ratio base; otherwise atmospheric pressure is used.

\begin{equation}\label{eq:compressor-assessment-pressure-power} P_{adj} = F_P \cdot \frac{P_{atm}}{P_{in}} \cdot BHP_{FL} \cdot \frac{0.746}{\eta} \cdot R_P \end{equation}

\begin{equation}\label{eq:compressor-assessment-pressure-flow} Q_{adj} = F_Q \cdot Q_{rated} \cdot \left(1 - 0.00075(P_d - P_{rated})\right) \end{equation}

\begin{equation}\label{eq:compressor-assessment-pressure-ratio} R_P = \frac{\left(\frac{P_d + P_b}{P_b}\right)^{(n-1)/n} - 1} {\left(\frac{P_{rated} + P_{in,rated}}{P_{in,rated}}\right)^{(n-1)/n} - 1} \end{equation}

When pressure adjustment is enabled, the empirical atmospheric correction factors are:

\begin{equation}\label{eq:compressor-assessment-pressure-factor-power} F_P = -0.0000577P_{atm}^3 + 0.000251P_{atm}^2 + 0.0466P_{atm} + 0.4442 \end{equation}

\begin{equation}\label{eq:compressor-assessment-pressure-factor-flow} F_Q = 0.000258P_{atm}^3 - 0.0116P_{atm}^2 + 0.176P_{atm} + 0.09992 \end{equation}

Symbols

\(P_{adj}\)	Adjusted package power \([\unit{ \kilo\watt}]\)
\(Q_{adj}\)	Adjusted airflow \([\unit{ \acfm}]\)
\(F_P\)	Atmospheric pressure correction for power \([\unit{ \unitless}]\)
\(F_Q\)	Atmospheric pressure correction for airflow \([\unit{ \unitless}]\)
\(P_{atm}\)	Atmospheric pressure \([\unit{ \psia}]\)
\(P_{in}\)	Measured inlet pressure \([\unit{ \psia}]\)
\(BHP_{FL}\)	Full-load brake horsepower \([\unit{ \horsepower}]\)
\(0.746\)	Horsepower-to-kilowatt conversion \([\unit{ \kilo\watt\per\horsepower}]\)
\(\eta\)	Motor efficiency \([\unit{ \unitless}]\)
\(R_P\)	Polytropic pressure-ratio correction \([\unit{ \unitless}]\)
\(Q_{rated}\)	Rated airflow capacity \([\unit{ \acfm}]\)
\(P_d\)	Operating discharge pressure \([\unit{ \psig}]\)
\(P_{rated}\)	Rated discharge pressure \([\unit{ \psig}]\)
\(P_b\)	Pressure-ratio base pressure \([\unit{ \psia}]\)
\(P_{in,rated}\)	Rated inlet pressure \([\unit{ \psia}]\)
\(n\)	Polytropic exponent \([\unit{ \unitless}]\)