A new formula for saturated water steam pressure within the temperature range −25 to 220°C

Instead of approximation formula ln( E ( t )/ E (0)) = [( a − bt ) t /( c + T )] commonly used at present for representing dependence of pressure of saturated streams of liquid water E upon temperature we suggested new approximation formula of greater accuracy in the form ln( E ( t )/ E (0)) = [( A − Bt + Ct 2 ) t / T ], where t and T are temperature in °C and K respectively. For this formula with parameters A = 19.846, B = 8.97 × 10 −3 , C = 1.248 × 10 −5 and E (0) = 6.1121 GPa with ITS-90 temperature scale and for temperature range from 0°C to 110°C relative difference of approximation applying six parameter formula by W. Wagner and A. Pruß 2002, developed for positive temperatures, is less than 0.005%, that is approximately 15 times less than accuracy obtained with the firs formula. Increase of temperature range results in relative difference increasing, but for even temperature range from 0°C to 220°C it does not higher than 0.1%. For negative temperatures relative difference between our formula and a formula of D. M. Murphy and T. Koop, 2005, is less than 0.1% for temperatures higher than −25°C. This paper also presents values of coefficients for approximation of Goff and Grach formula recommended by IMO. The procedure of finding dew point T d for known water steam pressure e n based on our formula adds up to solving an algebraic equation of a third degree, which coefficients are presented in this paper. For simplifying this procedure this paper also includes approximation ratio applying a coefficient A noted above, in the form T d ( e n ) = + 0.0866ɛ 2 + 0.0116ɛ 10/3 , where ɛ = ln( e n / E ( T 0 )). Error of dew point recovery in this ratio is less than 0.005 K within the range from 0 to 50°C.