A New formula for Saturated Water Steam Pressure within the Temperature Range -25 to 220°C

摘要

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 x 10~(-3), C = 1.248 x 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. Pruβ 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 ap- plying a coefficient A noted above, in the form T_d(e_n) = -(AT_0)/(A-ε) + 0.0866ε~2 + 0.0116ε~(10/3), where ε = ln(e_n/E(T0)). Error of dew point recovery in this ratio is less than 0.005 K within the range from 0 to 50°C.