RB3-7 The Effect of Pressure on the Nucleate Pool Boiling Mixture Correction - Initial Assessment Based on the System R22/R114
Published data from the University of Paderborn in West Germany were analyzed to determine the effect of operating pressure up to 0.9 times the thermodynamic critical on the mixture mass transfer correction to heat transfer in pool boiling. The data ranged from 0 to 100 percent R22 in a mixture of R22 and R114. Although the maximum boiling range was only about 16 °C, a significant mass transfer correction was seen which became severe as the critical pressure was approached.
The data show that the increase in heat transfer coefficient with pressure is much less for mixtures than for pure components. This means that if the mixture heat transfer coefficient is calculated as the pure component heat transfer coefficient times a mixture correction, this mixture correction must counteract the exponential increase in the heat transfer coefficient near the critical pressure, which occurs for pure components and not for mixtures. It was concluded that the reason for the difference in behavior of pure components and mixtures is due to diffusion limitations to bubble growth at high mass transfer rates.
It is strongly recommended that additional pool boiling tests at high reduced pressures be made using hydrocarbons with wider boiling ranges, in order to further develop correlations for this important effect, which is not now adequately taken into account by HTRI methods.