final report on the multiport dryer - Argonne National Laboratory

22Average Condensing Heat Transfer Coefficient (W/m 2 K)2500020000150001000050000System Pressure = 170 kPa - 620 kPaQuality = 0.10 - 0.8020 25 30 35 40 45 50Mass Flux (kg/m 2 s)Fig. 8. Average condensing heat transfer coefficients as function of mass fluxPressure Effect. In Fig. 9, the average condensing heat transfer coefficient for each test isplotted as a function of the system pressure. It can be seen that the condensing heat transfercoefficient is also approximately independent of system pressure over the experimental range of170-620 kPa (25-90 psi).Quality Effect. Figure 10, which is a plot of the local condensing heat transfer coefficientsas a function of quality in a representative multiport channel, shows that the heat transfercoefficient is approximately constant as quality changes over the experimental range of 0.8-0.1.This trend was also observed previously in ANL condensing tests with refrigerants at low flowrates. Although the heat transfer coefficient gradually reduces with the decrease of the qualityfrom 0.80 to 0.10, the reduction is not large. In fact, the results of wall-temperaturemeasurements demonstrate that very good uniformity of cylinder wall temperature distributioncan be achieved with multiport cylinder dryer technology (see Fig. 11). Thus our experimentaldata would suggest that at low steam flow rates there will not be a tendency for a wet streak todevelop in the sheet at one end of the dryer cylinder.Pressure Gradient. Figure 12 shows the two-phase pressure gradient as a function of massflux. The test section pressure gradient is shown to be relatively low, primarily because of thelow mass flow range of operation in multiport cylinder dryers. This means that for a 10-m (32.8-ft)-wide dryer shell and a mass flux of 50 kg m 2 s (36,867 lb ft 2 hr ), the total pressure dropwould be ≈15 kPa (2.2 psi). This is less than the maximum acceptable value of 27.6 kPa (4.0psi) for cylinder dryers. These low pressure-gradient values translate into almost negligiblechange in saturation temperature along the channels of multiport cylinder dryers. (Note that thedata scatter in Fig. 12 is a quality effect that is not taken into account in the figure.)