In order to determine the specific fuel requirement a total of 63 experiments were carried out during two campaigns on two oil fired pulp dryers which had the same dimensions (diameter 3,25 metres and length 17,0 metres) but slightly different mountings. In 31 experiments the retention time of pulp and cyclone dust was simultaneously ascertained. The specific evaporation Vs was varied between 90 and 210 kg/m^ . h and the rotational speed of the drum between 0,6 and 2,1 rpm whereas the amount of boiler flue gases added was set at 40.000, 50.000 and 60.000 Nm^/h respectively. The dust content of dryer flue gases was determined in 38 experiments through two parallel measurements in each case.
The experiments have shown that the heat efficiency is optimum in the range of Vs = 160 –
190 kg/m^ . h whereas the specific fuel requirement increases progressively with increasing dryer load at constant amount of boiler flue gas addition. A decrease in specific fuel requirement can be achieved by decreasing the drum rpm in low loading range but by increasing the drum rpm under high loading conditions. These as well as other experimental results lead to the conclusion that the heat transfer from hot air to pulp increases with increasing specific evaporation whereas the heat transfer from the mountings to the pulp decreases.
The average retention time for pulp was found to lie between 40 and 130 minutes and is determined mainly by the gas velocity inside the drum. It is therefore decreased by the addition of boiler flue gas. Since low retention time makes higher temperatures inside the drum necessary, there is an optimum for the amount of boiler flue gas which can be utilized. The influence of drum rpm on retention time decreases with increasing evaporation to become negligible in high load range. Increasing the retention time by decreasing drum rpm under low load conditions decreases the specific fuel requirement.
The amount of dust in clean air after multiple cyclones is not a function of drum rpm but depends on the throughput and the temperature at the drum entrance. The influence of temperature at the dryer entrance, the ratio of retention time of cyclone dust to pulp and the fact shown in these experiments i.e. that there is no correlation between dust content and dry matter of dried pulp, show that the first part of the drier is chiefly responsible for the production of dust.
The determination of retention time for pulp is an important criterion to explain the processes in inside a drum dryer and only an exact knowledge of these processes can enable one to control the whole process. The optimization of heat utilization is only possible if a cost analysis considering all factors is performed. The fixed costs component which can be tolerated increases with increasing energy costs. That means that the economical specific evaporation is as low the energy costs are high.
Proceedings: CITS 1975
pdf download: CITS1975-479-501.pdf