Evaluating Dryer Drainage System Energy Use
Steam flows are commonly measured to monitor dryer section energy consumption. The total steam flow, however, does not provide enough information to assess energy efficiency. Measures that consider steam consumption in terms of production or output can be used to evaluate the efficiency of steam use in the dryer section. There are three common metrics to evaluate steam use efficiency: pounds of steam per pound of paper produced, pounds of steam per pound of water evaporated, and percent of energy loss from the system.
Pounds of steam per pound of paper produced is not a good metric to use when attempting to benchmark energy efficiency. Dryer energy consumption is directly related to the amount of water evaporated. Differences in press moisture entering the dryer section will affect the benchmark. Each 1% difference in press moisture will change the dryer section energy consumption by approximately 4%. Machines with different pressing efficiencies will have different steam consumption benchmarks. Therefore, it is difficult to benchmark energy efficiency against other machines using this metric.
Pounds of steam per pound of water evaporated is a better way to assess energy efficiency. This metric requires an accurate value for the press moisture to calculate the water evaporation for a given production condition. Obtaining an accurate press moisture can be difficult. Grab samples and press section water balances are often used. The press moisture will also vary depending on grade, speed, furnish, and other unique characteristics of the machine. A good value for this metric is 1.2 lbs of steam per pound of water evaporated.
Energy losses from the system is a highly usable and appropriate metric to use to assess steam utilization and evaluate energy efficiency. Any steam that is not condensed in the dryers to dry paper should be considered a loss from the system. The losses typically consist of the steam that is vented to the condenser or atmosphere. This can be tracked by valve position in a process historian. The loss can also be calculated by measuring the water flow and temperature around the vacuum condenser. A well designed dryer drainage system will have an energy loss of 1% to 3% of the total steam supplied to the dryer section.