An Air Side Control System that can be retrofitted to
existing machines is a key element
BEN THORP, FRANK D. SORRELLS
Conventional Tissue Machine Drying (CTMD) technology was developed to reduce low carbon emissions by reducing the amount of natural gas burned, a significant reduction in energy costs. An Air Side Control System (ASCS) with the CTMD software are the key elements of the technology and the ASCS can be retrofitted to existing machines. In a recent computer study, a machine with hoods arranged in cascade was compared with a machine with hoods in parallel. The machines are typical two wide tissue machines with gas-fired hoods operating at 1,500 m/min producing 14.5-g/m2 tissue.
The parallel machine reduced CO2 emissions by 33 percent when the moisture of the drying air was increased from a humidity ratio of 0.1 to 0.4.
The parallel machine saved €479,000 ($564,700) per year in energy costs over the cascade machine.
The authors of this paper are Ben Thorp, who retired from Georgia Pacific where he served in top management positions including director, pulp and paper engineering; and, Frank Sorrells, who retired from Valmet Enerdry where he served as manager of technology transfer.
Some years ago, Thorp’s engineers attempted to optimize the moisture in the drying air on one of its machines. They had some success, but operators could not restore high humidity after machine changes. Thorp concluded that a control system would be required, but this was beyond the scope of work. Sorrells has spent more than 20 years developing the CTMD software programs and the ASCS for controlling the various air side devices. Ten copyrights have been issued to him.
It is easy to state how the energy is saved. Adjusting the exhaust air damper to decrease the amount of air leaving the system causes the moisture in the air to increase if the makeup air damper is adjusted to decrease the amount of fresh air entering the system. An air balance must be maintained. When those changes are made, there is less makeup air to heat to the blowing air temperature, so less burner energy is required. It is easy to state, but complex to do.
The CTMD software program includes an optimization routine where the entire drying spectrum is displayed on a graph for each hood section. The three controllable variables on the air side are the blowing air temperature, velocity, and humidity. An optimize graph (Fig. 1) is shown with all three variables plotted along with energy cost. Every potential operating set point for a machine is displayed in the graph.
These graphs permit mill management to select their desired operation. In most EU countries the cost of electricity is about twice the cost of natural gas when compared on an equal energybasis. That is reflected in the slope of the humidity lines.
In a 31-page paper that Thorp and Sorrells wrote, this graph is discussed in depth, including important information about how moisture influences the tissue drying rate.
The longer document also describes the computer study and includes 12 pages of computer printouts; verification using theoretical equations to calculate the energy to heat the makeup air and that differs by 1.9 percent from the program’s calculated value; CO2 emissions are calculated; the ASCS is described and the authors’ resumes are included.
For the computer study, at the starting point each machine has the same temperature, velocity, and a DE humidity ratio of 0.1. Savings from heat exchangers was added. For the parallel machine, both hood sections were increased from 0.1 to 0.4 humidity ratio. This cannot be done on the cascade machine. The dry end hood cannot be operated at the same moisture as the wet end. On cascade machines, the powerful circulating air fans are connected. It is not possible to have a control system with two masters.
The ending point for each machine is also equal, an exhaust humidity ratio of 0.47 on each. The 0.4 moisture for the parallel machine was selected because the exhaust air matches the 0.47. Operation at higher moistures can save even more. For a copy of the longer document, see Sorrells’ email address below.
Ben Thorp and Frank Sorrells are consultants with numerous years of experience in the tissue industry. A copy of the 31-page document will be sent to interested tissue mills. Reach Sorrells at [email protected]