Plants use mechanical draft fans to move airborne material through a system duct and components. They are utilized to force, induce, and boost fluid media throughout the system – and use a large amount of power in the process. Mechanical draft fans operating at peak efficiencywill show superior performance, reduce costs, and maximize ROI.
For this reason, understanding how to efficiently control and operate these draft fans is of paramount importance to protecting a plant’s bottom line.
Methods and devices to control flow can have an outsized impact on power savings. Different methods have different advantages – and drawbacks – when it comes to flow control with mechanical draft fans. Here, we’ll discuss the pros and cons of two particular methods: damper control and VSD/VFD control.
The Purpose of Draft Control
Draft control originated due to the way boilers were constructed decades ago. They were built with brick-set construction, which required operation at negative draft or balanced draft pressure. These boilers weren’t airtight, and negative pressure was used to keep the furnaces cool. The goal was to prevent positive pressure from being realized in the boiler, which could result in damage to the casing or injury to operators.
Even with today’s airtight boilers, though, draft control is needed – primarily due to flue gas recirculation for NOx control. Failure to maintain draft control can result in excessive NOx emissions, among other things (such as higher operating cost and lower efficiency).
Damper Control
One method is to use dampers to manage volumetric airflow. This is an older method, but one that is still common across various systems and boiler types.
Advantages
Damper control is a very cost-effective method when the only consideration is the purchase price. Additionally, installing damper control will not materially impact floor space in a control room, and it does not require a foundation structure – meaning a wider range of plants can incorporate damper control without having to significantly alter or install infrastructure.
Disadvantages
Damper control isn’t the most efficient method of controlling the flow of gas and air. Additionally, damper controls typically require more regular maintenance because they have more moving parts than variable speed/frequency drives.
VSD/VFD Control
Variable speed (VSD) and variable frequency (VFD) control are two newer methods of maintaining draft control for fan systems. While there are differences between these two methods, they are both used to describe the same type of equipment.
Advantages
VSD/VFD control results in more energy savings than damper control, which means the payback for these systems is typically quicker than with damper control. Also, using VSD/VFD controls will result in a lower purchase price for the motor, on average, as long as bypass isn’t needed. And the overall value of a VSD/VFD system exceeds that of a damper control when taking into account total cost of ownership over a given period of time.
Disadvantages
Initially, you’ll pay more for a VSD/VFD system than you would a damper system. They also take up more floor space and generally require foundation structures for the steam turbine drive and hydraulic coupling mechanisms.
Summary
Draft control for mechanical draft fans is highly important. The two systems explained above – damper control and VSD/VFD control – each have advantages and disadvantages compared to each other. Selecting the right system for mechanical draft fans in your plan involves analyzing your needs and choosing the appropriate method for your facilities.
Choosing the right system, though, can result in maximum ROI, lower costs, and enhanced performance across the board.