Like pumps, which we covered in the first column of this two-part series ("Don't Overwork Pumps and Fans, Part I,"), fans consume significant electrical energy while serving several applications. In many plants, the motor drives of fans together account for 50% to 60% of the total electricity used. Centrifugal fans are the most common but some applications also use axial fans and positive-displacement blowers. The following steps help identify optimization opportunities in systems that consume substantial energy running the fan drives.
Step 1: Install speed control on partially loaded fans, where applicable. Any fan that is throttled at the inlet or outlet may offer an opportunity to save energy. Most combustion-air-supply fans for boilers and furnaces are operated at partial loads compared to their design capacities. Some boilers and furnaces also rely on an induced-draft fan near their stack; it must be dampened to maintain the balanced draft during normal operation. Installing variable speed drives on these fans is worthy of consideration.
Similar to centrifugal pump operation, the affinity law applies here. Because constant-speed motors consume the same amount of energy regardless of damper position, using dampers to maintain the pressure or flow is an inefficient way to control fan operation.
Step 2: Switch to inlet vane dampers. These dampers are slightly more efficient than discharge dampers. When a variable speed drive can't be installed to control fan operation, shifting to inlet vane control could provide marginal energy savings.
Step 3: Replace the motor on heavily throttled fans with a lower speed one, if applicable. Smaller capacity fans with high-speed motor drives operate between 25% and 50% of their design capacity. Installing a low-speed motor drive could save considerable energy.
For example, a 2,900-rpm motor drove a plant's primary combustion air fan with the discharge side damper throttled to about 75–80%. Installing a variable speed drive on this motor would save considerable energy, but we recommended switching to a standard 1,450-rpm motor. This was implemented immediately, as 1,450-rpm motors are readily available. With the lower-speed motor, the damper can be left at near 90% open; the fan's power consumption dropped to less than 50% of the previous level.
Step 4: Control the speed when multiple fans operate together. Fans consume a significant amount of energy in industrial cooling and ventilation systems. Supply fans of HVAC [heating/ventilating/air conditioning] systems are good candidates for speed control, if not already present.
Step 5: Switch off ventilation fans when requirements drop. Ventilation systems usually run a single large centrifugal fan or several axial exhaust fans. A close look at their operation may indicate these fans could be optimized depending upon the actual ventilation needs of the building they serve.
Recently, we surveyed a medium-sized industrial facility where 26 axial-type exhaust fans were installed on the roof of one building. All fans were operating continuously, even though the building had many side wall openings and not much heat generated inside. To better conserve energy, we suggested the 26 fans be divided into four groups with separate switches for each group. As a result, energy consumption for the fans dropped by about 50%, as only the required fan groups now are switched on.
At another industrial site, the exhaust fan of a paint booth ran continuously but paint spraying was scheduled only about 50% of the time. Modifying fan operation with timer switches and delayed sequencing saved energy.
Pumps and fans are the most common energy-consuming devices. Running them more than required is a substantial waste of energy.
Ven V. Venkatesan is Chemical Processing's Energy Columnist. You can e-mail him at VVenkatesan@Putman.net.