Vary Drive Speed to Save on Plant Utility Costs

When a component is not in use, or is being used at a percentage of its capacity, running its motor at full capacity costs the same in resources and utility expenses as if you had the component in full operation. For this reason, original equipment manufacturers (OEMs) and parts suppliers can retrofit a variable frequency drive (VFD) onto a component to alter how much energy it’s draining from your bottom line. Not every component at the asphalt plant has had this option as a viable one in the past. VFDs were once expensive and cumbersome. The return-on-investment has improved in this century. Here’s how.

A spokesperson for Stansteel/Hotmix Parts and Accessories, Louisville, Ky., shared that the VFD helps the producer avoid the high peak demand charge that’s common with large horsepower motors starting and operating on a hot mix asphalt (HMA) plant. “A customer may have to run his plant only sporadically for a few of the colder months of the year, and this can trip or raise his cost of electricity because of the demand charge for the large horsepower start-up. In some cases, this could be several thousand dollars just for operating the plant one or two times within a given time period. With the VFD, this will reduce the large peak demand load and help reduce the overall cost of operations.”

From the power company’s standpoint, that’s called “peak demand” savings. A spokesperson from Maxam Equipment, Kansas City, explained, “Peak demand on an asphalt plant occurs during the plant start-up sequence when all the large motors such as the fan, burner and drum are all started close to the same time. A VFD can, with its reduced voltage starting characteristics, reduce the peak demand by as much as 70 percent. In turn, it lowers the peak demand and the overall rate used by the power company. During online plant production, you could see a 15 to 20 percent reduction in overall demand. Overall demand reduction is the primary approach to take with the power company to reduce their taxed generating systems during high load times.”

Producers don’t always have to foot the bill of the VFD alone. “Today, plant manufacturers are supplying VFDs on any components that require speed variation,” the Maxam spokesperson said. “Feeders, pumps, fans and burners are all generally outfitted with VFDs.”

Stansteel’s spokesman offered this good news: “There are a number of utilities around the country that will actually pay part of the capital cost of the VFD….In other cases, the producer could ask for a list of incentives from the utility and at what motor horsepower do they get into peak demand charges. Another question to ask is if they have any other green incentives or cost incentives for changing out a series of motor starters depending on horsepower.”

The horsepower of course depends on the component.

A spokesman from Tarmac International, Inc., Kansas City, shared, “Rotary mixers have frequency drives to allow for longer or shorter mix times; this improves mix quality. Cold feed bins and RAP bin feeder belts have frequency drives as a method to vary the belt speed and these are cost effective. For big cost savings, the baghouse fan is where we find largest savings (See Sidebar).”

“On many of the special and custom engineered items that Stansteel and Hotmix Parts provide, there are variable frequency drives installed on a number of the items when there is a need for changing the rate of flow on the device. Items such as this would include variable speed drives on cold feed bin additions, on rotary airlocks or even screw conveyors that need to change the rate of flow of additives or other material feeds.

In addition, we highly recommend the VFD on the main exhaust fan whether it is for a new Stansteel baghouse and air control module changeout or even if the customer just wants to update their existing technology, such as a new, more efficient fan with an improved drive. These are very common on many of the devices these days, but also need to be custom-sized based on the load that the unit sees both on start-up and over a course or range of operation, as well as service factor requirements.”

Stansteel offered a checklist to go over when considering components for VFD installation. “Here are a few examples of what Stansteel/Hotmix Parts would recommend a producer evaluate on a plant:

• Emission control fans that evacuate blue smoke and efficiently incinerate it in the main burner combustion process.
• Service requirements of the motors and if they need to be changed with the use of an inverter.
• The benefits of reduced wear and tear on mechanical parts such as bearings by running at lower RPM.
• Fugitive fans for batch towers or for general blue smoke recovery/reduction systems.
• Noise pollution reduction both from a standpoint of safety and being good neighbors.”

Quick Exhaust Fan Math
If the baghouse exhaust fan ammeter draws more than 400 amps when the motor starts, and you add a VFD  to the 200-hp motor, the exhaust fan ammeter should now draw about 100 amps when the motor starts.