Energy costs are a major operational expense for most food manufacturers — but you’re not at the mercy of your utility bills. With an advanced energy management system, you can control your energy use so that your facility runs at optimal efficiency, you pay the lowest possible rates, and you can participate in incentive programs that pay you for unused kilowatts. This article is the third in a four-part series on the fundamentals of electrical load shedding.
For many plant operators, load shedding sounds good in theory, but it’s hard to see how it could work in their food manufacturing facility. So let’s bring this discussion down to the shop floor — we’ll discuss some typical targets for load shedding and how they fit into major load-shedding strategies. Note that each load will have its own rules, and those rules can be different depending on the season or other production variables.
In plants with refrigeration systems, those systems usually account for 40 to 60 percent of the plant’s electrical load, so they’re a good place to start.
Compressors: These are typically 200 to 500 HP, so they’re big power users and consequently a prime target for load shedding. Most units are screw-type compressors with microprocessor controllers, which are easy to communicate with.
For demand control, you can reduce compressor power usage by force-unloading the slide valve or raising the suction setpoint, which also unloads the compressor. Installing variable frequency drives (VFDs) to provide capacity control via compressor speed allows for power shedding while maintaining compressor efficiency. For demand response, the only way to shed a large block of power is to systematically unload and turn off compressors. During this time period, minimum compressor capacity may be required, based on control system rules, to maintain critical space or product temperatures.
Evaporators: Evaporators use a lot of fan power to transfer heat from the product or space. Many evaporator fans now have VFDs for energy efficiency, and these provide a simple, convenient method of communicating with the energy management system (EMS).
For demand control, fans will be slowed down or turned off. For demand response, they will have to be turned off, though they can be cycled to stir air and avoid temperature stratification. The EMS rules should always protect production or space temperature.
Fans: All fans that are 10 HP and larger have good potential for shedding electrical load. Again, VFDs are common and provide power savings as well as making fans easy to control and communicate with. If fans don’t have VFDs, the EMS will stop them according to plant rules.
Vacuum tubes: For fresh-produce processors, vacuum tubes represent a large electrical load that can be controlled on a priority basis for demand response and time-of-use management. The EMS rules will delay the start time of this batch-type process until the peak cost period has passed.
Battery chargers: Many processors and cold storage facility operators are surprised by the amount of power required to charge forklift batteries — commonly more than 100 kW. The solution: move battery charging to off-peak times to reduce costs. It’s important to understand the different styles of battery chargers because each has a unique electrical profile that can impact load shedding.
Nitrogen generation: Many food manufacturers that use nitrogen to guarantee hygienic packaging and preservation have installed in-plant nitrogen generators to replace trucking nitrogen in to a storage tank. They typically maintain a back-up storage tank, though, and this can help with all load-shedding strategies: just shut down generation and run off stored nitrogen for as long as that is cost effective.
These are just some of the loads available for shedding in a processing plant. Any equipment that can be temporarily stopped or run at reduced power is available for demand control and, possibly, for demand response. The key to making it work: Let the EMS drive savings and balance it with the rules that should always be in place to protect production and environmental conditions (temperature, air quality, personnel safety, and so on).
Last week: Choosing the Right Load-Shedding Strategy
Next up: Take the Risk Out of Load Shedding
For more information, visit www.poweritsolutions.com.