Respond Right to Reduced Rates

Maintaining energy efficiency helps keep operating costs in check.

By Gary Faagau, Energy Columnist

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In today's economy, operating flexibility becomes extremely important. When production demand slumps, energy efficiency suffers. This usually isn't factored into operating costs or energy goals. Higher per unit energy demand can sometimes destroy all the good produced by energy efficiency programs.

Without a firm understanding of how rates affect energy consumption, a company may just entirely scrap its energy program, which has a domino effect. With de-emphasis on energy, operations become more relaxed about keeping equipment efficiency high. Adjustments to keep equipment at peak efficiency are delayed or not made. Maintenance is reduced and repair decisions postponed. Purchases are deferred and projects are killed.

Wasted efficiency takes a toll on operating costs . Also, when equipment production increases, energy efficiency often lags and years of effort to reduce energy costs can be wasted. So, how do you balance economic restriction with trying to maintain an energy efficient operation?

There's a psychological effect in getting the best energy efficiency at lower rates. First, accept that operating at lower rates is something that you may have to live with for a while. Once you accept downsizing (or right-sizing) of your unit, you will start seeing opportunities never thought of at higher rates.

By using an equation, you can set targets for energy based on the operating plan.

During any prolonged rate reduction, gather information on energy usage to see the effect of reduced rates. When enough data are gathered, turn your energy per unit number into an equation. To simplify calculations, develop a first order equation that yields a component that's based on rate and a constant that isn't based on rate. That constant is what I call core energy use of the plant.

Incorporate the energy equation into the plant's linear programming (LP) model so that energy usage can be part of the economic question. Now, there're new obtainable targets for the operators to achieve. Goals are then set to improve on historical energy usage during reduced rates.

To beat energy targets, understand how energy efficiency changes with production rate. Depending on how you reduced your rates, core energy use typically doesn't respond to rate changes. Some equipment have minimum rates that prevent them from being reduced. For example, some pumps have a minimum flow so that fluid is circulated if rates are reduced. This recycle raises the per unit energy costs.

This creates "hidden gems" that you normally wouldn't have. If minimum flow requirement is causing a lot of recycle, consider installing a pump with lower requirements or trimming the impeller in your exiting pump — a cost efficient way of saving on energy. If you have a spare pump, trim the impeller on one pump while having the spare available for higher rates.

Other equipment, such as those associated with some utility systems, controls, lighting and environmental equipment, typically will use the same amount of energy no matter what rate is run. However, sometimes components in these systems can be isolated and de-energized during reduced rates. Taking a closer look at these systems will help determine your needs.

Still others have equipment with step function loading that's triggered at certain rates. To lower energy bills always maximize usage to the limit of the step function. Alternating operation rates may produce a lower average energy cost.

For batch operators, the energy needed to keep equipment hot between batches will have a big effect on per unit costs. If different batch operations depend on different heaters for their heating medium but use the same source (like steam or hot oil), modifying reduced operations so that one or more heaters can be removed from service, by managing timing of batches, will decrease the need to keep equipment running on idle.

For distillation units, lower rates can cause a loss of tower traffic and make it difficult to get correct separation. Also, overhead cooling system design may hinder proper temperature control  at reduced rates, so operators may actually increase reboiler duty to maintain separation and tower temperature. This problem can be resolved by cutting tower pressure, which will reduce reboiler requirements.

Take a closer look at what you're doing at lower rates. Many opportunities become available at reduced rates that you can't take advantage of at normal operating rates. By first setting energy targets so that you can have realistic goals at reduced rates, you can still make energy savings a high priority.


Gary Faagau is Chemical Processing's Energy Columnist. You can e-mail him at GFaagau@putman.net.

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