Drives & Controls - Saving energy through drive system efficiency


Many pump and fan systems are designed with safety margins which can make them grossly oversized. This in turn leads to inefficient use of energy, but the efficiency can be vastly improved with the use of variable speed drives

Of all the resources used in modern manufacturing, energy is arguably the most fundamental. This resource has long been taken for granted, but rising energy prices and concerns over greenhouse gas emissions are increasingly leading users to critically assess their energy usage.

In many technology areas, significant energy savings are difficult to achieve and gains of a couple of percent are then celebrated as breakthroughs.
There are technologies, however, that can deliver very significant reductions in energy use. Foremost among these is the variable speed drive. It doesn't make much noise or develop extreme temperatures or go through complex motions. In fact it sits in a cabinet and usually doesn't even get a mention when the overall process is explained. However, it can deliver great cuts in energy consumption, frequently nearly halving the consumption, and if applied in all relevant plants worldwide, it can deliver energy savings that equate to the electrical consumption of a country such as Spain.

The principle is simple: In the past, the motors that powered pumps were usually run at full power all the time, with the output controlled with valves. A drive regulates flow through direct control of the electrical power fed to the motor, so eliminating friction-based controls and the associated losses.

The lack of system standards
However, a lack of system standards for energy efficiency may result in up to 90 percent of pump installations being incorrectly sized, leading to energy waste.
"We have standards for everything," you may argue. However, in the area of energy efficiency there are still important gaps. While there are standards for pump designs and many for the hydraulic data such as developed head, efficiency and net positive suction head, a search for standards providing guidance in system design is less likely to produce a result.

To use an analogy, if somebody were to buy a three-ton truck for use on shopping trips, it would do the job, but would not be a demonstration of energy efficiency - even if the truck selected boasted the best efficiency figures for three-ton trucks. 

When planning a system, there is a degree of uncertainty as to the shape of the system curves (friction, pipe cross section changes and the number of bends in the final pipe layout all take their toll). These factors all add to the risk that the expected operating conditions will not be met. There are three basic ways to address the changed operating conditions: 
- If the changed condition is permanent, then the pump or fan size should be changed to match the load. 
- This adds to the installation cost
- The pump or the fan speed can be changed, or the impeller can be modified. 
- A throttling device (such as a valve, damper or guide vane) can be added. 
- Both of these options can waste energy. 

The cost of energy is the all-dominating part of the lifecycle cost of a pump or a fan. Energy consumption is the best place for optimization to start.

How systems get over-dimensioned
Systems get oversized throughout the design process, but variable speed drives can be used to conserve energy.

Despite careful analysis and design, many systems do not operate optimally. One reason is that many systems are simply sized too large to start with, resulting in higher operating and investment cost. To illustrate this, the case is considered of a system with a fan in a process plant.

In this example, it is assumed that the ‘true' nominal condition for the application is 100 units of flow, requiring 4000 units of pressure. 
In order to be on the safe side concerning the maximum flow, the figure for the fan communicated to the engineer is 110 units of flow. With the assumed system curve, this would require a fan with a higher capacity that can deliver 110 flow units and 5000 units of pressure. 

When establishing the fan capacity, the engineer estimates the overall pressure drop that these 110 flow units will cause. The pressure drop value that is calculated is increased by a 10 % margin because is difficult to foresee whether the assumed number of bends in the duct will conform to that estimated (possibly the installation contractor will have to add bends to avoid other equipment). Also, the cross-section of the duct may be uncertain. A smaller cross section would lead to a higher pressure drop. This therefore means that a 10 percent margin is not unreasonable. 

So what data are finally sent out in the requests for tender? Flow: 110 units at a pressure of 5500 units. Even if the original assumptions were correct, the fan is now grossly oversized. At 100 units flow the necessary additional pressure drops over the damper must be about 2800 units. This corresponds to 70% of the assumed correct total pressure. However, it is rare that 100% of the design flow will be needed other than for very short bursts. Assuming that most of the time, 80% of the flow rate will be required; the additional throttling needed across the damper will be about 6000 units. This corresponds to 150% of the assumed correct total pressure.

The steps illustrated in this example are more common than they may seem. An additional factor is that, when it comes to the selection of a fan, this choice must be made from a standard range of fixed sizes. The next larger one will usually be chosen, with a motor sized to suit.

The correctly sized fan for this example at point ‘g' should be 100 × 4000 = 400,000 power units, and the normal running at point "l" will require 80 x 2500 = 200,000. The case above produces a requirement for a fan of at least 110 x 5500 = 605,000 power units (150% of the optimum). Correcting this with damper control leads to high levels of wasted energy. The additional losses at the 80% flow point amount to 80 x (8100 - 2500) = 448,000 power units (120% of the full power of a correctly sized fan). These figures will in practice become worse, because the fan will not be working at its optimum efficiency throughout the operating range. With a speed controlled fan instead of damper control, nearly all of this energy can be saved.

Old pumps given energy boost
The Corus site at Port Talbot in Wales is one of the biggest steelmaking plants in the UK with an annual output of 5 million tonnes. Energy is Corus' second highest cost after raw materials. The costs and revenues of the business are fairly fixed, so high productivity is crucial to stay competitive.

As part of a plant-wide energy saving programme, 24 ABB industrial drives, ranging from 140 to 400 kW, are being installed to control pumps on the hot strip and cold mills, plus three fans on the coke ovens. The pumps recirculate cooling water in the mills, while the fans are used for dust extraction at the coke ovens. The cost of the drives about £1m; the whole project including pumps, cabling etc. is around £2.5m.

"The pump and fan motors were clearly oversized and running longer hours than necessary," says Guy Simms, leader of the energy optimisation team. "Much of the equipment on the site was installed during the sixties, seventies and early eighties. At the time, it was common practice to oversize the equipment by as much as 50%, to make sure it was sufficiently robust. In many ways this was a successful policy - after all, it has lasted all these years. But with the ABB drives we are now installing, we can fine-tune the applications to a degree that just wasn't possible in those days."

The applications have varying demand but until now, the pumps and fans have been running continuously at full speed. Running to demand will not only reduce energy costs, a million pounds in annual energy savings is expected, but also save water and improve control, particularly of the cold mill, which could potentially result in better product quality.
In any process where a restriction is used to control flow, energy can be saved, and in any process where volume can vary, energy can be saved.

Picture top right: 24 ABB drives, controlling pumps on the hot strip and cold mills, plus fans on the coke ovens, will be saving a million pounds in energy annually at Corus Strip Products in Port Talbot

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