VARIABLE SPEED DRIVES AND THEIR USE IN RESOURCE EFFICIENCY AND CLEANER PRODUCTION

Forced draft fans in dry cooling devices
like the one pictured can be fitted with
VSD's  to allow them to  vary speed in
response to climatic conditions or
tower performance

The speed of an induction motor is a function of its number of poles and the frequency of the power supply. Variable speed drives (VSD’s) – also called variable frequency drives (VFD’s) – are used to vary motor speed by changing the frequency of the alternating current and voltage applied. They have many uses in industry, which is to be expected given the wide range of potential applications for continuous speed control within industrial processes.

VSD’s may be controlled through the intervention of a plant operator, using an interface such as a simple button/knob which can be used to increase and decrease speed, or through a SCADA system, which can be used to manually changed the frequency output. Generally however, they are used within the context of a control loop, which varies speed automatically in response to an output from some other measured variable. For example, pump speed could be varied in response to the output from a flow meter in the pipeline carrying the fluid being pumped. Alternatively, frequency setpoints could be written into control software based on the status of the process being controlled. For example, agitator speed could be set to a minimum value while a vessel is being filled, with the speed increased after filling has been completed. There are an infinite number of ways that VSD’s can be employed in control schemes such as this. Just a few examples of common applications are:

• Changing the speed of agitation in a mixing vessel
• Changing conveyor speed on a production line
• Changing the speed of a pump (and hence the flow) in fluid handling
• Changing the speed of a compressor
• Changing the speed at which racks are lifted from baths in electroplating
• Changing the pressure inside a boiler (on the fireside) by changing the speed of FD and ID fans.

So far I have discussed how VSD’s give us more control over industrial processes, but what then is the contribution of VSD’s to industrial sustainability? For one, improved control means reduced rework, which translates into reduced consumption of energy, water and materials. VSD’s are however probably more recognised for their growing use in industrial applications on the basis of the energy savings they provide. Simple examples include their use to control poorly loaded screw compressors (minimising “off-load” losses), and their use in controlling the speed of fans (where flow is directly proportional to speed, but power varies with the cube of speed). Controlling fluid flow by manipulating pressure drop (e.g. the use of a control valve or damper in conjunction with a pump or fan running at a fixed speed) consumes far more energy than that required were a VSD used to control pump or fan speed instead. VSD’s also provide exceptional “soft start” capabilities (while using less energy than traditional soft starters) and can also be used to control torque characteristics and to boost torque (e.g. at start-up).  While there are losses incurred in using a VSD (of the order of 1 – 3%) the efficiency gains at the system level arising from their use generally far exceed these. As far as energy efficiency projects involving induction motors go, I tend to find many more viable opportunities through VSD applications than I find in areas such as motor replacement.

VSD’s are however far more versatile from a sustainability point of view than simply being an electrical energy efficiency tool. Their use in material usage reduction can yield spectacular results where correctly applied. Consider the following drivers of material usage in the industries in the table below, and how VSD’s can be applied to improve material usage performance:

INDUSTRY	PROCESS	DRIVER OF MATERIAL USAGE	HOW TO EMPLOY A VSD
Drum manufacturing/reconditioning	Spray painting	The speed at which drums are rotated during spray painting is an important driver of paint thickness.	Install a VSD and allow the operator to manipulate drum speed to control paint thickness, along with other variables such as solvent ratio, air pressure and nozzle design.
Powder coating	Curing	Curing time and temperature affects curing quality and rework rates	Install a VSD on the conveyance system to permit variations in residence time in the curing ovens for items of different dimensions. To be used in conjunction with oven temperature profile.
Waste management	Oil recovery in a plate separator	Residence time in the plate separator impacts on efficiency of oil recovery	Use a VSD on the supply pump and vary the flow based on the composition of the incoming effluent, thereby changing the residence time. A turbidity meter in the separator outlet could be used for automatic feedback control.
Batch chemical manufacturing	Material dosing	Accuracy with which individual chemicals are added to a batch	Use VSD’s on dosing pumps (liquids) and screw conveyors (solids)  to vary dosing rate, slowing down the flow as the target value is reached and eliminating overshoots

How else could VSD’s be employed? The answer is really that the opportunities are limited by your creativity. There are few manufacturing processes in which speed is not an important variable, and the first thing to understand is: “what is the impact of speed on the process being analysed?”. Armed with this understanding, the next question would be: “does a VSD, with its ability to provide speed control over a continuous range, provide leverage in terms of reducing resource consumption and minimising pollution?” In my experience, the answer is very often a resounding “yes”. Your final consideration would be whether this leverage/benefit justifies the investment in the VSD. In real terms, VSD’s have become cheaper over time, despite gains in the features they offer. Combine this fact with the significant benefits they can provide, and they are often easy to justify from a financial and environmental perspective. 

HAVE YOU ESTABLISHED YOUR SITE'S RESOURCE EFFICIENCY BASELINE?

A Resource Efficiency Baseline is a 

point of departure for your efforts to 

save energy, water and materials.

Resource efficiency is an important aspect of industrial sustainability, not least because it is an area which delivers direct financial benefits by reducing the variable costs of production. The resources referred to here are energy, materials and water, all of which are undergoing significant price increases in most parts of the world. In my country, South Africa, electricity prices as at January 2013 have roughly tripled in the last 5 years, and are set to increase at rates above inflation for the foreseeable future. Rising energy prices increase extraction and transport costs, and spill over into the cost of materials. Water scarcity is a widespread global issue. Economics aside, the life cycle benefits of resource efficiency in terms of reducing emissions, improving water security and reducing the impacts of depletion are significant. No industrial organisation should therefore be without strategies and programmes to continuously improve the efficiency with which resources are used.

Site-specific resource efficiency is not the only aspect of resource management on the industrial sustainability agenda. Through the use of approaches such as life cycle analysis and the analysis of value chains, organisations can identify hot spots in their supply chains where environmental impacts are most severe, and target these to limit the extent of their footprints.  The results of such an exercise can be very surprising – for example, SABMiller’s South African operations found that water use in the agricultural part of their value chain far exceeded that at their breweries (see the report). Life cycle analysis should also prompt companies to find alternative materials, and in some cases fundamentally change their products and processes. The lead times for making such marked changes are however fairly long, unlike typical time-frames for site-specific resource efficiency projects. They should nevertheless be pursued in parallel with site-specific resource efficiency projects, given that the scale of the benefits is potentially very large.

In tackling site-specific resource efficiency, an important first step is to establish the RESOURCE EFFICIENCY BASELINE for your site. Establishing a baseline is primarily a quantitative process, involving an assessment of the quantities of each resource that are consumed on the site, with the data disaggregated as far as possible. These quantities should be expressed in absolute terms (e.g. total kWh of energy used per annum) and then also on the basis of activity (e.g. kWh/ton of production for a given time horizon). Since individual products can also use very different amounts of resources for their production, you may want to factor product mix into your baseline assessment. This is in my view an extremely important consideration, albeit one that is not necessarily simple to incorporate, particularly where sub-metering is not in place. A workable approach is perhaps to use a reasonable allocation procedure based on sensible calculations, and ensure that the individual usages tally to the total usage. 

To understand what I mean by "disaggregated data", consider energy as a simple example. Most industrial sites I visit use at least 3 different energy carriers, sometimes more. In addition to knowing the total energy consumption, one would also want to know how much of the total energy consumption was in the form of electrical energy, how much was due to the consumption of boiler fuel (and here you would want to disaggregate the data further into individual fuel types) and so on. Within each energy carrier, you would also want to understand how much energy was used by individual process areas, and possibly by individual types of energy-consuming equipment. Hence for electrical energy, it is useful to have a sense of how much energy is consumed by induction motors, how much by heating, how much by lighting etc. and to do this by area as far as possible. To do so may require some measurement (always the best option), or where this is not possible, some intelligent estimates. 

You may also want to delve deeper into specific areas, adding some qualitative data into the baselining exercise. For example you may choose to quantify the total number of induction motors on site, their capacities and other details such as the nature of their drive systems. Some of this information may already reside in your computerised maintenance management system (CMMS) should you have one, though often the information captured for preventive maintenance purposes may be short of details applicable to resource efficiency. This aspect of the baseline is not so much a case of using the data to establish trends as it is a case of building up a profile/technological footprint for the site, and providing context. Think of this profile as a living record that can be updated over time as circumstances change and more detailed information is gathered.  It really is up to you in terms of how detailed your analysis is, and as a rule, the more information you can gather, the better. Over time, you will begin to establish linkages between performance and the nature of the assets employed, facilitating problem solving.

The more meaningful you can make the baseline in terms of the richness of the data, the more focused you can be in choosing areas for the subsequent processes of opportunity identification and development. Consider that resource efficiency opportunities can involve technology, work practices, process optimisation, operating procedures and other factors. This should be reflected in the type of data you gather in constructing your baseline. The process is also instructive in terms of determining the nature of the routine reporting that should be taking place as regards resource efficiency. 

Many organisations propose and report on specific goals in terms of energy efficiency, emissions, water use and other indicators of resource efficiency in their sustainability reports. Their intentions are no doubt noble, but without a well-constructed baseline, formulating such goals, actively pursuing them and monitoring progress against them is virtually impossible. 

EXPLORING THE "ZERO LIQUID EFFLUENT DISCHARGE" WATER MANAGEMENT PHILOSOPHY

Industrial sites use water for various purposes, but in most instances an effluent stream is generated. The effluent is typically a composite of various individual effluent streams, and can vary with respect to volume, chemical characteristics and physical characteristics, such as temperature. There are a number of industrial organisations I have worked with who seek to eliminate this effluent discharge. This is known as a “zero liquid effluent discharge” approach. 

While that definition seems simple enough, implementing ZLED can be a costly and challenging exercise. In this post I will examine why organisations pursue the approach and what the implications for water management are.

It should be noted that while point effluent discharges from an industrial site can be eliminated, this does not necessarily mean that no liquid wastes are discharged to the environment. Contaminated aqueous waste streams can still arise due to seepage from impoundments, uncollected leachate from solid waste storage facilities or uncontrolled releases and spills. Water and effluent transfer infrastructure can also fail, leading to leakage of both contaminated effluents and water of good quality. In some cases, these spills could mean the loss of water of superior quality to that supplied (since water quality may have been upgraded using specialised treatment facilities), which can be very costly. The implementation of ZLED therefore requires consideration of a far broader range of issues than the elimination of known effluent streams produced on an industrial site, and must consider operations and maintenance factors in an integrated way.

Organisations seeking to implement ZLED generally have the following motivations:

• They wish to recycle as much water as possible, thereby reducing the amount of fresh water consumed. This reduces the strain on local water resources, and for large water users can also have a significant impact on the extent of the infrastructure needed to supply water and the impacts associated with its operation.
• They wish to completely avoid waterborne pollution arising from effluent streams. This reduces negative impacts on the environment and on downstream water users.

The pursuit of ZLED is therefore driven by considerations associated with water use efficiency and water quality impacts. One or more of these considerations may be in turn driven by a need for regulatory compliance. Alternatively, water management may simply be high on the organisation’s sustainability agenda. Economics will also be a factor as water prices and the penalties for pollutant discharges increase. Whatever the motivation, it has to be appreciated that ZLED comes at a cost. The question to be answered by organisations considering its implementation is whether the costs are less than the benefits. These costs and benefits have to be considered from a long term perspective, and are not only financial but also social and environmental.

The economic costs associated with ZLED typically concern the treatment costs associated with improving the quality of effluent streams to a level that is sufficient to allow their reuse. This could entail the use of a range of technologies as well as the ongoing operational costs associated with their use. The benefits to the implementing organisation are a reduction in water usage (and hence the costs of supply) and the avoidance of effluent charges. In a broader sense, there is clearly a significant environmental benefit in the form of reduced pressure on local water resources as well as reduced levels of water pollution, making more water available to other water users. In developing countries where many people may lack access to piped potable water supplies, pollution may have very serious social consequences through impacts on human health as vulnerable communities access raw water resources.  It should be clear therefore that the benefits of ZLED are many, and would have particular appeal for large water users and those that produce harmful effluent streams.

Be wary however of falling into the trap of thinking that ZLED is a panacea for industrial water management issues. An industrial site could implement ZLED, but still employ wasteful water management practices, such as excessive evaporation. Such a site could be placing more stress on local water resources than an equivalent site which discharges effluent of a quality level that makes the effluent available to downstream water users. Bear in mind also that water treatment processes all generate some form of waste, which could be in the form of concentrated contaminants, organic sludge, additional discharges (e.g. when regenerating treatment media) and/or local emissions of GHG’s. Contaminants removed or generated during treatment would need to be disposed of safely, failing which they could still enter local watercourses. Treatment processes also require energy and chemicals, with consequent life-cycle impacts. The point is that in evaluating whether ZLED is the right option to pursue, the risks need to acknowledged and mitigated.  You then need to reconcile the expected end-state of the ZLED implementation with the reasons you wish to implement ZLED in the first place.

A final thought is that not all industries can implement ZLED at acceptable financial cost, since recycling is ultimately about sources and sinks, and these have to be considered against the backdrop of the costs of treatment. It may however be possible to generate a partially treated effluent stream that is of acceptable quality for use as a supply to the processes of a nearby site, and in this regard, collaboration with local industries can assist with individual sites attaining ZLED, or at least significantly reducing discharges. Of course, you will then need to take a keen interest in the fate of the water supplied in order to ensure that the strategic objectives of your ZLED implementation are indeed met.