DON’T KNOW WHERE TO BEGIN WITH ENERGY EFFICIENCY FOR SERVICES? HERE’S YOUR ANSWER.

To have a chance at being efficient, a boiler must

first be adequately loaded. This is a system issue.

Services (sometimes called “utilities”) in manufacturing plants are typically a large contributor to site energy consumption. With these services, an energy carrier such as a fossil fuel or electricity is used to transfer energy into a fluid, which is then distributed to point of use.

Some examples of common services are compressed air, steam, chilled water, thermic oil and refrigerants. Of course, the supply of electrical power may itself be considered a service.

In general, manufacturers tend to focus more on core manufacturing processes than on services, and since often the services section of a manufacturing site is managed by someone other than the managers responsible for production processes, there is poor integration between services and manufacturing when it comes to energy efficiency. The result can be a piecemeal, component-level approach to energy efficiency, when what is required is a systems approach. I started this post by saying that services tend to be large users, but we need to take a step back to appreciate that while the energy input is to the service, the drivers of that energy use lie in various plant areas, and even include environmental factors. When resource efficiency practitioners talk about “systems” with respect to services, generally we mean the generation, distribution and user side of the service infrastructure, considered as an integrated whole. In the case of some services, there is an additional system element, that of recovery (e.g. returning condensate, thermic oil or refrigerant vapour).

Let’s consider for a moment why a systems approach is important. The first point to note is that by considering an individual element of a system in isolation, one can make small improvements but miss the far larger opportunities available. Hence there could be focus on boiler efficiency which could reduce steam system costs by 5%, when all the while an opportunity exists to reduce the quantity of steam consumed, potentially reducing system costs by 25%.  

A second problem with taking a component-level approach would be that in attempting to save energy, there could be unintended consequences which lead to a net decline in performance, either in energy use or in some other operational area. A simple example would be to reduce the fan speed on a refrigerant condenser in order to reduce the energy consumed by the fan motors, but to inadvertently increase the energy consumption of the vapour compressors, thereby increasing system-level refrigeration energy consumption.

A third and vital reason to take a systems approach to services optimisation is that the individual elements of the system interact with each other, and when implementing a basket of solutions comprising projects in each individual area, the implementation of each solution will influence the impact and viability of the others. A heat recovery project for a compressed air system will reduce in attractiveness should the amount of air required be significantly reduced, for example. The point here is that the benefit of implementing a portfolio of solutions for a given system will generally always be less than the sum of the individual solutions. This implies that it is useful to have a view of the full portfolio of planned solutions before proceeding with implementation, and while this is not universally true (e.g. one should not hesitate to fix air or steam leaks), it is important when considering capital-intensive options.

Reducing the pulse frequency for this bag filter

would reduce compressed air consumption. This

would reduce the loading of the air compressors.

Almost without exception, the approach to energy efficiency I see in most manufacturing plants as regards services is as follows:

         i.        Tinkering with the distribution network e.g. insulating distribution pipelines, fixing leaks and the like;

       ii.       Investing heavily in generation e.g. an “efficient” new air compressor, replacement of slide valve control with VSD control for refrigeration compressors etc.

There is no harm in dealing with obvious inefficiencies in distribution. The costs for implementation in this area tend to be low. At very large facilities, the savings can be significant. In general, however, projects involving the distribution network tend to yield only modest savings, and if this is all you are going to do, don’t expect breakthroughs on the bottom line. There are some distribution problems that do however require system-level considerations – an example would be a project to increase the size of air distribution pipelines to reduce pressure drop, which may become unnecessary should air use be significantly reduced.

The focus on generation I referred to above is in part due to the fact that there is a relentless focus on technological development in this area by OEM’s. Shiny new machines are attractive, and tend to come with assurances from suppliers with respect to increased levels of efficiency. Exercise extreme caution however. Many technologies only operate optimally when used in the correct context. Establishing this context requires a consideration of the system. As a simple example, a VSD air compressor as a replacement for a fully-loaded fixed-speed machine will not yield savings. A second reason that organisations tend to focus on the generation side of the system is the common misconception that efficiency needs to be “bought”, and that older installations cannot operate efficiently without investment. Since the generation side of the system is where capital tends to be concentrated, the tendency is to spend in this area. The reality is however that there are any number of no-cost and low-cost options that may be employed to drive energy efficiency.

The recovery aspect of some systems offers significant opportunities. In steam systems, condensate and flash steam recovery options can offer large savings at relatively low capital cost. Their viability is however impacted on by system-level considerations. A change in the steam pressure supplied to a user could significantly reduce the amount of flash steam produced, for example.

Given the above, you have most likely figured out by now that the place to start when optimising services is on the user end of the system. This means finding ways to reduce the amount of service fluid required. This comes with a lot of good news. In general, user-side opportunities abound. And in many cases, a common-sense approach can yield large benefits. Close those freezer doors. Insulate those process vessels. Recover energy from the leaving process stream. Automate the operation of that system so that switching off equipment is not dependent on someone remembering to do so. It may also be necessary to challenge paradigms in your business to reap these benefits. Does that process really need to operate at that temperature? Do we really need to wash this pipeline with hot water after each and every batch? Does the air pressure to that nozzle really have to be 8 barg. or can we get away with 5 barg?

When you take the time out to do this, you could be surprised by the quantum of the savings possible. That is one big reason to start here. The second is that there will be knock-on effects for the rest of the system. You may now only require two vapour compressors in your refrigeration plant room instead of three. The back-pressure turbine you thought was viable is now no longer so, since the amount of steam used has been significantly reduced. Your fully-utilised chiller plant is now a candidate for the implementation of a VSD compressor, since you now operate at part-load conditions for much of the time. You thought that you needed a VSD air compressor, but can now switch one of your compressors off. Had you made investments in generation, distribution and recovery before optimising your users, you could have ended up with a sub-optimal system.  

Copyright © 2016, Craig van Wyk, all rights reserved

LEVERAGING SHORT-INTERVAL CONTROLS TO DRIVE OPERATIONAL EXCELLENCE

Operations-orientated environments such as manufacturing are mastered by those who pay strict attention to detail. The organisations that do this tend to be the most successful, but the real champions in manufacturing are those who understand which details are most important. These organisations know immediately when an out-of-control situation (such as a quality or cost problem) arises, and also immediately understand why, and what to do about it. They then react quickly, rapidly restoring desired performance levels and maintaining high levels of delivery against a multitude of performance indicators. How is this possible? The answer lies in the short-interval control systems implemented on the shop floor.

 

As the name implies, "short-interval controls" refers to systems that deal with operational matters over short time-frames. Hence while senior management may be interested in tracking process yields for raw materials at monthly intervals or longer, the frequency for assessing these yields should be far higher on the shop floor. In a batch manufacturing environment, process yields may be calculated for every batch, for example. The principle is essentially that the monthly process yields are simply a result of the process yields for individual batches, and that by maximising the yields for each batch, monthly process yields will be maximised.  This approach can be followed for all indicators of operational performance, including product quality, throughput, safety, reliability and whatever indicators a business may choose.

 

It is one thing to measure performance at short intervals, but while this is a step in the right direction, and in my experience is itself often lacking, the best-performing organisations take it one step further. Instead of only measuring outcomes at short intervals, these organisations also track the drivers of performance at short intervals. These are tracked in one system so that the relationships between the drivers and the outcomes are readily apparent and can immediately be acted upon. The principle behind this approach is that by keeping the drivers of performance in control, performance will be maintained or where possible, improved. If the drivers have been comprehensively identified, then when performance deteriorates, it should be a simple process to scan the measured drivers, identify the one that is out of control, and then take steps to restore this driver to desired levels. Trends in the driver data can also be used to draw conclusions with regards to trends in outcomes, so that process owners can be proactive about performance management. If the reasons for poor performance do not show up in any of the drivers, this means you need to identify more drivers. You can use root cause analysis techniques to do this (WHY-WHY analysis is my personal favourite) or even do this by trial and error. I can tell you from experience in manufacturing management that this process works, and in my career has been one of my most powerful "secrets" to enhanced performance.

 

To make this less abstract, let me illustrate with a concrete (but hypothetical)  example. Say you are dissolving a solid in a liquid e.g. making a sugar solution in a beverage facility. The process needs to be completed in a given time period in order to meet the throughput requirements of the factory in which it is operated. The outcomes for this process would be the volume produced for each batch, the time taken to prepare each batch and the final concentration or density of the solution being prepared. Each of these would need to be monitored for each batch. The process is carried out in a heated, agitated vessel, into which the liquid is pumped. The solid is manually added after being weighed by the process operator. Typical inputs that would also be monitored using the short-interval control system would be:

• The volume of liquid added (affects turnaround time, throughput and yield)
• The time taken to pump the liquid into the vessel (affects turnaround time)
• The time taken to heat the liquid to dissolution temperature (affects turnaround time and possibly also product quality e.g. burn-on if rate is too high)
• The actual initial and final temperatures of the liquid (affects turnaround time and dissolution rate)
• The mass of solid material added (affects density of the solution and process yield)
• The speed of the agitator - this is equipped with a variable speed drive (affects turnaround time and dissolution rate)
• The time taken to pump the solution into the holding tank (affects turnaround time)
• The mass of solution produced (affects throughput and yield)

By monitoring these variables on a continuous basis, this process can be kept in control and this data can immediately be consulted should there be a problem with throughput, yield or product quality. Without it, management and the plant operator is pretty much in the dark with regards to the reasons for performance problems, and the time to correct these would be much longer, impacting negatively on the performance not just of this process, but of the wider processes in the facility impacted upon by it. If this process is the bottleneck in the facility, for example, turnaround problems would affect the throughput of the entire factory, with potentially disastrous consequences. We see therefore that something as seemingly small as the time taken to pump a liquid into a vessel can impact on the performance of en entire business. This is what I meant when at the start of this post I spoke about "details".

Copyright © 2016 Craig van Wyk, reproduction only with written permission.

Craig is the founder and Managing Consultant at VWG Consulting, a Johannesburg-based productivity and resource efficiency consulting and services company serving the industrial and commercial sectors.

ENERGY EFFICIENCY IN COMPRESSED AIR SYSTEMS

Compressed air is a common utility and  often a significant energy user in factories and commercial installations.  Compressed air systems comprise the compressed air source (the compressor), the distribution system and the users of compressed air. To save energy, review all 3 areas.

 

Why the system and not just the compressor?

A systems approach is necessary because the compressor interacts with the rest of the system, and changes made downstream affect compressor operation and energy efficiency. As a simple example, if the amount of air used is reduced, the loading of the compressor would reduce, with potentially significant implications for the approaches required to increase its efficiency.  It is commonly advised that when evaluating and optimising systems, one should start with users and then work back to the energy source. I will take the same approach with this post.

 

Air Users

Minimising compressed air use reduces the amount of air that has to be compressed and is a powerful way to reduce the energy required by a compressed air system. I am always amazed by the low level of awareness in factories with respect to the cost of compressed air, and how it is almost treated as being "free". In generic terms, one can reduce air use by reducing the time for which the air is used and/or the flow rate it is used at. What this means in practical terms varies widely between different facilities.

 

Reductions in flowrate are typically achieved through local pressure regulation or restrictions in flow at point of use (e.g. through use of a modulating valve), and this should be considered for larger users in particular. This effectively reduces the mass of air used, but clearly this can only be done if the process objectives are still met. In some instances, compressed air use can be eliminated and replaced with low-pressure blowers. Where more air is used than is required to perform a task, this is known in the industry as "artificial demand".

Reducing the time for which air is used is really about stopping the air flow when it is not required. This can either be done manually, in which case training and work practice optimisation is needed, or via some form of automation. A common example is an unscrambler, as found on packaging lines, which uses compressed air to move/orientate items (e.g. caps for aerosol cans) in a hopper prior to them being fed to the point of use. Solenoid valves fitted onto the air supply and hardwired to the conveyor switch would ensure than when the line is not producing, air supply is terminated. Be aware that such "open blowing" applications are often considered to be inappropriate air users, and should be minimised as far as possible.

Another inappropriate use for compressed air is the use of air-driven pumps (compressed air is very inefficient as an energy source, since most of the energy that goes into the production of compressed air is dissipated as heat). Since compressed air is easily distributed around factory sites, it is not surprising that some creative approaches are applied in exploiting its convenience. I once arrived at a factory for an energy assessment and was greeted by an employee cleaning the walkway with a custom-made "air broom" - a shaft and handle with an air supply used to blow dust away.

 

Distribution System

Compressed air distribution systems have a significant impact on energy use. It is important to ensure that pressure drops are minimised, otherwise higher source pressures are required to deliver the air at the pressure required by users. Line sizes are hence important (if lines are too small, pressure drops are higher), as is the minimisation of bends in pipework. It is also important to ensure that the condition of the pipework is maintained in good order. Corrosion roughens up pipe surfaces, increasing frictional losses. While I have seen some facilities with very large (in diameter) air distribution lines, and this is also good from a storage perspective, there are implications in terms of installation costs. I recommend welded pipework or even better, screw joints rather than flanged pipework for air distribution systems. It is not uncommon to find defunct equipment that is still in the compressed air network, and through which air is still passed, contributing to pressure drops and energy losses. Equipment such as air filters should be sized correctly for the pressures and flows required. I have seen a few plants in which incorrect equipment was specified with astronomical impacts on costs. Each item in the distribution network represents an additional pressure drop, so use correctly specified equipment and only use the equipment that is absolutely necessary to do the job.

Compressed air leaks can add hugely to operating costs, and should be minimised through an ongoing leak detection and repair programme. This should be integrated into routine maintenance practices rather than be a stand-alone initiative. Take care in selecting equipment when seeking to minimise leaks. Quick-release couplings are notorious sources of air leaks, for example. Look out also for "intentional" air leaks, such as drain ports that are left open in order to allow for the constant removal of moisture, and which leak air continuously as a consequence. Most leaks are audible and the best time to detect them is during breaks, when equipment is not running but the compressed air network is still pressurised. These days there is some very advanced condition monitoring equipment available (such as ultrasonic detectors) that can not only detect leaks but also quantify them.

Incoming air has a humidity level, and hence has to be dried, since compression concentrates this moisture. Driers are a topic all of their own, each with their own energy use implications. Most larger screw compressors come equipped with integrated refrigerated driers these days, and most sites provide further backup with stand-alone drying systems, which can themselves be refrigerated or be of the desiccant type (other types exist but these two are the most common). Be aware of the consequences of not drying air effectively. Liquid in the distribution pipelines not only accelerates corrosion, it also contributes directly to pressure drop. Pay attention to automatic drains, which can fail and become leak points, or can also be cycled too frequently, leading to excessive amounts of air being lost with the water removed.

 

Compressors

Efficiency in compressed air production is about minimising the amount of energy required to produce a given quantity of air, and different compressors have different inherent efficiency levels. Various compressor-specific factors impact on efficiency, including the compressor motor, drive and the air-end design. Loading is an important driver of efficiency, and like with most equipment, low loading levels lead to inefficient operation.  The pressure of the air produced is related to the efficiency with which it can be produced, and it is best to produce compressed air at as low a pressure as possible. The location of the compressor is important, and cool (and therefore dense) incoming air is better than warm air. This is not only about atmospheric conditions, but also about keeping the compressor location away from heat sources in your facility. This includes the heat generated by the compressor itself, which should be removed from the immediate vicinity of the compressor intake. Dusty areas should also be avoided, as dust blocks intake filters/screens and increases pressure drop.

 

Poorly loaded screw compressors are inefficient, and VSD's can assist in reducing energy consumption where air demands are variable. A further important consideration is that of heat recovery from oil-flooded screw compressors. Much of the input energy to a compressor is rejected as heat from the oil and the air produced, with some OEM's reporting recovery levels as high as 90%. The common practice is to reject this heat to the environment, either in an air stream or via cooling towers in the case of water-cooled compressors. This heat can be recovered and used to produce hot water (temperatures in excess of 70 deg.C are possible) or hot air. The hot air can be used for space heating (not so attractive for warmer/temperate climates as in my home country of South Africa, where this application is only needed for a few months of the year) or for processes requiring hot air. The idea is to choose a heat sink that requires more input energy than is rejected by the compressor, in order to maximise recovery levels. A good application would be to supply combustion air for a boiler or furnace. 

 

The reason I am mentioning heat recovery here is because it has a marked impact on whether to go with a VSD replacement, or whether to rather keep your existing fixed speed screw compressor and employ heat recovery. While a poorly loaded screw compressor is inefficient, a VSD replacement compressor is typically very expensive, and it may be more attractive to employ heat recovery, since this is cheaper to implement. The point is that while your poorly loaded screw compressor will be inefficient, most of these losses would be recovered with a heat recovery system. A case-by-case approach is however required. The economics of heat recovery depend on electricity and fuel prices (assuming you are not using an electrode boiler or electrical heating system), and an analysis specific to your circumstances is essential.

The above is but an introduction to the energy efficiency considerations associated with compressed air. Where multiple compressors are employed, system optimisation becomes more complex. Remember also to back up each opportunity with a quantitative assessment of savings when making decisions regarding implementation.

Copyright © 2015, Craig van Wyk, all rights reserved