Tuesday, June 2, 2015

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

 

Monday, April 13, 2015

BASIC OPPORTUNITIES IN INDUSTRIAL LIGHTING

Linear Fluorescents have largely been superseded by LED's,
but are still a good option in terms of efficiency. Ballast losses
are higher for older electromagnetic designs.
Factories are too diverse and complex for there to be meaningful benchmarks in terms of the contribution of lighting to overall energy consumption. In general however, except for very light industry (no pun intended), the proportion of a site's total energy that is employed for lighting tends to be small. Despite this fact, I always include lighting assessments when reviewing energy usage on any industrial site. My main reason for doing so is that this is often an area heavily laden with low hanging fruit from an energy efficiency perspective.
 
Often the basket of lighting opportunities identified has a payback of under 2 years, with many individual lighting solutions having an almost immediate payback. Of course one wants short paybacks for lighting projects, given that lighting retrofits will not have the lifespan of other larger investments. Savings with regards to lighting are about far more than lighting retrofits however, so be sure to include options such as work practice changes, maintenance and operational improvements when seeking to reduce lighting costs.
 
There is a huge amount of detail associated with the rigorous analysis of lighting opportunities, and I won't get into that here. What I want to highlight in this post is how simple it can be to identify and implement sustainable lighting savings. Firstly, the cost of lighting has to be appreciated along with the drivers of operating cost. The energy used for lighting is a function of input power to the lighting / luminaires and the operating hours involved. It is quite a simple exercise to carry out a lighting inventory which outlines, preferably on a "room-by-room" basis, the number of lights, their individual power consumption levels and their hours of operation. This can then be converted to an annual  energy value (in kWh), and then, depending on applicable tariffs, an annual energy cost can be calculated for each individual room. This is a simple enough exercise - don't forget to include ballast losses. It is also important to appreciate that lighting often contributes to maximum demand, and hence an apparent power value for individual lighting options must be determined. You might need to consider power factors should the site concerned not have a correction system in place, but to keep it simple, perhaps just take the power factor of individual fittings as unity and add the apparent power levels to assess the contribution to site demand. Ignoring demand can make a large difference to the estimated operating cost of lighting.
 
Once you have this inventory, you can set about identifying relevant solutions for each area. The plethora of modern lighting solutions can make this a minefield as you consider issues such as lumen output, lumen depreciation, lifespan, colour rendition, efficacy, lighting and installation costs, disposal considerations, human health impacts etc. The key point I want to convey with this post is that while the temptation often is to pursue the latest innovations in lighting technology, there are very simple ways that lighting costs can be reduced. Before embroiling yourself in cost benefit analyses for individual lighting technologies, take some time to apply some common sense to the matter of finding savings. Questions I often ask as I evaluate individual areas include:
  • Are there any obviously inefficient options in place e.g. mercury vapour lamps, T12 fluorescents with magnetic ballasts, incandescent lights etc? If so, what can they be replaced with? - there is no standard answer here, and it depends on the individual installation and characteristics of the room e.g. roof height, reflective surfaces etc.
  • Is there too much light available on working surfaces (as measured with a light meter) when the lights are on? This would be wasteful, and often savings are possible through a simple reduction in the number of luminaires. It is sometimes also possible to modify existing luminaires e.g. the control gear for mercury vapour lamps can be bypassed and the existing fittings can be used to house compact fluorescent lights. Be sure to adhere to regulatory lighting standards as a minimum.
  • Are daylight harvesting opportunities available? This may result in lights being switched off during the day, or to a reduction in the number of lights used during the day. Existing lighting may require supplementation with options such as transparent roof sheeting for example.
  • What are the switching arrangements? It is not uncommon to find a single switch for a large area, but with only part of the area used regularly. By applying a zoning strategy, lights can be switched on selectively, saving large amounts of energy. Switching options can also be used to provide flexibility in terms of the number of lights switched on e.g. all lights on dull days but only a few on typical sunny days.  
  • What are occupancy levels for individual areas? Motion sensors can be a useful solution when paired with the correct lighting types.
  • Are windows, roof sheeting and lamps themselves cleaned regularly? If not, can a simple cleaning regimen be easily implemented?
These simple solutions are every bit as important as the deployment of efficient lighting technologies, and most often cost very little to implement, if anything.
 
Copyright © 2015 Craig van Wyk, all rights reserved

Tuesday, September 16, 2014

MAKING THE SWITCH TO SIGNIFICANT FUEL SAVINGS

Fuel switching can be a huge
cost-reduction opportunity
Fossil fuels are used extensively for activities such as steam generation and for process heating applications such as furnaces and curing ovens. There are a range of ways to make these operations more efficient and cheaper to operate, but often the fuel employed is not given much attention. This could however be one of the most impactful ways to reduce energy costs. So what is fuel switching, and how does one determine the nature and quantum of the cost reduction opportunities it presents?
 
Fuel switching is simply a change in the fuel used to one of a different type e.g. a switch from coal to wood pellets. It is sometimes done for operational reasons. Heavy fuel oil is an example of a fuel that if not properly stored and handled, can cause blockages and downtime, particularly in cold weather. Safety could be another factor, for example volatile fuels in a hot climate come with risks if not stored and handled properly (not to mention losses). Some fuels are not freely available, and hence reliability problems could prompt a switch. In the absence of these challenges, the biggest motivation for a fuel switch is however that of cost reduction, and this is what this post is about.
 
In determining whether a fuel switch is financially viable, the first thing to understand is what the cost of the fuel you currently use is relative to the cost of the fuel it could be substituted with. I refer here of course to the cost per unit of energy, not per unit of fuel. You also need to have a sense of the efficiency with which you would be able to use the new fuel relative to current efficiency levels. By noting the energy content per unit of fuel and then applying the efficiency with which the fuel will be used, you will be able to determine the cost of delivering energy to the process you are operating, whether this be steam generation or a heating application.  This immediately conveys the quantum of the potential savings on offer, and these savings then have to be contrasted with the potential additional costs that come with making the switch to determine the net financial benefit.
 
The method I use is to determine the stack losses for the two different fuels, since this is the biggest loss to consider. This is a function of flue gas temperature, flue gas oxygen and ambient temperature, and different fuels typically require different levels of excess air for effective combustion. I would typically use a common flue gas temperature for the two fuels (usually just the one currently achieved), fix the flue gas oxygen content at the minimum required for the fuel being assessed and then determine the boiler/furnace efficiency I could expect when making the switch. If the plant involved is equipped to deliver low flue gas temperatures (e.g. thorough the use of economisers or air pre-heaters) you could fix the flue gas temperature for the analysis at the acid dewpoint temperature for each fuel plus a small margin of safety. Of course if you are switching to a very clean fuel and this allows use of a condensing economiser, the whole equation changes, but let's leave that for another day. The efficiency determination allows me to calculate the fuel rate required (based on heating value per unit of fuel and the heat load of my process), and I can then make a comparison of expected fuel costs to current fuel costs. Where there is a large difference, I move onto the next phase of the investigation, which is a risk assessment.
 
There are several issues to consider when deciding to switch fuels, and it is important that a thorough risk assessment is done before making this change. Some questions you should ask before making any proposed fuel switch include:
  • Can you lawfully use the fuel? - local air quality regulations could preclude the use of certain fuels and before doing anything, check this first.
  • Can your boiler handle the new proposed fuel, or can it be modified to do so?
  • Can your fuel handling infrastructure support the new fuel, and if not, what modifications are required and at what cost?
  • Is the supply of fuel going to be reliable, and what price changes are expected going forward relative to the price changes expected for the fuel you currently use? (you may need a crystal ball for that one)
  • What are the emission impacts associated with the new fuel, not just in terms of GHG's but also particulate matter, sulphur, mercury and other pollutants?
  • What are the water pollution risks?
  • What are the safety risks associated with the new fuel? - here characteristics such as flash point are a consideration, among others
  • What impact, if any, will the fuel have on manning requirements and operating and maintenance costs?
Note that you could spend money on mitigating some of these challenges if the economics allow. There are examples I have seen where even a change in boiler was financially viable, so keep an open mind. Once all of these issues have been addressed and you are satisfied that none of them is a barrier to the switch, you are in a position to start planning for implementation, but the process is far from finished. Ideally you should observe the fuel in operation at another location (if you haven't already seen one) and engage directly with users of the fuel and suppliers of fuel and equipment to ensure that there are no unexpected surprises. If no plant modifications are needed for the switch, you should also use the fuel on a trial basis before committing to any long-term supply contracts. All of this due diligence can sound like hard work, but I can assure you that this could be one of the biggest cost reduction opportunities at your facility, and if you haven't looked into it, you should do so without delay.

Copyright © 2014, Craig van Wyk, all rights reserved