PRACTICAL ELECTRICAL ENERGY CONSERVATION ON INDUSTRIAL SITES - ARTICLE 3: HEATING

Electrical heating systems are common in industrial environments. They may be assumed to operate at very high efficiency levels i.e.  close to 100% of the electrical energy supplied to heating elements is converted to heat. Of course, generation of electricity is not a process that operates at 100% efficiency, and this has to be considered when assessing the overall emission impacts of using electrical heating systems.They also can have significant demand impacts, and as resistive devices with power factor close to unity, demand reduction through power factor correction is not possible.

Typical applications are for the heating of process fluids (e.g. solutions of chemicals used for washing, or for air used in drying) or sometimes for the heating of materials (e.g. heating of plastics during extrusion). Electrical heating may also be used for producing hot water and even steam.

Given the fact that heating is itself a very efficient process, energy savings associated with heating are unlocked by taking a systems perspective. Hence if you are analysing a steam system, you would need to consider the distribution and usage of steam and the recovery of condensate in order to increase the efficiency of the steam system, thereby reducing the electrical energy input.

      

In smaller operations I often see manual control being employed, particularly in batch processing environments, and generally I always recommend some type of feedback temperature control system, even if in the form of a simple thermostat. Any overheating represents a waste of energy, and also accelerates heat losses to the environment due to an increase in driving force.

An obvious opportunity with heated systems is the use of insulation to limit losses to the environment. Basically you want to close the system as far as possible, and this includes any open surfaces, as would be found in things like heated process baths and the like. The use of covers helps to limit losses to the environment. Heat losses from agitated liquid surfaces tend to be significantly higher than from still surfaces, and hence any agitation should be limited as far as possible. Obviously many processes require agitation for legitimate reasons, and it’s about finding a happy medium between your process goals and heat losses.

Heat loss occurs not only through direct heat transfer, but can also happen as a result of carryover of hot process fluids into downstream processes. This can happen easily in process such as electroplating, or washing. It’s clear that such carryover should be limited as far as possible, since not only is heat lost with the fluid leaving, this fluid is generally replenished with fluid at room temperature which has to be heated to the process temperature.

Depending on the heat load and when the heating is done, the rate of heating could impact on maximum demand charges, and you may want to look at how heat-ups of individual process vessels are scheduled at start-up.

Lastly, heated processes should be located in areas free from drafts (either natural or induced) since these increase the rate of heat transfer from hot surfaces and increase heat losses.

Let’s briefly explore what I mean by taking a "systems approach" by considering a simple process in which a tank cleaning solution is heated on a batch basis using an in-line heating element, with the cleaning solution circulated through a sprayball in order to clean the tank. Water is added to the tank, cleaning chemicals are added and the resultant cleaning fluid is circulated through an in-line heater and back into the tank to be cleaned. After cleaning, the fluid, which becomes soiled, is drained. The system is shown in the diagram below. 

Important basic issues to consider would be:

Is there a temperature probe used for feedback control of the heating operation? Without control of the temperature, the fluid could be heated excessively, wasting energy. If a probe is installed, what is the temperature used for cleaning? This should be as low as is practically possible in order to reduce energy requirements for heating up the cold cleaning fluid. Lower temperatures will also translate into reduced heat losses to the environment due to a reduced driving force for heat transfer;  Is the system well insulated? This includes the vessel as well as the piping used to circulate the cleaning fluid;  What volume of fluid is used for each cleaning operation? The larger this volume, the greater the sensible energy requirement for heating the fluid up;  How often is the cleaning operation performed? – clearly the less frequently this can be done (while still meeting all other process goals, such as sterility for example) the lower the energy requirements for cleaning; Instead of a once-through cleaning system with the cleaning fluid discarded after each wash, is it possible to collect and reuse the fluid, if not for subsequent cleaning, then for other purposes in the facility? Integrating this washing process into other processes in the factory would lead to reduced facility-wide energy consumption.

    

     While heating elements are essentially simple devices, the systems within which they operate can be quite complex, and optimising these systems in an integrated fashion is the challenge when seeking to reduce their energy consumption.

HEAVY METAL POLLUTION OF WATER RESOURCES - CAUSES AND IMPACTS

The term “heavy metal” is not altogether clearly defined, but in the case of water pollution, these are metals such as arsenic, cadmium, iron, cobalt, chromium, copper, manganese, mercury, molybdenum, nickel, lead, selenium, vanadium and zinc. While heavy metals do tend to have a high atomic mass, and so are heavy in that sense, toxicity seems to be a further defining factor as to what constitutes a heavy metal and what does not.

Municipal treatment plants are generally ill-equipped 
to cope with significant heavy metal pollution, both 
in terms of removal and safe sludge disposal.

Heavy metals occur in the earth’s geological structures, and can therefore enter water resources through natural processes. For example, heavy rains or flowing water can leach heavy metals out of geological formations. Such processes are exacerbated when this geology is disturbed by economic activities such as mining. These processes expose the mined-out area to water and air, and can lead to consequences such as acid mine drainage (AMD). The low pH conditions associated with AMD mobilise heavy metals, including radionuclides where these are present.  Mineral processing operations can also generate significant heavy metal pollution, both from direct extraction processes (which typically entail size reduction - greatly increasing the surface area for mass transfer - and generate effluents) as well as through leaching from ore and tailings stockpiles.

While mining activity poses significant risks for heavy metal pollution, this sector is not the only culprit in the industrial sector. Many industrial processes can generate heavy metal pollution, and in a large number of ways. Clearly, some industries will be more likely to pollute than others. Hence the electroplating industry, which can produce large volumes of metal-rich effluents, will naturally be a more likely polluter than the food processing industry, for example. This is not to say that players in this industry will necessarily pollute, and it is in fact in the electroplating industry’s best economic interests to minimise metal discharges, since these are inversely proportional to resource efficiency. Reducing losses by minimising drag-out from plating baths leads to reduced metal discharges, for example. The lead-acid battery manufacturing industry is another example of an industry which can generate metal-rich effluents as well as airborne lead pollution which can subsequently be deposited in surface water resources (and of course on land). So clearly, where an industry uses heavy metals as key input materials, pollution risks increase.

  

An example of a large non-point source of heavy metal pollution is coal-fired power generation, which can contaminate water resources through aerial deposition of mercury emitted from boiler flues. Technologies such as wet scrubbing are available to remove much of this mercury, but of course the effluents produced have to be safely handled to prevent subsequent pollution. Some of these processes have the primary goal of removing sulphur dioxide, with heavy metal removal a welcome by-product of the scrubbing process. The industry also generates large amounts of ash which itself contains heavy metals, including uranium. 

The importance of minimising heavy metal pollution for industrial organisations extends beyond simple compliance. The impacts of heavy metal pollution on living organisms are very serious. Heavy metals are bio-accumulative, toxic at high concentrations, have neurological impacts, and some are carcinogenic. They can also interfere with chemical processes by poisoning chemical catalysts and can impact on biochemical processes by interfering with enzyme action. There are hence serious environmental, economic and social impacts associated with heavy metal pollution. 

As always, a detailed risk assessment, which must include include quantitative measurement, is recommended to help you to understand your heavy metal pollution baseline. Where problems are identified, the solutions you choose should focus on the source of the pollution as far as possible, in line with Cleaner Production principles. End-of-pipe treatment methods are unavoidable in many circumstances, but where they are employed, care should be taken to dispose of resultant concentrated metal wastes safely. For example, lime treatments, which raise pH and precipitate metals, produce concentrated wastes (sludge) requiring safe disposal, as do processes such as reverse osmosis (retentate). Take care then that you are not just moving the problem around through poor management of these wastes, which generally require disposal at certified hazardous waste installations.

PRACTICAL ELECTRICAL ENERGY CONSERVATION ON INDUSTRIAL SITES: ARTICLE 2 - INDUCTION MOTORS

Induction motors are the heart and soul of most industrial sites. As modern motors have become more and more efficient, opportunities have arisen to replace older motors with newer ones, thereby harvesting the energy benefits. This replacement, while guaranteed to save energy and possibly reduce maximum demand (for sites that do not have PF correction) is not necessarily always financially viable, and you will need to take some care in investigating opportunities in this area.

Motors driving dosing pumps at a water treatment works

The power factor and efficiency characteristics quoted for induction motors are based on full-load conditions i.e. conditions at which the power delivered by the motor at the shaft is equivalent to the rated capacity of that motor. Motors operated at part-load will have a reduced power factor and efficiency. Don’t therefore make the mistake of simply looking at power factor and efficiency characteristics of two motors for comparison when looking at replacement options. You will need to do field measurements to determine power consumption and ascertain loading, which will then provide additional input to the decision, which may even be that you need to install a smaller motor. An under-loaded energy-efficient motor will not achieve its full load efficiency, and savings may therefore be less than expected. It is also important to assess transient conditions, such as start-up, when power draw can be much higher than during stable running, and variations in power draw which result from the nature of the process being driven by the motor. Whatever motor you install would need to cope with those conditions. 

Aside from the technological issues associated with the motors themselves, there are other system-related issues which are in many cases much more important. Probably the first question I want answered when looking at motor replacement is: “what are the annual running hours of the motor?” Remember that in energy terms, we are looking here at kWh, and while efficiency reduces kW, if the hours (h) of running are small, the total energy savings from retrofitting will also be small. Low-efficiency motors that run continuously in plants that run around the clock are therefore always attractive retrofit candidates, while a motor that runs intermittently in a dayshift-only operation will probably not be interesting, unless it is so under-loaded that its efficiency is only a fraction of full-load efficiency.  In this latter instance, you may even want to install a smaller standard-efficiency motor that is being replaced elsewhere to limit costs while still saving electrical energy.

The next system-related issue to consider in terms of motors are the drive systems associated with each motor. These entail things such as gearboxes, chains, belts, couplings and shafts. You need to consider not only the technological issues here (for example, cogged belts are reported to use 2% less energy than standard v-belts without any changes required to pulley systems) but also the maintenance issues. Key maintenance issues from an energy efficiency perspective could include:

• ·         Lubrication of bearings, chains and gearboxes;
• ·         Alignment of shafts, couplings, sprockets and pulleys;
• ·         Tension levels in belts and chains

It is important to include checks of these types of issues as tasks in your preventive maintenance programme.

The final system-related issue is that of the process being driven by the motor. An efficient motor driving an inefficient process still constitutes an inefficient operation. Hence ask yourself if that agitator is of an efficient design, if the pressure drop in that pumping system as low as it could be or if that fan design is best for the application, as examples. Delve deep into your processes and look at the constraints around your plant, remembering that the overall process only has the capacity of the constraint. It makes no sense to run non-constraining processes at higher throughputs than the constraint, leading to unnecessary circulation, waiting times (with motors running in many cases) or higher flow rates than needed in the case of pumped systems (remember that when pumping fluids, pressure drop varies with the square of flow rate). These types of issues are unique to each individual industrial site, require comprehensive process knowledge, and in my experience are the most neglected when it comes to energy efficiency programmes and projects. They should in fact be addressed first, with motor replacement options only investigated once underlying processes have been optimised.