With focus intensifying on environmental issues, legislation increasing and likely to become more stringent, most businesses are considering their impact on the environment. Furthermore, public concern about pollution and global warming is increasing and the ‘PR cost’ to companies who cannot demonstrate their green credentials, is also rising. The PR costs of not being seen to be doing one’s bit as a company often dwarf the actual fiscal costs of suboptimal energy usage. Unsurprisingly, we now see the phenomena of marketing and PR departments taking an active interest in the efficiencies and waste management practices of their companies. Sustainable and efficient manufacturing processes are no longer only the preserve of accountants and senior engineering management – they now are a marketing and PR issue as well.As a result, engineers are receiving edicts from management to cut waste and increase the sustainability profile of the company because being seen to be green is probably high on the agenda for any progressive manufacturing business. Process engineers are now seeking any way they can to meet their sustainability targets and of course, water usage is one key metric by which this can be measured because any savings in water usage are easily convertible to carbon foot print savings. Here we look at methods of optimising tank cleaning systems as a way of meeting sustainability targets and it’s clear that some quick wins with significant efficiencies are possible by optimising tank cleaning processes.
The true cost of water
The true cost of water, both in environmental impact and in fiscal terms, is often under appreciated. Firstly, there is the cost of purchasing the water which is increasing and will continue to rise as pressure on the world’s water systems increase. The cost per m3 of new water to businesses is significant, but it’s only part of the true cost.
Heating cost
Many applications require water to be heated, especially in cleaning applications. Water takes 4.2 Kj of energy to heat each litre (Kg) and its high specific heat capacity means it takes a lot of energy to heat water to the temperatures needed for optimal cleaning.
Treatment cost
Once water has been used for a cleaning operation it needs to be disposed of and the cost of disposal varies greatly with water used for cleaning operations having particularly high disposal costs. The very nature of the application means the waste water will be dirty and often contain caustics or solvents to ensure effective cleaning. Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are metrics used by the water treatment industry to effectively measure how dirty waste water is. Both measure the oxygen required in reactions to break down the pollutants and the water used in food and dairy industry applications often has very high BOD levels resulting in higher waste water treatment costs per m3.
Chemical cost
Using alkali caustics to break down oils, fats or greases and acids to break down mineral deposits, is common in many tank cleaning systems. These chemicals are not cheap and their usage only adds to the cost of treating the waste water used.
Standard methods of optimisation / energy saving. There are two commonly deployed mehods of reducing energy / water usage in tank cleaning systems.
Heat recovery
Many factories produce excess heat in their processes and good heat management systems can divert waste heat to be used in other applications like cleaning. This “free” heat is a key method in reducing environmental impact and meeting sustainability targets, although there is a limit to how much “free” heat is available for cleaning.
Recycle water CIP
Most clean in place (CIP) systems will reuse waste water from previous cleans. The pre-rinse and even main clean can often be performed with dirty water. As long as a final rinse of clean water is used then a hygienic clean can be achieved using mostly soiled water.
Of course, there is a limit to how many times cleaning fluid can be recycled in this way and eventually all the waste liquid needs to be removed from the CIP system.
Other methods of saving
Once heat recovery and water recycling efficiencies have been realised to their full, where next for the environmentally responsible manufacturer?
Optimising the cleaning mix
One still often overlooked saving can be found by optimising the cleaning mix. The basic theory underpinning any cleaning operation is that there are four contributory factors in cleaning: heat, time, chemical action and mechanical action. An increase in one element will mean that the other elements can be reduced without compromising overall cleaning effectiveness.
From a sustainability perspective, there is a logical choice of which element to increase and Increasing heat is only going to increase the environmental costs. Chemical action of course should immediately be seen as a non-starter. Increasing time might seem beneficial but in reality, a time increase in cleaning often means using more water so, again, it’s a non-starter if our goal is to improve sustainability. This leaves mechanical action as the obvious “green” candidate to increase.
If we can improve the impact of the cleaning system, then the other elements can be reduced accordingly. This means the same level of cleaning can be achieved with less heat, chemicals, water and possibly in less time too.
Nozzle selection & Mechanical action
When cleaning with fluids the overall mechanical action delivered to the tank wall is dictated by the energy transferred from the pump to the wall. The more efficiently the energy from the pump is transferred to the wall, the greater the mechanical action component of the cleaning mix will be.
Nozzle / tank cleaner selection has a significant effect on the efficiency of this energy transfer process. With simple spray balls and spinners, most of the potential energy contained within the cleaning fluid is wasted in dispersing the fluid over a wide area. The motion of the liquid between the tank cleaning device and the wall is highly turbulent and dissipated. This has the advantage of spreading out the liquid to cover a wider area but comes at the sacrifice of losing most of the energy available to generate mechanical action.
In reality both these styles of tank cleaner have poor mechanical action.
Spinning spray balls are better than static spray balls and have a modest gain in mechanical action. Matters can be improved by increasing the flow rates by using tank cleaners with larger orifices but, if the goal is sustainability, increasing water usage is clearly counter-productive.
Increasing the fluid pressure on such devices is equally futile. Sure, the overall potential energy is increased but, due to the turbulent nature of the flow, this energy is simply wasted i.e. very little of it ends up contributing to the mechanical action component of the clean. Anything much above 2.5 bar fluid pressure supplying spray balls or spinning spray balls is wasted and will do nothing to improve sustainability. Indeed, it will more than likely decrease overall water and energy efficiency.
Rotary jet cleaners – the green upgrade
The type of tank cleaning heads that can significantly improve mechanical action are rotary jet cleaners. These produce laminar flow jets which deliver high impact cleaning to each part of the tank as they move through their cleaning cycle. The laminar jet is the key to delivering as much energy as possible from the pump to the tank wall. With this style of cleaner, increasing fluid pressure at the pump means more energy makes it to the tank wall rather than being wasted in generating chaotic and turbulent flow.
The advanced nozzles on these machines mean the fluid stays as a coherent jet for many metres at pressures in excess of 10 bar and increasing pressure makes perfect sense from a sustainability perspective. The modest increase in pumping costs to generate higher pressures is more than compensated for by the sustainability benefits. As the mechanical action component is so much higher in these machines, chemical action, heat and time can all be reduced.
Heat
When changing to impact cleaning it’s unlikely that the temperature of the various wash stages will be reduced. An 80 caustic wash will remain at 80º, but because less fluid will be required to achieve the same cleaning there is simply less to heat so the energy cost of heating is dramatically reduced.
Chemical action
Less fluid will be used overall so a corresponding reduction in caustics will occur. The percentage mix of caustics in the fluid will probably remain constant but as less fluid is used overall the amount of chemicals used is far lower. From a sustainability perspective this means less of a cost of water treatment and less of a cost associated with the production of chemicals in the first place.
Time
Another great benefit of rotary jet impact cleaners is that they often have much faster cleaning time resulting in more production time compared to other tank cleaning devices. Whilst this does not have any great sustainability benefits it does have some very practical and obvious production benefits for manufactures. Less cleaning time means more production time.
Cost as a proxy measure for environmental savings
Estimating the carbon foot print savings achieved by moving from less efficient methods of tank cleaning to more modern ones is quite difficult. An approximation of the levels of saving can be made if we use overall cost as proxy measure for environmental impact. Whilst not perfect in the case of tank cleaning it’s generally true that if one reduces cost then sustainability will be increased.
Cost saving calculations
Firstly, the overall water consumption needs to be calculated for each stage of the clean. Any chemicals used in the caustic stages need to be costed in as well and the cost of heating the water also needs to be calculated. This is straightforward enough using the energy cost to the business in Kwh multiplied by the Kwh needed to raise the water to the desired temperature. This can be modified downwards if heat recovered from another part of the factory is used. So, if 50% of the heat is recovered the overall cost can be halved.
Next the cost of water treatment and disposal needs to be calculated and this will vary greatly depending on the COD and BOD load of the waste water. When water recovery CIP systems are used, an estimate of the percentage of recovered water should be made to reduce this cost. So, if only 25% of the overall water used is dumped this cost can be similarly reduced.
This “recovery percentage” can, of course, also be applied to the cost of the initial water and, when caustics are recycled, the cost of chemicals. Finally, the energy costs required to pump the fluids through each stage of the clean need to be calculated. This can be derived from the wattages of the pump and the time taken. In practice, however, this has a very small effect on overall cost when compared to the other costs.
A 2.5Kw pump running for an hour’s cleaning cycle only has an energy cost of 35p (at 14p/ Kwh) this is likely less than 5% of the overall cost of cleaning. Any changes in pump duty to a higher pressure, but lower flow rate, are likely to balance out meaning changes in pumping costs will be negligible when compared to the other factors. When all this is added up together, we get an estimate of the cost of each cleaning cycle. This cost is a reasonable proxy measure for environmental impact.
Conclusions
Improving the water efficiency of tank cleaning operations can contribute towards an organisation achieving its sustainability targets. In some industries, like dairy and other similar processes, this contribution can be very large, while in other more modest. Spray balls and spinning nozzles are still commonly used which means there is a great opportunity for engineers to meet those sustainability targets. This is a relatively quick and painless win when trying to reduce the environmental footprint of an organisation. The really good news is that the costs of swapping can be quickly paid back and meeting the sustainability targets can also keep the purse string holders happy – a win, win situation.
