For over 100 years the boiler house has been centre stage in the modern industrial laundry, historically separating the professionals from the amateurs. This is now changing rapidly, with a large proportion of new builds and revamps using distributed energy systems and saving a significant chunk of investment capital in the process. This month we look at why this is happening and what the true benefits can be.
The modern steam boiler
It only seems a few years since every industrial laundry was constructed and constrained by a large steam boiler with a basic combustion efficiency of around 80%, meaning that the other 20% went out of the boiler chimney as waste heat. Only 80% of the power in the fuel purchased was converted into heat energy in a high-pressure steam main, which circled the plant. The latest designs, featuring four passes through the heated zone and flue gas heat recovery have improved the efficiency to over 90%, provided the maintenance regime is improved to achieve this consistently. Effective maintenance has long been the Achilles heel of the laundry sector, which at times has hindered the reliability of the entire laundry.
Laundries based on washer-extractors demand a central boiler with twice the capacity of the average steam requirement, solely to cope with the periodic surges in steam demand. These occur whenever more than one washer-extractor calls for steam simultaneously and cause a serious drop in steam pressure whenever the maximum rating of the boiler I exceeded, even for a short time. This can then cause ironers and steam-heated dryers to malfunction and production is seriously disrupted for a considerable period. Meeting the need for a large boiler is expensive and the investment has long been regarded as ‘dead money’ by forward thinking launderers.
Distribution design and efficiency
The central boiler investment is increased by the need for a high-pressure steam distribution system and condensate collection, each with appropriate modern insulation to prevent energy wastage. Leaks and heat loss from poorly insulated systems still account for a further 10% increase in the fuel account in many laundries. Even small and inevitable losses result in condensate accumulating in the steam mains, which has to be collected and trapped out to prevent operational issues arising. Looking back, it seems surprising now that the central steam boiler house has survived for so long! The different approaches to boiler house maintenance and management taken by individual laundries have accounted to a large extent for the significant variations in energy efficiency (and operating costs) from site to site.
One striking example of this lies in the differences in the handling of flash steam amongst laundries. When steam is condensed, the hot condensate gives up its heat, but it is still at steam pressure. When it passes through the steam trap into the condensate main, a portion of this re-evaporates to form low pressure steam, which is still released to atmosphere in some laundries, even though it can represent 10 – 14% of the fuel account! Recovery and re-use require careful laundry engineering, which has sometimes been lacking.
The energy saving advantages of direct gas firing
Direct gas firing has now become the favoured mechanism for heating tumble dryers and it is steadily gaining ground in the proportion of new ironers which use it. In the gas-heated tumble dryer, all of the products of combustion flow into the dryer airstream, so there is none of the flue gas loss found in the steam boiler (where some 20% of input heat energy is lost to the chimney). The difference becomes even more stark when the heat actually released from the high-pressure steam is taken into account. Only the latent heat portion of the energy in the steam is released in the dryer heater battery, leaving typically 10 – 17% in the liquid condensate to be returned to the boiler house via the condensate main (unless the flash steam associated with this is vented to atmosphere, in which case less than half of this heat is actually recovered for re-use).
The arithmetic for the gas-fired ironer is slightly different, because at present it appears that most gas-fired ironers operate indirectly. The gas is burned to heat thermal oil, with a typical thermal efficiency of 90-93%, which is then circulated through ironer chests where it gives up its heat. Despite this loss, the overall energy efficiency of the gas fired ironer is still significantly better than that of its steam-heated equivalent. Design of the ironer bed needs a different design to that made for steam heating, with the most effective designs achieving turbulent flow of thermal oil over all of the heating surface.
Operational flexibility
One big attraction of the move to distributed heating for the dryers and ironers is the ability of the Laundry Manager to operate part of the laundry without having to start up central boiler and run this (inefficiently) at reduced output for, say, an evening shift which is just washing and drying towels. With a distributed gas supply to each dryer and ironer, the laundry only pays for energy to the plant which is actually working. The significant standing losses from a central boiler are eliminated.
Another attraction is the removal of dependence on a single central boiler, where a single failure can delay the daily start-up by four hours or more. A laundry can usually survive and meet its daily demand with one dryer, or even one ironer, out of action, but failure of single central boiler brings the whole plant to a standstill.
Gas firing usually brings much greater flexibility with regard to operating temperature, because the exit temperature from the burner in the dryer and from the oil heater for the ironer are thermostatically controlled. The thermostat settings can be varied both up and down and this has resulted in much greater productivity for cotton towels and cotton sheets, for example. The downside of this has been the temptation to raise ironer temperatures for cotton-rich flatwork, which has often been counterproductive, if the safe working temperature for cotton-rich has been exceeded in a mistaken focus on productivity alone. The resultant distortion of sheets and pillowcases has led to severe customer complaints about sheets, pillowcases and duvet covers which no longer fit the bed, pillow or duvet!
Planning for the future
There are two scenarios for the transition from the central boiler house to distributed energy systems. The first involves the best way of converting a traditional central system to one based on individual gas heating. Early pioneers of this were sometimes insufficiently ambitious. Consider, for example, an historically ‘large’ steam laundry processing 300,000 pieces per 5-day, 60-hour week, with 20,000 kg/hr central steam raising capacity. Simply converting all ironers and dryers to direct gas firing would achieve considerable economy in boiler flue losses, steam distribution and condensate heat recovery, but the boiler would now have been operating less efficiently, at a fraction of its rating, serving the washer extractors, garment finishers and office heating. Market leaders recognised that with the elimination of the limitation imposed by boiler capacity, the average laundry could be profitably expanded (sometimes by two or three times). Several laundries moved from 300,000 right up to 1m pieces per week, with very significant reductions in unit production costs and carbon footprint. Washer-extractors and the peaks in steam demand imposed by these were largely eliminated from future planning, speeding the transition to plants based entirely on tunnel washers. Reliability of plants which originally had two large central boilers was improved by keeping both boilers and operating them efficiently, near to high fire, to support the greatly increased plant capacity. The thermal efficiency was improved further by the removal of the surge demand of the previous washer extractors.
The second scenario features the brand new, greenfield site laundry with no previous boiler house investment to have to utilise. All that is needed here is either a very small boiler room containing a pair of steam generators or a small steam generator alongside the tunnel washers and another alongside the garment finishers, with no boiler room at all. The attraction of steam generators operating with a steady steam demand is the small area these vertical units occupy, the high thermal efficiency (over 93%) and the removal of any need for condensate return. Free steam injected at the entry to the tunnel finisher and direct steam injection into the tunnel washers allows 100% utilisation of the heat energy in the steam, giving a much-improved overall carbon footprint for steam raising and usage.
Conclusion
The inescapable conclusion from the points made in this article is that the days of the central boiler house are almost certainly numbered and the benefits from this will be reflected in significant reductions in the carbon footprint of more laundries over the next few years. We are looking here at eliminating the flue loss of around 20% of the energy in the fuel purchased and replacing this with much lower losses from thermal oil heaters and small steam generators. The biggest savings will come at tumble drying, where the entire gas energy purchased will be used in the drying process.
In some plants, the associated benefit of increased capacity of the dryers will sometimes be worth even more than the financial saving. This can be maximised by routinely fitting every gas dryer with termination controls, to finish cycles automatically once the desired dryness has been reached. This benefit is usually accompanied by an improvement in towel colour, because these will eliminate over-drying and the greying associated with this.
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