Gas price crisis calls for shrewd investment12 April 2022
It is time to get savvy on minimising laundry gas demand in order to set your own cap on prices, says Richard Neale of LTC
In last month’s issue, we looked at steps that could be taken to reduce gas demand in a laundry without any significant investment, drawing attention to the fact that there is a wide disparity between the most energy efficient plants and the least, even between those with apparently identical equipment! This month we assume that all of the low-cost and nil-cost steps have been taken and the low hanging fruit has all been picked. For flatwork plants, this means that your energy consumption is now at or coming down towards or below 1.0 kWh/kg of dry textiles. For garment plants, it may be a little higher – say, 1.1 - 1.2 kWh/kg. Getting to this stage will have involved considerable time and effort by your production managers, supervisors and laundry engineers. This month we look at actions that are less labour intensive, but much more capital intensive. Getting the first part right is essential to minimise any capital expenditure and this month we now look at the investment options and getting your strategy right.
If you are still operating with a central boiler house (rather than using direct gas firing, thermal oil heating and distributed steam generation at point of use) and you intend to continue with this for at least the next four or five years, then you may have available two valuable sources of waste heat. The first is the boiler flue, which is emitting around 16% of the heat energy for which you are paying in the boiler fuel, and which is continuously being discharged to atmosphere. The second is in the plume of steam which may be coming from the vent on the boiler feed tank, representing unrecovered flash steam from the condensate main, together with live steam leakage past poorly chosen or poorly operating steam traps. This usually represents over 10% of the energy in your boiler fuel.
Heat recovery from the boiler flue
There are two types of system for recovering flue gas heat: the first involves a commercially available, flat tubular exchanger which slides across the boiler flue. Either clean or recovered water is pumped through the tubes which get warmed in the hot flue gas stream and so raise the water temperature. The flue needs to be widened just at the point of insertion, so as not to form a barrier to the flue gas flow. If recovered water is to be heated, then be sure that the water system includes an effective lint filter that is preferably self-cleaning. The maximum temperature achievable is often only 40C, but if this is insufficient, it can be increased by careful tube design. The thermal efficiency of this recovery system should approach 50%, so if you can use all of the recovered heat, you should drop your boiler fuel cost by around 8%; not a massive saving, but still well worth having.
The second system uses a spray tower, built into a section of the boiler flue. Softened water could be used to form the spray of liquid droplets distributed across the flue cross-section, rather like a 'drench' shower head in a domestic bathroom. Depending on the design, especially the length of flue available for the recovery, it should be possible to achieve a thermal efficiency of over 50% and increase the water temperature towards 70C (by reducing the flow down to the minimum flow the washhouse requires).
What about a stack economiser?
Flash steam recovery into wash water or tunnel washer main wash zone.
If your finishing equipment is still largely steam-heated (rather direct gas firing), then unrecovered heat coming off the boiler feed tank typically represents 10% of the laundry boiler fuel account. If there are leaking steam traps, then this can easily rise to 15 – 20%! Locating and replacing the leaking steam traps gives by far the quickest payback and a regular in-house trap monitoring system is an essential pre-requisite. Do not consider any larger investment until this is in place!
Once you have got the wastage down to 10% of the fuel account by eliminating the leakages, then you have further choices for wash-water pre-heating. You can either inject the flash steam directly into an intermediate soft water (or recovered water) storage tank; or you can inject it directly into your tunnel washer. If you choose to create a hot water storage tank, for your washer-extractors, then this can be used for the pre-wash and hot wash to reduce or eliminate steam injection and warm-up times. If you are already practising last rinse recovery on your washer-extractors, then you would use the heated water for the last rinse, gaining an additional advantage in the speed of ironing or conditioning. If you choose to inject the flash steam into the tunnel washer directly, then you need to design the system so that there is an automatic mains steam back-up permanently available, which is not difficult. You will also need to increase the size of your injectors and pipes, if you are operating at above 40C.
Whichever usage you choose, you may need to install a small flash steam separation vessel in the condensate main, to provide either a very low-pressure supply of flash steam to the hot water tank or a slightly higher one for direct use at the tunnel washer.
Waste wash water heat recovery
If you are still operating with main wash temperatures above 40C, then there is likely to be appreciable heat in the flow to drain. This could be at over 60C if you fit your washer extractors with dual dump valves and put in a separate drainage system to a collecting tank. It could be at around 40C if you are working with a tunnel washer. You could use the heat from either of these streams using well-proven proprietary equipment to create a flow of soft or recovered water at 25 - 45C, depending on your waste temperature. If you are using last rinse recovery and washing at 40C, then you should consider using warm water at 40C for all washes and rinses, which will save you the time and energy needed to warm the machine up from cold before every cycle.
Tumble dryer recycling
Tumble dryers can be inefficient users of energy.
Recent design innovations have seen the introduction of movable flaps operating to use increasing amounts of recycled air as the moisture content of that air diminishes within the load.
Tumble dryer endpoint detection
Automatic cycle terminators are now available for most dryers, usually relying on advanced infra-red detectors gauging the moisture content of the drying textiles. These can usually pay for themselves within twelve months, from the energy saving alone, but big benefits also come from improved dryer productivity, towel quality and textile life.
The ironer exhaust carries all of the water vapour from the drying textiles being finished, along with some hot air. The airflow from an ironer is quite low (it is only there to provide a vehicle for the water vapour) which makes the thermal efficiency of the ironer much higher than it is for a tumble dryer or tunnel finisher. There are different systems available for recovering this heat as part of an integrated heat recovery system
Tunnel finisher recycling
Tunnel finisher technology and design is also available to include air recycling and in addition to this the recovery of energy from the exhaust of a garment tunnel finisher can also be achieved. It uses a semi-permeable membrane to extract the water vapour from the exhaust airstream. This can be used to create a continuous high temperature water stream for the washhouse. This unit is still being developed, but initial results of projected savings could make it a game changer in the battle to further reduce significantly the kWh of energy required per kg of garments processed by the laundry.
Overall strategy – get the balance right
There is an array of options available for energy recovery and re-use in a modem laundry and success depends on making the right choices or combination of choices. There is no universal 'best' design, because every laundry is unique and the optimum for one could be a waste of precious investment capital in another. Here is one suggested route out of this maze:
Check that you have already implemented all of the low -cost or nil-cost actions suggested in the previous issue of LCN, which should have already reduced your energy demand down towards 1.0 kWh/kg dry textiles. Seek urgent help with this if you need it, because energy prices are set to double, and this step is not that difficult.
If you are operating with a central boiler and intend to continue with this, then get your boiler efficiency, system insulation and condensate recovery optimised, and plan your flash steam separation.
Get your Engineers and Production Management together and draft an outline of how you might best combine some of the systems described in this article to effectively halve your energy consumption. Then get some detailed quotes for the installation feasibility, capital cost and payback. Check that the payback only includes the energy that you can productively use - not necessarily the saving claimed by the supplier.
Then revise your draft plan and seek expert help if necessary. You should end up with a plan for, as a minimum, heating the wash-water at nil cost in prime energy, plus a scheme to reduce significantly the energy demand at the tumble dryers. The costs might seem high, but this step will enable you to survive in a marketplace where only those who get this right will prosper.