Maximising the efficiency of a steam-based system

19 November 2013



Richard Neale explains how following best practice to optimise the use of the energy in flash steam and condensate can produce savings at minimal cost


The laundry and rental sector has seen big advances in processing machinery and washing systems over the last twenty years. In contrast steam boiler design has changed very little and most laundries still rely on a steam boiler that is often the same one they used twenty years ago and certainly of the same design.
In this time, the only significant advance has been in the efficiency with which the boiler is used. The industry has seen a transition from laundries that relied solely on washer-extractors to plants where the workload is split roughly 50/50 between washer-extractors and tunnel washers and more recently to a state where many laundries process all work in the tunnel washer.
The result is that the twenty year old boiler now handles twice the workload that it used to do. This is because washer-extractors required regular surges in steam while tunnel washers only call for steam in small frequent bursts.
Laundry/textile rental businesses have also become more efficient in their use of condensate energy and recognised the considerable value of doing so.
The biggest source of energy loss is the boiler flue and the second is the wasted heat that results from failure to make efficient use of condensate and flash steam. Although the energy lost through hot flue gases is inevitable, energy losses through poor handling is almost entirely avoidable.

Best practice saves energy and costs
By following best practice, laundries can successfully recycle all the energy in the condensate system. This can reduce the fuel account by up to 20% and the cost involved is usually only a fraction of that needed to invest in other energy recycling systems.

Flash steam
A hard-working ironer releases its condensate through steam traps under each bed and this condensate is now a liquid at 8, 10, or 12 bar pressure with a temperature of 175, 184 or 192C. However, the condensate cannot retain its liquid form in the condensate pipework where the pressure is much lower so around 10 - 13% flashes off to form low pressure steam, typically 1 - 2 bar. The remaining 87 - 90% stays in liquid form at about 125C.
This mixture would once have been returned to the top of the boiler feed tank, where much of its energy would have been lost as a plume of steam from the large vent pipe on the feed tank. This plume was the hallmark of the steam laundry.
Most laundries now feed the contents of the condensate main to a flash steam separation vessel sited just before the boiler feed tank.
The vessel separates out the valuable flash steam so it can be re-used. If the flash steam is mixed into the feed tank its heat will make the feed water too hot to pump safely.
Once the flash steam has been removed, the remaining hot liquid condensate can be mixed with the cold make-up water to produce a boiler feed temperature of around 75 - 80C.
The feed water is now cool enough to pump safely and there is now room in the feed tank for more recovered heat, for example, heat from the boiler blow-down.
The separation of the flash steam also means that there is no trademark plume of steam coming from the roof of the boiler-house, providing visual evidence that the system is working. If a plume of steam is visible, this means that there is a problem in the system, such as steam traps that are leaking live steam into the condensate.

Keeping the heat in the condensate
As the condensate heat can be worth up to 20% of the fuel account it is sensible to conserve this with insulation. The condensate main should be protected in exactly the same way as the main steam supply to the processing equipment. Typically, at least 50mm of resin impregnated fibreglass with aluminium backing foil should be applied to elbows and joints, to the flash steam separation vessel and to the large float trap or condensate pump used to divert the liquid condensate to the boiler feed tank.
The losses from the boiler feed tank itself need to be controlled with at least 75mm of insulation on all four sides plus top and bottom.
It is vital that this insulating material is not allowed to become damp, because a continuous moisture track through the insulation will cause it to lose over 90% of its insulating properties.
The pipework from the feed tank to the boiler feed pump requires at least 50mm insulation. This pipe carries an intermittent flow of hot water to the boiler and it is important to retain as much heat as possible to avoid injecting a slug of cold water and so inflicting a thermal shock on the boiler.

Making optimum use of flash steam
Flash steam carries latent heat, that is the amount of heat released when it is condensed. For example, flash steam at 1.5bar will have a latent heat of 2,181kj/kg of flash steam. This compares with 1,972 kj/kg for steam at 12bar. Flash steam actually contains more heat than high pressure steam but the flash has a much lower temperature, 127C compared with 192C for 12bar steam. So although flash steam contains a considerable amount of energy, this cannot be used for heating an ironer or tumble dryer.
Flash steam will directly reduce the use of mains pressure steam (ie, in like-for-like amounts) if it is used for direct heating of wash water or in a boiler feed pre-heater.
Modern tunnel washer designs allow either mains or flash steam to be injected into the hot wash compartments. However, to use flash steam it may be necessary to fit enlarged control valves and injectors to cope with the considerably higher volume flow of the low pressure steam. Most suppliers of tunnel washers now offer this as a low-cost option for a new machine, enabling this to be run on "free" recovered heat.
So the tunnel washers, which are the main drivers of turnover in a modern laundry, can be run on very little energy, because the flash will provide over 90% of the typical requirement. The remaining top-up can be dribbled in from the main steam supply via a reducing valve, set to open whenever the flash supply pressure drops below around 1.5 - 2.0 bar.
For a washer-extractor plant, the flash should be injected into a new hot water system running at about 70C. This can be blended with cold to make the pre-wash dip and used unblended for the main wash.
Although it may be possible to maintain this with flash alone (by careful tuning of the dip levels), it works best with a mains steam top-up under thermostatic control. The advantages of this type of system include the ability to combine it with last rinse recovery to achieve water savings of around 30% and an improvement in washroom productivity as it eliminates the time needed for warm-up.
Using the flash steam for boiler feed pre-heating requires the purchase of a small heat-exchanger to be fitted between the boiler feed pump and the boiler. Once the feed water is downstream of the feed pump it will be at boiler pressure and in a temperature range of 70 - 90C. It can then be heated to 115 - 120C, using the flash steam from the flash separation vessel.
This is one way of raising the output rating of a boiler (and hence the amount of laundry equipment that it can support), thus avoiding the need to invest in a new boiler.
Boiler feed water pre-heaters work best with modulating boiler feed pumps, as these pumps continuously control the flow rate to match instantaneous boiler demand, rather than simply switching on and off. The cost of the pump can often be justified by arguing that it will avoid the need for purging whenever the boiler reaches top pressure and switches off the burner.
Using flash steam to pre-heat the hot wash compartments on the tunnel washer will allow other heat recovery systems (such as recycling heat from hot liquid laundry effluent) to still be used to warm the cold soft water supply to the tunnel washer. There is not much point in using valuable flash heat to warm the cold water because only about 40% of heat recycled into the cold feed water results in energy savings in the hot wash.
Injecting flash heat directly into the tunnel washer leads to a 100% saving of the energy it represents.

Blow down heat recovery
Modern boilers need a continuous bleed to drain off the very hot water from the boiler. This prevents impurities concentrating in the boiler water that can result in foaming, priming of the boiler and wet steam.
The heat in this stream running to waste can be cheaply and simply recovered with a separate tiny flash vessel and heat-exchanger. These are sold as simple proprietary kits by boiler equipment suppliers. They recover the heat into the boiler feed tank, which is why it is important to create this recovery capacity when designing the condensate system. They also improve boiler economy and increase boiler capacity, typically by 2 - 4%.
Energy costs are set to continue their former rate of increase as the world comes out of recession, so following best practice to conserve energy is essential for laundry/textile rental businesses.
Before considering further investment, look carefully at energy strategy and see how better use of existing equipment can become part of a five-year energy plan.

BOILER FEED: The boiler feed pre-heater condenses the waste flash and uses this to raise the feed temperature



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