Extracting efficiency from washers

30 November 2000



Integrated heat and water recovery will allow washer- extractors to profit from recycling says Richard Neale.


Continuous batch washers have efficient internal recycling which reduces their water consumption to less than a third of that required by an equivalent bank of washer-extractors and cuts the steam consumption by a half as well. How can a garment rental operator who only has washer-extractors hope to compete with a competitor who is now processing garments through a continuous batch washer?

The answer lies in the use of an integrated heat and water recovery system, which brings a simple form of recycling to washer-extractor based washhouses. But how does this work and what are the benefits?

Last rinse

The scheme shown in the diagram is based on simple recovery of last rinse water from non-coloured work, coupled with flash steam separation from the condensate main. The proven economies from this type of design are a 30% reduction in steam for washing and a 30% reduction in water. While these savings are not as great as can be achieved with a continuous batch washer, they can be installed relatively cheaply with a payback period of around 12 months, especially if the majority of the installation is done inhouse.

The liquid condensate in the condensate main was originally a high pressure liquid at a temperature of around 170degC. As it passes through the steam trap, the pressure reduces and some 15% of the liquid re-evaporates to form low pressure flash steam. If the entire contents of the condensate main, liquid and flash steam together, are returned to the boiler feedtank, then most of the flash is lost via the vent from the feedtank and can be seen as a plume of steam above the laundry

When steam traps start to leak, the plume of steam becomes even larger. So the steam saving in this scheme comes from separating out the flash steam from the condensate main and using this to generate washhouse hot water.

Water is saved by collecting the last rinse liquor through separate dump valves and drain lines from each machine, filtering out any lint by using a wedge wire screen and then pumping this liquor into an overhead storage tank. The flash steam is injected into the tank to create warm recovered-water and used to maintain a gravity-fed supply, through a second recovered-water inlet, to each washer-extractor.

Flash steam

Separating out the flash steam requires a single flash-steam separation vessel. The flash is taken off a line from the top of the vessel (typically 60-70mm). The liquid condensate is trapped out at the base for pumping to the boiler feedtank as at present. It is important to site the flash vessel so as to pick up as much condensate as possible whilst still minimising pipe runs to the feedtank and recovered-water tank.

It should not be necessary to dig up a laundry floor, in order to recover final rinse water from a second set of dump valves. Most washer-extractors can accept a second dump valve and a piped offtake in plastic piping laid on the floor behind a row of machines. These can usually be ducted into a common manifold and dropped into a small pumping pit via a sloping wedge wire screen that discharges any lint continuously via the overflow to drain. It will be found that if the last rinse liquor is collected for reuse, there is usually a small overflow to keep the filter clear and the pit sweet.

The recovery water in the pit is pumped up to an overhead tank, usually sized to give sufficient capacity for wet-out, first wash and main wash simultaneously, for about a third to a half of the washer-extractors installed.

Most washer-extractors are equipped with two inlet valves and it is the second of these which should be piped into the gravity-feed system from the recovered-water tank. Do not stint on pipe diameter here, as it is much quicker to gravity fill from a 75mm main from which 50mm branches are taken.

The diagram shows the principal valves and connections. The only controls and indicators needed are safety valve and pressure gauge on the flash vessel, a temperature indicator on the warm recovered-water supply and a high level/low level control for the recovered-water pump from the pumping pit in the floor. This pit will obviously have to be sized to accept two or three rinse dips simultaneously to avoid unintentional overflow.

Providing warm recovered water will reduce warm-up times both for the first wash and the main wash. If the soiled work carries heavy protein staining such as abattoir blood, then it will be necessary to control the incoming temperature, possibly by blending recovered water with mains water for wetting-out. This is generally more cost effective than deliberately keeping the recovered-water temperature below 38degC (the temperature above which stain-setting accelerates).

It will be found that the recovered-water temperature will usually rise above 60deg C and get the main wash off to a flying start, saving useful process time.

If the boiler is already struggling to cope with the odd peaks in demand from two or three washer-extractors calling for steam simultaneously, then having a warm recovered-water supply will greatly reduce these peaks and enable the laundry boiler to support significantly more plant than it does at the moment.

Some laundries have found that a struggling boiler can actually supply double the throughput with this type of system, allowing scarce capital to be put to better use with productive plant rather than spent buying bigger boilers. In this respect, the greatest benefits are achieved by putting a throttling valve in the steam main, because then, all that should be needed in future will be steam injection to maintain wash temperatures rather than to raise them from cold.

Even in today’s competitive market it is possible for smaller rental operators to trim processing costs and offer quite competitive prices in the face of competition from operators with continuous batch washers. When the potential quality benefits of a washer-extractor based process are combined with this improved cost control, it will be found that it is quite possible to maintain a competitive edge whilst deferring further investment until volumes rise.



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