Technology for recovery

Heating of water,which is later discharged to the drain from cold to operating temperature accounts for a major part of any process bill. The cost of the energy used in this way can be 25% to 50% of the energy cost of process. Recovering this energy would therefore produce great savings.

Heat exchange technology has been available for many years but traditional plate heat exchangers are not successful when used in a waste water environment. This is due to problems associated with blocking and loss of thermal efficiency due to fouling.

However, these can be overcome by using a specialised tube in a shell heat exchanger and using such technology guarantees that systems will not block and will remain thermally efficient throughout their life of 20 years.

This technology uses turbulent flow so that the solids in the effluent prevent the tubing from becoming fouled. This, coupled with the use of specialist trash pumps, allows effluents which contain high volumes of suspended solids, chemicals, lints and so on, to be pumped successfully through the heat exchanger without the need for any cleaning. These have been installed without problems in around 2,000 plants throughout the USA and Europe.

The units are slightly less efficient than plate heat exchangers when both are clean, but average efficiencies are much greater over the lifetime of the system. We would expect a recovery of approximately 80% of the waste water energy currently dumped. The saving is 975kWh.

Direct contact water heating

Traditionally in Europe, industrial water heating has been carried out by producing steam in a pressurised boiler. This steam is then distributed throughout the factory and directly or indirectly injected into a water bath.

Boiler efficiency has improved over the years but would not be expected to be above 85%. Transmission efficiencies through long steam lines even if insulated is not efficient and a loss of 15% could be expected. This equates to an efficiency of heating at the point of heating of approximately 72%. If indirect heating is used, a further 10% could be lost. This means for every 100kW of energy used in water heating only 72kW of hot water is produced.

A revolutionary system of water heating was developed during the energy crisis of 1974 by the University of Florida under a government sponsored scheme. This technology has been refined over the years and is now available in Europe under the name of Direct Contact Water Heater.

This technology works on the simple premise of removing the indirect energy transfer interfaces and using the gas flame to act directly on the fresh water. Water is injected down one end of a cylinder and a gas flame is injected up the other.. This obviously removes the gas to steel, steel to water, water to steam and steam to water interfaces. It gives a 99.7% efficiency. In other words, for every 100kW of energy used in water heating produces 99.7kW of hot water, with a saving of 341kWh.

Flue gas heat recovery

Even with the most efficient boilers available, we are still losing 15% of energy up the boiler stack and into the atmosphere. Measure the stack temperature on a typical boiler and it will be between 200C and 250C. In the past heat exchangers have been put in this environment but suffer from the same problems as waste water heat exchangers, namely high maintenance. More importantly as they block up they cause a back pressure on the boiler which allows the efficiency of the boiler to drop off. Therefore the system is self defeating.

Using the technology of the direct contact water heater - spraying water directly through a gas flame - this energy can be recovered and converted to hot water. This has the effect of making the boiler 99.7% efficient, similar to direct contact water heating, but is only used on existing gas-fired steam boilers (natural or propane). This has the advantage of recovering energy from all the steam not just that used from hot water. The saving is 895kWh.

Vent condensing

All traditional steam boiler systems use the same principle of generating steam, using that steam in the process indirectly or directly and - in the case of indirect use - trapping the condense and returning it to the boiler feed tank. This works fine in theory if all the steam traps are serviced and operational. In practice, condense is returned to the boiler feed tank at a higher temperature than boiling point, flashes of through the vent when it reaches the unpressurised boiler feed tank. This can be seen as the "tell tale" plume of steam drifting above nearly all process factories.

This is a waste not only of energy but also of treated boiler water and it can have a serious effect on the build-up of TDS in the boiler water stream, which can have a knock-on affect on the corrosion of the boiler if not treated. Using the same technology as the waste water heat recovery, a simple tube in shell heat exchanger can be installed on the vent of the boiler feed tank and vented to atmosphere. Water is pumped through the system and condenses the flash steam which normally is vented to the atmosphere. The energy value of this is 8% in a well-maintained factory.

The advantage of these over condense coolers is that only the excess flash steam is condensed and it is self-regulating. The boiler feed tank operates at a temperature of 85C. No controls are required.The saving is 500kWh.

Summary

It is not practical to use all these technologies together but by careful analysis of:

• boiler efficiency

• boiler utilisation

• waste water temperature

• boiler feed tank temperature and

• flue gas temperature

it is possible to do an energy balance for any industrial plant or process. Having achieved this energy balance it is possible to apply the above solutions to the various problems and ascertain the most cost beneficial effective way of installing suitable equipment to recover the large amount of wasted energy. Some, if not all, of these technologies have been approved for the Enhanced Capital Allowance under the terms of the Climate Change Levy. This means in effect that 100% of the capital employed in buying, installing and commissioning any of the above systems would be recoverable against Corporation Tax in the first year.

These systems have been installed in many textile companies including Levis, Wranglers and numerous textile laundries throughout the UK and Europe. The technology has been available for the past 20 to 30 years and if the right choices are made in the selection of the equipment, the equipment itself should last 20 to 30 years.

Most if not all textile companies in the USA have some form of energy saving and heat recovery system installed. This has been normal for the past 30 years and, for some, this saving in energy cost is the difference between profit and loss. Payback is inside two years. Selection of an appropriate equipment supplier with an understanding of the technology involved in a complex industrial environment is vital. Most modern equipment suppliers will conduct a survey and produce an energy picture of the processes involved. This picture should give non-technical people a clear understanding of where money can be saved.