Reducing heating losses from process solutions in the surface treatment industries.
Best Available Technique (BAT)
It is BAT to reduce heating losses by:
- Seeking opportunities for heat recovery.
- Reducing the amount of air extracted across the heated solutions.
- Optimising the process solution composition and working temperature range. Monitor temperature of processes and control within these optimised process ranges.
- Insulating heated solution tanks by one or more of the following techniques:
- Using double skinned tanks
- Using pre-insulated tanks
- Applying insulation
- Insulating the surface of heated tanks by using floating insulation sections such as spheres or hexagonals. Exceptions are where:
- Workpieces on racks are small, light and may be displaced by the insulation.
- Workpieces are sufficiently large to trap the insulation sections (such as vehicle bodies).
- The insulation sections can mask or otherwise interfere with the treatment in the tank.
It is not BAT to use air agitation with heated process solutions where the evaporation caused increases the energy demand.
Brief technical description
It is normal practice to minimise heating losses from process solutions but actual techniques used may depend on the options to re-use heat, the availability of renewable energy supplies and local climatic conditions.
Temperatures of heated processes can be monitored manually or automatically (according to the size and energy demand of the vat being heated), with automatic and/or lockable controls.
Energy losses from the surface area of heated process solutions related to processing temperatures demonstrates that the highest energy loss occurs from the solution surface with air extraction and with liquid agitation. Air extraction above the surface of process solutions enhances evaporation and thus the energy loss. Techniques to reduce the volume of warm air extracted and reduce energy losses by evaporation.
Where there is a temperature range for a process, the temperature can be controlled to minimise the energy input:
- operating temperature of process solutions that require heating may be reduced,
- processes that require cooling may be operated at higher temperatures.
Heated process tanks can be insulated to reduce heating losses by:
- using double skinned tanks
- using pre-insulated tanks
- applying insulation.
Floating spheres are widely used to insulate the solution surface without restricting the access of workpieces or substrates. They allow jigs, barrels, coils or individual components to pass between them.
Process solutions may be heated by energy coming from process steps generating energy. Water from the cooling circuit of various process solutions may be used to heat lower temperature solutions, incoming air, etc. Alternatively, the hot cooling water is collected in a central tank and cooled through a suitable heat pump. The gain in energy may be used to heat process solutions with process temperatures up to 65 °C, or to heat up water for other purposes.
Achieved environmental benefits
Seek technical support when changing operating temperatures changes to processes.
To all heated solutions.
Reducing the operating temperature of solutions will depend on support from the proprietary process suppliers or in-house expertise in developing solutions or processes that are viable at lower or higher temperature ranges. It may also be a factor in choice of process solution chemistry.
Many solutions have a narrow operating range, and cannot be operated outside of these. Other optimal operating factors may have to be considered, such as processing time.
In anodising, the heat of spent seal solutions can be used to heat the water used for a new sealing process, using a heat exchanger or piping the incoming cold water through the hot seal solution.
In automatic lines, floating spheres may be carried to the rinsing tanks by barrels or by components. The spheres may block pipes and cause malfunctions for pumps and transport tubes. This can be limited to some extent by the choice of size of spheres and installing simple coarse screens to critical pipework and equipment. The spheres can cause tidiness problems in the workplace by being carried outside of the tanks. The system can be used in manual lines, and in automatic plants.
Attention needs to be paid to energy efficiency in all installations using air extraction. Process control is feasible for all installations. Other options will be site-specific.
Where the processing line is enclosed, the maintenance of the plant and solutions may become more complicated and time-consuming This technique is likely to be most effective with new installations, rather than retrofitting.
Applicable to all heated solutions.
Floating spheres are cheap.
Capital investment for sophisticated heat-exchange systems may be high.
Driving force for implementation
Cost saving and process quality control.
Surface treatment of metals plants
REDUCTION OF THE VOLUME OF EXTRACTED AIR
The most common system uses extraction hoods located laterally to the entrance area for plating jigs on flight bars and plating barrels above processing vats.
The efficiency of the air extraction is determined by the minimal air velocity (vx) necessary to capture the uprising vapour, fumes or aerosols at the most distant point from the extraction hood.
There are three options to reduce the volume of extracted air:
- Reduction of free surface area above tanks : Lids hinged to the tank, driven individually and automatically opening and closing when jigs and barrels enter and leave the processing tank are appropriate but more expensive design. Usually this system is combined with a device designed to automatically increase the volume of extracted air when the lids are opened. A reduction in extraction rate of up to 90 % may be achieved.
- Push-pull system : This method is designed to create an airflow over the surface of the processing bath. It works with an extraction hood opposite a blowing duct. The surface of the processing solution must not have any frame or obstacle to the airflow. Therefore its application remains quite limited.
- Enclosure of the plating line : Recently, the complete segregation of the process plant has been achieved in some installations. The plating line is installed inside an enclosure, while all plant operations, the plant management systems, and the loading/unloading stations are located outside. Since a substantial amount of extracted air is still necessary to prevent corrosion of the equipment within the enclosure, an energy saving higher than the figures for other techniques cannot be expected.