With a worldwide overcapacity of steel production it is important that each thermal system for a continuous coil coating line is tailored to the exact needs of the user to ensure maximum productivity, highest quality and minimum overhead cost with the minimum necessary capital expenditure.
Successful implementation of anti-dumping regulations has seen many countries benefit from protection against cheap imports, however, good equipment selection is still imperative to ensure that the competitive advantage is maintained while at the same time ensuring a high quality product.
It is important to think carefully when compiling a specification for any capital equipment. In particular, the thermal equipment associated with a continuous coil coating line has to be specified with the greatest of care such that it can accommodate the requirement of both today’s and tomorrow’s product ranges and emissions criteria.
There are many different options that can be selected to build up the most optimised thermal system for a continuous coil coating line for a given application.
The most optimised system is determined by many factors and there is no one solution that will best satisfy all plants. Each customer is unique due to geographical location, size of factory, product type, production rate, available budget, skill level of operators, quantity of staff, environmental legislation and in-house regulations.
A typical continuous coil coating line will comprise the following main items and it is the responsibility of the thermal engineer to size and then integrate these pieces of equipment into a safe working thermal system that meets all the local health & safety and emissions legislation and the production requirements of the end user while at the same time installing best available technology to ensure maximum energy efficiency.
One of the first issues to consider is whether or not local regulations require destruction of volatile organic compounds (VOC’s), emitted from the continuous coil coating line. In many countries there is a requirement to destroy volatile organic compounds (VOC’s) within a given destruction efficiency and hence a method of VOC destruction is required.
Even in countries where VOC destruction is still not a legal requirement it would be wise to install VOC destruction equipment as emissions legislation is rapidly being harmonised globally and any plant not fitted with a VOC destruction system could be left wanting within a few years of installation.
There are many types of systems available for the removal of VOC’s from a paint curing oven exhaust stream but by far the most proven and reliable system for a solvent based coil coating line is by thermal oxidation.
To adequately destroy the VOC’s, the contaminated exhaust stream has to be held at a minimum temperature of 760oC for a minimum of 0.5 seconds. However, as emission regulations are becoming more stringent it is common for thermal oxidisers to be designed with dwell times of up to 1.5 seconds.
There are two commonly accepted types of oxidiser that are used on coil paint lines; namely a recuperative thermal oxidiser and a regenerative thermal oxidiser.
The recuperative thermal oxidiser uses an air-to-air heat exchanger of metallic construction (usually referred to as the primary heat exchanger) to pre-heat the curing oven exhaust gases before they enter the VOC destruction chamber.
Pre-heat temperatures can vary dependent upon the size of primary heat exchanger used and typically range from 300oC to 500oC.
The higher pre-heat temperatures, although requiring more capital expenditure, do reduce fuel consumption and overall plant running cost.
After pre-heating the oven exhaust gasses in the primary heat exchanger the exhaust enters the oxidiser VOC destruction (or dwell) chamber via the burner chamber where it is heated to 760oC and held for the required dwell time.
In a regenerative thermal oxidiser (often referred to as an RTO) the oven exhaust gases flow over hot refractory bricks prior to entering the VOC destruction chamber. The direct contact between the exhaust stream and the hot refractory bricks provides a very efficient method of pre-heating the oven exhaust gases and pre-heat temperatures close to the oxidation temperature of 760oC can be achieved.
On a typical coil coating line application there will be sufficient solvent within the oven exhaust air stream for the auto-ignition of the solvent to generate sufficient energy to maintain the minimum required oxidation temperature without the burner being switch on.
This point is referred to as the Auto-Thermal condition.
After passing through the dwell chamber of the regenerative oxidiser the gases then exit via a second canister full of refractory brick identical to the first. As the exhaust gasses flow over the bricks, heat will be transferred from the exhaust gases into the bricks.
After a period of about 90 seconds the bricks in the first canister will have reduced in temperature and the bricks in the second canister will have become hot and therefore the direction of the flow of the exhaust gases is reversed such that the bricks in the second canister are used for pre-heating the oven exhaust gases while the heat generated by the oxidation process is used to start re-heating the bricks in the first canister.
This cycle will continue to repeat itself during the operation of the regenerative oxidiser.
Due to the strict method of stack emission sampling used in some regions such as Europe and Australia, a third canister of bricks has to be added to the regenerative oxidiser to avoid the ‘spike’ of VOC contaminated exhaust that occurs during the flow reversal process.
However, in some countries such as the USA the sampling method differs and although the overall VOC emission limits are the same as Europe the ‘spike’ is not detected during sampling and thus a two canister regenerative oxidiser can be used.
If we assume that a three canister regenerative oxidiser is required (which is the most likely scenario) then the capital cost of a basic recuperative thermal oxidiser (i.e. no additional heat recovery to supply systems outside the coil coating line) that is used with a direct fired convective oven will be approximately half the capital cost and a regenerative thermal oxidiser used for the same application.
However the thermal efficiency of the regenerative oxidiser is around 95% and when compared with the thermal efficiency of a basic recuperative oxidiser which is less than 50% (again assuming no external heat recovery systems) then the overall running cost of the regenerative oxidiser based coil coating line will be on average 30% less and thus the initial extra capital cost of the regenerative system can be recovered within a relatively short period of time.
It is a common misconception that regenerative thermal oxidisers are much larger than recuperative thermal oxidisers because although regenerative thermal oxidisers are taller they do not have a footprint any larger than recuperative thermal oxidisers when the primary heat exchanger is included within the dimensions of the recuperative system.
Taking into account the thermal efficiency and the short payback period of the regenerative thermal oxidiser the regenerative thermal oxidiser based system is often the system of choice.
We should however mention that there is a well proven recuperative oxidiser based system available that takes advantage of the poor thermal efficiency of the recuperative thermal oxidiser in a way that allows all burners on the coil coating line with the exception of the oxidiser burner to be removed.
The system uses the high temperature recuperative oxidiser exhaust gases to indirectly pre-heat clean air to 650oC. The pre-heated air is subsequently used to provide all the heating requirements of convective ovens and then additional heat recovery is used to provide hot water for heating of the cleaning/pre-treatment section holding tanks.
Thus neither the oven(s) or the cleaning/pre-treatment section require the installation of burners as all the heat needed to run the line is recovered from the oxidation process of which the majority of the heat used comes from the combustion of the solvent that has been evaporated from the paint coating. This system is highly thermally efficient (assuming minimal duration at standby) but would normally only be applied to faster lines.
With the method of thermal oxidation now in place the thermal system can then be designed around the type of thermal oxidiser used. The next decision to make is what type of paint curing oven to use.
Disclaimer: This information is for general information purposes only and should be viewed as such. For detailed, precise information for your upcoming coil coating line, its best to speak to a Bronx Thermal Technologist.