The Viable Alternative

A new breed of batch heaters from Mixlance

Mixlance have developed a new breed of batch heaters designed to mix the complete range of coated products including hot-rolled asphalt (HRA) and heavy-duty materials in meaningful quantities. The plants are proving to be a viable alternative to conventional asphalt plants, operating under the high demands of major contract requirements but with the proven flexibility to supply local authority wok and the ‘call and collect’ market.

Asphalt plants with large rotary dryers and burners, large exhaust systems and integral screening towers will always play a vital role in the production of hot asphalt in large quantities for contract work, but in many cases these plants have been adapted to also produce a wide range of low-temperature materials for the local sales environment.

Under these trying conditions asphalt plants are well able to cope with high-temperature mixes with wet sand and rock fines, but are stretched when handling many low-temperature materials. When the burner is turned down to match the lower mix temperature, heat transfer within the dryer is considerably impaired and can lead to excess filler problems, high fuel consumption and unhealthy exhaust gas conditions being presented to the exhaust plant, causing condensation in the filter.

In addition, when a typical 150 tonnes/h asphalt plant is in normal full-load production, some 15 tonnes of stone and sand is ‘in flow’ through the drying and screening sections of the plant and a further 50 tonnes may be held in the hot bins awaiting final mixing. With these quantities in transit it is very difficult to match the draw from the cold-feed system with the final product mix demand and invariably excess material needs to be purged from the mixing tower.

Heat-balance tests on established plants have shown that upwards of 20% of burner fuel may be lost because of these operational problems during a typical day’s coating programme.

Conversely, batch heaters have traditionally only been considered by producing low- and mid-range coated products in small quantities from low-moisture aggregates. This design was invariably used in conjunction with conventional quarry plant, especially where a pre-dryer was installed to dry the aggregates before screening. This ensured that most of the aggregates going forward to the heater were already well dried and heated.

As a general rule, relatively small burners and exhaust plants were installed which could not cope with hot mixes like HRA and heavy-duty products because of the moisture problems involved. Batch heaters in their traditional form were flexible and reasonably efficient but were never considered suitable for the production of the wider range of coated products and thus almost became a design of the past.

It had been noted, however, that although the design of batch heaters restricted their flexibility to produce a wider range of coated products, they were nevertheless reasonably thermally efficient, having a wide-angle radiant flame zone always in intimate contact with the product. In addition, heat losses were minimized because only the aggregate batch weight needed to be preheated before being immediately transferred into the mixer for final coating.

In recent years the industry’s need for an all-purpose urban plant design has brought new challenges to bear on plant manufacturers. Urban plants are often sited in small but remote locations, away from other coating plants, to serve a local market and therefore need to be flexible enough to produce the whole range of heavy-duty and high-temperature products as well as conventional low-temperature materials.

They are also required to operate with little or no wastage to prevent environmental problems and to minimize the operational costs of both fuel and power.

Inspired by these challenges, Mixlance have developed a number of techniques and improvements to allow the entire range of coated products to be produced from a single batch-heater installation, offering a viable alternative to conventional asphalt plant designs. This can be achieved with less plant wear and maintenance together with little or no aggregate wastage, resulting in considerably lower fuel and electrical costs.

A range of plant sizes is available from Mixlance, from a low-level modular construction capable of 70 tonnes/h of low-temperature materials and 30 tonnes/h of HRA, up to a twin 5-tonne batch heater capable of 300 tonnes/h of low- and medium-temperature materials and 180 tonnes/h of HRA.

During design exercises at the development stage, many conclusions were reached, including:

1. The thermal efficiency of a batch heater with a conical-flame burner is at least equal to that of a rotary dryer system forced to operate under a wide range of thermal conditions.
   A batch heater enjoys constant combustion conditions and rapid heat transfer into the stone for all mixes. The final temperature is achieved during a predetermined heating cycle with an ideal aggregate batch weight for the product, depending on the mix temperature and moisture.
   The burner fuel flow and exhaust air are predicted and trimmed for stable combustion for each operating condition. The rate of moisture removal is predictable and air flow is automatically adjusted to enhance the safe removal of steam. In addition, fuel flow is monitored and profiled to ensure minimum temperature rise to the filtration plant at the end of each heating cycle.
2. Only the final aggregate content of the design batch weight is heated and dried during the process on a batch-by-batch basis. This ensures maximum production flexibility, with product changes taking only seconds without wastage, and keeps heating costs to an absolute minimum.
   Product repeatability is well proven through fixed-gated and constant-speed belt feeders with time controls for each aggregate size. The feed system can be backed-up with an aggregate-screening system if required, discharging into cold-feed bins to stabilize nominal sizes for each aggregate.
3. Recycling is achieved by feeding recycled asphalt planings from a dedicated reception hopper through the feed system and the aggregate weigh hopper for addition to the batch in the heater at the end of the heating cycle.
4. Figures indicate that an overall operating cost saving of some 25% may be expected from a batch heater operating in a typical urban environment.
   These savings arise from:

•  a plant design which ensures that all fuel is used to heat only the aggregate component of the batch. Although the burner is supported by a small pilot burner, it is not possible to run the main flame on tick over or standby. The plant design prevents fuel wastage between batches and the need for compensation for product temperature loss
• a plant design which prevents the need to purge excess aggregates
• a robust plant design which features minimal working parts, providing extended life expectancy both in the short term, by reducing revenue-consuming costs, and in the long term, by reducing capital spares costs and depreciation
• reduced electrical power and running costs, complemented by variable-speed motors where necessary.

In short, the Mixlance batch-heater design provides what is said to be the best available option to meet the high demands of the urban coating market.