Configuration Selection Logic of Mobile Asphalt Plants

1 Introduction
2 Related articles on Designing Buildings

[edit] Introduction

The selection and configuration of an asphalt mixing plant can significantly affect the cost, productivity, quality and environmental performance of road construction projects. Mobile asphalt plants are designed to be relocated between sites and may be suitable where projects are temporary, geographically dispersed or located in areas where transporting asphalt from a permanent plant would be impractical.

The appropriate configuration depends on a range of factors, including the required production capacity, project duration, asphalt specifications, availability of aggregates and bitumen, transport constraints, site conditions, fuel supply, environmental requirements and the anticipated frequency of relocation.

Mobile plants are available in different sizes and configurations. Selection should be based on the requirements of the project rather than on production capacity alone.

[edit] Mobile and stationary asphalt plants

A stationary asphalt plant is generally installed at a fixed location and may be intended for long-term or continuous production. Mobile plants are designed to be dismantled, transported and reassembled more readily, although the degree of mobility varies between systems.

Mobile plants can be advantageous where:

Construction projects are located in remote areas.
Plant needs to be relocated between projects.
Roadworks are temporary or geographically dispersed.
The transport distance between a permanent plant and the paving site would be excessive.
Local production capacity is required for a limited period.

However, mobility may involve compromises in storage capacity, production capacity or the range of available configurations. The time required for transport, assembly, commissioning and dismantling should be included in programme planning.

The choice between a mobile and stationary plant should therefore consider the whole project lifecycle, including installation, operation, relocation, maintenance and eventual removal.

[edit] Production capacity and mixing configuration

Production capacity is generally expressed as tonnes of asphalt produced per hour. The required capacity should be determined from the anticipated paving programme, daily production requirements, operating hours, storage arrangements and the reliability required to maintain construction activities.

Selecting an excessively large plant can increase capital, transport and operating costs, while insufficient capacity may affect the construction programme. Allowance should also be made for maintenance, breakdowns, changes in production conditions and variations in demand.

Asphalt mixing plants can broadly be divided into batch and continuous mixing systems.

[edit] Batch mixing plants

Batch plants produce asphalt in separate batches. This can provide flexibility when different mix designs are required and can allow individual batches to be controlled and adjusted.

Batch plants may be suitable for projects requiring a range of asphalt specifications or frequent changes between products. However, the equipment can be more complex than some continuous systems, and the appropriate configuration depends on the required output and operational requirements.

[edit] Continuous and drum mixing plants

Continuous plants produce asphalt through an ongoing process rather than in individual batches. Drum mixing plants commonly combine drying, heating and mixing processes within a rotating drum.

Continuous systems may be appropriate where a consistent asphalt specification and sustained production are required. Their suitability depends on the required mix flexibility, production capacity and quality control arrangements.

Neither batch nor continuous production is inherently preferable in all circumstances. The selection should be based on the range of asphalt mixtures required, expected output, project duration, maintenance requirements and available resources.

[edit] Site, material and logistical considerations

The location of an asphalt plant can affect the efficiency and cost of asphalt production. Site selection should consider access for deliveries, the availability of aggregates and bitumen, fuel supplies, electrical power where required, water, drainage and sufficient space for plant operation and material storage.

Aggregate properties and moisture content can affect drying requirements, fuel consumption and plant productivity. The plant configuration should therefore be compatible with the available materials and the required asphalt specifications.

The transport of plant components between sites may be affected by road width, bridge capacities, gradients, height restrictions and requirements for abnormal load permits. Mobile plant should be configured with transport limitations in mind, particularly where access routes include constrained or remote roads.

The availability of spare parts, maintenance services and suitably trained operators should also be considered. A plant that is technically suitable may still create operational risks if specialist components or support cannot be obtained within an acceptable timeframe.

[edit] Environmental and regulatory requirements

Asphalt plants can generate emissions, dust, noise and odours. Their design and operation should comply with applicable environmental legislation, permits and local planning requirements.

Environmental considerations may include:

Particulate emissions from aggregate handling and drying.
Combustion emissions from burners and other equipment.
Dust collection and filtration systems.
Noise from plant and associated vehicles.
Storage and handling of bitumen and other materials.
Management of waste and surplus materials.
Fuel consumption and energy efficiency.
Site drainage and the prevention of pollution.

The required level of environmental control varies according to the location and applicable regulatory requirements. Equipment such as dust collection and filtration systems should be selected according to the characteristics of the plant and the relevant emissions requirements.

Plant operators should also consider how changing regulations or project requirements may affect the suitability of equipment over its anticipated service life.

[edit] Cost and ownership considerations

The cost of an asphalt plant should be assessed over its expected operational life rather than solely on the initial purchase price. Relevant costs may include:

Leasing or hiring equipment may be appropriate for short-duration projects or where future workloads are uncertain. Ownership may be more appropriate where equipment is expected to be used regularly over a longer period.

The financial assessment should also consider the cost of purchasing asphalt from external suppliers, transport distances and the potential value of maintaining direct control over production.

[edit] Developing a configuration strategy

The configuration of a mobile asphalt plant should be based on a systematic assessment of project requirements. Relevant information should include the expected project duration, annual and daily asphalt demand, required mix designs, material sources, transport routes and site constraints.

Key decisions may include:

Required production capacity.
Batch or continuous mixing.
Aggregate storage and handling capacity.
Bitumen storage and supply arrangements.
Drying and heating requirements.
Fuel type and availability.
Dust and emissions control equipment.
Control and monitoring systems.
Asphalt storage capacity.
Transportable module dimensions and weights.
Provision for future modifications or additional equipment.

Future requirements should be considered where a plant is expected to serve a number of projects. However, unnecessary capacity or equipment can increase capital costs and reduce the practical advantages of a mobile configuration.

[edit] Operation and quality control

The performance of an asphalt plant depends on more than its nominal production capacity. Effective operation requires appropriate control of aggregate grading, moisture content, temperatures, bitumen content and other characteristics specified for the asphalt mixture.

Quality control procedures should be established to confirm that the asphalt produced meets the relevant specification. Plant configuration should support appropriate sampling, testing and monitoring.

Regular inspection and maintenance are also necessary to maintain productivity, quality and safety. Particular attention may be required for burners, dryers, mixers, conveyors, filters, control systems and material handling equipment.

A suitable configuration should therefore balance mobility, production capacity, mix flexibility, operational reliability, environmental performance and lifecycle cost.