Process Heating - All fired up01 October 2006

Eliminating bottlenecks in the heat-curing phase of any assembly process, thereby increasing the efficiency of the whole operation, is an essential goal for any business - nowhere more so than at Hepworth Building Products.

A key objective for the company - a supplier of drainage and water supply pipe systems to some 40 countries worldwide - is to achieve a deeper understanding of kiln aerodynamics. This Hepworth regards as critical, as it constantly seeks to derive the maximum efficiencies for the business and highest quality of product for its customers, which include water companies, large civil engineering contractors and builders' merchants.

Hepworth operates four roller kilns, making clay pipes from 100mm to 300mm diameter, at its Hazlehead site in south Yorkshire. Through the application of computational fluid dynamics (CFD), Hepworth has made rapid strides forward, using modelling, verification and modification techniques to optimise its kilns to the most exacting standards, embracing:
- Increased OEE (Overall Equipment Effectiveness) = Rate x Yield x Availability
- Greater thermal efficiency
- Improved pipe-size tolerance and control
- Improved design, in terms of cost and longevity.

Dependent on pipe diameter and wall thickness, roller kilns handle between 16 and 260 pipes per hour, with a total firing time of between 1.5 hours and 12 hours, and a thermal capacity of 2 to 5MW.

To understand the kilns' main firing zone, an internal solid kiln model was developed, complete with all thermal inlet/outlets, which involved around 40 known thermal boundary conditions.

"Firing zone aerodynamics and flame radiation characteristics dictate the final pipe detailed geometry," says Russell Kenworthy, Hepworth's process and engineering manager. "The effective hot and/or cold spots on the pipe surface result in variations in pipe diameter, so detailed control is required at all times."

Upstream boundary conditions derived from the full kiln model were imported into a more detailed main firing zone model, including the burner head detail, to model the combustion/radiation in much finer detail.

"Through modelling, a modified burner and quarl arrangement was derived, which has already proved successful in production on one kiln," states Kenworthy, "with a seven per cent energy saving. Also, there has been improved dimensional control through burner set-up, widening the process window and improving yields, especially around product changes."

At Hepworth, the greater understanding of kiln thermodynamics gained through CFD modelling has created significant tangible savings during both steady state and transient/re-feed conditions. "One of the significant benefits of using this method is that many trials can be conducted effectively off line. Therefore the improvements are delivered with much reduced risk to the process and negating much of the experimentation required on the plant. Process trial costs, as a result, are vastly reduced both in trial time, and product and energy wastage."

The new generation of advanced ovens now available is also proving highly valuable to PEI-Genesis, an assembling international distributor of commercial, industrial and military connectors. It has recently replaced three static ovens with a new Caltherm conveyor oven at its European headquarters facility in Basingstoke, Hampshire.
PEI-Genesis stocks thousands of piece-parts for a wide range of connectors and ovens, which are required to cure the adhesive that bonds contacts into inserts, and the inserts themselves into connector shells. Most products are assembled within 48 hours, so any hold-ups in the process can have a potentially significant impact on throughput and delivery times.

"The old static ovens had to be kept closed for an hour while the adhesive was cured," comments operations manager Tony Houghton, "and this sometimes caused bottlenecks in the assembly process. With the new conveyor oven, we now have a level flow through the heat-curing phase and the entire process is more efficient."

The bespoke variable-speed oven is 8.5m-long and 900mm wide - a larger dimension than standard versions - which means that four tote boxes can be accommodated across the width of the conveyor, while providing room for future expansion of production capacity. The use of heat-resistant paperwork and customised fibre-glass tote boxes means that, even during a one-hour pass through the conveyor oven at 163°C, the parts and paperwork for each order are never separated throughout the assembly process.

One of the key deliverables expected from any modern process heating procedure is for heat to be applied with the highest degree of accuracy and consistency. This is critical for GE Infrastructure Sensing, a leading manufacturer of thermisters, which has recently increased its production facilities with the addition of a fourth Carbolite top-hat furnace for processing ceramic sheet at temperatures up to 1,300°C.

The thermal sensors manufactured by the company - which are widely used in the automotive industry, and in heating, ventilation and refrigeration equipment - incorporate several types of ceramics to suit different applications. The materials are produced from metal oxides mixed with binders, which are formed into thin sheets and then fired in the Carbolite furnaces. Heating the sheets to 400°C over four hours burns off the polymer binders, after which the material is sintered at between 1200°C and 1300°C to form the new crystal structure. Sintering can last up to 20 hours, depending on the formulation and the performance characteristics required. After cooling, the sheets are cut with a diamond saw to the size required for the finished assembly.

The hearth, which can hold many sheets at a time, runs on a track and is driven by an electric motor, so it can be moved in and out of the furnace. The heating elements are in a 'top hat' structure, which is lowered over the hearth when a charge is being processed and raised at the end of a cycle.

The success of the sintering process depends on heating the material to very accurate and uniform temperatures, so the furnaces have been designed to provide a working area within the chamber 200mm wide x 600mm long x 350mm high where temperature uniformity is ±2°C at 1200°C. Access ports allow thermocouples to monitor load temperatures in the chamber itself.

As ways are sought to accelerate further the efficient operation of furnaces and conveyor ovens, is there a role to be played here by microwave and radiant heating?

James Roper, product manager, Carbolite, concedes these have been little used in combination in the industrial sector in the past, despite the theoretical advantages they might bring in the form of faster processing times, energy savings and final product performance.

This may well be about to change, according to Roper. "Microwaves and radiant elements heat objects by two completely different means," he points out, "the first by conduction from the external surface, the second through generation of energy within the body of the material."

The main advantages of microwave heating are:
- Energy-efficiency, because power is only applied to the material
- Higher quality through avoidance of case-hardening and other surface damage
- Selective heating, giving processing benefits, in some cases
- Direct heating of the sample body, reducing process times.

Despite these advantages, microwave heating alone can sometimes be less successful at temperatures above 800°C, because of the temperature gradients caused by the external surface being cooler than the centre - the reverse of those associated with radiant heating. This limiting factor can be particularly significant for materials requiring high structural integrity.

"Combining radiant and microwave heating has been found to overcome the temperature profile problem," says Roper. "C-Tech Innovation Ltd has been at the forefront of microwave-assisted heating technology (MAT), in which microwaves provide an additional heating mechanism in support of conventional gas or electric radiant heating.

With this technology, while the microwaves provide a thermal equalising effect, the radiant heating retains the controllability essential for many advanced materials.

"This combined approach has been found to have significant advantages over both radiant-only and microwave-only systems."

Carbolite has now concluded a technology transfer and licence agreement with C-Tech to manufacture and sell equipment with the MAT heating technology in Europe. Whilst the first models produced have been for laboratory use, Roper stresses they can be scaled up relatively easily for production work.

"More consistent product properties, greater strength, improved yield, reduced formation of undesirable phases and lower quantities of harmful emissions," he states, "can all be achieved through the use of MAT heating."

Time will tell which technologies hold greatest sway in the marketplace, but one thing is certain. The new breed of furnaces and conveyor ovens they are helping to produce are, collectively, transforming the way in which the process heating sector is working. And that is providing the industry with a powerful tool in the battle to hold its own against the most hard-line of global competitors.

New Carbolite ovens at Greene,Tweed & Co
Greene, Tweed and Co - a world leader in the supply of polymer-based engineered solutions to market leaders in industries such as defence, aerospace, chemical processing, medical equipment and oil and gas production - has installed two high-specification Carbolite ovens to increase the capacity and consistency of the raw material sintering operation at its UK manufacturing facility. The company uses a wide variety of proprietary PTFE formulations for its products, which are moulded in tube and bar form in the facility before being sintered and then machined to custom designs required by customers.

The new ovens, which join two existing units that have been in the factory for more than 10 years, have a maximum volumetric capacity of 1.73m and can operate to 425°C maximum. Temperature uniformity is guaranteed to be better than ±5°C and is typically only ±3°C.

SOE

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