Graphite and Baking Belts
High humidity in baking ovens, caused either by ambient conditions or particular baking processes, can lead to rust forming on the underside of carbon steel baking belts. Similar problems may be created by condensation in dry atmospheres at high altitudes. Improved methods of oven belt lubrication are now available.
To ensure the best working conditions at the high temperatures experienced in baking ovens, a layer of graphite should be deposited on the inside of the belt at regular intervals. This will decrease friction and avoid mechanical damage to the belt surface while providing protection from rusting.
Lubrication is often carried out by means of a suspension of colloidal graphite in water. This is, however, time consuming and requires careful application, drying and a final meticulous cleaning of the belt itself. The consistency of application depends upon the skill of the maintenance personnel. Too much or too little graphite, combined with uneven application, can lead to production problems - band slip or tracking difficulties caused by build-up on skids or pulleys. Although the danger of product contamination can be minimised by applying the solution out of production hours, the oil can dry off leaving free graphite to blow around the system.
A new graphite station allows a homogenous and gradual deposit to be applied automatically during normal production. Graphite bars, having an appropriate granularity and hardness are installed close to the oven terminal drum. The structure incorporates springs that ensure constant contact between the graphite bars and the underside of the belt. A safety device prevents contact with the belt when the graphite bars are worn down.
Graphite skids have another application in baking ovens. Extra long baking ovens, or when existing ovens are to be extended, can require rollers to be fitted, rather than conventional cast iron skids, to reduce friction. This is an expensive option. An alternative approach is to replace some, or all, of the cast iron skids with skids made from graphite bars. A 1200mm wide baking oven, extended to a total baking chamber length of 98 meters plus 9 metres run-out, was fitted with graphite skids. It performed perfectly, with no tracking or drive problems, at belt speeds in excess of 20 metres/minute.
This oven has been in production for over 12 months. Comparison was made by visual inspection, and by taking rubbings from the inner surface of the belt running on graphite skids and three other ovens in the same plant running on cast iron skids. The rubbings gave a clear indication of the degree of rust, grease, product dirt and graphite build-up on the underside of the baking belt. It was very apparent that the oven fitted with graphite skids remained much cleaner. The rubbings showed only a light coating of graphite. No graphite build-up was evident on the driving drum. The belt was burnished to a deep, even colour, creating a virtually non-stick surface. The result is better heat transfer, lower friction and a very evenly baked product.
Where ovens are fitted with belt support rollers, the underside of the band is often left unlubricated because of the need for friction between the belt and the rollers. The underside can become very dirty and rusty as the rollers tend to press any debris into the belt surface.
Experience suggests that graphite skids can be considered both as a band support and as a controlled form of lubrication and can therefore replace some rollers and/or cast iron skids in existing ovens. Better lubrication, less cleaning and smoother operation should result.
NOTE: It is suggested that graphite belt lubrication should not be used in ovens with baking temperatures over 370°C (700°F). Overwide skids can also cause problems with build-up on the edges creating a fire hazard.
Obviously, lubrication is not the only determinant of belt performance. Ambient conditions and the location of the oven relative to other ovens or to cold air inlets can significantly affect belt tracking, rusting, etc., leading to a fall in product quality.