Lower priority is usually given to durability and maintenance, spares, and other lifetime costs, whilst ‘performance risk’ is rarely considered; although this is one of the potentially highest expenses and difficult to cover by guarantee. This may be due to the equipment being unable to achieve its design rate, affecting product quality, having a high operating cost, or completely failing to work properly; resulting in loss of production loss of customers, or even loss of market. Into this category must rest the response time and the degree of support that can and will be provided by the supplier in the event of operating difficulties arising, irrespective of responsibility. This feature is important for two reasons:
- Time is of the essence to affect a cure and secure production.
- as the supplier the competence to produce a solution.
It is well established that ‘Performance risk’ is higher on solids handling contracts than of those handling parcels, liquids, or gasses. The experience of and with the equipment supplier should therefore be a key factor in the evaluation of potential cost. A pointer in this direction is the care taken by the equipment supplier to establish the relevance and range of handling properties of the bulk material that is to be handled, as the condition of the bulk material is central to the most appropriate extraction profile for the duty and any performance shortfall.
As an example, screw feeders are widely used to control the rate of discharge from bulk storage hoppers, but they also can be sized to exceed ‘critical arching’ and ‘rathole’ dimensions, enable extra capacity to be stored, save headroom and improve flow performance by generating the most appropriate flow regime. When designed as an integral part of the storage system they can also rectify segregation that occurred during the filling of the hopper, provide consistent and optimum discharge density conditions and counter any tendency for the contents to ‘flush’ during discharge.
However, to secure all these operating benefits, the extraction pattern per unit length of the feeder should develop a uniform flow across the cross-sectional area of the hopper that it serves. As a feeder is usually shorter than the hopper body dimension, the extraction rate demand at the start and end of the feeder opening can vary to many times that of the intermediate sections and also vary across the width of the hopper outlet. A common technique employed intended to generate progressive extraction along the length of the outlet slot is to increase the pitch of the screw along its axis.
Apart from the fact that the initial section of the screw exposed to the hopper contents will extract the full transfer capacity, whereas the subsequent section extract only the differential increase in capacity, the effect of pitch increase is limited as very short pitches tend to ‘log’ and ultra-long pitches generate high radial pressures. The efficiency of axial transfer of feed screws depends on the contact friction value of the material handled on the face of the screw flight. Not only is the axial transfer efficiency reduced and can become zero, but longer pitches have to serve longer sections of the outlet, so the extraction rate is actually lower. Optimum screw construction is therefore unique to the application and can be the domain of specialist suppliers.
Ajax Equipment Ltd is a specialist manufacturer of solids handling and processing equipment, incorporating Lynflow technology. It has over 40 years experience in the design and manufacture of equipment for a wide range of international organisations involved in food and confectionery processing, pharmaceutical, fine chemicals, waste to energy and ...