This paper compares the use of pneumatic and mechanical unloaders mainly for grain. Therefore it gives a brief overview of what options are available in the market but also goes into more detail with a comparison between chain type mechanical vs. pneumatic unloaders.
Efficiency = average capacity/maximum capacity
The goal is to develop an unloader that achieves higher performance by increasing the average capacity, therefore making better use of the complete installation.
The ideal case would be average = maximum, but this is impossible to achieve due to a series of variables, including some of which that are not even equipment related, including rain delays or the amount of visits to each hatch for ship stability.
Pneumatic CSU developments
The development of software which uses frequency inverters to control turbo blower rotation speed has resulted in a more constant capacity during operations. It has also increased the ability to control the unloaded capacity. This helps when materials with different specific weights are unloaded or capacity needs to be adjusted to match receiving conveyors.
By traditional design the boom should reach the centre of the hatch to achieve ship stability. However, a longer boom would allow one to reach the hatch corners resulting in the need to move less material with payloaders, while bigger payloaders for the clean up work would also help. With these measurements, it is possible to achieve 75 per cent capacity with pneumatic unloaders.
As you can see, it is not only the unloader itself but all of the receiving conveyors which determine the maximum capacity of the system; i.e. the system is only as strong as the weakest link in the chain.
CSU principles
The principle of CSU is to avoid peak unloading by distributing materials in time at the receiving conveyors.
CSU types
The main CSU groups are mechanical and pneumatic. The mechanical group can be divided into belt, screw, chain and bucket conveying types.
Average compared with batch
CSU is normally compared with a traditional grab batch unloading system. The grab capacity is calculated on the basis of the movement cycle. A buffer is needed to distribute the material load along the cycle, avoiding the extreme peak. CSU does not need a buffer because the peak is not distributed in time cycles, but instead it is continuously transporting material. The consequence of this is normally a lower weight and the fact that operations are environmental enclosed.
Chain X Pneumatic
A regular chain unloader is used in the same application as apneumatic unloader because they do not have a feeder, but instead use a free flowing characteristic to feed the unloader. However, this is where the similarities end. The pneumatic unloader is the only CSU that picks up material from the bottom of the hatch. Figure 1 and Figure 3 show it can act like a vacuum cleaner, and remove all products from the hatch.
The chain however relies on product pressure to introduce the material in the chain through. The product pressure is achieved by a product column. Looking at Figure 2, the material height is shown as h1 while the remaining height which does not to hit the bottom of the hatch is shown as h2. The distance of the chain unloader from the bottom is dependent on the chain size (2-3m). The design characteristic of the chain system makes it difficult to achieve better average capacities, especially when only one unloader is available and no pneumatic system is available. In many cases portable pneumatic units are used to get better results, although they increase the operating cost.
Capacity definitions
There are many types of capacity, including, for example, nominal, project, peak and average.
Nominal
Nominal capacity is defined by the equipment, normally the maximum capacity it can reach.
