In a selection discussion the throughput is usually the first number to be settled: 6000 L/h or 12000 L/h. What really decides the result is often the number nobody discusses, namely the size of the oil tank to be paired with it. The tank volume and the flow rate hold each other back; if only one of the two is fixed, the capability of the machine cannot be brought out.
First calculate the filling time
Taking the 6000 L/h of the ZYD-100 as the basis, the filling time for a single charge differs a great deal with the volume of the tank.
| Tank volume | Filling time for one charge | Number of loading and unloading cycles to process 6000 L of oil | Direct consequence |
|---|---|---|---|
| 1000 L | approx. 10 minutes | 6 times | Frequent loading and unloading, and at a batch change the oil has not yet circulated fully in the system |
| 3000 L | approx. 30 minutes | 2 times | The batches are easy to manage and the oil is mixed evenly in the tank |
| 6000 L | approx. 60 minutes | 1 time | It is filled in one go, but the residence time of the oil in the system becomes noticeably longer |
The two problems of a tank that is too small and a tank that is too large
The problem with a tank that is too small is not the flow rate, it is the batch. A tank that is changed every ten minutes means that, from the entry of the oil into the machine to its exit, the number of times the oil is actually circulated in the system is limited, and the result depends more on the accumulation of several cycles. Many loading and unloading operations, much manual intervention, and having to establish the temperature and the vacuum again at every changeover all mean that more time is lost and that more chances of error are opened.
The problem with a tank that is too large is the long residence time of the oil. The oil waits in the tank to be drawn off, the temperature of the heating section of the machine is maintained for a long time, and during that period the oil neither takes part in the degassing nor goes back to the storage tank, it simply stays in a heated condition. The longer it stays, the greater the risk of oxidation and of a rising acid value, and that is why, when the water content is high and the temperature control limit is set down to 70 °C, the oil should not be left hanging in the system for a long time either.
There is a further variable that is often left out, the mixing and the temperature uniformity inside the tank. If the oil stands in layers in the tank, with the upper layer already treated and the lower layer still as it was, the mixed oil that is drawn off is neither uniform nor fully dehydrated. Where the inlet and the outlet are placed, whether the return oil is directed at the lower part of the tank and whether the tank needs a circulation stirrer all affect the actual result. The same tank with a stirrer and the same tank without one give quite different results on the same batch of oil, and the cost of that stirrer is far below the cost of a larger machine.
Three criteria
The first is the constraint of the working batch. The tank volume has to hold at least the smallest batch to be treated on site, otherwise the machine is always being taken apart and filled with small batches and there is no efficiency to speak of.
The second is the constraint of the cycle time. The filling time for one charge should be clearly shorter than the cycle time the oil needs in the system to complete the degassing, otherwise a large part of the oil is only waiting in the tank and never takes part in the treatment.
The third is the constraint of the heat exposure time. The longest residence time of the oil in the system has to be kept within what the oil can accept, and a batch of oil must not be left hanging in a heated state for a long time just to reduce the number of changes.
Only when the three criteria are met at the same time is the tank volume settled. A practical way is to set the lower limit from the working batch, then to work back the upper limit of the tank volume from the upper limit of the cycle time, and to take the value from the range in between.
- Measure the actual working batch on site first, rather than working the tank volume back from the flow rate of the machine.
- Calculate the filling time for one charge, then compare it with the cycle time that the degassing needs.
- Write the longest residence time of the oil in the system into the working procedure.
- Leave time at a batch change for the temperature and the vacuum to be established again, this time produces no throughput.
My way of putting it: the flow rate decides how much oil this machine can treat and the tank volume decides how cleanly this machine treats it, and the two have to be settled together.