Putting the electricity consumption of an oil purifier into the cost table as a separate item usually happens after the machine has already been bought. Many customers only work out the account after the first continuous run: the total power on the nameplate is 72 + 5 kW, the meter runs up a long way over a whole night, and purifying a few tonnes of oil should not cost that much. The answer to this question has to be looked at in three parts, rather than ending at the total power multiplied by the number of hours.
The total power is not the power of each section
A two-stage vacuum oil purifier does three things at the same time while it runs: it heats the oil up to the working temperature with far-infrared carbon fibre heating, it draws the gas off the oil surface with an X-100 vacuum pump working together with a JRP-500 Roots pump, and it circulates the oil between the tank and the filter elements with a ZH screw oil pump. Each of the three has its own pattern of electricity consumption: the heating section follows the temperature control, the vacuum pump section runs almost throughout, and the circulation pump section varies with the resistance of the oil circuit. Averaging 72 + 5 kW over the three sections produces a conclusion with no meaning.
One point has to be made first: the nameplate gives the rated total power, which is the electrical capacity that has to be reserved for the machine at full load. It is not the consumption of each section, and it is not the reading at any particular moment. The only way to know the real distribution is to measure on site section by section, or to look at the ammeter for each operating stage. A three-phase 380V, 50Hz supply is available in most industrial plants, but the circuit that the distribution board reserves for a new machine is often not sufficient and has to be checked in advance.
The running time decides the consumption, the batch size decides the consumption per unit
Taking the process from start-up to shutdown apart, the consumption falls into two parts. The first is the warm-up, where the heater works at full load to bring the temperature of the oil and of the tank up to the set value; the consumption of this part depends mainly on the quantity of oil and its starting temperature, and hardly on the throughput. The second is the steady processing, where the heating changes to holding control and the vacuum pump and the circulation pump keep working; here the consumption grows in proportion to the processing time, and so does the output.
The key inference is that the consumption of the warm-up is a one-off, and it is spread over the whole quantity of oil in that batch. The larger the batch, the more thinly the warm-up cost is spread and the lower the consumption per unit of throughput; the smaller the batch, the larger the share of the warm-up. The same machine processing three small batches on three occasions and processing all three in one run therefore differ in their consumption per unit. The usual practice on site is to concentrate the oil for a machine into a single working window rather than processing it as it arrives.
| Operating stage | The part that consumes the power | Its relation to the throughput |
|---|---|---|
| Warm-up and preheating | Far-infrared carbon fibre heating | Essentially fixed, independent of the batch size |
| Steady processing | The vacuum pump, the Roots pump and the circulation pump | Proportional to the running time |
| Holding and standby | The holding heating, the control and monitoring circuits | Present throughout, whether or not the machine is processing |
| Changeover and shutdown | Valve operation and a small standby consumption | The more often it happens, the more it adds up |
How the consumption per unit is estimated on site
The method is not complicated; the point is to record three figures completely: the cumulative meter reading at the start and at the end of the processing, the quantity of oil actually processed in that batch, and how long the batch took. The consumption per unit is the first of these divided by the second, converted to each litre or each tonne of oil. After two or three batches have been recorded this figure becomes worth using, because the differences between the seasons and between the starting temperatures of the oil show up clearly in the figures.
- Record the meter reading once before and once after the processing, and do not reset the meter in between, otherwise the difference between the two readings will be calculated wrongly.
- Cross-check the throughput against the tank level difference and the oil output metering; relying on a single figure easily leads to an error.
- Record the oil type and the starting oil temperature of the run as well; the warm-up consumption in winter and in summer cannot be compared directly.
- During continuous running, watch the amplitude of the ammeter fluctuation; as the elements gradually block, the current of the circulation pump rises.
Two items to be costed besides the electricity
The electricity is only part of the running cost. Two others are easily overlooked. The first is the consumables: in the standard configuration the fine filter elements 150-500-5 are five in parallel, and the actual life of the elements directly determines the unit cost. The second is the cost of downtime: the working window is a period of shutdown for maintenance, and every hour taken inside that window is a loss of production. Putting the electricity consumption, the consumables and the occupation of the window on one sheet is what gives a basis for choosing the number of machines and the models.
A common mistake in the judgement is to take the total power, multiply it by the running hours and call that the electricity cost. That figure only reflects the load running at the same time: it excludes the fixed element of the warm-up and does not reflect the influence of the batch size. The figure that really has decision value is the consumption per unit of throughput, because it normalises both the differences between the equipment and the differences between the processes, and only then can different models be compared on one sheet.
Once the machine has been bought, the cost is not on the meter; it is in how much time, how many units of electricity and how many filter elements each litre of oil costs.