To find out whether a given oil purifier is power-hungry, the most direct method is to look at the electricity bill, but the bill gives the total load of the substation over a given period, and the machine is only part of it. To work the machine out separately, the operating record and the metering data have to be used together; neither can be left out. Once the figures are clear, the questions they answer are quite specific: how many kilowatt-hours does it take to treat one litre of qualified oil, and how much of that energy goes into heating.
Start with where the power goes. The total power of the machine is broken down: the ZYD-100 is 72 + 5 kW, the ZYD-300 is 110 + 11 kW, and the ZYD-200 is 175 kW of which 160 kW is heating. The heating circuit is switched on and off by the temperature control, while the pumping and the vacuum circuits run continuously with the process; the two behave in completely different ways. Only with metering per circuit can the two parts be looked at separately.
The four sets of data to record
| Quantity to record | Why it has to be recorded | Where it is recorded |
|---|---|---|
| Energy per circuit | Separate the heating from the pumping and the vacuum | Energy meter for each circuit |
| Start-up and shutdown times | Separate the heating-up period, the steady period and the standby period | Operating record |
| Oil volume treated over the same period | Calculate the energy consumption per litre of oil | Flow meter with a totalising function |
| Ambient temperature | Normalisation is needed for comparisons across seasons | Operating record |
The energy consumption of the three periods must not be calculated together
The first period after start-up is the heating-up period, where the power is most concentrated; this is a one-off cost. The steady period is the real production period, and its duty cycle is determined by the temperature control and the process. The standby period is a different matter again: temperature-holding standby, idling standby and complete power-down differ widely in electricity cost. An average calculated with the three periods mixed together serves neither to judge the efficiency of the machine nor to compare two machines.
Seasonal difference is a fourth factor that is easily overlooked. When the ambient temperature varies within the range of -20–45 °C, the heat loss from the tank changes with it, the duty cycle of the heating changes with it, and the steady-state energy consumption in winter and in summer is in any case different. When data is compared across seasons, the ambient temperature has to be recorded alongside it, otherwise the difference that comes out of the comparison is put down to the machine.
Fix the basis of calculation before doing the accounting
The basis on which the energy consumption is calculated has to be settled first: whether the denominator is this litre of oil or this litre of qualified oil makes a large difference to the figure, and whether the numerator is the consumption of the whole machine or only of the heating circuit makes a larger difference still. With the throughput as the denominator and the total energy over the same period as the numerator, the result is the overall energy consumption per litre of oil; with the qualified throughput as the denominator, the result is the energy consumption per litre of qualified oil. The two bases each have their use, and mixing them produces contradictory conclusions, so the basis has to be written down before the counting starts.
What can be done with the result
- Calculate the energy consumption per litre of qualified oil, as the basis for the electricity tariff terms and for the acceptance criterion. The measurement boundary, the operating conditions and the statistical basis have to be stated clearly when the terms are written, otherwise this figure cannot be used.
- Calculate the marginal return on purification. What is the difference in cost per litre between treating to NAS 1638 class 5 and doing nothing beyond coarse filtration? That difference decides how far the purification has to go to be sufficient.
- Calculate the standby strategy. The difference in electricity cost between temperature-holding standby and complete power-down, converted into the number of shutdown periods, is worth arranging separately when the production schedule is set.
- Calculate the case for modifications. Only with metering per circuit does it become visible which segment an energy-saving measure applies to; without itemised data, savings can only be guessed at.
The electricity bill gives the total, and the operating record gives the starting points of the individual periods. Only when the two are reconciled is there a comparable energy consumption per litre of oil.
