An oil purifier is typical of equipment that runs intermittently. In the sequence of starting up, warming up, pulling a vacuum, feeding in the oil, running steadily and finally shutting down, the electricity consumption of the individual stages is not evenly distributed. Taking these stages apart is what keeps the energy account from being calculated wrongly, particularly on stations that run intermittently, where the cost of repeated starts may be larger than the steady running part.
The three stages where the electricity goes
The first stage is pulling the vacuum. The vacuum system of the ZYD-100 consists of an X-100 vacuum pump and a JRP-500 Rootes pump, and the working vacuum is -0.08 ~ -0.0999 MPa. The load during the vacuum stage depends on the space that has to be evacuated and on the leakage that has to be overcome; the stage does not take long, but the power is at full load. The second stage is the warm-up. The heating elements bring the oil in the tank up to the set value; the temperature control range of the TYA series is 45–80 °C, with the upper limit pulled down to 70 °C for oil with a high water content. The length of this stage depends on the oil charge, the initial temperature of the oil and the ambient temperature; with an ambient temperature between -20–45 °C the warm-up when starting up in winter is clearly longer.
The third stage is steady processing. The heating load falls away here and the main load moves to the oil pump and the filtration section. The total power of the ZYD-100 is 72 + 5 kW, three-phase 380 V / 50 Hz; the flow rate of the TYA-50 is 2000 L/h, of the TYA-100 is 4000 L/h and of the ZYD-100 is 6000 L/h. The energy consumption per unit of throughput during the steady stage depends on the flow rate and on the temperature control set value actually reached.
| Stage | Main load | The characteristic of this stage |
|---|---|---|
| Pulling the vacuum | Vacuum pump and Rootes pump | Short in time, high in power, dependent on the tank volume and the leakage |
| Warm-up | Heating elements | The longest stage, varying with the oil charge and the ambient temperature |
| Steady processing | Oil pump and filtration section | A stable load, directly related to the flow rate |
Why intermittent running costs more
The cost of intermittent running does not lie in the shutdown itself but in the fact that every restart has to pay for the warm-up again. If a batch of oil takes ten minutes to process, the processing time is only part of the run and the rest is spent on pulling the vacuum and warming up. If the machine is stopped and started repeatedly, for instance processing only a small batch per shift, then the warm-up cost of each run is spread over a very small throughput and the electricity consumption per unit of oil is clearly high.
Continuous running spreads the warm-up once and then only the steady processing is needed for the second batch and those after it, so from the point of view of electricity consumption, as long as the site allows it, concentrating the processing into one continuous period is usually more economical. The condition is that the site allows it: the outage window, the oil scheduling and whether the transformer can wait; where these conditions are not met, intermittent running is the only option.
Another easily overlooked point is the waiting time. The machine does not run entirely idle while it waits, but the energy used for insulation and the control circuits still exists; this part is unrelated to output and is a fixed cost. When it is included in the energy consumption per tonne, the denominator has to be the quantity of oil actually processed, and not the hours the machine was switched on.
- Record the start and end of every run and how long the warm-up took; over a week it becomes clear where the habit lies.
- Process each batch of oil as far as possible in one go, so that the repeated warm-ups caused by frequent stopping and starting are reduced.
- Use the quantity actually processed as the denominator when calculating the electricity consumption per tonne, not the hours the machine was switched on.
- The warm-up time differs between winter and summer, so consumption data spanning different quarters has to be looked at separately.
- Record the total power and the flow rate together, so that the energy consumption per unit of throughput can be calculated and not just a single power figure kept.
Bring the account down to figures that can be measured on site
There are only a few measurable items on site: the quantity actually processed in each run, the total running time, how much of it the warm-up took, and the quality the batch of oil had reached when processing finished. Record these figures per batch and after a while you have a unit energy consumption that belongs to your own site, instead of one copied from somebody else's experience. Once this record is being kept, the judgement on saving electricity has a basis, and so does the judgement on costs other than the electricity bill.
The long tail of the electricity consumption does not lie in the steady running, but in the part before every restart.
