With the same machine, the same batch of oil and the same set temperature, the time taken to bring the oil up to temperature is not the same in summer and in winter, and the site tends to report this as a fault. The heating elements have not changed, the oil quantity has not changed and the temperature control range is still 45–80 °C; what actually changes is two variables that are easily overlooked: the temperature the oil starts at, and the heat the tank loses to the environment.
To heat a tank of oil from the starting temperature to the set temperature, the heat that has to be put in equals the heat capacity of the oil multiplied by the temperature difference, and this part varies with the starting temperature: in winter the oil starts cold and more heat has to be put in, in summer the oil starts warmer and less has to be put in. At the same time, the temperature of the outer wall of the tank lies between the oil temperature and the ambient temperature and keeps losing heat to the environment, and this part of the heat increases as the ambient temperature rises.
Two variables that work in opposite directions
The influence of the season is therefore not simply one-directional. In winter the ambient temperature is low and there is less heat loss, so the duty cycle of the heating element falls and holding the set temperature costs less electricity; but bringing oil that has already cooled to the ambient temperature back up to 60 °C takes more heat than it does in summer, so the heating-up process takes longer. Summer is the other way round: the duty cycle is high and the continuous consumption is larger, but the oil itself is close to the set temperature and heats up faster.
Which of the two dominates depends on the heat capacity of the tank, the insulation condition and the setpoint itself. On a machine with good insulation and a small oil quantity the heat loss can be disregarded, and the seasonal difference shows up mainly in the heat needed for heating up; on a machine that is filled up and poorly insulated the heat loss cannot be disregarded. Both cases exist on site, so the heating speed cannot be judged from the season alone; comparative records have to be made on the same machine.
| Change | Effect on the heating-up process | Effect on steady-state operation |
|---|---|---|
| Ambient temperature rises | The oil starts at a higher temperature, so less heat has to be put in | The heat loss from the tank increases and the heating duty cycle rises |
| Ambient temperature falls | The oil starts at a lower temperature, so more heat has to be put in | The heat loss from the tank decreases and the heating duty cycle falls |
| The setpoint is raised | The heat to be put in increases with the temperature difference | The difference with the environment grows and the heat loss increases with it |
| The oil quantity is increased | The heat capacity grows and the heating-up time gets clearly longer | The heat transfer area grows and the steady-state heat loss increases |
How the seasonal comparison has to be recorded to mean anything
- Record the starting oil temperature and the ambient temperature. If only the minutes taken to heat up are recorded, without the starting point and the external conditions, the figure cannot be used again next time.
- Record the setpoint and the actual time taken to reach it. The temperature control is a continuous range and not a few fixed steps, so the time taken to reach 45 °C and the time taken to reach 80 °C are naturally different; the comparison has to be made at the same setpoint.
- Separate the heating-up stage from the steady-state stage. The first period after the machine is switched on is the heating-up, and the power there is not the same as in steady-state running; only by working it out separately can it be reconciled with the energy consumption records.
- For oil with a high water content the upper limit has to be pressed down to 70 °C. When the ambient temperature is high it is especially important not to push the upper limit up, and enough margin has to be left between the degassing result and the protection of the oil.
- An ambient temperature outside -20–45 °C is outside the working range; data obtained under such conditions cannot be used directly to judge the condition of the machine, and the site conditions have to be confirmed first.
Do not jump to the heating system when the temperature control reads high
When the temperature display reads higher than the actual temperature, the usual cause is not the heating element but the position of the measurement. If the probe is fitted close to the heat source, the reading is higher than the average temperature in the tank; if the gauge value is normal but the oil heats slowly, the usual cause is scale on the heat transfer surface. Both of these lead the temperature control to a wrong conclusion, so when a displayed temperature is compared with a hand-held measurement, the position of the measuring point has to be recorded.
The difference brought by the season comes from two things at once, the starting temperature and the heat loss; record the two separately before deciding whether there is a problem with the machine.
