On many sites the preheating is treated as waiting time after the machine has been started: as soon as the temperature display reaches the setpoint, the oil feed begins. The figures show no difference, but a latent risk is built into the degassing process. The oil has not really been heated through, the liquid in front of the nozzle is still relatively viscous, the particles produced by atomisation are larger, and the oil surface in the tank becomes rougher as a result. This additional waiting time appears to cost only energy, but it actually determines how long this batch of oil takes to reach a stable state.
Temperature determines the viscosity, the viscosity determines the atomised particle size
Each step up in oil temperature is followed by a drop in viscosity, and the flow condition in front of the nozzle changes accordingly. At high viscosity the same flow rate forms a more concentrated jet at the nozzle and that jet breaks up more slowly, so what enters the tank is a series of lumps rather than a uniform mist. Once the viscosity falls into the appropriate range, the jet disperses into fine mist shortly after leaving the nozzle. Fine mist has a large surface area, so in the same volume of oil more interface is exposed to the vacuum, and the path along which gas escapes from the oil phase becomes short.
The converse is equally important. When the atomised particles are too large, the proportion of oil that settles and clings to the tank walls rises, and more oil hangs on the nozzle and on the pipework. Oil clinging to the walls does not take part in the subsequent degassing, which means that part of the throughput of this batch is wasted. The feed flow reading stays normal, yet the qualified oil can only be made up by running several more batches, so the progress and the quality accounts do not agree here.
The release of dissolved gas depends on temperature
Gas dissolved in the oil does not come out on its own at ambient temperature, and its release rate is directly related to temperature. When the temperature is low the surface tension of the oil is higher, so bubbles formed inside the oil phase neither grow nor detach easily; the interface only hands molecules over to the gas phase slowly. Once the temperature rises to near the setpoint, both the nucleation and the detachment of bubbles become easier, and the rate at which gas collects at the exhaust end increases noticeably.
This explains a common difference. With the same batch of oil, preheating first and degassing afterwards settles the pumping speed and the airflow sound at the exhaust end quickly. Heating and degassing at the same time keeps the pumping speed low in the first stage, and once the temperature has caught up it takes a further period before the pressure starts to fall. The first arrangement concentrates the gas release in the degassing stage, while the second leaves part of the degassing effect to batches that have already been counted as production.
Why the upper limit is reduced to 70 °C for oil with a high water content
The temperature control range of the TYA series is 45–80 °C. This is the general range; it is not the case that every oil should be pushed towards 80 °C. When the water content is high the upper limit has to be reduced to 70 °C, because with more water and more dissolved gas the difficulty of oil-gas separation rises with the temperature, while the energy consumption and the equipment load caused by heating increase at the same time. The temperature has to be selected as a range according to the condition of the oil, not taken from the upper end of the range.
| Item | Value |
|---|---|
| TYA series temperature control range | 45–80 °C |
| Temperature upper limit with high water content | 70 °C |
| Working vacuum | -0.08 MPa |
| ZYD-100 processing capacity | 6000 L/h |
| Ultimate vacuum | ≤5 Pa |
Why the gas content is high at the start of a batch
Data collected over consecutive batches often shows a pattern: in the first batches after a changeover, the average gas content is clearly higher than in the stable batches that follow. The cause is often not in the vacuum system but in the fact that the temperature has not yet reached the setpoint. When the oil first enters the tank it carries the viscosity and the dissolved gas of the oil at ambient temperature, and degassing takes place while the temperature is still rising, so both the gas release rate and the atomisation state are changing. The results from the first stage therefore pull the average of the whole batch down.
Carrying out the preheating before the oil feed means that the first stage of the batch is processed under stable temperature and stable atomisation conditions, so the gas content of the first stage does not pull the average of the whole batch up. The difference is visible in the data: the first batches after a changeover are no longer clearly higher than the batches that follow, the comparability between batches improves, and there is no need to explain the first-stage data separately at acceptance.
How to confirm on site that the preheating is complete
- The temperature display stays steady over a continuous period and the reading no longer fluctuates, which means the system temperature has evened out.
- Distinct movement can be seen in the tank and the oil surface is in a circulating state rather than a static layer.
- The airflow sound at the exhaust end is continuous and even, with a stable sound character rather than an intermittent hissing.
- The oil inlet valve is confirmed to be in the closed position before the formal degassing stage begins.
Preheating is not heating the equipment up; it is adjusting the state of the oil to a state that can be processed. Once the temperature is in place, the viscosity and the atomisation are stable and the gas release has a defined rate. Preheating before the oil feed gives the first batch a qualified level that can be compared with the batches that follow. If this step is skipped, the time saved is given back later in the additional waiting for the temperature to rise and in repeated processing.
Preheating before the oil feed is not about waiting for a longer period; it is about keeping the atomisation state and the gas release rate on the same baseline throughout the batch.
