There is a common instinct on site: when the throughput will not go up, open the inlet valve further and speed the pump up, on the assumption that the machine still has capacity. In most cases this does give more oil in the short term, but at the same time the vacuum level starts to fall, the water content no longer comes down to the pass line of 3 ppm (GB/T 7600), and the motor temperature climbs as well. This is not an equipment fault; it is treating degassing, a process with a limited rate, as a capacity that can be scaled up linearly.
Why the throughput doubles and the vacuum level still does not rise
Degassing is a limited-rate process, and the rate is set by three quantities: the evaporation area, the oil temperature and the vacuum level. The geometry of the machine fixes the evaporation area, the heating provides the temperature and the vacuum level provides the pressure difference. As long as these three conditions do not change, the total amount of gas that can be removed per unit of time has a ceiling.
Increasing the feed rate has two effects that work against each other. First, the volume of oil in the tank rises, so the time each drop of oil spends in the tank is spread thinner and it is pumped out before it has had time to degas, and the residual gas content rises. Second, the volume of vapourised gas increases, the gas phase load exceeds the pumping speed of the vacuum pump, the pump cannot keep up, and the pressure in the chamber rises, which shows as a falling vacuum level. So after the throughput is increased, the vacuum level often does not stay the same but gets worse; the two are not in conflict, they have simply been put on the same sheet for comparison.
For a reference scale: the flow rate of the ZYD-100 is 6000 L/h with an installed power of 72 + 5 kW; the ZYD-200 in the same series is 12000 L/h and the ZYD-300 is 18000 L/h. When selecting a machine, if the throughput actually required is approaching or already above the rated flow of a given size, the correct action is to move up to a larger model rather than push the existing machine into overload.
The relation between motor temperature rise and load current
The temperature rise of the motor follows the load. At no load the current is small, the windings generate little heat and the fan has cooling to spare; as the load rises the current increases, the copper loss grows with the square of the current and the winding temperature rises with it. The cooling capability of the machine is fixed: the heat dissipation area of the casing, the air volume moved by the fan, the ambient temperature and the dust inside the cabinet. Once the heat generated exceeds the cooling capability, the temperature keeps climbing.
For that reason, whether the motor is overloaded is judged more directly by the current than by the temperature. The temperature is a lagging indicator: by the time the casing is hot to the touch, the winding insulation has already been overheating for some time. Continued overheating leads to ageing of the winding insulation and carbonisation of the varnish film, and the life of the motor is clearly shortened; when the bearing lubrication fails the temperature rises further, and a vicious circle sets in.
How to decide whether to keep increasing the rate
Use the step method and do not go to the limit in one move. Starting from the current feed rate, increase it by about a fifth at each step, hold for ten to fifteen minutes and watch three readings: the motor running current, the vacuum level and the oil temperature. If all three stay inside the stable range there is still capacity and one more step can be added. If any one of them deteriorates clearly, go back to the previous step, and that is the throughput actually available under the present conditions.
The environmental conditions have to be checked at the same time: whether the cabinet air inlet is obstructed, whether the filter is full of dust and whether the fan is turning. When the ambient temperature rises, the motor runs hotter at the same load, and that part of the heat generation has nothing to do with the load, so exclude it before drawing a conclusion.
| Symptoms seen together | What it points to | How to confirm | Action |
|---|---|---|---|
| The feed rate increases and the vacuum level falls at the same time | The degassing capacity has been spread thin; the usable throughput for the present oil condition has been exceeded | Reduce the feed rate by one step and watch whether the vacuum level recovers | Go back to the previous step, or move to a model with a larger throughput |
| The motor temperature rises and the current rises with it | The load is on the heavy side | Read the running current on a clamp meter and compare it with the temperature rise | Run at a lower load and check the heat dissipation and the dust |
| The motor temperature is high but the current is normal | Poor heat dissipation or a high ambient temperature | Look at the air inlet, the fan and the material stacked around the machine | Clear the cooling path and improve the ventilation |
| The output has gone up but the water content has risen | Insufficient residence time | Compare the water content of the samples taken before and after the reduction | Reduce the feed rate and, if necessary, extend the processing time of a single batch |
Safety and the stop criteria
The machine has to be stopped if any one of these holds: the motor casing temperature is clearly high and still rising, a burnt smell appears, the current clearly exceeds the rated value and jumps, or an abnormal noise is accompanied by an abnormal position of the coupling. The stopping order is to reduce the feed rate first, then stop the heating, then stop the main unit; cutting the power off in the middle leaves oil standing in the line and can easily cause mechanical damage when the pump runs backwards.
Throughput and vacuum level are not related linearly. Before increasing the rate, ask one question: is the vacuum level not deep enough, or is the feed rate not high enough.