Both of them filter the oil clean, but the experience on site with an open system and with a closed one is very different: one leaves oil stains and a smell on the floor and the other is more or less clean; one has to worry about air being drawn into the oil and the other does not. The difference comes from a structural choice and not from the operating habits.
What the positive pressure closed circuit is as a structure
The positive pressure here means that the pressure inside the oil circuit is higher than the atmospheric pressure, and the oil is pushed along the pipework by a screw oil pump, in a closed loop the whole way. What is held under negative pressure inside the machine is the vacuum chamber, not the whole oil circuit. This is worth understanding, because quite a few sites talk about the positive and the negative pressure together and end up with the two the wrong way round.
The closed arrangement has three direct consequences. The oil and the gas do not escape, the hot oil and the oil vapour stay inside the pipework and the machine, and the operator on site is not exposed to the hot oil that splashes out. The machine also does not draw the air in from the workshop, so the dust and the moisture in the air are not carried into the oil. For an oil with a high gas content to begin with this point is especially important: as soon as an open return line is not properly sealed, the ambient air is drawn into the oil continuously and the more it is treated the worse the problem becomes.
Why an open return is more dangerous when the gas content is high
Suppose the return opening is exposed to a negative pressure. The air above the oil surface is drawn into the oil through the return pipe and forms free bubbles. Once these bubbles are in the vacuum chamber they expand violently, they take up the volume of the chamber and the pumping speed is wasted; at the same time the entrainment of gas and liquid carries the oil mist into the vacuum pump and the pump oil emulsifies and carbonises more quickly. The higher the gas content of the oil, the stronger this positive feedback and the more the treatment makes the oil worse.
An open system has a further hidden problem: the oil is exposed to the air, moisture and oxygen keep entering it and the oxidation rate of the hot oil goes up with it.
Apart from the oil and the gas phases, the dust in the environment also gets into the oil through the open ends. For an insulating oil with strict cleanliness requirements the opening is in itself a new source of contamination, and the cleanliness is precisely the parameter that is hardest to put right at short notice at the acceptance.
Where the pressure monitoring and the protection are placed
It is not enough to fit one pressure gauge; what matters is which section it is fitted in. The inlet side, the downstream of the filtration section and the return side reflect different problems: a high pressure on the inlet usually points to a blocked filter element or a blocked coarse strainer; an abnormal pressure on the return side has more to do with the back pressure, the valves and the routing of the pipework. The protection logic should set the alarm values and the trip values separately and distinguish the direction: a high pressure is a blockage signal and a low pressure is a dry running signal, and the two are handled in completely opposite ways.
The overpressure trip with an audible and visual alarm is the last line of defence, used to cut off the heating and the pumping when there is a blockage or a valve operated in error, and so to avoid a continuing pressure rise. The phase loss and the phase sequence protection deal with a different class of problem: with the motor wired incorrectly or with a phase missing, the output of the pump becomes abnormal and the machine may run for a long time under the wrong conditions.
| Item | Open return | Positive pressure closed circulation |
|---|---|---|
| Escape of the oil and the gas | Oil stains and a smell on site | No escape, the personnel are not in contact with the hot oil |
| Ingress of the ambient air | The negative pressure return draws in air continuously | The oil circuit is closed, no air is drawn in |
| Oxidation of the oil | Accelerated by the exposure to the air | Relatively slower |
| Contamination of the pump oil | Oil mist is entrained into the vacuum pump | Less entrainment |
| Cost of the structure | Simple, low cost | Higher demands on the pressure rating and the sealing of the pipework |
- If continuous bubbles appear on the return side, check the seals and the return pipework first, and the gas content of the oil itself after that.
- The pressure protection has to be set in both directions, with a different handling logic for a high pressure and for a low pressure.
- The valves, the pipe fittings and the flanges of a closed system have to be inspected regularly, this is the maintenance cost that a closed design has to pay.
- Combine it with the infrared level detection and the foam detection with the automatic defoaming, to keep the foam out of the vacuum chamber.
In my judgement the positive pressure protects the people and the closed circuit protects the oil; both statements hold, and that is why the closed version is worth the little extra in structural cost.