Open up a machine of the ZYD series and you will see the vacuum pump set and the oil pump set at the same time. Many sites ask: if there is a vacuum pump drawing off, how does the oil get fed in? The machine in fact has two pressure systems, and each of them governs one part of the process.
Two pumps, two pressures
The vacuum pump and the Roots blower as a set establish the negative pressure; what they draw off is the water vapour and the dissolved gas released from the oil, and they act on the tank side. The screw oil pump ZH establishes a positive pressure; it pushes the oil from the oil tank through the coarse filter element into the flash evaporation tank and acts on the pipework side. One draws the medium out, the other feeds it in. The presence of a positive and a negative pressure system on the same machine is a basic feature of this kind of equipment.
Where the driving force of the oil circuit comes from
The circulation of the oil through the whole system is driven by the energy the oil pump supplies. The oil enters at the oil inlet, is stopped by the coarse filter element from the large particles, is pressurised by the screw oil pump and fed into the flash evaporation tank, where under the negative pressure it is heated, vapourised and separated into water and gas; the gas and oil mixture is drawn off by the vacuum pump set, and the dry oil left after the separation returns to the clean oil tank and finally passes through the fine filter element before it leaves the machine. The pump is the source of the whole circulation: without it there is no driving force on the positive pressure side, the oil does not move, and there is nothing on the negative pressure side to separate.
- Positive pressure side: the screw oil pump ZH drives the oil forward in the pipework.
- Negative pressure side: the X-100 rotary vane pump with the JRP-500 Roots blower draws off the water vapour and the gas.
- The two sides are decoupled: fluctuations in the pumping do not directly disturb the vacuum level, and the intermittency of the extraction does not take the thrust out of the pipework.
Why a screw pump is used
The characteristic of a screw pump is that the volume of the meshing cavity between the rotor and the stator changes periodically, squeezing the oil out. Its two advantages match the duty of an oil purifier exactly: the first is good self-priming, so that when there is air in the pipework the delivery can be built up step by step, without having to prime the pump first as some centrifugal pumps do; the second is that the flow rate is relatively insensitive to viscosity, so that the proportion by which the flow falls when the viscosity of the oil rises is smaller than with a centrifugal pump. For an oil purifier this is a practical point, because the viscosity of the oil changes with temperature and the temperature on site cannot be held at the optimum value at every moment.
The further benefit of separating the pumping stage from the degassing stage
Separating the oil delivery from the gas extraction into two stages has, besides making the oil path clear, several direct benefits. The first is protection: the coarse filter element is fitted ahead of the pump, so particles cannot be driven into the meshing surfaces of the pump, which extends both the mechanical life and the sealing life of the pump. The second is adjustment: the oil feed rate can be matched to the viscosity and the degree of contamination of the oil, and a higher feed rate can be matched by a correspondingly higher extraction capacity, without touching the vacuum system. The third is fault isolation: when the vacuum system has to be serviced, the pipework side can be isolated first, so that dirty oil does not flood into the chambers that have to be opened while the machine is stopped.
| Item of comparison | Screw oil pump ZH | Vacuum pump set |
|---|---|---|
| What it acts on | The oil itself | The water vapour and the dissolved gas |
| Direction of pressure | Positive pressure, driving the oil forward | Negative pressure, drawing off the vapourised components |
| Position | After the coarse filtration, before the tank | The gas phase side of the tank |
| Behaviour with viscosity | The flow rate falls only gradually as the viscosity rises | Governed by the vapourisation rate, a high viscosity slows the vapourisation |
| Consequence of a fault | The pump body heats up, the seals leak | The pumping speed falls, the pump oil emulsifies |
Separating the pumping from the degassing is so that each stage does one job only: the pipework that carries the oil is not disturbed by the fluctuations of the extraction, and the vacuum used for the extraction is not contaminated by the dirt in the pipework.