The simplest practice on site is to connect all the oil straight into the vacuum machine and leave out the pre-filtration stage. In the short term this does save effort, but after the machine has run for a while a set of hard-to-explain symptoms appears: the differential pressure rises quickly, the pump oil emulsifies early, the vacuum level falls and the same batch of oil takes longer and longer to process. The root cause is usually that the pre-treatment has been left out.
Particles stand in the way of degassing efficiency
The essence of vacuum degassing is to separate the water vapour out of the oil and draw it away. But particles get in the way: they occupy the pores of the medium, so the filtration resistance rises and the condition of the oil as it passes the element changes from "steady flow" to "an alternation of flow break and turbulence", and the spray pattern from the atomiser deteriorates accordingly. More directly, as soon as the separated solids and the condensate are drawn into the pump chamber by the vacuum pump, the quality of the pump oil falls and the pumping speed drops, the degassing depth of the whole machine follows it down, and the site tends to put the cause down to "the machine is not good enough".
Pre-treatment buys three things
- Consumable life: the coarse filter element 150-250-50 of the ZYD-100 is in itself the first barrier, and adding one pre-filtration stage in front of it noticeably extends its service life.
- Condition of the machine: with the pump oil and the atomising system kept free of solid contamination, the ultimate vacuum is easier to hold, and only then does the working range of -0.08 ~ -0.0999 MPa stand up.
- Rework cost: a batch that fails has to go through the process again, which consumes power, filter media and labour, and it may also contaminate the batch that has already been treated.
How the filtration chain should be arranged
The normal sequence is coarse filtration first, then degassing, and fine filtration last. The element configuration of the TYA series follows this logic: coarse filter element 110-250-50, second-stage element 110-400-20 and fine filter element 110-600-5 in turn, with the oil pump kcb55 and the vacuum pump X-21 working together with the medium heat-absorbing condensing system and the automatic separation of the condensate for online draining. For insulating oil, the ZYD-100 completes the water and gas separation by three-dimensional flash evaporation, and five fine filter elements 150-500-5 close the chain, with a filtration accuracy of ≤1 μm (GB/T 511).
The point is that each stage deals only with the contaminant it is there for: the coarse stage handles solids, the degassing stage handles water and gas, and the fine stage handles the particle accuracy. If they are mixed up, every stage can only just meet its figure.
There are cases where pre-filtration is not needed
New oil delivered to the works, of known origin and with a complete factory test report, is only lightly contaminated and goes into the vacuum machine without any obvious loss. The basis for the decision is not a feeling but sampling and visual inspection: whether the colour of the oil is abnormal, whether visible particles or sediment are present, and whether it has been stored in the open for a long time. Deciding on pre-filtration by feel tends to produce one of two extremes: a stage is added for nothing, or heavily contaminated oil is sent straight into the vacuum stage.
Pre-treatment is not an optional step; it is a deliberate choice that moves the cost from equipment failures onto the consumables.