The fine filter stage of the ZYD-100 is fitted as standard with 5 elements of 150-500-5 in parallel, and that configuration is not the only possibility. There is also a fine element in the size 110-600-5, and the outer diameter and the length are different. Swapping the fine element on the same machine has a cost and a benefit that both have to be worked out, and it is not a question of looking at the accuracy figure alone.
What the three numbers in a specification mean
The element specifications in use are written in three parts: the outer diameter, the length and the filtration accuracy. The coarse elements 150-250-50 and 110-250-50 carry 50 in the third position, meaning 50 μm. The second stage 110-400-20 is 20 μm. The fine elements 110-600-5 and 150-500-5 carry 5, meaning 5 μm. The accuracy of the two fine elements is the same grade, and they differ in the outer diameter and the length, so when an element is ordered against the wrong number, it is the dimensions and the connection that are wrong rather than the accuracy.
The filtration accuracy of the machine is ≤1 μm, and that is the result of several stages in series: the upstream elements hold back the large particles and the last stage does the fine separation. The cleanliness is assessed as NAS 1638 class 5, and that also depends on the stages in series. A single fine element on its own cannot bring the accuracy of the machine down to 1 μm.
The difference between the two fine element specifications
The 150-500-5 has an outer diameter of 150 and a length of 500, and the 110-600-5 has an outer diameter of 110 and a length of 600. The outer diameter sets the filter area per unit length, the length sets the total amount of filter medium, and the two of them together are what the element can really hold. An element with a large diameter but a short length and one with a small diameter but a long length take the same space in the same housing, yet their dirt holding capacity and their differential pressure behave differently.
On the ZYD-100 there are 5 elements in parallel, so the area and the dirt holding capacity of the five are added together and then shared out over a flow rate of 6000 L/h. Each element therefore carries less flow and the differential pressure rises more slowly. What the parallel arrangement buys is steadier running and fewer interruptions because a single element has reached its differential pressure limit, and that is a direct benefit on site.
The trade between accuracy and throughput
Set the accuracy high and the throughput has to give way; set the accuracy wider and the throughput can be taken instead. That is the most concrete exchange in selecting a filter element. There are three conditions on site that decide it. The first is what the oil type requires, because transformer oil and hydraulic oil do not tolerate particles in the same way. The second is the actual size distribution of the particles in the oil: when the particles are generally coarse, the extra life gained by narrowing the accuracy is far less than the flow given up. The third is the installation conditions of the housing, because the outer diameter of the element is limited by the inner diameter of the housing and the length is limited by the tank and by the space needed for maintenance, and these two often settle the choice before the accuracy is discussed at all.
What to decide the selection on
The order of the decisions is this: first fix the machine accuracy that the oil type and the test requirement demand, then fix the outer diameter and the length from the installation conditions of the housing, and only then compare the dirt holding capacity and the differential pressure among the specifications that are left. Once that is settled, the number of parallel elements has to be worked out with it: more elements is not automatically better, because more elements means less flow for each one and a slower differential pressure, while the amount of dirt a single element can hold does not increase and neither does the replacement cost fall.
| Element specification | Used in which section | Filtration accuracy | Notes |
|---|---|---|---|
| 150-250-50 | Coarse section | 50 μm | Holds back the large particles and protects the sections behind it |
| 110-250-50 | Coarse section | 50 μm | Chosen against the first one depending on the housing |
| 110-400-20 | Second stage | 20 μm | One more layer of filtration, taking the load off the fine section |
| 110-600-5 | Fine section | 5 μm | Small diameter, long, chosen against the installation space |
| 150-500-5 | Fine section | 5 μm | Standard configuration of the ZYD-100, 5 elements in parallel |
- Give the full three part number when ordering an element; reporting only the accuracy is where the errors come from.
- Check the inner diameter of the housing and the installed length before changing the specification; do not choose from the element catalogue alone.
- The number of parallel elements follows from the flow rate and the installation conditions, so do not add elements to stretch the service interval.
- Rebuild the differential pressure baseline after changing the specification; the curve of the old and the new element cannot be compared directly.
Selecting a filter element is not a matter of taking the highest accuracy available, but of finding the combination that fits the oil, the housing and the share of the flow.
