When Buying Filters, Here Are The Things That Actually Matter
We've all been there - a large-scale operation with large amounts of lubricants or fuels being used annually and then the salesman arrives. And those salespeople will push anything on you because well, they live on commission. However, when trying to make an informed decision about the filtration of lubricants or fuels, what are the actual metrics that make an actual difference to the process?
Firstly, however, I think it's important that we put these catchy sales terms to bed because they might be good at selling you something, but they don't contribute at all to your bottom line.
Dirt-Holding Capacity
This is a favourite of the salesman, and for good reason - it's a relatable metric that helps them sell. In all honestly, this metric is outdated and does not mean much.
Dirt-holding capacity (DHC) is a metric that tells you how many grams of dirt a filter can hold until a filter element reaches its terminal pressure drop. DHC should be determined in line with ISO 16889. However, even in this case flow rates and terminal pressure drops are left to the decision of the relevant laboratory.
Why do I say this is a rather meaningless sales term? Well, simply put - it does not matter when it comes to saving your process. For example, a bag filter has a much higher DHC than a high-efficiency filter, but the actual particle capture capability varies widely.
In a critical system such as a hydraulic piston, I care less about the run rate of filters, and more about the cleanliness of lubricant that reaches my piston. DHC is a measure of run rate (which no one can really predict) but tells you nothing about how clean a filter can make your lubricant or diesel.
Special Fixed Media Technology
This one actually has a grain of truth, but still, the idea irritates me. Many manufacturers will tout the technology they have that will decrease media migration (which is a very good thing).
My objection is as follows: this shouldn't be a selling point, it should be what you aim for.
When media migrates in a filter medium, the beta efficiency (we'll discuss this later) decreases rapidly. This is because when the media moves it creates areas of higher pressure and flow and larger holes for particulates to move through.
Simply put - if a salesman says they have some special media anti-migration technology, reply with, "Well, I surely hope so."
The Things That Actually Matter
Now that that's out the way, what are the actual variables that you should consider when selecting a filter for your application? I design filtration systems daily and at it's bare-bones, as the client you should be looking at the following 4 things:
Let's look at each of these briefly
1 Pressure
Pressure is a system input parameter so should remain fairly constant throughout service life (excluding applications with cyclic loading). This aspect will mainly impact the housing you receive from the manufacturer - if you have a high-pressure application, make sure you are receiving a filter housing that can handle the pressure and vice versa.
Cyclic loading is trickier to quantify, but if your application is prone to system shocks then you will want to look at the media itself. For this make sure the media has been tested in accordance with a standard such as SAE ARP 4205 which looks specifically at how media reacts under cyclic loading.
Recommended by LinkedIn
2 Flow Rate
Flow rates are often a misunderstood, albeit important part of filtration. Many people think if I have higher flow rates, it automatically means I need large filter housings, but this isn't always true.
The better way to discuss this with your filtration supplier is by looking at what flow rate you require at the interaction point and then looking at what the pressure drop will be over your filter. This pressure drop is influenced by the fluid viscosity, flow rate, and pore density of your filter.
And don't forget to make sure that your pressure drop can be overcome by your pump. You might specify a very small pore size filter with a high-pressure drop and find your pump trips or your filter housing is continually in bypass.
A good filtration supplier should be able to explain and assist you with this process.
3 Beta Ratio
This is so important and should really be your main selling point when discussing filtration. The beta ratio of a filter is simply a way of quantifying how much contamination a filter media can remove from a fluid stream in one pass.
As you'll see in the picture below, this means that if I challenge my filter with 1,000,000 particles of a certain size upstream, how many can I expect to remain downstream. A beta ratio of 1,000 is depicted below and says that if I challenge the filter with 1,000,000 particles with a size of 5 microns, then I can expect 1,000 to make it through to the other side.
As a rule of thumb, the higher the beta ratio the better because it will have higher cleaning efficiency. This does however come at a cost, so it's often beneficial to look at the criticality and tolerances of the system under consideration and then decide from there about the type of resources that should be dedicated to it.
4 Terminal Pressures
A terminal pressure is denoted as differential pressure and relates to the blockage of flow through a filter medium. As a filter medium slowly becomes blocked, the flow will become more and more restricted and pressure will increase on the one side of the filter creating a pressure gradient across the media structure.
This differential pressure is actually what causes a filter to fail. The inequality of forces causes the media to shift and the structure of the element itself to distort until the media ruptures and releases all the captured contamination back into the system.
Filter manufacturers should indicate the change-out differential pressure on the filters datasheet. This is how we tell when a filter has reached its useful life, compared to the old time-based measurement methods. Higher change-out differential pressures ultimately mean longer filter life.
Conclusion
And there you have it. Obviously, there are a few more nuances we could discuss. However, if you had to come in as a client to me and discuss these specific variables, I would know that you are serious about your operation.
PS - I didn't mean to bag on any salespeople, I just think there are better ways to prove the effectiveness of their product.