The fluid flow rate is the first and most important parameter when installing an industrial filtration system.
If you want to ensure that when investing in a new filtration system, it will do everything it can for your business. You need to know what you’re getting yourself into precisely. You may know how much it will cost, and you’ve probably got an estimate of return on your investment. But while you may have decided the price, one thing that could make or break your entire investment: is the fluid flow rate.
If you have questions about how much fluid should go through your new filter or how many liters per minute (lpm) it can handle, this post is for you!
What is the flow rate?
In physics, the flow rate or the volumetric flow rate is the volume of fluid that passes per unit of time; usually, the symbol Q represents it.
Flow rate unit
The SI unit is cubic meters per second (m3/s). However, depending on the industry or the application, engineers also use- m3/hr, lit/hr, lit/min, lit/sec
Flow rate unit conversion
1 m3/second =
- 3,600 m3/hour
- 1,000 liters/second
- 60,000 liters/minute
- 36,00,000 liters/hour
Example – To Convert from m3/h to lit/minute (lpm)
Volume flow in m3/h must be multiplied by 17 to convert to lpm.
Alternatively, use the online fluid flow calculator here.
The equation for flow rate
In simple words, the tendency of the fluid to move from one place to another is its flow rate. Now, let us understand what is fluid and what is not.
Solid objects like potatoes retain shape when they move from one point to another. On the other hand, the water does not keep its form while flowing in a pipe.
In addition, there is a volume change when fluids move, but their mass remains the same. However, in the case of solid objects, the volume and mass both stay the same when they move.
Flow Rate Formula
Flow rate is the volume of fluid that moves in a certain amount of time. It also depends upon the area of the pipe in which the fluid flows and the liquid’s velocity. Hence, the formula is
Fluid flow rate = area of the pipe or channel × velocity of the liquid
Q = Av
Q = fluid flow rate measured in m3/s or L/s
A = area of the pipe or channel in m2
v = velocity of the fluid in m/s
Estimating your flow rate
Sometimes, you don’t know your flow rate or can’t measure it. Sometimes, your flow rate varies so much that getting an accurate measurement is impossible.
In those cases, knowing the actual flow is reasonable but unnecessary—you can use an estimated flow for design calculations instead of an exact number. The difference between estimated and actual may be as much as 10%.
Estimating the order of magnitude is very important to get a correct design. If this is wrong, then your system will be faulty too!
Visual estimation
We can easily calculate the flow in a filtration system using either method.
The first method is to estimate the cross-sectional area and velocity and then multiply them. You can estimate the speed by looking at how fast the fluid moves through the pipe. Then you can multiply these two values to get your estimated flow rate. The method is appropriate when estimating the flow in line with an easily measurable cross-sectional area.
This method is helpful to estimate flow when the “order of magnitude” of the flow is all that is needed. Or when the flow is so low, it is impossible to measure with a flow meter.
The second method uses a stopwatch to measure how long it takes to fill a container of known volume. In this case, you can use your stopwatch to time how long it takes for a known volume of fluid to fill up to calculate your flow rate. This method is appropriate when measuring lower flow rates (up to 200 liters per minute).
Calculating based on machine requirements
We have seen many of our customers take the approach of trying to estimate the fluid flow rate based on their experiments.
However, this is not a good approach since it requires much time and effort. Hence, getting an estimate from the machine manufacturer who designs and builds the machine is better. Usually, this information is available in the machine manual.
Why is it better to get an estimate from the machine manufacturer?
The machine manufacturer understands the machining requirements and designs the machine based on them. They are the best judge of how much fluid they need to machine a component successfully.
Since they understand fluid dynamics inside their machines, they can provide the most accurate estimate of the flow rate required.
Estimating based on pump flow rate
Suppose you’re looking to estimate the fluid flow rate entering a machine, and you have access to the pumps used on that machine. In that case, one approach is to add up the flow rates mentioned on the pump nameplates.
However, it’s essential to remember that these are ideal duty points on the pump curve. In reality, the flow will change with any change in head or resistance from the machine. It will also change if there is any undetected fault in the pump.
Factors affecting the flow rate
A filtration system is an automated process that helps separate the smaller particles in a liquid and discharge the clear liquid. Several factors can have a significant impact on the rate of flow in a filtration system.
Flow rate and pressure relationship
Pressure and flow rate are two of the most critical factors in determining the performance of a system. If the pressure is too low, it will not be able to overcome resistance to flow, resulting in a meager flow rate. If the pressure is too high, it may cause damage to the tubing or equipment.
Choosing a pump for your filtration system is not as simple as it looks. You have to take into account the relationship between flow rate and pressure.
This relationship is vital in designing a filtration system since filters are a form of resistance to the fluid flow and exert pressure.
If we do not correctly estimate the resistance or pressure created by the filters, it would mean wrong pump selection, leading to an incorrect estimate of operational costs.
Viscosity
Many customers ask us this question: What’s the relationship between viscosity and flow rate in a filtration system?
The short answer is that as viscosity increases, flow rates decrease. For example, viscosity is a significant property of oils and lubricants used in many machines to reduce friction. Hence, heavy machines require lubricants with high viscosity and meager flow rates. Thus the machines can be used for a long duration, with very few wear and tear movements.
On the other hand, we need comparatively higher flow rates in machining or metal-cutting operations like tool and cutter grinding. Since the fluids (among many other functions) have to carry away the debris and heat from the machining location. Hence, machinists prefer low-viscosity liquids for these applications.
The flow rate in a pipe
When designing a filtration, two things matter the flow rate and the pipe sizing.
The flow rate is how much liquid you can move through your piping system at a given temperature. The pipe sizing is how big or small your pipes are, affecting how much pressure is required to move the liquid through them.
Keep everything else constant, and double the length of your pipe while keeping its diameter consistent. You will get roughly half as much fluid flow.
On the other hand, keep everything else constant and double your diameter. You will get roughly twice as much flow.
Temperature
Temperature affects both viscosity and density. Fluids tend to expand when heated and contract when cooled, so if you increase the temperature of a liquid, its viscosity will decrease. Conversely, if you reduce the temperature of the fluid, its viscosity increases.
The higher the fluid temperature, the easier it is for its molecules to move around—therefore, its viscosity decreases. In other words, hot fluid flows more freely through your pipes than cold fluid!
But what if you’re using cold fluid? In that case, you must keep an eye on how much pressure you’re putting on those pipes because cold liquids are more viscous than warm ones! Hence, they require more force to push them through your pipes at any speed.
How an incorrect estimation of flow rate affects your system design?
By now, you must have understood that the flow rate is one of the most critical factors in designing a filtration system. A wrong estimate can lead to disastrous effects, so it’s essential to get it right.
Under-estimating the flow rate
If you base your design on a lower rate than what’s required, the following will happen:
- Pumps will be undersized, which can cause them to draw more electricity and wear out faster—leading to increased operational costs. It can also lead to frequent breakdowns and increased downtimes.
- Tanks holding the fluid will be undersized so they will empty more frequently. Hence, the possibility of the dry running of pumps increases- leading to higher operational costs.
- Pipe sizing will be wrong, leading to unexpected pressure rises at random places. If a pipe is too small for its job, it won’t be able to handle the flow it’s supposed to take and will experience damage.
- Filtration area calculations will be wrong, leading to frequent filter changes that add up fast when running a facility 24/7/365! That means you spend more money on filters.
- Machines won’t receive the expected flow, which might affect their working conditions and how well they perform.
Over-estimating the flow rate
When investing in a system, it’s tempting to overestimate your flow rate. You want to ensure that your system will work well and last a long time. Hence, it’s natural to think “bigger is better.”
But don’t do it! There are three reasons why overestimating your flow rate is a bad idea:
Firstly, you will need higher-capacity pumps to pass the fluid through your system. Hence, they will draw more electricity. Consequently, it means it will cost more than necessary.
Firstly, you will need higher-capacity pumps to pass the fluid through your system. Hence, they will draw more electricity. Consequently, it means it will cost more than necessary.
And finally—and most importantly—the filtration area will need to be significantly oversized to accommodate this higher operational volume leading to a higher footprint.