Flow Performance
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This flow bench is so simple that it makes it very easy to understand what a flow bench is and how it works. An air flow bench is a device that measures the volume or mass of air that will pass through a bounded path or conduit in relation to an applied pressure differential. The pressure differential is usually relative to ambient air pressure and is often produced by some  mechanical means such as an electric blower or at least one electric vacuum motor. The flow bench air source creates a pressure differential across the test object that causes air to flow through not only the test object but through the entire flow bench. At some point in the flow bench the air flow velocity is measured. From the measured air velocity, the volume or mass of air that is flowing through the test object can be calculated.

A flow bench with an air source that can apply a pressure differential to the object being flow tested is referred to as an active flow bench. A passive flow bench has no air source, the object being tested supplies its own pressure differential to promote measurable air flow, such objects would be electric blowers or fans and vacuum motors.
It is often believed that flow benches are only used for flow testing internal combustion engine cylinder heads, but in fact many other items like carburetors, intake manifolds, exhaust manifolds, turbocharger housings, air filters, pneumatic couplings and fittings, AC grills and condensers, and nozzles of different types often find there way onto a flow bench. Early flow benches were mostly used to test aircraft carburetors.

The pressure differential applied to the test object is important to know since air flow is relative to the applied pressure differential. This pressure is usually measured in Inches of Water Column, or "wc. If you place a drinking straw into a glass of water and draw the water up the straw 1 inch above the water level, that would be 1"wc pressure less than ambient air pressure, or -1"wc. If you apply a pressure to the drinking straw that causes the water level in the straw to be below the water level in the glass by 1 inch, that would be 1"wc greater than ambient or +1"wc. 28"wc is about 1 PSI (Pounds per Square Inch) pressure. Internal combustion engine parts are often flow tested at 28"wc.

Air flow through a bounded path or conduit is often measured as a volume of Cubic Feet per Minute, or CFM. 

Most all flow bench designs include the major components shown above. While some flow benches may have an orifice inside a larger settling chamber instead of a flow element, the basic design and layouts are still very similar.
       An active flow bench usually has an air source that can apply both a negative pressure to the test object (vacuum) or a positive pressure to the test object. A shop vac with a blower port (sometimes refered to as a leaf blower port) is a good example of a bi-directional air source. Some test objects, like cylinder heads, need to have bi-directional air sources so that both intake ports can be flow tested with a negative pressure and exhaust ports can be tested with a positive pressure.
       The air source exerts an air pressure to the flow element, receptacle and test object, causing air to flow through the entire flow bench and test object. The pressure exerted on the test object is measured at the receptacle, and the velocity of the air that is flowing through the flow element is measured and calculated to CFM by the flow computer.
Electronic flow computers are replacing liquid manometers as the price of these devices and personal computers continue to drop. Flow computers can measure test pressures and velocities within a flow element, calculate the CFM and display this data on a computer screen in real time as the flow test is being performed. Computers can also store this information in data bases for later retrieval and analysis.
Automobile engine cylinder heads are often flow tested on an active flow bench. An adaptor plate with a cylinder that is the same diameter of the cylinder(s) that the head will be bolted onto is used to adapt the cylinder head to the flow bench. The adaptor cylinder simulates the actual cylinder of the engine so that the effects of the cylinder surfaces on the flow through the cylinder head ports can be realized.
A typical flow computer takes the place of liquid manometers, and can also calculate and display CFM in real time as the flow test is in progress. A flow computer can also read both the test pressure and the air flow velocity through the flow element at the same instant. This may not seem like a great deal unless you have tried to control and read the test pressure and the pressure at the flow element on liquid manometers, and write these values down on a piece of paper.
Here is a professionally built active flow bench used by race car engine builders. Notice the aluminum cylinder bore adaptor with the automotive cylinder head on top. This type of adaptor is often referred to as a test stand.
This flow bench uses multiple vacuum motors to create very high flow rates and test pressures from its air source.
Inside this flow bench you can see the air source with a test pressure control valve (very upper left corner) attached to the air source plenum box. This test pressure valve is controlled by the flow computer, and will open or close, bleeding off excess pressure to achieve a desired test pressure on the test object. You can also see the 2.5" diameter flow element (gray pipe) that is connected to the air source with the 4" PVC white pipe.
Here is an example of a Flow Performance passive flow bench. While an active flow bench has a powered air source to apply an air pressure to the test object, a passive flow bench has no powered air source. The test object supplies the air pressure on a passive flow bench.
       This passive flow bench is used to test electric blowers.  The electrical power used by the blower motor is monitored during these tests. The static pressure and flow rate that the blower can produce is measured and recorded along with the electrical power consumed for performance analysis.
       A variety of different sized orifices that create a restriction on the blower motor can be installed to test the blower under different loads.
Just when you thought you knew everything about flow benches, now we look at the two types of active flow benches. One type of flow bench locates the air source at either end of the flow bench to create the pressure required to move air through the system and test piece. This type of system sees approximately the same air temperature throughout the entire system. Another type of active flow bench locates the air source between the flow element and test piece. This is often called a "center blower" bench, and the test object and flow element usually do not see the same air temperature. This is because blowers often heat the air as the air passes through the blower. This type of flow bench will often require a temperature compensation applied to the flow readings.
There are two popular methods of measuring the velocity of air flow through a flow bench, one method is by restricting the air flow through an orifice. Another method is by using a Pitot tube. Both methods are classified as "Differential Producers" and are square root flow devices. This means that each method create a pressure differential between two measurement points. This pressure differential is used to calculate the velocity of the air flow by the square root of the differential pressure, and from the velocity, the volume of air can be calculated. Most experts in fluid mechanics will agree that the possible accuracy of both these methods is about the same.

Orifice velocity measurement
Pitot tube velocity measurement
Air planes and race cars use Pitot tubes to measure air speed. Here you can see the silver L-shaped Pitot tube extending from the side of the fuselage just above the landing gear. This air craft uses two of these Pitot tubes, the other is on the other side of the fuselage and if you look closely you can just see the tip of that Pitot.
FlowBench 101     What is a Flow Bench     How does a Flow Bench Work
Got more questions? Give us a call or email your questions to us, we would be glad to answer any questions we can.
You can see the air speed Pitot tube on this Formula 1 car protruding up from the body work and angled forward just between the front wheels.
Methods of Controlling Flow Bench Test Pressure

A flow computer can estimate the flow rate of one test pressure to another test pressure, eliminating the need to control test pressure in low powered flow benches. If test pressure control is desired, and is usually needed on higher power flow benches, there are three basic ways to accomplish test pressure control.
Test pressure can be controlled by regulating the electrical power to the air pressure source, a vacuum motor in this example.
Test pressure can be controlled by restricting the air flow to and from the air source.

Test pressure can be controlled by bleeding-off, or venting excess pressure to the atmosphere.

Test Object or Test Piece
An Automobile engine cylinder head is often flow tested on a flow bench.
Test Port
Automobile engine cylinder head ports are conduits that transfer air into the engine for combustion. By flow testing these ports on a flow bench and making improvements to the ports, engine efficiency and power can often be improved.
Cylinder Bore Adaptor or Test Stand
A flow bench often requires an adaptor of some type to be fabricated to adapt the test object to the flow bench surface in an air tight fashion. Automobile engine cylinder head adaptors often include a section of cylinder about the same diameter of the engine cylinders that the head will be bolted onto. This section of cylinder will help simulate the effects that the cylinder surface will have on the flow of the cylinder head.
Bench Surface
Most flow benches have some sort of a bench surface that provides a platform to help locate test objects on the flow bench. The flow bench surface will have a round hole, often called a discharge port, that is 5 to 4.5 inches in diameter. This discharge port is a conduit that connects the air flow from the flow bench to the test piece.
Bench Receptacle or Settling Chamber
The flow bench receptacle or settling chamber provides an air flow  transition from the bench discharge port to the air flow measuring device. It is also where the test pressure that is applied to the test piece is measured. Air flow from some test pieces can be very turbulent. The flow bench receptacle or settling chamber also helps reduce this turbulence, hence the name "settling chamber".
Flow Element
The flow bench flow element measures the velocity of the air that is passing through the element. From the air flow velocity through the flow element, the volume of air that is flowing through the test piece can be calculated.
Air Source or Air Supply
Most flow benches have an air source that will exert an air pressure on the test piece that will cause air to flow through the flow bench and test piece. Most flow benches will have a bi-directional air source that will cause air to flow in both directions through the test piece. This is often required for automobile engine cylinder heads that require a vacuum, or negative pressure, for testing intake ports, and a positive pressure for testing exhaust ports.
First, we must determine the fluid density of the liquid in your mechanical manometer. Your mechanical manometer is only correct at one ambient temperature. This is because the density of your manometer indicating fluid changes with temperature. We determine the fluid density with this equation:

Ho = (Pt /Po) * Ht   where Ho is the corrected height of fluid, Pt is density of fluid at current temp, Po is density of fluid at standard temp, Ht is the height of fluid

Now we must determine the local gravity. This can be found on your local government geographical charts, or if you have a GPS or you know your GPS coordinates, you can look it up on the internet. The gravity of your local area will have an effect on your manometer and you may need to make corrections to your readings. The equation for making gravity corrections is:

Ho = ((Gt * Pt) / (Go * Po)) * Ht   where Ho is the corrected height of the fluid, Gt is local gravity, Go is standard gravity.

One more thought on mechanical manometers is drainage time. When the fluid of a mechanical manometer is evacuated, or lowered to a lower level in the glass tube, it takes a little time for all of the indicating fluid to drain from the surface of the glass tube. This can be a matter of minutes for some manometers.

While mechanical manometer are probably incapable of "lying", they can certainly give an untrained user a false or incorrect reading.

Velocity probes are usually Static/Pitot tubes, and are used to map local velocity profiles inside intake and exhaust ports.
Pitot tubes, or simple variations, are used to sense static pressure. Static sensing Pitots are often found in orifice style flow benches, to measure the test pressure and the pressure differential across the measuring orifice.
The question often arises about which is better, a mechanical manometer (fluid) or digital manometers? Some argue that mechanical manometers are never wrong. I once read a post on an internet forum that "[mechanical] manometers never lie". Well, I would agree to that, but let's look at the setup procedure for a mechanical manometer.
Manometers are devices used to measure air pressure, and are often used to measure flow bench test pressure, and the pressure created by the air velocity measuring device. A vertical manometer is often used to measure the test pressure, while an incline manometer is used to measure the pressure created by the measuring device. While a vertical manometer has a large range, the incline manometer has better resolution, but limited range.
Pitot Tubes are found everywhere, and it is estimated that more Pitots are in use for measuring air velocity than all other types of pressure differential producers combined.