One of the most difficult things to design and build for a flow bench is the air source. What are some attributes to the perfect air source?

As you can see, we require a lot from an air source for flow benches. In this installment we will consider solutions to many of these requirements, as we concentrate on a flow bench air source.

In Macro Bench part 1, we will examine a solution for a flow bench air source.

Let's look at some of the problems often encountered in typical air sources:

If large panels are used in the construction of an air source, they must be extemely strong and often require bracing. Flow bench implosions and explosions are not too uncommon when sudden over pressure is caused by something obstructing the bench dischage port. If your bench is producing 28" of water column (wc) pressure, this is close to 1 pound per square inch of pressure (1 psi). Now, if you have a panel that is, say 2 feet x 3 feet in size, that panel is 824 square inches. 864 inches times 1 psi = 864 pounds of pressure exerted on that panel. Some people flow test at 36"wc, 48"wc and 60"wc. You can see that limiting the size of the panels in an air source can go a long way in making a structure safe at higher test pressures.

Often, an air source uses multiple vacuum motors to create the air displacement needed in a flow bench air source.  It is often helpfull to be able to turn on only the number of vacuum motors required to produce the desired test pressure. This ability helps in the adjustment of the test pressure by not overpowering test pressure adjustment devices such as bleed valves or motor speed controllers. It also reduces noise and power consumption, both of which can be quite high for typical vacuum motors. The problem with being able to turn on individual motors in a multiple motor flow bench is that the motors that are not turned on will leak air, or worse, begin to spin backwards. So the motors need check valves to keep air from moving through them in the reverse direction.

If it were not for the need to reverse air flow, air sources would be much easier to design. Reversing the air flow in an air source is probably the most difficult thing to try to do in a simple and efficient way.

If large panels are used in an air source, they are not only subject to enormous pressures as mentioned above, but they have a tendancy to resonate, creating very loud noises. This can be very annoying, to you, to anyone near you, or in your house, shop or even your neighborhood. Again, the smaller the panels the better off you will be in your design.

The diagram above shows how this air source addresses many of the problems encountered when designing an air source.

Multiple vacuum motors are installed into an air box (grey) that slides from side to side inside a frame (red). By sliding the box containing the motors over a discharge port located on the bottom of the frame, the air direction is easily changed without having to operate any valves or doors.

The air box has two channels, the intake and exhaust channels. The motors are mounted in the exhaust channel, and the exhaust is vented out the top when air is being drawn from the flow bench (forward flow).

Simple flapper style check valves are installed in the intake channel that allow air to flow in only one direction. If the motors are turned on from bottom to top, the check valves will not allow air to leak through non-operating motors above.

When the air flow is reversed by sliding the air box to the right side of the frame, the air box pushes up on a lever, at the top of the frame, that exerts pressure on a panel that rests on top of the air box. This floating panel blocks off the exhaust air vent at the top of the frame, and forces the exhaust air to exit through the frame discharge port at the bottom. The motors also take in air at the bottom of the fram through an inlet port.

The air box panels are relatively small, so they will not accumulate excessive pressure and they will not tend to resonate.

Let's do a quick overview of this solution so that you can get an idea of how this idea has been executed.

The material used for this flow bench is 3/4" melamine. Melamine is a plastic coated partical board, and is a very good material for this project. It is assembled using coarse thread deck screws.

This side view shows how the air box is moved from side to side inside the frame. Handles are mounted on the air box and protrude through openings in the frame. The operator pushes or pulls on these handles to slide the air box to either side of the frame, changing the air direction.

This rear view shows the air box located inside the frame. You can also see one of the pull handles on the left side of the air box. A flexible conduit routes the wires from the control panel to the vacuum motors inside the air box.

This view from above shows very important features on the top crown piece: A hole for routing the laptop wires through the top and also an aluminum window screen over the opening. This screen is a spark arrestor in case any of the vacuum motors throw off any sparks or hot pieces of material.

The two photos above show an above view with the top crown piece removed. The top photo shows the air box in the forward flow position (flow bench is in vacuum mode). The lower photo shows the air box moved to the left for reverse flow (bench is in positive pressure mode). When the air box is to the left, the top of the air box pushes up on the lever, which pushes down on a panle that rests on top of the air box, and tightly seals off the motor exuast vent opening.

This "floating" panel seal is needed because the length of the air box changes with humidity and temperature.

This photo looking up at the top of the frame and air box, shows the floating panel that rests on top of the air box, and the lever protruding through the panel. When the air box is moved to the left, it will push up on the lever, and the lever will push down on the panel that is resting on top of the air box, sealing off the air box exhaust vent.

A 1/4" lag bolt threaded into the lever pushes down on a metal strap that is screwed onto the floating panel that rests on top of the air box. The pressure exerted by the lever on the floating panel can be adjusted by turning the lag bolt in or out.

This view looks down on the bottom of the frame, showing the panel that the air box rests, and slides on. The intake port for reverse flow mode is visible. The bottom panel and the bottom panel of the air box are melamine, a plastic coated particle board. These surfaces slide across each other very easily, especially if they have been waxed with an automotive wax, and kept clean.

This is a view looking up from the floor to the bottom of the frame. It shows a 4" toilet flange mounted over the frame's discharge port, with some 4" pipe connected to the flange. The upper 3" pipe opening is for a test pressure bleed valve, and the lower 4" pipe opening goes to the flow bench. The intake port for reverse flow mode is also visable to the right of the discharge port.

This flow bench uses an "open" bench surface that allows the use of C clamps and other clamping devices to easily be used to secure the test piece to the bench.

Here is where the actual flow bench "guts" are located, inside this bay. Some 3/4" angle iron brackets on each side are used to secure the open bench surface.

For digital electronics, there is nothing better than a lap top computer. These brackets are easy to make and secure to the main front panel of the frame. These brackets allow the lap top to simply rest on the brackets, and for the lap top to easily be removed for use in other places. Thick foam rubber strips on the surfaces that contact the lap top isolate the lap top from vacuum motor vibration.

In Part 2 we will take apart the air source, see how it is built and get some measurements

The top crown piece is just a simple structure that rests on top of the frame to give it a finished appearance. It can also house a light if desired. But the most important thing this crown does is supply a spark arresting screen across the top of the air source incase one of the vacuum motors start to come apart.