E: Airslides

E1: Description and Characteristics

The airslide consists of two main sections. One being the material trough, which

is the upper section and is used for material flow and the lower section which is

used for the air trough. Both sections are firmly bolted together and divided by a

layer of canvas. A wire fabric is installed at times on the top side of the canvas

and at impact points and high wear areas.

The normal slope of an airslide from inlet to outlet is 7° minimum to about 12°

maximum. Air is supplied by a fan or blower to the lower section via duct work.

Air should injected at the top of the slide with midway boosts for very long

installations. Injection points at the bottom are not recommended. Also Fuller

recommends about 10 scfm for each square foot of canvas and an undercanvas

pressure of 16 to 17 inwc. However under certain circumstances some plants

are discovering that this pressure can be reduced to as little as 8 inwc. Some

experimenting is required to find the correct setting. Note also that over-aeration

can adversely affect volumetric equipment such as Bucket Elevators and F.K.

Pumps.

Material flow is accomplished by a combination of air flow through the canvas,

slope of the airslide and the weight of the material. Air essential fluidizes the

powdered material so it slides down the trough or box. However there must be a

minimum thickness of material above the canvas for this to happen. If it's too

thin, air will rifle up through in localized spots. This will appears as bubbling or

boiling with no movement of material. For this reason airslides can never be

purged 100%, unless keeched (scraped) out. This may be an important

consideration where cement contamination is an issue.

On the other hand over filling the airslide can cause problems as well.

Essentially there is no room above the material for air movement. Thus air will

not/can not fluidize material because it's dead headed. Unfortunately there are

many airslides that are equipped with flow trim gates or flow control valves.

Effectively material is backed-up creating the situation just described. To

compensate many installation use higher pressure blowers or compressed air

jets in this section, but even this will not always work. Ideally such flow control

devices should be placed in the chute feeding the airslide and not the airslide

itself. Interestingly some suppliers build airslides that step up in height

progressively down hill. In theory air expands as it de-compresses. As well, on

long installations, more room must be provided for multiple injection points of air.

Such "high boy" units are designed to meet these requirements.

Glass ports may be installed on the sides of the upper section to monitor material

flow and inspection doors may be installed on the top section for clean out

purposes and sampling. Airslide venting should be done downstream.

NOTE: When using an air hose to aid in material flow or cleaning do not leave

the air blowing directly on the canvas as the air/material combination

will cut through the canvas.

Airslide Design Considerations

Figure E1:

Pressurized should be introduced at the top of the airslide, where it can fluidize

the material coming in. The dedusting duct should be at the bottom of the

airslide run. This helps promote the downward flow of material.

In the arrangement shown in Figure E2, pressurized air is introduced at the

bottom of the airslide run, which does not fluidize material except at the last

minute. This is particularly bad if you are feeding a FK pump or bucket elevator.

Dedusting at the top should only be done if large amounts of air are needed to

get material flowing into the airslide (e.g. silo). A straight dedusting ducts usually

has high intake velocities, promoting wear and picks up excessive quantities of

dust instead of air. Flared transitions are a must.

Figure E2:

Figure E3:

Shown Figure E3 is a "high boy" airslide. For long runs, the high pressure air

injected at the top, de-pressurizes and expands. The "high boy" ensures room

for this expansion. Without it airflow may become restrictive, causing the flow to

"deadhead". As a result the material may not fluidize and flow properly. Over

filling an airslide will have the same effect.

Many airslides are designed with flow control gates and valves as part of the

assembly, shown in Figure E4. This works OK if the airslide is in near constant

use. Despite constant use, lumps can still cause the chamber above the gate to

overfill, resulting in deadheading. Typically, after a temporary shutdown, the

airslide is never fully purged and material accumulates ahead of the gate. (This

is especially bad in silo applications where the silo shut-off is leaky.) Given

enough time, the material de-aerates and becomes difficult to move the next time

the airslide is started up. Again the chamber can overfill, resulting in

deadheading.

Figure E4:

A better arrangement is shown in Figure E5, whereby the flow control gate is

located at the airslide inlet - not as part of it.

Figure E5:

Airslide Trivia:

1) Invented in Alpena.

2) Airslides not mix materials. Segregation has been known to happen.

3) Material in airslides does not flow evenly. If the canvas is rippled or

buckled, material can flow in rivers. Loaded off-center, material flow can be

deeper on one versus the other - even after 50'.

4) If the bed is not thick enough, air will take the path of least resistance and

rifle through the material. The rest of the material will not flow. Surging can

occur. Increasing the air pressure can have the same effect.

E2: Trouble Shooting Airslides

Condition: Plugged, material not flowing.

1. Check equipment in front of it for proper operation. If at fault, fix and repair.

2. If okay, check discharge of slide for plugging. If plugged, clear plug up.

3. If discharge is clear, check airslide fan to ensure it is running, filter is clean, and the duct work to the airslide is not plugged or closed off.

4. If the above is okay, check the air chamber of the slide for material build-up by tapping lightly with a hammer. If plugged, check for hole in the canvas. If the air chamber is okay, check for restriction on top of canvas and clear canvas of all restrictions and resume operation.

WARNING: DO NOT HAMMER ON THE SIDES OF THE AIRSLIDE.Denting the airslide box will cause the canvas to buckle or ripple which will retard the airflow and affecting the transport rate. The uneven flowrate will also cause holes to wear into the canvas.