Hopper Discharge Devices

A discharge device is necessary for emptying the hopper and can be manual or automatic. The simplest manual discharge device is the slide gate, a plate held in place by a frame and sealed with gaskets (Figure 1). When the hopper needs to be emptied, the plate is removed and the material is discharged. Other manual discharge devices include hinged doors and drawers. The collector must be shut down before opening any manual discharge device. Thus, manual discharge devices are used only on very small units that operate on a periodic basis.

Automatic continuous discharge devices are installed on ESPs that operate continuously. Some devices include double-dump valves (also called double flap or trickle valves), and rotary airlock valves. Double-dump valves are shown in Figure 2. As dust collects in the hopper, the weight of the dust pushes down the counterweight of the top flap and dust discharges downward. The top flap then closes, the bottom flap opens, and the material falls out. This type of valve is available in gravity-operated and motorized versions

Rotary airlock valves are used on medium or large-sized ESPs. The valve is designed with a paddle wheel that is shaft mounted and driven by a motor (Figure 3). The rotary valve is similar to a revolving door; the paddles or blades form an airtight seal with the housing, and the motor slowly moves the blades to allow the dust to discharge from the hopper.

After the dust leaves the discharge device it is transported to the final disposal destination by screw, drag, or pneumatic conveyers. Screw conveyors can be used as discharge devices when located in the bottom of the hopper as shown in Figure 4 or as a separate conveyor to move dust after it is discharged. Screw conveyers employ a revolving screw feeder to move the dust through the conveyor. Drag conveyors use paddles, or flaps, that are connected to a drag chain to pull the dust through the conveyor trough (Figure 5). Drag conveyors are used frequently for conveying sticky or hygroscopic dusts such as calcium chloride dust generated from municipal waste combustors (collected fly ash/acid gas products). Pneumatic conveyers use blowers to blow or move the dust through the conveyor (Figure 6). Pneumatic conveyors can be positive pressure (dust is moved by a blower) or vacuum type systems (dust is pulled by a vacuum). In large ESPs, dust is usually discharged from hoppers by using a combination of devices. Either rotary airlock or double dump valves empty dust into screw, drag, or pneumatic conveyers that move dust for final disposal into trucks or storage bins.

                                          figure 1- slide gate

Figure 2- Double-dump discharge device

Figure 3- Rotary airlock discharge device

Figure 4- Screw conveyor

Figure 5- Drag conveyor

Figure 6 - Pneumatic conveyor for transporting dust from ESP