Discharging rate

Because mass flow bins have stable flow patterns that mimic the shape of the bin, permeabihty values can be used to calculate critical, steady-state discharge rates from mass flow hoppers. Permeabihty values can also be used to calculate the time required for fine powders to settle in bins and silos. In general, permeabihty is affected by particle size and shape, ie, permeabihty decreases as particle size decreases and the better the fit between individual particles, the lower the permeabihty moisture content, ie, as moisture content increases, many materials tend to agglomerate which increases permeabihty and temperature, ie, because the permeabihty factor, K, is inversely proportional to the viscosity of the air or gas in the void spaces, heating causes the gas to become more viscous, making the sohd less permeable. 

Outlet Size Determination. The second consideration for proper design of a mass flow bin is the size of the outlet required to prevent arching and to achieve the required discharge rate. 

Usually the rate, Q, is far in excess of the required rate, especially if the bulk material consists primarily of coarse particles. Slowing down the discharge rate requires a feeder. Fine powders, on the other hand, have considerably lower maximum discharge rates when exiting from a mass flow bin, because of the interaction between air (or gas) and solid particles as reflected in the permeabiUty of the material. 

To be consistent with a mass flow pattern in the bin above it, a feeder must be designed to maintain uniform flow across the entire cross-sectional area of the hopper outlet. In addition, the loads appHed to a feeder by the bulk soHd must be minimised. Accuracy and control over discharge rate ate critical as well. Knowledge of the bulk soHd s flow properties is essential. 

Rotary V lve Feeders. Devices known as rotary valve feeders are commonly used for circular or square configured outlets. These are particularly useful when discharging materials to a pneumatic conveying system where a seal is required to prevent air flow through the hopper outlet. The discharge rate is set by the speed of rotation of the vanes or pockets of the valve. 

The key to solving these problems is to design the vessel for a mass flow pattern. This involves consideration of both the hopper angle and surface finish, the effect of inserts used to introduce gas and control the soHds flow pattern, and sizing the outlet valve to avoid arching and discharge rate limitations. In addition, the gas or Hquid must be injected such that the soHd particles ate uniformly exposed to it, and flow instabiHties such as fluidization in localized regions are avoided. 

Fig. 8. Sedimentation equipment performance where the particles have a A5 value of 1.0 g/cm and a viscosity, p., value of 1 mPa-s(=cP). The value of is twice the settling velocity at G = 1, and Q = overflow discharge rate in measurements given.

Magnetic Discharge Kate. In order to maintain high magnetic media recovery, the magnetic discharge rates should not exceed 9 MTPH/m for 762 mm dia dmms or 15 MTPH/m for 914 mm dia dmms. 

The effect of the discharge rate is especially pronounced for the general purpose cells. On intermittent tests, the heavy-duty cell operates at high efficiency even at high rate. On continuous test at high rate, heavy-duty cells provide 60—70% of the intermittent service, whereas general purpose cells give only 30—50% of the intermittent service values. 

The chemistry of the Li—FeS2 system is quite complex. There are at least two steps to the reaction at low discharge rates. The first reaction is an approximately two-electron reduction to a new phase which is a hthiated FeS2 compound. 

Lower cost and lower weight cylindrical cells have been made using plastic bound or pasted actwe material pressed into a metal screen. Tliese cells suffer slightly in utilization at high rates compared to a sintered-plate cylindrical cell, but they may be adequate for most applications. Tlie effect of temperature and discharge rate on the capacity of sealed nickel-cadmium cells is illustrated in Figure 8 and Table 3. 

Siatered nickel electrodes used in nickel iron ceUs are usually thicker than those used in Ni/Cd ceUs. These result in high energy density ceUs, because very high discharge rates are usually not required. 

Some efforts toward sealed battery development (76) were made. However, a third electrode, an oxygen recombination electrode was required to reduce the cost of the system. High rate appHcations such as torpedo propulsion were investigated (77) and moderate success achieved using experimental nickel—zinc ceUs yielding energy densities of 35 W-h/kg at discharge rates of 8 C. A commercial nickel—zinc battery is considered to be the most likely... 

Self-Discharge Processes. The shelf life of the lead—acid battery is limited by self-discharge reactions, first reported in 1882 (46), which proceed slowly at room temperature. High temperatures reduce shelf life significantly. The reactions which can occur are well defined (47) and self-discharge rates in lead—acid batteries having immobilized electrolyte (48) and limited acid volumes (49) have been measured. 

When it is desired to determine the discharge rate through a nozzle... 

Example 8 Compressible Flow with Friction Losses Calculate the discharge rate of air to the atmosphere from a reservoir at 10 Pa gauge and 20 G through 10 m of straight 2-in Schedule 40 steel pipe (inside diameter = 0.0525 m), and 3 standard radius, flanged 90 elhows. Assume 0.5 velocity heads lost for the elhows. 

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