Drum speed

At eonstant pressure drop, Ap, the volume of filtrate eolleeted depends on the drum speed aeeording to the relation... 

Thus the volume of filtrate eolleeted per revolution is inversely proportional to the square root of drum speed. But at higher speeds there are more revolutions made in unit time. Thus... 

Therefore, the elear filtrate flowrate varies directly with the square root of the drum speed. 

Thus, cake depth / varies inversely with the square root of drum speed N. In practice a minimum cake thickness is reached, typically 5 mm. 

On the assumption that the cake is incompressible and that 5 mm of cake is left behind on the drum, determine the theoretical maximum flowrate of filtrate obtainable. What drum speed will give a filtration rate of 80 per cent of the maximum ... 

Allows use of high drum speed and is capable of very high flow rates. 

As the drum rotates, each compartment undergoes the same cycle of operations and the duration of each of these is determined by the drum speed, the submergence of the drum and the arrangement of the valve. The normal cycle of operations consists of filtration, drying and discharge. It is also possible, however, to introduce other operations into the basic cycle, including ... 

Under all conditions, it is essential to operate with combinations of drum speed, submergence and vacuum, which, for the feed conditions that apply, will ensure that a fully dischargeable cake is formed. If this is not done then progressive deterioration in the effectiveness of the filter medium will occur and this will adversely affect the performance of the machine. Typical filtration cycles for drum filters are given in Table 7.4. 

Type of Dryer and Feed Size by Letter, A, B, or C Drum Speed (rev/min) Steam Press (bar, g) Type of Material Physical Form of Feed Solids in Feed (%) HzO in Product (%) Output of Dried Product (g/secm2) Evaporation Rate of Watar (g/sec m2)... 

Method of Feed Moisture Content, (% Wet Basis) Steam Pressure, (Ib/sqin.) Drum Speed (rpm) Feed Temp. (°F) Capacity [lb product/ (hr)(sq ft)) Vacuum (in. Hg)... 

Clearly, viscous dissipation is not significant in the experimental range given for the 168°C drum temperature experiments. Neither is it significant for the lower drum temperature experiments, which were conducted at lower drum speeds. It follows from the theoretical models (Eqs. 5.7-38 and 5.7-55) that the rate of melting in this case is proportional to the square root of drum speed and the temperature difference (7q — Tm)... 

It is easy to verify that the curves in Fig. E5.6 follow quite well the predicted increase in rate of melting with drum speed. For example, the predicted rate of melting at 1.6 in/s from the corresponding value at 0.2 in/s is 0.4 /1.6/0.2 = 1.13 in3/s, which is very close to the measured value. Similarly, selecting a fixed drum speed of 0.5 in/s, the measured rate of melting at 154°C is 0.5 in3/s. The predicted value at 168°C is 0.5y(168 I27)/(I54 127) = 0.616 in3/s, which once again is very close to the measured... 

Some characteristics of drum dryers are given in Table 6.8. In a given application, testing is usually required to establish drum speed, temperature, retention time, product characteristics and feeding equipment. The short time that solid is in contact with hot metal makes these units suitable for heat-sensitive materials. As illustrated in Fig. 6.4, many different feed... 

Increased drum speed Increase Decrease Decrease Increase... 

The filter press is to be replaced by a rotary vacuum-drum filter with negligible filter-medium resistance. This rotary filter can deliver the filtrate at a rate of 1000 lb/h when the drum speed is 0.3 r/min. Assuming the fraction submerged and the pressure drop are unchanged, what drum speed in r/min is necessary to make the amount of filtrate delivered in 24 h from the rotary filter exactly equal to the maximum amount of filtrate obtainable per 24 h from the plate-and-frame filter ... 

A drum speed of 0.33 rpm will be used, and a vacuum of 10.2 in. mercury below atmospheric pressure will be used. ... 

The specific cake resistance and the specific air-suction cake resistance can be assumed to be independent of pressure drop, drum speed, temperature, fraction of drum submerged, fraction of drum available for suction, and slurry concentration. However, to eliminate possible errors due to this assumption, a lab test should be run at conditions approximating the planned design. The results of these tests can be used as a basis for the design (i.e., cake and filtrate compositions and densities can be assumed to be the same for the design as those found in the lab). 

Cloth resistance is Rf — l(E10)m Normal peripheral speed is about 1 m/min. Filtering surface is 1/3 of the drum surface and washing surface is 1/6 of the drum surface. The amount of wash equals the pore space of the cake. The cake thickness is to be limited to 1 cm. At suitable operating pressures, find the drum speed in rph and the drum diameter ... 

Higher drum speeds, however, mean more frequent shaving of the precoat by the blade. To maintain rates, the precoat must be cut to a depth below that which solids have penetrated, and therefore partially blinded, the precoat. A grade of precoat material must be selected, then, to minimize this cake penetration. If this is done, it is possible to take a small enough cut to maintain precoat filter aid economy at high drum speeds and filtration rates. 

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