Transactions A.S.M.E.

274 TRANSACTIONS OF THE A.S.M.E. MAY, 1940tluced into the cylindrical chamber mentioned with an air injector,making it possible to secure a uniform coating of coal on thecloth, the amount of which was determined by weighing the clothafter each run.The results of these tests are indicated in Fig. 6, which showsthe effect of coal loading on the pressure drop for an air flow of3 cfm free air per sq ft. The curves are plotted for cloths A, Ai,Bi, and B2 and have the same letter designation given in Fig. 5.If, as an example, for cloth A a point at 0.05 lb per sq ft coal-dustloading is taken the pressure drop is 3.4 in. Comparing withFig. 5, a pressure loss of 3.4 in. will pass 102 cfm free air per sqft or 34 times as much air. It will be noted that, in line with thetests of Fig. 5, cloths Bi and B2 will allow more coal accumulationon the cloth for the same pressure-drop and flow conditions thancloths A and Ai. Thus for a 3-in. pressure drop, cloth Ai willallow a coal loading of 0.056 lb per sq ft, whereas cloths Bt and B2will allow 0.078 and 0.083 lb per sq ft, respectively. This, how-than with cloth Ax. No tests were made with cloth B (untreatedcloth) because it had been decided to use flameproofed cloth in theinstallation.Cloth Ai was initially selected for the installation because of itssuperior mechanical properties. It was believed that the heaviercloth would withstand the effects of temperature and beater rodsbetter than the lighter cloths Bi or B2. Flow curves were constructedfor cloth Ai, as in Fig. 7.The design for cloth-filter surface for both mills is contained inTable 3.From the figures given in Table 3, it is evident that the cleaning-periodcycle should be less than 37 min for the 25-ton mill andless than 25 min for the 40-ton mill. It was recognized that thecalculated figures are only approximations because it had beenassumed that the filter cloths were thoroughly clean at the beginningof each cycle. The limited value of the tests, because ofthe number of uncontrolled variables, such as temperature, humidity,dust and sizing, did not justify more accurate determinationof the cleaning cycle. To simplify the control for theseoperations, it was decided to operate both mills with the sameinitial cleaning cycle, a compromise between the two calculatedcleaning cycles.Each filtering unit has four compartments and is so constructedthat one compartment at a time is removed from service forcleaning. It is, of course, important that the cleaning operationoccur only while the compartment is dampered off. The settingadopted is given in Table 4.F i g . 7P r e s s u r e D r o p T h r o u g h C l o t h A i V e r s u s C o a l - D u s tL o a d in gTABLE 4 CYCLE SETTING ADOPTED25-Ton mill 40-Ton millNumber of complete cycles per h r............................ 2 2Time required for 1 cycle, m in.................................. 30 30Total time, unit operating with 100 per cent filteringcapacity, m in.................... . ........................... . 10 10Total time per cycle when unit is operating with75 per cent filtering capacity, m in........................ 20 20Actual shaking time per compartment per cycle,m in...................................................................... .. • • • ^ ^Actual shaking time per compartment per hr, min 6 6Actual shaking time all compartments per cycle,m in........................................................................ 12 12Actual shaking time all compartments per hr, min 24 24N o t e : Time control for the cycle was obtained from a synchronous motorthrough reduction gears driving a shaft onto which were fastened bakelitecams opening and closing contact switches in proper sequence for damperand shaker motor operation. Provision was also made for manual operationof the control limit switches.ever, does not mean that the normal operating pressure of thecloth filters will be less with cloth Bi or B2 than with cloth Ai,but rather that cloths Bi and B2 will retain more coal on theirsurfaces than cloth Ai and that the residual loading, after thecleaning period of cloths Bi and B2, is more than with cloth Ai.The normal operating pressure is approximately the same witheither cloth as borne out by actual operating experience with bothcloths.Observations made during these tests indicate that coal dustpasses through the cloth when it is clean until the pores becomefilled with dust particles to a point when no more coal dust passesthrough. The dust retained on the cloth provides a mat filteringmedium. This effect is more noticeable with cloths Bi and B2TABLE 3 CLOTH-FILTER SURFACE AS DESIGNED25-Ton mill 40-Ton millCoal milled, lb per h r............................................ 50000 100000Coal to cloth filters, lb per h r.............................. 800 1600Vented air, cfm........................................................ 15000 27500Free air cfm per sq ft cloth area (100 per centarea assumed)...................................................... 2.1 2.6Free air cfm per sq ft cloth area (75 per centarea)................................... ................................... 2.8Total cloth area, sq f t ............................................ 7040 *^710Reduced area, 75 per cent of total, sq f t......... 5280 8030Coal-dust loading, lb per M sq ft to give 4 in.normal operating pressure at reduced area... 71 62Total coal on cloth filters, lb ................................ 500 665Shaking interval, m in.....................................................37 25The validity of such testing with respect to the resistance ofdifferent cloths has a good degree of plausibility, since it is possibleto test the cloths under practically identical Reynolds-numberconditions as exist in the full-scale apparatus. These tests indicatethat the effective filter resistance is principally determinedby the filter mat which is formed by the cloth and the so-calledresidual dust loading. To a considerable degree, independenceof filter resistance to the textile specification of the cloth exists,a fact of interest because cloths may be selected principally fortheir mechanical qualities.If it should be found that such tests furnish an approximatebasis for the design, they would assume considerable importance.The authors’ results indicate that this is only roughly true.Since such correlation was not of primary interest at the timeof carrying out the work, necessity for further and more extensivedata is indicated, especially on different materials.The laboratory tests indicate two apparent flow-resistancerules. First, at constant loading of the cloth the draft loss variesdirectly as the flow. This is reasonable enough as the flowthrough the filter itself must be viscous. Second, at constant airflow, the draft loss varies as the 1.5 power of the loading, a statementby no means so obvious and which must be confined to thelimits of the experiments, because it involves several variables