ENGINEERING, DESIGN, AND EQUIPMENT
Operating Economics of Air-Cleaning Equipment Utilizing the Reverse Jet Principle WILLIAM B. HARRIS Health a n d S a f e t y l a b o r a t o r y ,
U. S .
Atomic Energy Commission, New York, N. Y.
MONT G. MASON H e a l t h Physics Department, Mallinckrodt Chemical W o r k s , St. louis, Mo.
W
ITH the obsolescence of the war-built equipment for the refining and processing of uranium, i t has been necessary to design replacement facilities. While the heavy stress of production was being carried by existing plants, it was possible to give adequate study t o the many problems before settling on new plant designs. Among the areas requiring special attehtion, the control of in-plant and out-plant pollution received intensive engineering consideration. This included design of: Process controls and equipment t o reduce exposures to potential toxic materials to within specified limits. Adequate facilities for replacing air removed by ventilation. Air-cleaning equipment to provide for minimum process losses and R clean external environment. Experiences gained in the many plants that cooperated in the production of uranium materials were carefully examined in every design area. On the basis of these experiences, it became obvious that the major problem in the choice of air-cleaning equipment for operations of this type was to find equipment that would efficiently remove air-borne dust from exhaust system effluents. The process and the material were such as to dictate dry collection as the preferable means of dust separation. Particle size and dust concentrations in all cases were comparable to usual industrial loadings.
dust collector requires either exhaust system overdesign or the operation of the system a t low efficiency during a portion of the cycle. This results in a diminished economy and generally in some loss of product through either increased carry-off or increased dispersion into the working environment. A study of plant effluents revealed that large bursts of dust found their way outside the plant immediately after filter cleaning.
Choice of equipment was based on experience with dust collection
Operating conditions are summarized
On the basis of experience with the collection of this type of dust, the following fundamental decisions vere made regarding the choice of equipment: T o attain the high efficiencies required by both health standards and process accountability, electrostatic precipitation is uneconomical. Inertial and scrubber-type air cleaners are inherently of too low efficiency for most of the materials to be removed. Deep-bed filters do not have sufficient holding capacity nor permit satisfactory recovery of the material for reprocessing. Any kind of well designed and constructed cloth filter arrestor is adequate for this job.
The follon-ing data summarize the conditions of operation: Collectors are operated a t housing pressures varying from 2 inches of water to 10 inches of mercury, vacuum. The individual capacity range is from 700 cubic feet per minute on a pneumatic conveying system t o 12,000 on a simple dustcontrol application. The total design capacity of all these machines is about 110,000 cubic feet per minute. The average operating dust load for the individual collectors ranges from a minimum of 0.002 to a maximum of 32.0 grains per cubic foot. A peak dust load in excess of 100 grains per cubic foot occurs in one pneumatic conveying system. The over-all operating average dust load is about 5 grains per cubic foot. The discbarge air from the individual collectors under full dust load conditions contains dust concentrations ranging from a minimum of 0,0001 to a maximum of 0.41 grain per 1000 cubic ieet, the over-all average being about 0.16 grain per 1000 cubic feet. The data include the filter for the pneumatic conveying system as well as all process dust-control filters. They represent normal operating conditions (including cleaning cycles), but do not take into account unusual losses through the collector from abnormal operations such as excessive seepage or bag failure. The average cleaning efficiency found during the 2-year study 01:individual collectors in the group ranged from a minimum of 99.946% to a maximum of 99.9996Oj,, TIith an average efficiency
Alter a careful investigation of commercially available cloth collectors, i t appeared that conventional equipment had several basic disadvantages for the type of operation under investigation. Under conditions of use, this equipment requires a degree of maintenance in man-hours per year which results in unacceptably high radiation dosage t o maintenance personnel. The only protection against exposure of this type is uneconomical shifting of personnel t o reduce the duration of exposure. The same is true of exposure to dust from these toxic materials. Although in most cases this type of exposure could be reasonably well controlled through the use of personal respiratory protection, this type of protection is undesirable. Experience showed relatively high out-of-service time resulting in process or sometimes plant Bhutdomn, with the alternative of large unnecessary loss of valuable product. The fluctuating collector pressure drop of the conventional December 1955
In an attempt to reduce these deficiencies, the authors investigated the use of reverse jet air-cleaning equipment ( 2 , 3). Installations were made on small extremely difficult units and considerable experience was gained. As a result of experiences with the operation of these few early dust collectors, the decision was made to standardize on the use of wool felt, reverse jet types of air-cleaning equipment in all cases where high efficiency of collection was required and where dry dust was handled. Other types of collectors have been used under other conditions. However, this report describes the experience of one plant in the use of this type of air-cleaning equipment. The information presented in this report covers the operation of 18 dust collectors built by two different manufacturers, all under the Hersey patent. These dust collectors are in continuous operation a t the Atomic Energy Commission plants of the Mallinckrodt Chemical Ll’orks.
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ENGINEERING, DESIGN, AND EQUIPMENT of 99.986% for all machines under all conditions of test for the same period. The over-all costs for operating this equipment, including a 5-year write-off on initial installed cost and all labor and material maintenance, comes to $0.32 per year per cubic foot per minute. This, however, is not an accurate presentation of the facts, as it includes a cost of over $3 per year per cubic foot per minute for a single grossly undersized dust collector, compared to an average of $0.23 per cubic foot per minute per year for equipment of adequate design. Maintenance costs alone amount to $0.12 per year per cubic foot per minute when all units are included, and $0.042 for equipment of adequate desigu. Eighteen collectors have been installed
The first Hersey-type filter installed a t the plant was designed for an air-bag ratio of 20 cubic feet per minute per square foot of filter surface. A number of small mechanical problems required correction before this machine gave satisfactory service, b u t once these corrections were made, it did a very good job of sir cleaning. Measurements made under operating conditions showed an average grain loading of 2.02 grains per cubic foot with an average cleaning efficicncy of 99.977%. Within 12 months after the beginning of successful operations with this machine, two more machines were installed. The second installation was also designed for 20 cubic feet per minute per square foot, but before it could be completed, the process equipment was revised so that it became necessary to operate this machine a t about 28 cubic feet per minute per square foot in order to obtain satisfactory dust control. After start-up this machine was found to have dust loadings as high as 32 grains per cubic foot. The third machine was installed as a final filter on a pneumatic conveying operation; it operates a t an air-bag ratio of 17 cubic feet per minute per square foot and an average grain loading of 14 grains per cubic foot, with peaks exceeding 100 grains per cubic foot. This machine has given good cleaning efficiency, but excessive maintenance problems indicate same deficiency in design. Certainly, for a collector of this design the dust and pressure loads are too high for the available filter. I n both of these latter filters the differential pressure across the filter ranged from 4 t o 10 inches water gage, even with continual operation of the reverse jet blow ring. Under the conditions as stated, bag life on both of these machines averaged about 3 weeks of operating time. There was excessive stretching of the bags from the high differential pressure. This, combined with continuous operation of blow rings, caused both the bags and the blow rings to wear excessively. The experience gained with these three machines indicated that satisfactory cleaning could be done a t an air-bag ratio of 20 cubic feet per minute per square foot. However, it was apparent that when dust loadings were high enough to cause excessive pressure drops across the bag, the life of the filter would be shortened and maintenance would be high. First Production Group. The next seven machines installed were designed to operate a t a dust loading of approximately 1 grain per cubic foot of air, with air-bag ratios not to exceed 20 to 1. Many additional features were incorporated in this group of seven machines to eliminate some of the shortcomings which had developed with the first three installations. Performance tests on these seven machines under operating conditions showed that six were doing a very satisfactory job of cleaning; the lowest efficiency found was 99.990%. The seventh machine, however, did not give completely satisfactory service, despite the fact that the air-bag ratio was only 17.5 to 1, with a dust loading of 4.2 grains per cubic foot. Extensive experimental work with this last machine established the fact
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that the dust handled is a “seeper,” which migrates through the filter medium, resulting in excessive losses. After several changes, a special resin-treated felt which resulted in satisfactory operation was obtained from the supplier of the collector. However, this machine still gives as much trouble from a maintenance standpoint as any two other collectors of this group of seven. Second Production Group. Experience gained with the first group of machines resulted in the selection of a lower air-bag ratio for subsequent installations. Most of the eight collectors installed since that time have been designed to have an air-bag ratio not t o exceed 15 cubic feet per minute per square foot, and have given very satisfactory long-term operation. Maintenance Program. A preventive maintenance schedule provides for a daily inspection of all collectors and charts by production personnel. The Maintenance Department inspects each machine biweekly for mechanical conditions of bags, blow rings, suspension chains, drive sprockets, blow ring air supply tubes, etc., paying special attention to the following: Blow rings must remain smooth and level t o avoid excessive bag wear. Bags must be maintained taut to avoid sagging or bulging. Contact between blow ring and bag must be correct The air supply hose of the blow ring must be good, to assure that bags are properly cleaned. Canvas wear strips over sewed seams in the bags must remain in place, so that blow ring wear will not cause the bag to split. The maintenance department has assigned to one man the sole responsibility for all dust collectors; he has learned the problems of each individual machine and usually anticipates trouble before it happens. This policy has proved most advantageous. Maintenance requirements for the 18 collectors average 2.5 man-days per week over a 2-year period. If only 14 machines are included, this is 1 man-day per week. This covers both repairs and preventive maintenance. With the exception of the three troublesome units previously discussed, maintenance problems have been minimal; however, none of these collectors can be expected to give continual good operation over long periods if allowed to go completely untended. It has been found desirable to provide safeguards in the form of instrumentation, a thorough inspection program, and a preventive maintenance program in order to assure good continuous operations. Instrumentation. All reverse jet collectors a t the plant are now provided with pressure-control instruments to provide intermittent operation of blow rings; this instrumentation is of the recording type so that inspection of the charts immediately reveals abnormalities in operation. Optimum pressure setting maintains a pressure differential across the filters of between 3 and 4 inches water gage. Electric eye dust detectors have been installed in the discharge stack of all collectors to detect bag failure. Thermocouples are installed in the housing of all collectors handling heated gases to provide an alarm and to safeguard against rises above permissible filter temperatures (175OF.). Maintenance costs compare favorably with other collectors
The average down time for 16 of the collectors, including preventive maintenance, has been less than 2 hours per month per machine. For the two remaining collectors down time has averaged about 2 hours per week per machine; these machines are the pneumatic conveying system collector and the one other heavily loaded machine. Average bag life for all machines included in these data was 8 months per bag. However, this number does not correctly illustrate the true usage picture because it includes the high usage of the underdesigned pneumatic system and ore-crushing
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING, DESIGN, AND EQUIPMENT system collectors, as well as the high usage on the seeper before the special resin-treated felt was installed. The following breakdown shows actual usage by groups of machines:
Total bag cost for 180 bags $16,500 Blow ring hose 1,800 Miscellaneous parts 2,000 Maintenance labor 6,300 526,600 per 2 years or $13,300 per year
Machine Pneumatic system Ore Black Orange Others Total
No. of
No.
Machines
of Bags
1 1 1
1 14
1
2
4 4 52
Bags Replaced/ 2 Years 70
49 16 16 29
-
-
__
18
63
180
Bag Life, Months/ Bag 0.33
=
1.5wk. 1 6 6 43 8
110,000 cubic feet per minute a t $13,300 per year cubic foot per minute
.
December 1955
$0.12 per gear per
Operating costs may be computed on the basis of $13.90 per 1000 cubic feet per minute or $1400 for 110,000 cubic feet per minute. Assuming a 5-year write-off and using $1.00 per cubic foot per minute as installed collector cost, the total annual cost for all machines is: 22,000 -I- 1,400
Since these data were collected, a new collector has been installed on the pneumatic conveying system. Although there are still some bugs in this system, bag life is now approximately 2 months. Further improvements to the system, now under way, are expected to extend this to 6 months. Plans to increase the size of the ore room collector were canceled because recent process changes reduced both dust load and usage of this machine, so that bag life now exceeds 6 months. The special resin-treated felt has produced satisfactory operations on the seeper and it is not planned to make further changes to this system. Routine Maintenance Problems. Wear of supporting chains and drive sprockets results from excessive blow ring operation, from misalignment, and from faulty equipment design. Chain or sprocket slippage will cause cocking of the blow ring, which in turn may tear up the filter medium and cause breakage of the blow ring. Failure of blow ring air supply hose due to excessive operation of the blow ring and/or poor alignment of air outlets on the side of the collector housing will result in failure to clean the filter medium, which in turn causes excessive pressure drop across the bag, with resultant bursting of the bag. Excessive operation of blow rings may be due to underdesign of equipment or to changes in ductwork. Continuous operation of blow rings causes unnecessary bag wear and low collection efficiency This in turn causes frequent bag changes and high effluent dust loadings. Faulty blow rings-Le., warping of the blow ring, improper manufacture, and poor selection of blow ring material, erosion of the blowring surface, or build-up of residue on the face of the ringcause localized wear of the bag, which eventually results in splitting. The material of choice for blow rings is stainless steel with overlapping staggered slots. Very few dusty materials will adhere to stainless steel and the hardness of stainless steel minimixes surface flaws. High temperature will result in rapid degeneration of the wool fibers, which in turn causes frequent bag failure. Exhaust systems should be designed so that temperatures of collector housing do not exceed 175" F. Some chemical fumes may result in splitting of the bags a t the seam owing to acid or alkaline action on the material used to sew the seam, Wool felt is moderately resistant to both mild acids and mild alkalies. However, the material used for stitching the seam should be selected to resist the particular chemical fume present, Nylon stitching has been found satisfactory for alkaline fumes and orlon for acid fumes. Poor clamping of the bag to the bag collar may cause the bag to tear loose a t high pressure differentials, with a resultant high loss of material in the effluent air stream. Stretching of bags usually is due to excessive temperature or excessive pressure drop across the bag; the bag should be pulled tight a t frequent intervals to avoid lapping of the filter medium beneath the blow ring. This eventually results in creasing and splitting of the bag. Costs. Total maintenance cost for all machines during the 2-year period is summarized as follows:
=
+ 13,300 = 836,700
or $0.335 per year per cubic foot per minute. Cost for maintaining new design equipment for I4 machine8 is obtained as follows: Total bag cost for 29 bags Miscellaneous parts Maintenance labor
S3?00 lo00
2400
___
$7800 per 2 years or $3900 per year 93,000 cubic feet per minute a t $3900 per year = $0.042 per year per cubic foot per minute
A calculation similar t o that made previously shows for the 14 well designed machines: Operating costs Maintenance costs Amortization
$
1,300 3,900 18,600
$23,800
or $0.256 per year per cubic foot per minute. The following conclusions may be drawn from the above data: 411 Collectors 2 '/2 3
Man-hours/week for maintenance Down-time. hours/month/machine
Well Designed Units
1 2
Recently ( I ) , Bloomfield described the operating characteristics of three large wet collectors and presented some excellent cost data. An interesting comparison can be drawn between the data presented by Bloomiield and those reported above. The three units described were high efficiency wet collectors with a cumulative capacity of approximately 52,000 cubic feet per minute. Installed cost of these collectors is $38,600 or $0.74 per cubic foot per minute. On the basis of the data given, the annual cost of these collectors, neglecting maintenance, is: Operating cost (assuming power a t an average cost of 5 mils) Maintenance Amortization Total
$
2,250 8: 000
$10,250 or $0.197 per year per cubic foot per minute
The over-all average efficiency of these collectors operating on an average dust load of 1.70 grains per cubic foot is 93.5y0. Any cost advantage is lost when the material being collected can be valued a t $16 a ton or more. The installed cost of the collector operating a t 89% was $0.64 per cubic foot per minute while the collector that had an average efficiency of 97y0 cost $1.20 per cubic foot per minute. literature cited (1) Bloomfield, B. D., Heating and Ventilating, 51, No. 4, 89 (1954). ( 2 ) Caplan. K. J., Ibid., 4, No. 2 , 79 (1907). (3) Hersey, H. J., Jr., Ibid., 51, No. 2 , 109 (1954). RECEIVED for review April 20, 1955. ACCEPTED September 15, 1955. Divisions of Industrial and Engineering Chemistry, Analytical Chemistry, and Water, Sewage, and Sanitation Chemistry, Symposium on Processes and Equipment in Air Pollution Control, 126th Meeting, ACS. New York, N. Y., September 1954.
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