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Air Filtration for the Spray Drying of Dairy Products1
OUR
INDUSTRY
TODAY
Air Filtration for the Spray Drying of Dairy Products D. R. HELDMAN, ¢. W. HALL, and T. I. HEDRICK Departments of Agricultural Engineering and Food Science Michigan State University, East Lansing Abstract The production and manufacturing of dry milk which is completely safe for human consumption requires the product to be free of any pathogenic microorganisms. The attainment of this goal requires complete control of product contamination during manufacturing. The large number of product-air contact points during spray drying, instantizing, and packaging indicates that air-borne contamination may be of signifieant importanee. A considerable portion of the air-borne contamination during dry milk manufacturing can be eliminated by direct air filtration. Ultra-high efficiency air filters will remove essentially all microorganisms from an air supply. I-Iowever, the selection of the filter to be used in dry milk plant applications is dependent on a balance between efficiency desired and filter costs, which include initial cost and costs of operation and maintenance.
The recent encounters with Sahnonella organisms in dry milk available for human consumption have resulted in a sequence of unpleasant events for the dr5' milk industry. The need for preventing contamination of dry milk from the potential sources presents many unsolved problems. Since air is brought into contact with the product at several points in the dry milk plant, the quality of this air may be of particular significance. The objective of this presentation is to provide information on air purification and cleaning, with emphasis on air filtration and the efficieneies to be expected. The extent to which air may play a role in contamination of dry milk is illustrated in Figure 1. This illustration would apply to normal dry milk plants using spray dryers and instantizers to produce the consumer product. As is evident, new air (a different source of air) is brought into contact with the product at nine
different locations. The relative role played by the different air sources is not known, but some comparisons can be made. Air mixed with fuel and heated air which enters the drying chamber should not be sources of microorganisms as long as the heating process is adequate to eliminate viability. Although there may be some doubt concerning the effectiveness of the heating process at all times and situations, the need for air filtration to remove microorganisms at this point is questionable. Air for redrying, cooling, and conveying is brought in direct contact with the product for a considerable length of time. These could very likely be sources of contamination unless precautions are taken to treat the air. The instantizing process introduces additional points of air-product contact. One widely used type of instantizer has three different locations where air is introduced into the system and in turn contacts the product. In addition, the product is exposed to the ambient air space during conditioning and transport to redD'ers after agglomeration. The final point of air-product contact is during filling and packaging. Although there is no attempt to bring product and air into contact at this point, other factors emphasize the importance of contamination. I n most cases, plant workers are very close to filling and packaging operations, resulting in situations which may increase the chance of air-borne contamination by undesirable microorganisms. The method used for preventing air-borne contamination will vary depending on its location in the plant. At the present time, filtered air is being used for drying, redrying, cooling, conveying, and instantizing. The effectiveness of this approach will be discussed later. Air filtration is not the answer to preventing air-borne contamination at filling and packaging stations, and possible solutions are being investigated. It is beyond the scope of this paper to discuss these investigations. Since air filtration is being used to considerable extent at several locations throughout the dry milk manufacturing process, it should be of value to discuss basic principles of air filtration with special reference to factors which influence efficiency. Filtration mechanisms. According to Whitby and Lundgren (3), there are four basic mechanisms which contribute to collection of particles
Preparation and presentation of this paper was sapported in part by the Public Health Service tiesearch Grant EF00624 from the Division of Environmental Engineering and Food Protection. Published with the approval of the Director of the Michigan Agricultural Experiment Station as Journal article no. 4071. 466
OUI¢ I N D U S T R Y
Air Mixed with Fuelif Direct Flame Heated Product before Drying
467
TODAY
Air for Redwing l
Heated Air to DryingChamber
Air for Cooling and conviying
2
I
Spray Dryin~l Heated Air to Complete Redwing
Ambient Air Contact
Heated Air for Partial
AmbientAir to
Agglomerator
Instantizin9 Air Contact During Filling 8= Packaging
Final 1 lnstantized Dry Milk
FIG. 1. Points and relative magnitudes of product-air contact during dry milk manufacturing. by filter fibers: a) Brownian diffusion, b) interception, c) inertial impaetion, and d) electrical attraction. An additional mechanism (deposition in accordance with Stokes' Law) is discussed by Decker et al. (1). The relative contribution of each mechanism is dependent on the particle size to be removed and the over-all filter efficiency. ]3rownian diffusion contributes to particle collection by causing the particles to deviate from streamline flow around the filter fibers and come in contact with the fiber. This particular mechanism will apply primarily to very small particles of less than 0.5 ~. The removal of particles by interception results when the particle does not deviate from the air streamline and the streamline brings the particle into contact with the fiber. Inertial impaction causes renloval of particles from air when the particles are too large to follow the air streamline around a filter fiber. This mechanism applies primarily to particles which are above 1 t~. I n some cases, the airborne particle and the filter fiber will have opposing electrical charges, causing the particle to deviate from the air streamline and deposit on the fiber. The deposition of particles on fibers due to gravitational settling will apply only to
larger particles and the efficiency of small particle removal by this mechanism will be very low. Particle size distribution considerations. I t is evident when discussing filtration mechanisms that the size of the particle to be removed has a significant influence on filter efficiency. The problem of predicting filter efficiency becomes more complex when considering the variation in particle size of air-borne contaminants. Any sample of air will contain particles ranging from subnficron to much larger particles, limited by the influence by gravity. The distribution of particle size in air will valT somewhat with location and the point at which the sample is taken. The distribution of all air-borne particles might approach the normat or bell-shaped distribution in which the number of particles with sizes greater or less than the mean size are equal. Whitby et al. (2) reported the distribution of air-borne dust which approached a normal distribution, Distributions of viable air-borne particles will be less, similar to normal, due to the smaller numbers. The distribution of particle size in air and the manner in which it is expressed may have a significant influence on the response expected from a filter. F i l t e r efficiencies are normally deterJ . DAIRY SOIENCE "VOL. 51, NO. 3
468
JOURNAL
OF
mined by some standard method. The National Bureau of Standards (NBS) method measures discoloration of filter p a p e r by various types of dust used to challenge the filter. Standard dust, fly ash, or atmospheric dusts are types normally used. The American Society of Heating, Refrigeration and A i r Conditioning Evaluation and A i r Filter Institute methods measure efficiency based on the weight of dust which passes through the test filter. The dioctyl-phtbalate (DOP) test measures the concentration of 0.3-~ particles passing through a test filter by light scattering in a smoke penetrometer. None of the standard testing methods for filter efficiency measm~es actual numbers which may pass through a filter. The NBS and DOP methods measure surface area or stain, while the A S H R A E or A F I methods measure efficiency on a weight basis. The DOP test can be interpreted in terms of numbers, since the aerosol particle size is a constant 0.3 ix; however, the method is used primarily on high and ultra-high efficiency filters. Co~zsideratio~zs i~ sir filter selectio~. There arc at least five basic factors which must be considered when selecting a filter for any application : a) air cleanliness required, b) characteristics of the air-borne particles, c) concentrations of particles in the air, d) volume of air to be cleaned, and e) costs of installation, operation, and maintenance. In present-day dry milk operations, the degree of air cleanliness may be somewhat difficult to establish without consideration of operation requirements. There are air filters available which will remove essentially all (99.97%) 0.3-~ particles; however, the initial cost and cost of operation may not be justified. The concentration of air-borne particles becomes important in two situations. First of all, high concentrations tend to load air filters rapidly, resulting in high maintenan~e costs. This can be overcome by using the p r o p e r types and numbers of filters in sequence. The concentration of particles in air also influences the quality of filtered air. Since filters will remove a percentage of particles, higher concentrations in air entering the filter will result in higher concentrations in the filtered air. The ]attar situation is not of particular consequence when considering undesirable air-borne bacteria. Air-borne bacterial concentrations in most food plants are not high and the probability of one passing through a filter with reasonable efficiency is low. Any problem related to the volume of air to be filtered can be overcome by p r o p e r design of the selected filter. Characteristics of the particulate aerosol must be considered in filter selection. Since any physJ. DAIRY SCIENCE VOL. 51, NO. 3
DAIRY
SCIENCE
ical characteristic of the particle may influence filter efficiency, knowledge of the particle properties is essential. One of the more important characteristics is the particle size distribution. Since the filter efficiency varies with particle size, the over-all efficiency of a filter will depend directly on the particle size distribution in the air being filtered. Decker et al. (1) and Whitby and Lundgren (3) both classify air filters into four categories: a) roughing filters, b) medium efficiency, or performance filters, c) high efficiency, or performance filters, and d) ultra-high efficiency, or absolute filters. Roughing filters are the common type o£ air filter found in home air-conditioners and furnaces. They may be the viscous coated fibers of metal, hair, or glass wool; or they may be the dry type composed of loosely packed glass, cotton, or similar fibers. In general, the primary purpose of the roughing filter is to remove relatively large particles from the air, and these filters will not have high efficiencies for small particles such as air-borne microorganisms. The medium efficiency or performance filter provides an improved efficiency for small particles by using compressed glass fibers, high quality p a p e r fibers, and pleating of the media to maintain low media velocities at high flow rates. This type of filter is used to remove large particles and provide relatively clean air, but may be used as a prefilter for higher efficiency filters in some applications. High efficiency or performance filters will usually have efficieneies of greater than 90%. The increased effÉciency over the roughing and medium performance filters is attained by using smaller diameter fibers, decreasing porosity, and increasing" the media pleating to maintain low media velocity. The ultra-high efficiency or absolute type filter has an efficiency of at least 99.97% for 0.3-~ particles. Absolute filters are eonstrneted of cellulose-asbestos fiber paper, glass and glassasbestos fiber paper, and other similar materiMs. Although efficiencies of removM arc high, prefilters must be used to prevent excessive loading of the ultra-high efficiency filter. To increase filter efficiency from the level of the roughing filter to the efficiency of the ultrahigh efficiency filter, more filter media pleating is required to maintain low media velocities at high air-flow rates. This results in higher filter costs and increased pressure drop. Pressure drop is the difference between the static pressure at the inlet and outlet of a filter as measm'ed by draft gauges. I t can be used as an indication of power requirements, since the
OUI%
INDUSTRY
I00
90
~
o
~To 0(~" 6 0
..~
~ ~o
~2
°3°
-%
:~_
1.01u PARTICLES
~ / ~
o.s~, ~RTIC,~S
~- / = ~ l
o
|///
,o o
~/
i/
'
I
/
~7~u.v..,=o,
~ Right
,orthi,
A~i. on Curve
0.2 0.4 0.6 0.8 1.0 INITIAL PRESSURE DROP, IN. H=O
,ooo
L6o~ ° /
;~--
I- 40 g-u~o: --
~IG. 2. Influence of filter efficiency on filter pressure drop. horsepower of the fan moving air through the filter is directly related to the pressure drop. The increase in filter pressure drop encountered to attain higher filter efficiencies is illustrated in Figure 2. The filter efficiencies and pressure drops were presented by Whitby and Lnndgren (3) for each of the four filter categories. The pressure drop values represent the response at the time of il~stallation, and the pressure drop of all filters will increase during use. In general, the filter efficiencies attained increase rapidly from less than 5 to over 90% as pressure drop increases from 0.1 to about 0.4 in. water. Itowever, the pressure drop doubles while efficiency increases from 90 to 99.9%. The important influence of particle size is illustrated in Figure 2, also. As was indicated earlier, filter efficiency is dependent on particle size and Figure 2 reveals the extent to which this relationship varies with the type or category of filter. Roughing filters have very low efficiency for 0.5- and 1-~ particles, but their efficiency does increase to over 98% for 10-/x particles. The influence of particle size is very pronounced when considering the medium efficiency filter, which has only 15% efficiency for
469
TODAY
0.5-F particles and 58% efficiency for 1-/x particles. The efficiency of high efficiency filters is influenced significantly when considering particle sizes less than 1 tL. A decrease in efficiency of nearly 10% is encountered when comparing 1- and 0.5-t~ particles, respectively. Ultra-high efficiency filters are not influenced significantly by particle size. When considering the factors involved in selection of the filters to be used in dry milk plants, characteristics of particles to be removed may be most important. Since the primary concern is with air-borne bacteria and other microorganisms, the characteristics of biological aersols existing in dry milk plants must be considered. According to W o l f et al. (4), microorganisms may be present in air in any of the following forms: a) single unattached cells, b) clumps of more than one cell, c) a cell attached to a dust particle, and d) a free-floating cell sun'ounded by a fihn of dried organic or inorganic material. The microorganisms may exist in the vegetative or spore state. F i l t e r selection must be based on the form which represents the smallest particle size, the single unattached cell or spore. The actual cell size of bacteria will vary with the type and species considered, but it is recognized that spore size may be less than 0.5 t~. Based on this observation, removal effieiencies of greater than 90% cannot be expected without using a high efficiency filter. This efficiency can be attained at an initial pressure drop of just over 0.4 in. water. To receive the benefits of the ultra-high efficiency filter, pressure drops in excess of I in. water are encountered. A somewhat better basis for evaluation may be attained by consideration of initial filter costs and horsepower requirements presented in Figure 2 and Table 1. This infoI~mtion will a p p l y only to a filter handling 1,200 cfm of air. The initial costs are for the filter only and do not include installation. The horsepower calculation is based on an approxhnate operating pressure drop, which is some value between the initial pressure drop and the pressure drop at which the filter is replaced. These results indicate that both initial costs and power requirements increase consistently with increasing filter efficiency. An increase from medium to high efficiency filter required an increase of about 30% for horsepower and 133% for initial cost. The ultra-high efficiency filter required approximately 86% more horsepower and 29% higher initial cost than the high efficiency filter. Summary and Conclusions
The many points of product-air contact during spray drying, instantizing, and packaging of dry J. DAIRY SCIENCE ~O,L. 51, NO. 3
M)UR,NAL OF DAIRY SCIENCE
470
TABLE 1 Characteristics of air filters to handle 1,200 cfm
:Filter type Roughing Medium efficiency High efficiency Ultra-high efficiency
Initial pressure drop
Approximate operating pressure drop
(in. 13,0)
(in. tt,0)
0.09 0.25 0.43 0.94
0.4 0.6 0.75 1.5
milks dictate t h a t definite p r e c a u t i o n s be used to control p r o d u c t c o n t a m i n a t i o n . The m e c h a n i s m of a i r filtration includes diffusion, i n t e r c e p t i o n , i n e r t i a l impaction, electrical a t t r a c t i o n , a n d deposition. T h e r e are f o u r comm o n types of filters: r o u g h i n g , medium, high, a n d u l t r a - h i g h efficiency. The efficiency f r o m each of these a i r filters is d e p e n d e n t on the size a n d size d i s t r i b u t i o n of the a i r - b o r n e p a r t i c l e s to be removed. Only the u l t r a - h i g h efficiency filt e r will remove 99.97% of the 0.3-~ particles. The selection of filter depends on a balance between efficiency desired a n d filter costs. I n i t i a l filter costs a n d p o w e r r e q u i r e m e n t s increased consistently with i n c r e a s i n g filter efficiency.
Acknowledgment The authors acknowledge the assistance of Thorn Simmons of C. E. Rogers Company in obtaining some of the information presented.
ft. DAIRY SOIESC~ "V-eL.51, NO. 3
Approximate horsepower of fan motor
Approximate initial cost of filter
($) 0.15 0.23 0.3 0.56
10 30 70 90
References (1) Decker, H. M., Buchanan, L. M., Hall, L. ]~., and Goddard, K. R. 1962. Air Filtration of Microbial Particles. Public l=[ealth Service Publ. 953. U. S. Government P r i n t i n g Office, Washington, D. C. (2) Whitbv, K. T., Algren, A. B., and Jorden, R. C. 1958. The A S H R A E Air-Borne Dust Survey. Heating, Piping and Air Conditioning. J. Sec., 30: 171. (3) Whitby, K. T., and Lundgren, D. A. 1965. The Mechanics of Air Cleaning. Trans. A S A E 8(3) : 342. (4) Wolf, H W., Skaliy, P., Hall, L. B., Harris, M. M., Decker, H. M., Buchanan, L. M., and Dahlgron, C. M. 1959. Sampling MiarobiologicaJ Aerosols. Public Health Monograph 60. U. S. Government P r i n t i n g Office, Washington 25, D. C.
INDUSTRY
TODAY
Air Filtration for the Spray Drying of Dairy Products D. R. HELDMAN, ¢. W. HALL, and T. I. HEDRICK Departments of Agricultural Engineering and Food Science Michigan State University, East Lansing Abstract The production and manufacturing of dry milk which is completely safe for human consumption requires the product to be free of any pathogenic microorganisms. The attainment of this goal requires complete control of product contamination during manufacturing. The large number of product-air contact points during spray drying, instantizing, and packaging indicates that air-borne contamination may be of signifieant importanee. A considerable portion of the air-borne contamination during dry milk manufacturing can be eliminated by direct air filtration. Ultra-high efficiency air filters will remove essentially all microorganisms from an air supply. I-Iowever, the selection of the filter to be used in dry milk plant applications is dependent on a balance between efficiency desired and filter costs, which include initial cost and costs of operation and maintenance.
The recent encounters with Sahnonella organisms in dry milk available for human consumption have resulted in a sequence of unpleasant events for the dr5' milk industry. The need for preventing contamination of dry milk from the potential sources presents many unsolved problems. Since air is brought into contact with the product at several points in the dry milk plant, the quality of this air may be of particular significance. The objective of this presentation is to provide information on air purification and cleaning, with emphasis on air filtration and the efficieneies to be expected. The extent to which air may play a role in contamination of dry milk is illustrated in Figure 1. This illustration would apply to normal dry milk plants using spray dryers and instantizers to produce the consumer product. As is evident, new air (a different source of air) is brought into contact with the product at nine
different locations. The relative role played by the different air sources is not known, but some comparisons can be made. Air mixed with fuel and heated air which enters the drying chamber should not be sources of microorganisms as long as the heating process is adequate to eliminate viability. Although there may be some doubt concerning the effectiveness of the heating process at all times and situations, the need for air filtration to remove microorganisms at this point is questionable. Air for redrying, cooling, and conveying is brought in direct contact with the product for a considerable length of time. These could very likely be sources of contamination unless precautions are taken to treat the air. The instantizing process introduces additional points of air-product contact. One widely used type of instantizer has three different locations where air is introduced into the system and in turn contacts the product. In addition, the product is exposed to the ambient air space during conditioning and transport to redD'ers after agglomeration. The final point of air-product contact is during filling and packaging. Although there is no attempt to bring product and air into contact at this point, other factors emphasize the importance of contamination. I n most cases, plant workers are very close to filling and packaging operations, resulting in situations which may increase the chance of air-borne contamination by undesirable microorganisms. The method used for preventing air-borne contamination will vary depending on its location in the plant. At the present time, filtered air is being used for drying, redrying, cooling, conveying, and instantizing. The effectiveness of this approach will be discussed later. Air filtration is not the answer to preventing air-borne contamination at filling and packaging stations, and possible solutions are being investigated. It is beyond the scope of this paper to discuss these investigations. Since air filtration is being used to considerable extent at several locations throughout the dry milk manufacturing process, it should be of value to discuss basic principles of air filtration with special reference to factors which influence efficiency. Filtration mechanisms. According to Whitby and Lundgren (3), there are four basic mechanisms which contribute to collection of particles
Preparation and presentation of this paper was sapported in part by the Public Health Service tiesearch Grant EF00624 from the Division of Environmental Engineering and Food Protection. Published with the approval of the Director of the Michigan Agricultural Experiment Station as Journal article no. 4071. 466
OUI¢ I N D U S T R Y
Air Mixed with Fuelif Direct Flame Heated Product before Drying
467
TODAY
Air for Redwing l
Heated Air to DryingChamber
Air for Cooling and conviying
2
I
Spray Dryin~l Heated Air to Complete Redwing
Ambient Air Contact
Heated Air for Partial
AmbientAir to
Agglomerator
Instantizin9 Air Contact During Filling 8= Packaging
Final 1 lnstantized Dry Milk
FIG. 1. Points and relative magnitudes of product-air contact during dry milk manufacturing. by filter fibers: a) Brownian diffusion, b) interception, c) inertial impaetion, and d) electrical attraction. An additional mechanism (deposition in accordance with Stokes' Law) is discussed by Decker et al. (1). The relative contribution of each mechanism is dependent on the particle size to be removed and the over-all filter efficiency. ]3rownian diffusion contributes to particle collection by causing the particles to deviate from streamline flow around the filter fibers and come in contact with the fiber. This particular mechanism will apply primarily to very small particles of less than 0.5 ~. The removal of particles by interception results when the particle does not deviate from the air streamline and the streamline brings the particle into contact with the fiber. Inertial impaction causes renloval of particles from air when the particles are too large to follow the air streamline around a filter fiber. This mechanism applies primarily to particles which are above 1 t~. I n some cases, the airborne particle and the filter fiber will have opposing electrical charges, causing the particle to deviate from the air streamline and deposit on the fiber. The deposition of particles on fibers due to gravitational settling will apply only to
larger particles and the efficiency of small particle removal by this mechanism will be very low. Particle size distribution considerations. I t is evident when discussing filtration mechanisms that the size of the particle to be removed has a significant influence on filter efficiency. The problem of predicting filter efficiency becomes more complex when considering the variation in particle size of air-borne contaminants. Any sample of air will contain particles ranging from subnficron to much larger particles, limited by the influence by gravity. The distribution of particle size in air will valT somewhat with location and the point at which the sample is taken. The distribution of all air-borne particles might approach the normat or bell-shaped distribution in which the number of particles with sizes greater or less than the mean size are equal. Whitby et al. (2) reported the distribution of air-borne dust which approached a normal distribution, Distributions of viable air-borne particles will be less, similar to normal, due to the smaller numbers. The distribution of particle size in air and the manner in which it is expressed may have a significant influence on the response expected from a filter. F i l t e r efficiencies are normally deterJ . DAIRY SOIENCE "VOL. 51, NO. 3
468
JOURNAL
OF
mined by some standard method. The National Bureau of Standards (NBS) method measures discoloration of filter p a p e r by various types of dust used to challenge the filter. Standard dust, fly ash, or atmospheric dusts are types normally used. The American Society of Heating, Refrigeration and A i r Conditioning Evaluation and A i r Filter Institute methods measure efficiency based on the weight of dust which passes through the test filter. The dioctyl-phtbalate (DOP) test measures the concentration of 0.3-~ particles passing through a test filter by light scattering in a smoke penetrometer. None of the standard testing methods for filter efficiency measm~es actual numbers which may pass through a filter. The NBS and DOP methods measure surface area or stain, while the A S H R A E or A F I methods measure efficiency on a weight basis. The DOP test can be interpreted in terms of numbers, since the aerosol particle size is a constant 0.3 ix; however, the method is used primarily on high and ultra-high efficiency filters. Co~zsideratio~zs i~ sir filter selectio~. There arc at least five basic factors which must be considered when selecting a filter for any application : a) air cleanliness required, b) characteristics of the air-borne particles, c) concentrations of particles in the air, d) volume of air to be cleaned, and e) costs of installation, operation, and maintenance. In present-day dry milk operations, the degree of air cleanliness may be somewhat difficult to establish without consideration of operation requirements. There are air filters available which will remove essentially all (99.97%) 0.3-~ particles; however, the initial cost and cost of operation may not be justified. The concentration of air-borne particles becomes important in two situations. First of all, high concentrations tend to load air filters rapidly, resulting in high maintenan~e costs. This can be overcome by using the p r o p e r types and numbers of filters in sequence. The concentration of particles in air also influences the quality of filtered air. Since filters will remove a percentage of particles, higher concentrations in air entering the filter will result in higher concentrations in the filtered air. The ]attar situation is not of particular consequence when considering undesirable air-borne bacteria. Air-borne bacterial concentrations in most food plants are not high and the probability of one passing through a filter with reasonable efficiency is low. Any problem related to the volume of air to be filtered can be overcome by p r o p e r design of the selected filter. Characteristics of the particulate aerosol must be considered in filter selection. Since any physJ. DAIRY SCIENCE VOL. 51, NO. 3
DAIRY
SCIENCE
ical characteristic of the particle may influence filter efficiency, knowledge of the particle properties is essential. One of the more important characteristics is the particle size distribution. Since the filter efficiency varies with particle size, the over-all efficiency of a filter will depend directly on the particle size distribution in the air being filtered. Decker et al. (1) and Whitby and Lundgren (3) both classify air filters into four categories: a) roughing filters, b) medium efficiency, or performance filters, c) high efficiency, or performance filters, and d) ultra-high efficiency, or absolute filters. Roughing filters are the common type o£ air filter found in home air-conditioners and furnaces. They may be the viscous coated fibers of metal, hair, or glass wool; or they may be the dry type composed of loosely packed glass, cotton, or similar fibers. In general, the primary purpose of the roughing filter is to remove relatively large particles from the air, and these filters will not have high efficiencies for small particles such as air-borne microorganisms. The medium efficiency or performance filter provides an improved efficiency for small particles by using compressed glass fibers, high quality p a p e r fibers, and pleating of the media to maintain low media velocities at high flow rates. This type of filter is used to remove large particles and provide relatively clean air, but may be used as a prefilter for higher efficiency filters in some applications. High efficiency or performance filters will usually have efficieneies of greater than 90%. The increased effÉciency over the roughing and medium performance filters is attained by using smaller diameter fibers, decreasing porosity, and increasing" the media pleating to maintain low media velocity. The ultra-high efficiency or absolute type filter has an efficiency of at least 99.97% for 0.3-~ particles. Absolute filters are eonstrneted of cellulose-asbestos fiber paper, glass and glassasbestos fiber paper, and other similar materiMs. Although efficiencies of removM arc high, prefilters must be used to prevent excessive loading of the ultra-high efficiency filter. To increase filter efficiency from the level of the roughing filter to the efficiency of the ultrahigh efficiency filter, more filter media pleating is required to maintain low media velocities at high air-flow rates. This results in higher filter costs and increased pressure drop. Pressure drop is the difference between the static pressure at the inlet and outlet of a filter as measm'ed by draft gauges. I t can be used as an indication of power requirements, since the
OUI%
INDUSTRY
I00
90
~
o
~To 0(~" 6 0
..~
~ ~o
~2
°3°
-%
:~_
1.01u PARTICLES
~ / ~
o.s~, ~RTIC,~S
~- / = ~ l
o
|///
,o o
~/
i/
'
I
/
~7~u.v..,=o,
~ Right
,orthi,
A~i. on Curve
0.2 0.4 0.6 0.8 1.0 INITIAL PRESSURE DROP, IN. H=O
,ooo
L6o~ ° /
;~--
I- 40 g-u~o: --
~IG. 2. Influence of filter efficiency on filter pressure drop. horsepower of the fan moving air through the filter is directly related to the pressure drop. The increase in filter pressure drop encountered to attain higher filter efficiencies is illustrated in Figure 2. The filter efficiencies and pressure drops were presented by Whitby and Lnndgren (3) for each of the four filter categories. The pressure drop values represent the response at the time of il~stallation, and the pressure drop of all filters will increase during use. In general, the filter efficiencies attained increase rapidly from less than 5 to over 90% as pressure drop increases from 0.1 to about 0.4 in. water. Itowever, the pressure drop doubles while efficiency increases from 90 to 99.9%. The important influence of particle size is illustrated in Figure 2, also. As was indicated earlier, filter efficiency is dependent on particle size and Figure 2 reveals the extent to which this relationship varies with the type or category of filter. Roughing filters have very low efficiency for 0.5- and 1-~ particles, but their efficiency does increase to over 98% for 10-/x particles. The influence of particle size is very pronounced when considering the medium efficiency filter, which has only 15% efficiency for
469
TODAY
0.5-F particles and 58% efficiency for 1-/x particles. The efficiency of high efficiency filters is influenced significantly when considering particle sizes less than 1 tL. A decrease in efficiency of nearly 10% is encountered when comparing 1- and 0.5-t~ particles, respectively. Ultra-high efficiency filters are not influenced significantly by particle size. When considering the factors involved in selection of the filters to be used in dry milk plants, characteristics of particles to be removed may be most important. Since the primary concern is with air-borne bacteria and other microorganisms, the characteristics of biological aersols existing in dry milk plants must be considered. According to W o l f et al. (4), microorganisms may be present in air in any of the following forms: a) single unattached cells, b) clumps of more than one cell, c) a cell attached to a dust particle, and d) a free-floating cell sun'ounded by a fihn of dried organic or inorganic material. The microorganisms may exist in the vegetative or spore state. F i l t e r selection must be based on the form which represents the smallest particle size, the single unattached cell or spore. The actual cell size of bacteria will vary with the type and species considered, but it is recognized that spore size may be less than 0.5 t~. Based on this observation, removal effieiencies of greater than 90% cannot be expected without using a high efficiency filter. This efficiency can be attained at an initial pressure drop of just over 0.4 in. water. To receive the benefits of the ultra-high efficiency filter, pressure drops in excess of I in. water are encountered. A somewhat better basis for evaluation may be attained by consideration of initial filter costs and horsepower requirements presented in Figure 2 and Table 1. This infoI~mtion will a p p l y only to a filter handling 1,200 cfm of air. The initial costs are for the filter only and do not include installation. The horsepower calculation is based on an approxhnate operating pressure drop, which is some value between the initial pressure drop and the pressure drop at which the filter is replaced. These results indicate that both initial costs and power requirements increase consistently with increasing filter efficiency. An increase from medium to high efficiency filter required an increase of about 30% for horsepower and 133% for initial cost. The ultra-high efficiency filter required approximately 86% more horsepower and 29% higher initial cost than the high efficiency filter. Summary and Conclusions
The many points of product-air contact during spray drying, instantizing, and packaging of dry J. DAIRY SCIENCE ~O,L. 51, NO. 3
M)UR,NAL OF DAIRY SCIENCE
470
TABLE 1 Characteristics of air filters to handle 1,200 cfm
:Filter type Roughing Medium efficiency High efficiency Ultra-high efficiency
Initial pressure drop
Approximate operating pressure drop
(in. 13,0)
(in. tt,0)
0.09 0.25 0.43 0.94
0.4 0.6 0.75 1.5
milks dictate t h a t definite p r e c a u t i o n s be used to control p r o d u c t c o n t a m i n a t i o n . The m e c h a n i s m of a i r filtration includes diffusion, i n t e r c e p t i o n , i n e r t i a l impaction, electrical a t t r a c t i o n , a n d deposition. T h e r e are f o u r comm o n types of filters: r o u g h i n g , medium, high, a n d u l t r a - h i g h efficiency. The efficiency f r o m each of these a i r filters is d e p e n d e n t on the size a n d size d i s t r i b u t i o n of the a i r - b o r n e p a r t i c l e s to be removed. Only the u l t r a - h i g h efficiency filt e r will remove 99.97% of the 0.3-~ particles. The selection of filter depends on a balance between efficiency desired a n d filter costs. I n i t i a l filter costs a n d p o w e r r e q u i r e m e n t s increased consistently with i n c r e a s i n g filter efficiency.
Acknowledgment The authors acknowledge the assistance of Thorn Simmons of C. E. Rogers Company in obtaining some of the information presented.
ft. DAIRY SOIESC~ "V-eL.51, NO. 3
Approximate horsepower of fan motor
Approximate initial cost of filter
($) 0.15 0.23 0.3 0.56
10 30 70 90
References (1) Decker, H. M., Buchanan, L. M., Hall, L. ]~., and Goddard, K. R. 1962. Air Filtration of Microbial Particles. Public l=[ealth Service Publ. 953. U. S. Government P r i n t i n g Office, Washington, D. C. (2) Whitbv, K. T., Algren, A. B., and Jorden, R. C. 1958. The A S H R A E Air-Borne Dust Survey. Heating, Piping and Air Conditioning. J. Sec., 30: 171. (3) Whitby, K. T., and Lundgren, D. A. 1965. The Mechanics of Air Cleaning. Trans. A S A E 8(3) : 342. (4) Wolf, H W., Skaliy, P., Hall, L. B., Harris, M. M., Decker, H. M., Buchanan, L. M., and Dahlgron, C. M. 1959. Sampling MiarobiologicaJ Aerosols. Public Health Monograph 60. U. S. Government P r i n t i n g Office, Washington 25, D. C.