Most organic materials we want to actually use for something hold too much moisture in their natural state to do us any good. A rotary dryer has the distinguished roll of removing that unwanted water to turn the organic material into a useable, more desired product. Rotary drum dryers can be the most affordable, economic and versatile choice when considering a new or used industrial dryer. Rotary dryers are the workhorses of the drying industry because of their capacity, efficiency, and ability to adapt to new purposes.
Rotary drum dryers can be chosen for projects large and small. They come in all sizes to accommodate many products, quantities, and moisture content ranges. Although they can be much larger, a 14 diameter drums will accommodate many different needs and is the most practical for manufacture, transport, and installation. Depending on length, a 14 drum could produce 289,000 tons/year (263,000 tonnes/year) of 10%mcwb product from 50% mcwb product.
Rotary drum dryers can be designed to withstand extremely high temperatures and/or corrosive materials. A basic carbon steel drum is designed to withstand inlet temperatures from 1,000F 1,100F. With specialty materials, rotary drum dryers can handle much higher inlet temperatures. Stainless steel can be used in place of carbon steel if the product is known to be corrosive.
Rotary drum flighting is the primary material handling mechanism in a drum. Flighting can be designed to accommodate different flow characteristics of the materials to be dried. Some materials clump and potentially form balls which need to be broken up, while others flow easily and must be separated at a higher frequency per flight. Other pieces might be long and stringy and can easily get trapped. Different flighting designs can solve these, and other problems.
The flighting in a rotary drum dryer system can be changed to repurpose a used or existing dryer. A flighting designer can look at the flow characteristics of the material in question and determine what kind of flights are necessary, thereby converting the dryer to a new purpose at a fraction of the cost of buying a new dryer. Hopefully.
What products do best in a rotary drum dryer? Rotary drum dryers can dry many different types of products. Not only are they great for uniform solid particles, but they are the best solution for non-uniform solid particles. The number one factor in deciding what type of dryer to purchase is whether the material handling of the dryer matches what the product needs.
Products that have non-uniform particle sizes are perfect candidates for a rotary drum dryer, like the wood in the picture above. However, the flighting system would be different for the sample on the left than for the sample on the right than a sample with the two combined. It is important to know the characteristics of the infeed product to choose the best flighting design.
A rotary drum dryers greatest asset is its ability to accommodate a product that has multiple size particles and moisture contents by utilizing the right flighting package. As seen in the picture above the wood is in a variety of shapes and sizes. The flights can be designed to keep the larger pieces in the drum longer and allow the smaller pieces to flow more quickly. The right flighting package can help achieve uniform outfeed product moisture.
Who uses rotary drum dryers? We believe everyone should! Assuming you need to dry something. Almost everyone does use one at home a clothes dryer. But thats not exactly what were talking about. Rotary dryers are popular in the biomass and forestry industries, as well as the ethanol industry to dry WDGS. Thompson Dehydrating Co., Inc. made its start in the alfalfa drying industry. Sugar processing plants have used rotary dryers to dry bagasse. There are food and pharmaceutical companies that use rotary dryers. Ethanol producers arent the only ones that want to dry the by-product of alcohol production. There is a push for microbrewers to dry and repurpose their byproducts. An up-and-coming technology, torrefaction, is being made possible on an industrial scale by rotary dryers.
Conclusion Organic materials in a raw state typically have too much water to be further processed. Removing the excess water is the job of a dryer. And because of their capacity, efficiency, and all-around-good-looks, rotary drum dryers are the best choice. For projects large and small rotary dryers can handle them all!
Rotary drum dryers are a central component in a wide range of industrial processes. These highly flexible industrial dryers can be configured to create a custom processing solution for optimal dryer performance.
While not necessary in every setting and still critical in others, combustion chambers offer several benefits that can contribute significantly to product quality and overall process efficiency. The following provides an overview on combustion chambers, including the many benefits they offer and when they are an essential add-on to the rotary dryer.
A combustion chamber consists of a static vessel (usually cylindrical), typically constructed from steel, with multiple air inlets for efficient combustion. A burner is mounted on the end of the combustion chamber, which is connected to the end of the dryer. The chamber is refractory lined to protect the shell walls from the high temperatures. A single shell is typical, but in some cases double shell designs are available to pre heat the air.
The combustion chamber is used to house the combustion reaction that feeds heat into the dryer, preventing direct contact between the flame and the material being processed. It can be used on either a co-current or counter-current design.
When a combustion chamber is not used, the burner is simply mounted directly onto the end of the dryer, so that the flame is present in the drum itself, allowing direct contact between the flame and the material being processed.
Some consider these two variations as subtypes of the direct dryer category; direct-fired, referring to when the flame is in direct contact with the material, and direct-heat referring to when the flame is kept away from the material through the use of a combustion chamber.
In some cases, depending on the product undergoing drying, a combustion chamber may be critical to the process. Products that are heat sensitive, flammable, or might break down under a flame, should not be processed in direct contact with a flame. Many materials are sensitive to heat and could break down or be damaged by flame, resulting in off-spec product and a high amount of attrition (the breakdown of product into fines and dust).
In cases where the material being processed is especially heat sensitive, the combustion chamber design can be used to reduce the amount of radiative heat transfer imparted on the material. For this, right angle designs are often employed, eliminating the direct path between the products of combustion and the material being processed.
Additionally, some materials, such as potash, will form undesirable pollutants if exposed to a flame, increasing the requirements of the air pollution control system and potentially presenting regulatory hurdles.
Combustion chambers are not always necessary. When product quality is not a concern and the presence of a direct flame does not produce any undesirable byproducts, producers can avoid the extra expense of a combustion chamber.
This is frequently seen in industries processing low-value products, such as asphalt or low-grade aggregates. The flame presents no problem to the material and essentially flashes off moisture by direct contact, reducing the required retention time.
The use of a combustion chamber promotes improved product quality. By preventing the material from being exposed to the flame, attrition is minimized and materials sensitive to heat are not over-exposed.
The inclusion of a combustion chamber also increases process control. Through the use of a combustion chamber and dilution air fan, the process gas temperature entering the dryer is more readily controllable, as well as more uniform. This enhanced control is often used to optimize the temperatures entering the drum according to the specific needs of the material being processed, while uniform heat yields more uniform drying results.
The ability to more readily control the temperatures entering the dryer improves overall process efficiency. Efficiency is further enhanced because dilution air can be incorporated into the process, promoting a more complete combustion of the fuel source and providing an outlet for recovered waste heat to be reused, reducing energy requirements.
As a result of the benefits that can be garnered, it is often desirable to retrofit an existing dryer system with a combustion chamber; many producers who initially opted to go without are finding themselves turning to combustion chambers to improve process efficiency, increase capacity, reduce pollutants, and/or improve product quality.
Combustion chambers are easily retrofitted onto existing rotary dryers; while space is often a limiting factor in incorporating additional equipment into a system, combustion chambers are flexible by design. Manufacturers can build combustion chambers to fit into a variety of different spaces. This might mean utilizing space above or below the dryer to direct combustion gases into the unit. The image below, for example, shows the custom ducting FEECO designed to allow the use of a combustion chamber on a separate floor from the rotary dryer at the customers plant.
Combustion chambers are essential to maintaining product quality and improving efficiency when drying material with a rotary dryer. They are not, however, necessary in all settings, and are often left off when product quality and pollutant byproducts are not a concern. Combustion chambers offer a range of benefits to industrial drying and are easily retrofitted onto existing dryer systems for improved efficiency.
FEECO is the worlds leading rotary dryer manufacturer. We can engineer a custom combustion chamber as part of a new dryer design, or for retrofitting an existing system. All FEECO combustion chambers are built to the highest quality standards and feature unique designs to maximize efficiency and combustion chamber lifespan. For more information on our combustion chambers or rotary dryers, contact us today!
The effects of drum drying on the physical and antioxidant properties of pregelatinzed riceberry flour were investigated. The drum drying temperature was varied (110C, 120C and 130C) and unheated riceberry flour was used as the control. The results showed that all pregelatinized riceberry flour samples had lower (p0.05) L* value, but higher (p0.05) a* and b* values than the control. The water absorption index and swelling power of all pregelatinized riceberry flour samples were also significantly (p0.05) higher than those of the control. The results from rapid visco analysis indicated that the pasting time, pasting temperature, peak viscosity, trough viscosity, final viscosity and set back of riceberry flour decreased (p0.05) after drum drying. Moreover, the total phenolic content and antioxidant capacity, as measured using 2,2-diphenylpicrylhydrazyl and 2,2azinobis (3-ethylbenzothiozoline-6-sulfonic acid) disodium salt radical assays, respectively, also decreased (p0.05) after drum drying. Such changes were more evident with increased drum drying temperature in the range 110130C. In addition, increasing the drum drying temperature led to poorer stability to withstand the thermal treatment and stress than in pregelatinized riceberry flour. The results suggested that special attention should be given to the drum drying temperature as it affects not only the physical but also the antioxidant properties of the pregelatinized riceberry flour. Pregelatinized riceberry flour produced using drum drying at 110C could be applied to formulate an instant soup product because it had high values for water absorption capacity, total phenolic content and antioxidant activity and was completely gelatinized and stable against thermal treatment and stress.
In this study, the minimum quality loss of 2-phase olive pomace and the maximum systems energy efficiency is targeted during pre-drying of olive pomace in a drum dryer. Vapor pressure and valse rotational speed were selected as the independent variables of drum dryer. For each vapor pressure value (1, 2, 3, 3.5 and 4bar), drying of 2-phase olive pomace was performed at different valse rotational speeds (0.5, 1, 2, 3, 4.5 and 6rpm). Drum dryer conditions were optimized with desirability function approach by targeting the moisture content range as 3050%, minimum peroxide value and maximum specific moisture extraction rate (SMER). The optimum drum dryer conditions were determined as 3.27bar for vapor pressure and 6rpm for valse rotational speed. Moreover, the effects of vapor pressure and valse rotational speed on the physical and chemical properties of pre-dried 2-phase olive pomace and the systems energy efficiency were examined.
Lin, C.S.K., Koutinas, A.A., Stamatelatou, K., Mubofu, E.B., Matharu, A.S., Kopsahelis, N., Pfaltzgraff, L.A., Clark, J.H., Papanikolaou, S., Kwan, T.H.: Current and future trends in food waste valorization for the production of chemicals, materials and fuels: a global perspective. Biofuels Bioprod. Bioref. 8, 686715 (2014)
Koutinas, A.A., Vlysidis, A., Pleissner, D., Kopsahelis, N., Garcia, I.L., Kookos, I.K., Papanikolaou, S., Kwan, T.H., Lin, C.S.K.: Valorization of industrial waste and by-product streams via fermentation for the production of chemicals and biopolymers. Chem. Soc. Rev. 43, 25872627 (2014)
Oktav, E., zer, A.: Zeytinya endstrisi atksularnn zellikleri ve artm alternatifleri, 1. Zeytinya retiminde evre Sorunlar ve zmleri altay, Zeytinli/Edremit-Balkesir, Bildiriler Kitab. 5165 (2002)
Kotsou, M., Kyriacou, A., Lasaridi, K., Pilidis, G.: Integrated aerobic biological treatment and chemical oxidation with Fentons reagent for the processing of green table olive wastewater. Process Biochem. 39, 16531660 (2004)
De Rosa, S., Giordano, G., Granato, T., Katovic, A., Siciliano, A., Tripicchio, F.: Chemical pretreatment of olive oil mill wastewater using a metal-organic framework catalyst. J. Agric. Food Chem. 53, 83068309 (2005)
Ginos, A., Manios, T., Mantzavinos, D.: Treatment of olive mill effluents by coagulationflocculationhydrogen peroxide oxidation and effect on phytotoxicity. Journal of Hazardous Materials. 133, 135142 (2006)
Minkova, V., Razvigorova, M., Bjornbom, E., Zanzi, R., Budinova, T., Petrov, N.: Effect of water vapour and biomass nature on the yield and quality of the pyrolysis products from biomass. Fuel Process. Technol. 70, 5361 (2001)
Hernndez, V., Romero-Garca, J.M., Dvila, J.A., Castro, E., Cardona, C.A.: Techno-economic and environmental assessment of an olive stone based biorefinery. Resour. Conserv. Recycl. 92, 145150 (2014)
Zungur, A., Ko, M., Yaln, B., Kaymak-Ertekin, F., tle, S.: Storage stability of microencapsulated extra virgin olive oil powder. In: 9th Baltic Conference on Food Science and Technology Food for Consumer Well-Being. p.257 (2014)
Demirok, E., Damar, , Hastaolu, E., Ekim, M., Turhan, , Denge, A., Muhacir, N., Akgz, E.: Avrupa lkelerinde ticari sofralk zeytin ile zeytin ya retim teknikleri konusunda eitim-2sofralk zeytin ve zeytin ya kalitesi. 1. Ulusal Zeytin renci Kongresi, Balkesir (2008)
Gmez-Alonso, S., Mancebo-Campos, V., Desamparados Salvador, M., Fregapane, G.: Oxidation kinetics in olive oil triacylglycerols under accelerated shelf-life testing (2575 C). Eur. J. Lipid Sci. Technol. 106, 369375 (2004)
Baysan, U., Ko, M., Gngr, A., Kaymak-Ertekin, F.: Effect of tray dryers independent variables (drying temperature and air velocity) on the quality of olive pomace and systems energy efficiency. In: IDS 2018. 21st International Drying Symposium Proceedings. pp.10051012. Editorial Universitat Politcnica de Valncia (2018)
Phahom, T., Phoungchandang, S., Kerr, W.L.: Effects of steam-microwave blanching and different drying processes on drying characteristics and quality attributes of Thunbergia laurifolia Linn. leaves. J. Sci. Food Agric. (2017). https://doi.org/10.1002/jsfa.8167
Baysan, U., Ko, M., Gngr, A. et al. Pre-drying of 2-Phase Olive Pomace by Drum Dryer to Improve Processability. Waste Biomass Valor 12, 24952506 (2021). https://doi.org/10.1007/s12649-020-01202-2
The characteristics of a small domestic tumbler dryer have been investigated using performance tests and a computer model of the dryer. This model, which was partially developed using the tests results, encompasses all of the energy and mass transfer routes within the complete dryer. The tests demonstrated that the mass and energy transport mechanisms within the dryer drum are not well defined, and a key feature of the model is the simplified equations used to represent these flows. There is a strong correlation between the results from the performance tests and the results predicted by the computer model.
The performance test results show that the dryer examined in this study operates at much higher efficiencies than those previously reported. The dryers in the earlier studies were slightly larger and operated with larger air flow rates and higher energy inputs.
The drying process generates lint, and the model has been used to demonstrate the effects which the localised accumulation of lint in a trap has on the dryers performance. Other operational effects investigated using the model include the influence of fabric type on the performance of the dryer, the sensitivity of the dryer performance to operating conditions and the possibility of energy recovery from the exhaust stream by recirculating part of the stream through the dryer.
Adjustment and control of moisture levels in solid materials through drying is a critical process in the manufacture of many types of chemical products. As a unit operation, drying solid materials is one of the most common and important in the chemical process industries (CPI), since it is used in practically every plant and facility that manufactures or handles solid materials, in the form of powders and granules.
The effectiveness of drying processes can have a large impact on product quality and process efficiency in the CPI. For example, in the pharmaceutical industry, where drying normally occurs as a batch process, drying is a key manufacturing step. The drying process can impact subsequent manufacturing steps, including tableting or encapsulation and can influence critical quality attributes of the final dosage form.
Apart from the obvious requirement of drying solids for a subsequent operation, drying may also be carried out to improve handling characteristics, as in bulk powder filling and other operations involving powder flow; and to stabilize moisture-sensitive materials, such as pharmaceuticals.
Drying may be defined as the vaporization and removal of water or other liquids from a solution, suspension, or other solid-liquid mixture to form a dry solid. It is a complicated process that involves simultaneous heat and mass transfer, accompanied by physicochemical transformations. Drying occurs as a result of the vaporization of liquid by supplying heat to wet feedstock, granules, filter cakes and so on. Based on the mechanism of heat transfer that is employed, drying is categorized into direct (convection), indirect or contact (conduction), radiant (radiation) and dielectric or microwave (radio frequency) drying.
Heat transfer and mass transfer are critical aspects in drying processes. Heat is transferred to the product to evaporate liquid, and mass is transferred as a vapor into the surrounding gas. The drying rate is determined by the set of factors that affect heat and mass transfer. Solids drying is generally understood to follow two distinct drying zones, known as the constant-rate period and the falling-rate period. The two zones are demarcated by a break point called the critical moisture content.
In a typical graph of moisture content versus drying rate and moisture content versus time (Figure 1), section AB represents the constant-rate period. In that zone, moisture is considered to be evaporating from a saturated surface at a rate governed by diffusion from the surface through the stationary air film that is in contact with it. This period depends on the air temperature, humidity and speed of moisture to the surface, which in turn determine the temperature of the saturated surface. During the constant rate period, liquid must be transported to the surface at a rate sufficient to maintain saturation.
At the end of the constant rate period, (point B, Figure 1), a break in the drying curve occurs. This point is called the critical moisture content, and a linear fall in the drying rate occurs with further drying. This section, segment BC, is called the first falling-rate period. As drying proceeds, moisture reaches the surface at a decreasing rate and the mechanism that controls its transfer will influence the rate of drying. Since the surface is no longer saturated, it will tend to rise above the wet bulb temperature. This section, represented by segment CD in Figure 1 is called the second falling-rate period, and is controlled by vapor diffusion. Movement of liquid may occur by diffusion under the concentration gradient created by the depletion of water at the surface. The gradient can be caused by evaporation, or as a result of capillary forces, or through a cycle of vaporization and condensation, or by osmotic effects.
The capacity of the air (gas) stream to absorb and carry away moisture determines the drying rate and establishes the duration of the drying cycle. The two elements essential to this process are inlet air temperature and air flowrate. The higher the temperature of the drying air, the greater its vapor holding capacity. Since the temperature of the wet granules in a hot gas depends on the rate of evaporation, the key to analyzing the drying process is psychrometry, defined as the study of the relationships between the material and energy balances of water vapor and air mixture.
There are a number of approaches to determine the end of the drying process. The most common one is to construct a drying curve by taking samples during different stages of drying cycle against the drying time and establish a drying curve. When the drying is complete, the product temperature will start to increase, indicating the completion of drying at a specific, desired product-moisture content. Karl Fischer titration and loss on drying (LOD) moisture analyzers are also routinely used in batch processes. The water vapor sorption isotherms are measured using a gravimetric moisture-sorption apparatus with vacuum-drying capability.
For measuring moisture content in grain, wood, food, textiles, pulp, paper, chemicals, mortar, soil, coffee, jute, tobacco, rice and concrete, electrical-resistance-type meters are used. This type of instrument operates on the principle of electrical resistance, which varies minutely in accordance with the moisture content of the item measured. Dielectric moisture meters are also used. They rely on surface contact with a flat plate electrode that does not penetrate the product.
For measuring moisture content in paper rolls or stacks of paper, advanced methods include the use of the radio frequency (RF) capacitance method. This type of instrument measures the loss, or change, in RF dielectric constant, which is affected by the presence or absence of moisture.
Adiabatic dryers are the type where the solids are dried by direct contact with gases, usually forced air. With these dryers, moisture is on the surface of the solid. Non-adiabatic dryers involve situations where a dryer does not use heated air or other gases to provide the energy required for the drying process
Non-adiabatic dryers (contact dryers) involve an indirect method of removal of a liquid phase from the solid material through the application of heat, such that the heat-transfer medium is separated from the product to be dried by a metal wall. Heat transfer to the product is predominantly by conduction through the metal wall and the impeller. Therefore, these units are also called conductive dryers.
Although more than 85% of the industrial dryers are of the convective type, contact dryers offer higher thermal efficiency and have economic and environmental advantages over convective dryers. Table 1 compares direct and indirect dryers, while Table 2 shows the classification of dryers based on various criteria.
Tray dryers. This dryer type operates by passing hot air over the surface of a wet solid that is spread over trays arranged in racks. Tray dryers are the simplest and least-expensive dryer type. This type is most widely used in the food and pharmaceutical industries. The chief advantage of tray dryers, apart from their low initial cost, is their versatility. With the exception of dusty solids, materials of almost any other physical form may be dried. Drying times are typically long (usually 12 to 48 h).
Vacuum dryers. Vacuum dryers offer low-temperature drying of thermolabile materials or the recovery of solvents from a bed. Heat is usually supplied by passing steam or hot water through hollow shelves. Drying temperatures can be carefully controlled and, for the major part of the drying cycle, the solid material remains at the boiling point of the wetting substance. Drying times are typically long (usually 12 to 48 h).
Fluidized-bed dryers. A gas-fluidized bed may have the appearance of a boiling liquid. It has bubbles, which rise and appear to burst. The bubbles result in vigorous mixing. A preheated stream of air enters from the bottom of the product container holding the product to be dried and fluidizes it. The resultant mixture of solids and gas behave like a liquid, and thus the solids are said to be fluidized. The solid particles are continually caught up in eddies and fall back in a random boiling motion so that each fluidized particle is surrounded by the gas stream for efficient drying, granulation or coating purposes. In the process of fluidization, intense mixing occurs between the solids and air, resulting in uniform conditions of temperature, composition and particle size distribution throughout the bed.
Freeze dryers. Freeze-drying is an extreme form of vacuum drying in which the water or other solvent is frozen and drying takes place by subliming the solid phase. Freeze-drying is extensively used in two situations: (1) when high rates of decomposition occur during normal drying; and (2) with substances that can be dried at higher temperatures, and that are thereby changed in some way.
Microwave vacuum dryers. High-frequency radio waves with frequencies from 300 to 30,000 MHz are utilized in microwave drying (2,450 MHz is used in batch microwave processes). Combined microwave-convective drying has been used for a range of applications at both laboratory and industrial scales. The bulk heating effect of microwave radiation causes the solvent to vaporize in the pores of the material. Mass transfer is predominantly due to a pressure gradient established within the sample. The temperature of the solvent component is elevated above the air temperature by the microwave heat input, but at a low level, such that convective and evaporative cooling effects keep the equilibrium temperature below saturation. Such a drying regime is of particular interest for drying temperature-sensitive materials. Microwave-convective processing typically facilitates a 50% reduction in drying time, compared to vacuum drying.
Continuous dryers are mainly used in chemical and food industries, due to the large volume of product that needs to be processed. Most common are continuous fluid-bed dryers and spray dryers. There are other dryers, depending on the product, that can be used in certain industries for example, rotary dryers, drum dryers, kiln dryers, flash dryers, tunnel dryers and so on. Spray dryers are the most widely used in chemical, dairy, agrochemical, ceramic and pharmaceutical industries.
Spray dryer. The spray-drying process can be divided into four sections: atomization of the fluid, mixing of the droplets, drying, and, removal and collection of the dry particles (Figure 2). Atomization may be achieved by means of single-fluid or two-fluid nozzles, or by spinning-disk atomizers. The flow of the drying gas may be concurrent or countercurrent with respect to the movement of droplets. Good mixing of droplets and gas occurs, and the heat- and mass-transfer rates are high. In conjunction with the large interfacial area conferred by atomization, these factors give rise to very high evaporation rates. The residence time of a droplet in the dryer is only a few seconds (530 s). Since the material is at wet-bulb temperature for much of this time, high gas temperatures of 1,508 to 2,008C may be used, even with thermolabile materials. For these reasons, it is possible to dry complex vegetable extracts, such as coffee or digitalis, milk products, and other labile materials without significant loss of potency or flavor. The capital and running costs of spray dryers are high, but if the scale is sufficiently large, they may provide the cheapest method.
With increasing concern about environmental degradation, it is desirable to decrease energy consumption in all sectors. Drying has been reported to account for anywhere from 12 to 20% of the energy consumption in the industrial sector. Drying processes are one of the most energy-intensive unit operations in the CPI.
One measure of efficiency is the ratio of the minimum quantity of heat that will remove the required water to the energy actually provided for the process. Sensible heat can also be added to the minimum, as this added heat in the material often cannot be economically recovered. Other newer technologies have been developed, such as sonic drying, superheated steam, heat-pump-assisted drying and others.
Drying is an essential unit operation used in various process industries. The mechanism of drying is well understood as a two-stage process and depends on the drying medium and the moisture content of the product being dried.
Batch dryers are common in chemical and pharmaceutical industries, while continuous dryers are routinely used where large production is required. Since the cost of drying is a significant portion of the cost of manufacturing a product, improving efficiency or finding alternative drying routes is essential.
1. Sverine, Thrse, Mortier, F.C., De Beer, Thomas, Gernaey, Krist V., Vercruysse, Jurgen, et al. Mechanistic modelling of the drying behavior of single pharmaceutical granules, European Journal of Pharmaceutics and Biopharmaceutics 80, pp. 682689, 2012.
6. Raghavan, G.S.V., Rennie, T.J., Sunjka, P.S., Orsat, V., Phaphuangwittayakul, W. and Terdtoon, P., Overview of new techniques for drying biological materials, with emphasis on energy aspects, Brazilian Journal of Chemical Engineering, 22(2), pp. 195201, 2005.
Dilip M. Parikh is president of the pharmaceutical technology development and consulting group DPharma Group Inc. (Ellicott City, MD 21042; Email: [email protected]). As an industrial pharmacist, Parikh has more than 35 years of experience in product development, manufacturing, plant operations and process engineering at various major pharmaceutical companies in Canada and the U.S. Prior to staring DPharma Group, he held the position of vice president of operations and technology at Synthon Pharmaceuticals in North Carolina and vice president and general manager at Atlantic Pharmaceuticals Services in Maryland. He is the editor of Handbook of Pharmaceutical Granulation 3rd ed. He has authored several book chapters and articles on various pharmaceutical technologies, including quality by design, process assessment and contract manufacturing. He has been an invited speaker at scientific conferences worldwide on solid-dosage technologies development and manufacturing.
A double drum dryer working under atmospheric pressure was developed for water evaporation rate of 20kg/h. Potato slurry of 12% solid concentration was dried to obtain potato flakes. Experiments were carried out at drum speed of 5 to 30rpm, steam pressure 2 to7kg/cm2 gauge (saturation temperature 120164C) and liquid level 5 to 10cm at the nip of drums. The responses obtained were, moisture content of the potato flakes: 1.1844.15% (db), dry matter output rate: 1.332.87kg dry solid/h and L value of colour: 30.966.4. Steam pressure and drum speed were the most influencing parameters affecting all 3 responses. Optimum combination of operating variables for obtaining potato flakes of 8% (db) moisture content, high dry solid output rate and high L value of colour was: drum speed 19.6rpm, steam pressure 4.3kg/cm2 gauge (saturation temperature 145C) and liquid level at the drum nip 6.3cm. At this combination of independent variables, residence time of the product on drum surface would be 2s, final moisture content of product 8% (db), product output rate 2.4kg dry solid/h and L value of colour 53.
Trystram G (1988) Contribution lautomatisation dun procd industrial de schage sur cylindre. In: Renard M, Bimbenet JJ (eds) Automatic control and optimisation of food processes. Elsevier Appl. Sci, London, pp 265283
Kakade, R.H., Das, H. & Ali, S. Performance evaluation of a double drum dryer for potato flake production. J Food Sci Technol 48, 432439 (2011). https://doi.org/10.1007/s13197-010-0184-0
When we say we are tuning a drum, what does that really mean? With the exception of kettledrums and tympani, drums do not make single identifiable notes when played. Drums actually make several different notes simultaneously.
What we call tuning mainly consists of several steps. 1) voicing the batter head tension for the initial lug note sounded when struck. 2) voicing the resonant head for agreeable after-ring decay after the drum strike. 3) clearing the lugs to perfectly agree with each other. This third step is the secret sauce that makes the difference between a drum that measures like it should be in tune, and one that sounds like it is in tune.
Even if a note sniffer says every lug is voicing the same lug resonance, they can still disagree at higher overtones. When a drum sounds multiple closely spaced higher overtones they interfere against each other and create dissonance. When those higher overtones are also in tune the result is a simpler, cleaner drum sound without the harsh dissonance.
Typical musical instruments play only one note at a time, one note per guitar string or one note per piano key. These notes can contain multiple higher frequency harmonics that define their complete sound character. These overtones generally fall on perfect harmonic ratios due to how these musical sounds are created. Even though they are different frequencies, they all combine to sound like a single musical note with complex sound character.
In a musical instrument that vibrates a string under tension, or a wind instrument that vibrates a column of air, the wavelength of the sound generated is defined by a physical dimension like the length of the string or length of the air column. Wavelength, just like the word sounds, is the length of a single waveform, beginning at zero, looping up before reversing to pass down through zero to loop below then return back up to zero again to start over. These zero points in the wavelength are called nodes, and these points of zero movement line up with hard physical boundaries. The endpoints of a string where it is firmly attached or the closed end of an organ pipe form natural nodes. The open end of an organ pipe is where the natural anti-node (peak amplitude) occurs so that length is for a 1/4 wavelength. String length and drumhead dimension nodes define full or 1/2 wavelengths.
The frequency or pitch of the fundamental note is determined by how fast sound travels in the vibrating medium and how far that sound must travel. The period of a waveform is the time it takes to form one complete wavelength. The frequency or pitch of this waveform is the mathematical reciprocal of this period or 1/period. To make different pitch notes in a pipe organ you use different length pipes. When playing a guitar you can make the effective length of the vibrating string shorter and therefore shift the note pitch up higher by pressing the string hard against a fret to form a new end point or node there. The shorter the length of vibrating string, the higher the note pitch it makes. The multiple guitar strings on one guitar are made using different diameter (weight) wire, sowhen the same length, they make a musically useful ratio or spread of notes.String tension is also varied to fine tune the actual notes.
An important difference about how higher pitch overtones develop in these conventional musical instruments, as compared to drums, is that conventional musical instrument overtones travel over the exact same physical path as the fundamental note. Since the overtone wavelength nodes must line up at the same physical boundaries, the overtones only meet these criteria for full multiples, like 2, 3, or N complete wavelengths fitting neatly into the same fundamental wavelength path. Doubles of the fundamental pitch make harmonic overtones that fall on the same musical notes but in higher octaves (2nd, 4th, 8th etc).
Unlike conventional musical instruments where the sound energy vibration is limited to a one-dimensional path, sound vibrations in a drumhead can travel anywhere within a round two-dimensional space. The distance straight across the drumhead defines one wavelength series. A round zero movement node is established by the fixed rim-edge drumhead attachment. As we can see in the far left hand animation below viewing a drumhead vibrating at the fundamental resonance, from the side looks like one half of a sine wave, with alternating up and down motion.
Loudspeaker diaphragms generally move in and out as a single rigid surface operating in what is called piston motion. When a loudspeaker driver diaphragm starts vibrating across its diameter like a drumhead, making higher frequency overtones, these are called break-up modes and undesirable. When drumheads do this its just normal drum behavior.
The wavelength for other series of resonances traces circular paths around and across the middle of the drumhead. Since the circumference or distance around a circle is pi (3.14) times the distance across the same circle, there are multiple different non-integer ratios between the drum fundamental pitch and higher pitched drumhead overtones. These non-integer ratios cause drumhead overtones to fall on different notes. Vibration mode 1,1 and 2,1 have a fixed node at the midpoint and additional nodes and anti-nodes that fall on a circle mid-way between the center point and rim edge. Other vibration series fall on other smaller circles, and one higher overtone series has an anti-node (peak) dead center again, similar to the lowest fundamental.
When only one head is mounted, like on a concert tom, mode (1,1) is weak and mode (2,1) dominates the first overtone. The chart below shows a series of typical notes and levels with the fundamental (in red), first overtone (in green), and higher resonances (in blue). These readings were made using a RESOTUNE II to excite the drumhead with sound energy from its internal loudspeakers. Resonances excited by drumstick hits will preferentially excite the resonances louder that have anti-nodes (local maximums) directly under where the drumhead is hit. For example the fundamental note is most excited by striking the drumhead dead center where it has the largest up/down movement or excursion. Higher pitch resonances generally have peak activity away from the drumhead center point, so respond more when the head is struck off-center. Hitting even closer to the rim edge excites even higher pitched overtones.
Observe that these notes are not on useful musical intervals and do not make some euphonious drum chord. Instead they are a characteristic mix of different notes that drummers can play louder or softer selectively by how and where they strike the drumhead. The concert tom is preferred by some drummers for its characteristic sound. With only one head, the sound is also more repeatable than from using two headed drums because there isnt a second different tensioned head interacting with the first head differently at different resonances. All concert toms will exhibit the same characteristic ratio between notes, while two headed drums will vary more with voicing.
The most important sound characteristic of a one-headed or concert tom is the spacing between the lowest note fundamental and the first (lug) overtone. The span between G and Ab is musically just the next note higher but in reality it is over one full octave higher. If we look at the standard vibration modes for a round flat vibrating disc we see this spacing agrees with the lug overtone being vibration mode (2,1) at 2.135x above the fundamental. Vibration mode (1,1) falls in the middle between them at 1.593x, but is not active in concert toms. That vibration mode apparently needs the bottom head to support a strong resonance. When we tension a one headed drum tighter, this increases the speed of sound traveling across and around the drumhead, like tightening a guitar string, so all the note pitches shift up higher together, while maintaining the same ratio between resonances and makes the same characteristic sound just at higher pitch.
We are most familiar with two headed drums. Adding a second head to the system doubles the vibrating mass since the two heads will both vibrate together for some resonances even though only the batter head is the one being hit. This extra vibrating mass sustains longer after being hit. There are multiple mechanical and acoustic pathways between these two heads to transfer energy back and forth. The most obvious is the air path coupling the two heads together. Another path for sound energy transfer is to mechanically couple vibration from the batter head through the rim into the shell, through the shell down into the bottom rim and into the resonant head. These different pathways will exhibit different transit times, different coupling efficiency, and even resonances of their own. Coupling between the heads can be constructive, destructive, or neither for different pitch head resonances. If a drum shells natural resonant pitch agrees with one or both drumhead resonances, the coupling between them at that resonance could be enhanced. In practice most shells have so much hardware bolted to them that any natural free vibration mode will not be very pure, while simple mechanical sound transmission will clearly occur. Investigating the full effects of shell interaction is beyond this general overview of tuning and the shells are not adjustable anyhow. Drums made with different shell depth and from different shell materials will behave and sound different.
One relatively simple mechanism to inspect is the acoustic coupling directly through the air path between the two heads. When the top batter head is struck dead center, it bows inward and compresses the air inside the drum. This momentary pressure increase pushes down on the bottom resonant head, moving it down in the same direction. As the top head swings back up, due to the restoring force of its spring compliance, it generates a reduced internal pressure, sucking the bottom head up with it. At the fundamental (lowest) note resonance, these two heads move up and down together as a single connected system. The mass and tension of both heads influence the pitch of this combination fundamental note. Changing the tension of the bottom head will shift the fundamental pitch of the top head because they both vibrate together. In theory there is a resonance mode where these two heads move in the exact opposite direction from each other, but this mode, dissipates energy quickly from squeezing and stretching the air inside the drum, so should decay quickly. It could be supported by a shell resonance wavelength that has a node at one rim edge and anti-node at the other rim edge of the shell.
Many of the higher overtone series do not compress the air inside the drum between the two heads, because after the initial pulse, a standing wave forms in the drumhead that causes one half of the drumhead to be moving up whenever the other half is moving down, and vice versa. When one half of the batter head is always going up at the same time as the other half of the head is going down, the air inside the drum is not being squeezed and less sound energy couples through this pathway to the opposite head. The initial pulse, before the standing wave builds up, will cause some direct energy transfer to the opposite head. The impact of this depends on how the top and bottom head resonances are set relative to each other. Unlike the fundamental resonance that involves both heads, the lug overtone and most higher overtones are local to just each head independently. If the two heads are tensioned precisely alike, this pulse from a strong hit will excite the resonant head sympathetically. If the two heads are tensioned differently, or have different mass, causing a different resonance pitch, the sympathetic resonance is less pronounced.
Often the two heads are intentionally tensioned to slightly different lug resonances to reduce the amount and duration of sustain. This can also subtly shift the after-ring. All drums will experience a slight drop in pitch between the initial attack and the after-ring, because the head deflection from the initial high amplitude vibration transiently increases the head tension shifting the pitch up. As this amplitude drops and the tension relaxes, the pitch glides lower.
If we re-inspect the resonances in that exact same tom batter head as measured above, but this time with a similar weight resonant head mounted to the bottom of the shell, and tensioned to the same lug resonance note pitch, we see an interesting change. The top head lug resonance remains the same note pitch, as before, since that head tension has not changed, but now the fundamental note pitch has been pulled up to a higher note pitch. It is not immediately obvious that the higher mass of two heads vibrating together would vibrate faster, but this is not a simple relationship.
Resonance in any mechanical system is characterized as the conversion back and forth between the kinetic energy of the drumhead mass moving up and down into the potential energy of a spring stretching and relaxing. The head stretching is this spring. The frequency of such resonances is influenced by one divided by 2 pi times the square root of the ratio between this spring rate divided by the mass.The head mass for a two head drum is a simple double, and now even the air mass captured between the two heads adds to and becomes part of the combined system increasing the vibrating mass, but there are now two springs or edge compliances between the head and rim system in parallel. The effective spring rate for two same size heads in parallel is stiffer than one proportionately larger head and the resonant note reflects that by being a higher pitch.
The note spacing now between fundamental and lug overtones are consistent with disc vibration mode (1,1) or 1.593x. Instead of the fundamental being the same and all the higher resonances shifting, the lug resonance stays the same as before and the fundamental note shifts up.
This tighter spacing between fundamental and lug overtone is the first significant difference between using one head and two, but there is another important difference. The ratio between lug overtone and fundamental is no longer rigidly fixed to follow a hard ratio based on the head dimension, but the relative mass and tension of the two heads can now shift the fundamental and associated resonances up or down, relative to the other resonance series. We now have a much wider palate of voicing possibilities defined by the relative tension and weight of the two heads.
For similar weight heads, you can completely characterize the voicing of a two-headed drum by measuring the top and bottom head lug resonance notes. Noting these two Lug notes (and head weights) will allow you to always return to a favorite drum voicing. By replicating these Lug notes and heads used, you can make different drums sound more alike (if desired), while mechanical characteristics of the shell and hardware between different drums can affect the higher resonances subtly.
We do not believe that there is only one ideal voicing for all drums so decline to give specific voicing advice. We observe that it is popular to de-tune the bottom resonant head slightly higher or slightly lower pitch than the batter head lug note, but even this is not universal. Here is a link to one pretty comprehensive general discussion of tuning and voicing drums Prof sound-Drum tuning bible . We are inclined to downplay any tuning advice that argues that there is only one way to voice a drum that sounds good. There are multiple different opinions published. We suggest that you try them all and take time to experiment and find your own personal voice.
Now we come to the major source of confusion surrounding this topic. Why can drums be adjusted to make the same note pitch at a given resonance, but exhibit such different sound quality?The attachment method of using multiple lugs spaced around the drumhead circumference to tension the drumhead can cause very audible sound quality differences when the lug tensions are not very closely matched to each other. The fundamental note pitch tracks with the average head tension so half of the lugs could be too tight and the other half too loose while still making the same fundamental note pitch. However as we inspect the behavior at higher vibration modes we find progressively more significance attached to individual lug tension and the matching between these lugs. These slight tension differences along with tiny mechanical imperfections in the drum construction can cause significant audible differences.
If the lugs are not in perfect agreement with each other, instead of just making the lowest possible number of overtone notes, sounding pure when played, we hear instead a much more complex dissonant sound character from multiple closely spaced overtones. This state of perfect tune when all the lugs are in complete agreement with each other is called clear or clearing the head, because the sound character of the drum appears to open up, or clean up, sounding more pure, or as pure as it can possibly sound from that mix of non-related drum notes.
The Bass/Kick drum: is generally just a larger tom, laying on its side. One import distinction about Kick drum sound is related to how it is struck. The fixed foot pedal beater strikes the drumhead dead center. That preferentially excites the fundamental resonance. As we recall from the concert tom discussion, when the resonant head is not mounted, the fundamental note pitch generated is lower frequency, perhaps desirable for a kick drum.
There are also after-market products that form a port with a hole in the kick drum resonant head. Without doing a detailed analysis of the physic involved, this is probably a combination of adding mass to the head with some acoustic effect from the port. Ill leave it to others to determine what is really controlling resonances and note pitch with those.
The Snare drum: is another variant on the two head tom for dominant head resonances, but in addition there are wire snares strung across the resonant head. While these snares do not vibrate like a musical instrument string, instead they rattle up against resonant head, but in a musically pleasing way. The shell of typical snare drums is shorter than most toms, and far more rigid, often made of dense metal or sometimes very thick wood. This increases the mechanical transmission though the shell from the top batter head down into the resonant end rim and snares.
The conga drum is a variant on the one-headed concert tom with similar resonant series note ratios. In addition the longer shell forms a tuned resonant air column, like a very short organ pipe, or a long loudspeaker port. This extra acoustic structure can enhance a lower note resonance.
The tympani and kettledrum are special cases of one headed drums designed for orchestral use that are perceived as making single note pitches when played. Their sealed back air cavity suppresses resonances like the typical lowest fundamental mode (0,1), that would compress the internal captive volume of air. Their sound character is dominated by vibration modes (1,1), (2,1), (3,1), etc. where half the drum head area is moving up while the other half is moving down so dont increase or decrease the internal air pressure.
The tympani/kettle drum is designed so that these lower resonances fall on a pitch spacing that while not perfect harmonics of each other, this spacing mimics them being upper harmonics above a missing lower phantom fundamental note. Our brain is trained to interpret that specific overtone spacing, based on our experience with naturally occurring musical sounds. Our brain assumes this missing lower note must be present, and hears the complex note as if the missing fundamental was present. This musical psycho-acoustic trickery allows the tympani/kettle drum to appear to make notes at lower note pitch than their physical dimensions can actually support.
Only one major drum manufacturer, that we are aware of (DW) provides a note target for their drums. They call this Timbre-Match (r) and identify this as the shell note pitch. From discussion with DW techs this should be coordinated with the lug resonance so it can also be targeted while using tap tuning, or any of the electronic sniffer variants providing specific pitch information. Timbre-Match (r) is a registered mark or Drum Workshop, Inc.
While discussing all of the head variants is also beyond the scope of this overview I have some general observations. Many of the exotic drumhead designs attempt to squelch the higher overtones to reduce the apparent dissonance of a poorly cleared drumhead. If the upper overtones are damped you cant hear how un-clear they are. Damping also reduces sustain, if the drumhead doesnt sound great when you hit it, why let it sound that way even longer? So my suspicion is that many of these trick drumheads are designed to conceal the sound of not being well tuned or cleared.
There is one alternate head technology with some actual physics behind it. The dot heads have a small mass added right at the center mid-point of the drumhead. For the vibration modes (like 1,1 and 2,1) where the exact mid-point node is sitting dead still, this added mass has no effect. However for the fundamental mode (0,1), this midpoint is now an anti-node and moving vigorously up and down. As we should recall from the mass/spring rate discussion, mass is in the denominator of the natural frequency equation, so more mass makes a lower note. The goal is to shift the fundamental note of a two head drum down from the nominal 1/1.593x, to 1/2x or one full octave spacing a useful ratio that makes the same note only in a different octave.
We still decline to give specific voicing advice but in general the tension and mass of a head affects stick feel as well as rate of sound decay. Using different weight heads on the top and bottom can make subtle voicing differences similar to de-tuning.Experiment and try different combinations, if you like the way some combination sounds use it. If you dont like the sound, dont use it.
Before you lose too much sleep trying to decide which resonant series to target to be precisely on note, since they all cant all be tuned exactly on full notes, lets look at the popular practice of establishing a musical spread of note pitches across your different toms. If all the batter and resonant heads are the same weight or same ratio of weights, with similar relative tuning, a musical note spread of lug resonances will map out directly to the same note spread of fundamental resonances. While only one or the other set of resonances can be tuned to fall precisely on full notes. Dont lose any sleep over this.One clue about whether you might favor tuning the fundamental or lug resonance to be on note, when you play a run across your toms do you hit them dead center making the thud fundamental sound, or do you hit them off center exciting more of the lug overtone ring? Again do not lose one moment of sleep over this.
A well-tuned drum kit that sounds great is more fun to play and sounds better to everybody else too. If a drum is worth playing it is worth tuning as well as you can. Despite writing far too much about specific notes, the perceived sound quality of a drum kit is most influenced by clear quality, and how pure it sounds, Not making just one of the several different notes fall precisely on some note target. Adjust the lug tension so it feels good when you hit it. Clear the lugs to each other so the head is only making the smallest number of different notes instead of a complex dissonant mess, and enjoy yourself. Even if you dont have access to the latest technology, do the best job you can using whatever tools you have available (even if just your ears and a well placed finger). We hope this information is helpful.
Batter Head: This is the top head for toms or snare, and the head on the bass/kick drum closest to you that you strike with the beater. The batter head is the one you hit, makes the loudest sound, and has the most influence on your sound quality. Since it is the one you are constantly hitting, it is also the one most likely to drift out of adjustment from playing so needs more re-tuning and re-clearing attention.
Resonant Head: This is the bottom head for toms, and front head on bass/kick drums. The one you dont hit. As the name suggests this head mostly vibrates sympathetically with the batter head to shape the total sound envelope including sustain and decay after the batter head is struck. These should be cleared when changed, but generally do not require as much touch up as the head being hit.Concert toms do not use a resonant head, and many kick drums have holes (for microphones) in the resonant head.
Clear: This describes the state of agreement between the multiple lug tensions on a given drumhead. When all of the lugs are precisely matched to apply the exact same influence on the drumhead standing waves, the drumhead will make a single set of resonant notes. This single set of resonances will sound more pure and open sounding compared to the same drumhead that isnt clear. Clear quality is different from note pitch targeting and a drumhead can be cleared for any arbitrary pitch.
Resonance : In mechanical systems with moving mass and spring compliances, the efficient transfer back and forth of kinetic energy between the movement of the mass into the potential energy of a coiled spring and back again can express as a sustained natural frequency, only diminished by damping that dissipates the energy and causes the sound to decay. Drumhead/ drum systems can express multiple resonances.
Voicing: this describes the subjective personal selection of head weights, absolute and relative lug tension or resonance target pitches between both heads to realize a desired overall sound character. This is mainly a consideration for two-head drums because one head drums like concert toms have fixed resonance ratios, so can only be tuned higher or lower as a group, not relative to each other.