3 REVIEW OF MAJOR DRYER TYPES

3.1 Rotary Dryers

Rotary dryers potentially represent the oldest continuous and undoubtedly the most common high volume dryer used in industry, and it has evolved more adaptations of the technology than any other dryer classification.

All rotary dryers have the feed materials passing through a rotating cylinder termed a drum. It is a cylindrical shell usually constructed from steel plates, slightly inclined, typically 0.3-5 m in diameter, 5-90 m in length and rotating at 1-5 rpm. It is operated in some cases with a negative internal pressure (vacuum) to prevent dust escape. Solids introduced at the upper end move towards the lower or discharge end. Depending on the arrangement for the contact between the drying gas and the solids, a dryer may be classified as direct or indirect, con-current or counter-current.

The drum is mounted to large steel rings, termed riding rings, or tires that are supported on fixed trunnion roller assemblies. The rotation is achieved by either a direct drive or chain drive, which require a girth gear or sprocket gear, respectively, on the drum.

As the dryer rotates, solids are picked up by the flights, lifted for a certain distance around the drum and showered through the air in a cascading curtain. Most of the drying occurs at this time, as the solids are in close contact with the gas. Flight action is also partly responsible for the transport of solids through the drum.

Refer fig 3.1 for schematic of rotary dryers.

Figure 3-1 Indirect Rotary Dryer

Typical performance data of direct heated rotary dryers is given below in table 3.1.

Table 3-1: Performance data of rotary dryers for various feed materials

Details	Sugar Beet pulp	Calcium Carbo nate	Blast Furnace Slag	Lead Concen tration	Sand	Zinc Concen tration	Al2 Sulphate	Fine Salt	Crystals
Air flow	Parallel	Parallel	Parallel	Parallel	Parallel	Parallel	Counter	Counter	Counter
Dryer Dia (m)	2.79	1.91	2.19	1.37	1.37	2.13	2.74	1.52	3.05
Length (m)	14	10.4	12.2	10.7	9.91	18.29	12.19	12.19	18.29
Method of Heating	Oil	Oil	Oil	Oil	Gas	Oil	Gas	Steam	Steam
Method of feed	Screw	Belt	Belt	Screw	Chute	Screw	Conveyor	Feeder	Screw
% of Moisture	456	15.6	49.2	16.3	6	21.9	2.5	5.3	7.5
(bony dry basis))	11.1	0.5	nil	8.7	0.043	8.7	0.2	0.1	9.9
Evaporation,./kg/	15426	2722	5263	632	318	3656	508	181	522
capacity, kg evpn/m3 of dryer vol.	176	96	112	40	22	37	8	8.3	3.9
Kcal supplied/ kg. Water Evpn.	788	1078	949	1166	1416	1028	1066	1166	916
Air Temp. inlet	849	849	849	704	899	816	204	138	150
outlet	110	104	120	93	106	93	31	77	62
Avg. Residence time in min	20	25	30	20	12	20	15	40	70
Fan H.P	70	40	50	20	5	75	25	8
Motive H.P	15	20	25	10	10	55	60	15	60

3.2 Pneumatic/Flash Dryer

The pneumatic or ‘flash’ dryer is used with products that dry rapidly owing to the easy removal of free moisture or where any required diffusion to the surface occurs readily. Drying takes place in a matter of seconds. Wet material is mixed with a stream of heated air (or other gas), which conveys it through a drying duct where high heat and mass transfer rates rapidly dry the product. Applications include the drying of filter cakes, crystals, granules, pastes, sludges and slurries; in fact almost any material where a powdered product is required. Salient features are as follows.

_ Particulate matter can be dispersed, entrained and pneumatically conveyed in air. If this air is hot, material is dried.

_ Pre-forming or mixing with dried material may be needed feed the moist material

_ The dried product is separated in a cyclone. This is followed by separation in further cyclones, fabric sleeve filters or wet scrubbers.

_ This is suitable for rapidly drying heat sensitive materials. Sticky, greasy material or that which may cause attrition (dust generation) is not suitable.

Table 3-2: Performance data of Pneumatic dryers

Details		Metallic	Starch	Adipic acid	Adipic acid
		Stearate
Method of feed		Sling	Sling	Screw	Distributor
Material size		Fine	Fine	30 mesh	6 mm
Product rate kg/hr.		127	6005	4537	1184
Moisture %	Initial	66.7	51.5	11.1	165.9
(bone dry basis)	Final	0.5	14.9	0.2	11.1
Air Temperature	Inlet	140	150	160	400
	Outlet	54.4	50	65	110
Method of Heating		Steam	Steam	Steam	Oil
Heat Consumption, Kcal/kg. Evpn.		1205	1014	1333	955
Air Circulation		No	No	No	No
Material Circulation		Yes	No	Yes	Yes
Fan Capacity std.m3/hr		2448	45058	16153	21254
Installed Fan HP		15	220	65	60
Product Exit Temp.( oc)		40	35	48.9	60

Fig 3.2 shows schematic of a pneumatic/flash dryer.

Figure 3-2: Pneumatic /Flash Dryer

3.3 Spray Dryers:

Spray drying has been one of the most energy-consuming drying processes, yet it remains one that is essential to the production of dairy and food product powders. Basically, spray drying is accomplished by atomizing feed liquid into a drying chamber, where the small droplets are subjected to a stream of hot air and converted to powder particles. As the powder is discharged from the drying chamber, it is passed through a powder/air separator and collected for packaging.

Most spray dryers are equipped for primary powder collection at efficiency of about 99.5%, and most can be supplied with secondary collection equipment if necessary. Salient features of Spray dryers are as follows.

 Solutions, suspensions, slurries and pastes, which can be pumped, can be dried on spray dryers. The advantage of spray dryer is rapid and non-contact drying.

 Much higher initial temperature of drying medium can be used. High evaporation rates and thermal efficiencies are achieved.

 It can be quickly started and shut down.

 It is capable of handling volatile or inflammable solvents in a closed cycle.

Figure 3-3: Spray Dryer

Spray drying of Milk is one common use in Dairy Industry. In general, there are two ways of drying milk. i.e. one-stage spray drying system with pneumatic conveying system and multistage spray drying system with an external vibrated fluidized bed dryer.

The advantages of multistage spray drying system are as follows:

Higher capacity per unit drying air

Better efficiency due to lower outlet temperature

Better product quality in terms of solubility, flow ability

Figure 3-4: Closed cycle spray dryer lay out

The feed liquid, which can be a solution, suspension or an emulsion, is pumped to an atomizer located in the air disperser at the top of the drying chamber. The atomizer sprays the liquid into a high velocity stream of drying air and the resulting spray droplets are dried as they are carried downwards in the central air jet towards the integrated fluid bed. Particles enter the fluid bed while the air flow reverses upwards to be exhausted from the top of the drying chamber. The finer particles separated from the exhaust air are recycled to the drying chamber. The fluidization of particles in the fluid bed, fines recycle, and particle movement in the air flow result in spray drying taking place in a powder-laden atmosphere which is much denser than in conventional drying systems. Particles of higher moisture content can then be handled as the resulting powdering effect overcomes problems of surface stickiness of the drying particles. The moisture content of particles entering the fluid bed can be controlled to the level required for achieving the desired particle size increase and structural change (agglomerating or granulating). When required, final drying and cooling of the product takes place in a fluid bed connected to the outlet of the integrated fluid bed.

3.4 Fluidised Bed Dryers

Fluid bed dryers are found throughout all industries, from heavy mining through food, fine chemicals and pharmaceuticals. They provide an effective method of drying relatively free flowing particles with a reasonably narrow particle size distribution. In general, fluid bed dryers operate on a through-the-bed flow pattern with the gas passing through the product perpendicular to the direction of travel. The dry product is discharged from the same section.

Refer figure 3.5.

 With a certain velocity of gas at the base of a bed of particles, the bed expands and particles move within the bed.

 High rate of heat transfer is achieved with almost instant evaporation.

 Batch/continuous flow of materials is possible.

 The hot gas stream is introduced at the base of the bed through a dispersion/distribution plate.

Figure 3-5: Fluidised bed dryer

3.5 Hot Air Dryer- Stenter

Fabric drying is usually carried out on either drying cylinders (intermediate drying) or on stenters (final drying). Drying cylinders are basically a series of steam-heated drums over which the fabric passes. It has the drawback of pulling the fabric and effectively reducing its width. For this reason it tends to be used for intermediate drying.

The stenter is a gas fired oven, with the fabric passing through on a chain drive, held in place by either clips or pins. Air is circulated above and below the fabric, before being exhausted to atmosphere. As well as for drying processes, the stenter is used for pulling fabric to width, chemical finishing and heat setting and curing. It is a very versatile piece of equipment. Refer fig 3.6 for a schematic diagram.

Figure 3-6: Schematic of a stenter

Modern stenters are designed with improved air circulation, which helps to improve drying performance, and with integrated heat recovery and environmental abatement systems. Infrared drying is used for both curing and drying. It is used as either a stand-alone piece of equipment, or as a pre-dryer to increase drying rates and hence fabric speed through a stenter.

In the carpet industry there are a number of different types of drying/curing machine used. Wool wash dryers at the end of scouring machines for drying the loose stock wool; wool drying ranges for drying wool hanks prior to weaving; and wide 4 and 5-metre latexing or backing machines used to apply and dry/cure the latex backing on to carpets. Low level VOC emissions are produced by this process.

3.6 Contact Drying- Steam Cylinders/Cans

This is the simplest and cheapest mode of drying woven fabrics. It is mainly used for intermediate drying rather than final drying (since there is no means of controlling fabric width) and for pre-drying prior to stentering. Fabric is passed around a series of steam heated cylinders using steam at pressures varying from 35 psi to 65 psi. Cylinders can be used to dry down a wide range of fabrics, but it does give a finish similar to an iron and is therefore unsuitable where a surface effect is present or required. In stenters, the fabric is width wise stretched for width fixation by a series of holding clips or pins mounted on a pair of endless chains.

Fig 3.7 shows schematic of a textile cylinder dryer. The drying section consists of a series of high velocity jets. Large quantities of air is re-circulated and reused to conserve heat. Exhaust fans exhaust a certain amount of air. The rate of drying is influenced by the velocity of air jet as well as the difference between dry bulb and wet bulb temperatures of air.

Figure 3-7: Cylinder Dryer

In Paper industry, steam cylinders are 4 – 5 feet in diameter and slightly longer than the width of the paper sheet. A typical paper machine has 40 to over 100 steam cylinders, depending on the line speed; the faster the line speed, the longer the drying section.

The performance of steam cylinders can be enhanced by the use of directed air either at ambient or elevated temperatures. In the latter case the air is a second means of heat transfer and the process is a combination of contact and hot air drying. The ATIRA Rapidry system , an Indian development, which uses air jets and claims increased drying rates of the order of 25 -30%. It is common for steam cylinders to have problems such as leaks at vacuum breakers, air vents, rotating joints and steam traps. This is a direct result of the design of the heating system which relies on passing steam and condensate into and out of each cylinder via a rotating joint. When you have upto maybe 32 such cylinders in a single bank then the potential for leakage is considerable. It is therefore important to initiate a good maintainance regime. For example, periodically checking steam traps using an ultrasonic steam leak detector.

3.7 Infra red drying

One way to improve drying operations is to add or use infrared energy. Infrared energy can be generated by electric or gas infrared heaters or emitters. Each energy source has advantages and disadvantages. Typically, gas infrared systems are more expensive to buy because they require safety controls and gas-handling equipment, but they often are less expensive to run because gas usually is cheaper than electricity. Gas infrared is often a good choice for applications that require a lot of energy. Products such as nonwoven and textile webs are examples where gas often is a good choice.

Gas IR heaters produce an infrared wavelength that is readily absorbed by the water in the sheet. This leads to a higher temperature and a drying efficiency increase that cannot be duplicated by conduction and convection temperatures alone.

Table 3-3: Drying rates for dryers

Method	Type of Drying	Drying Rate (lbs water/hr/ft2)
Steam Cans	Conduction	2-6
Air Hoods Impingement	Convection	4-8
Gas IR	Radiation + Convection	30+

By contrast, electric infrared is likely better for sensitive substrates such as film and certain fabrics, where extreme control and uniformity is required. Electric infrared heaters can be divided into multiple, separately controlled temperature zones with tolerances as tight as +/-1oF. Both electric and gas infrared typically are controlled by thermocouple feedback control loops that regulate the electrical power or fuel mixture going to the infrared heaters. For more precise control, temperature feedback from the product using an optical pyrometer is used. In paper drying, gas fired infrared heating can be used, as given below in fig 3.7. An increase in

speed of 20-25% is possible due to this.

Figure 3-8: Infrared heating for paper machine

ABB has developed a unique in-drum radiant heater system that increases drying capacity by increasing the surface temperature of a drying drum/can over what is possible with a steam system. A Radiant Burner inside cylinder acts as Heat Source. A schematic is given below.

Figure 3-9: Cylinder Dryer with radiant burner inside

3.8 Radio frequency drying:

In a radio frequency drying system, the RF generator creates an alternating electric field between two electrodes. The material to be dried is conveyed between the electrodes, where the alternating energy causes polar molecules in the water to continuously re-orient themselves to face opposite poles—much in the same way magnets move in an alternating magnetic field. The friction of this movement causes the water in the material to rapidly heat throughout the material’s entire mass. RF drying offers numerous benefits to ceramic and glass manufacturers, including moisture control and uniformity; reduction in surface cracking; and savings in energy, drying time and plant space. Precise Control of Moisture Content and Uniformity. Heating in an RF dryer occurs selectively in those areas where heat is needed because water is much more responsive to RF energy than most other dielectric materials. Since wetter areas absorb more RF power than dryer areas, more water is automatically removed from wet areas, resulting in a more uniform moisture distribution.

Energy Savings. The efficiency of convection dryer drops significantly as lower moisture levels are reached and the dried product surface becomes a greater thermal insulator. At this point, the RF dryer provides an energy-efficient means of achieving the desired moisture objectives. Typically, 1 kW of RF energy will evaporate 1 kg of water per hour. Additionally, because RF is a “direct” form of applying heat, no heat is wasted in the drying process.