Powder and Bulk Engineering, May 2000 47

How to select and operate a conveyor dryer Dan Poirier Aeroglide Corp.

Many dry bulk solids processors need to find a way to dry their product to reach an appropriate level of moisture content. One solution is to use a conveyor dryer, a versatile convection-based piece of equip- ment. This article discusses the drying process, how a conveyor dryer works, and how to select and use one efficiently.

ry bulk solids processors have many reasons for drying their products. D

Drying can ensure the quality and shelf life of certain prod- ucts. Drying keeps consumers from finding soggy snack- foods, clumped detergent powders, and moldy pet food in the bags and boxes they bring home from the store.

Saving money is another reason to dry. Most materials weigh less when moisture is removed, so in many in- stances it makes sense to dry them before shipping or land- filling them, since both shippers and landfillers generally charge by weight.

Drying adds new, desirable qualities to still other products. Without drying, we wouldn’t have instant coffee, cocoa, and soup. Dried pigments have enabled paint manufactur- ers to more easily blend pigments into base materials, and dried ingredients have enabled pharmaceutical companies to more easily produce tablets and capsules with uniform amounts of active ingredients.

The versatile conveyor dryer can effectiveEy handle a wide mnge of makrials, fmm hgile, heapsensitive cereal flakes and pet food to rugged coal chunks and heavy briquets.

The drying solution There are many ways to dry -to remove moisture from bulk materials. These range from laying the wet materials in a sunny location to baking them in an oven. By far the most efficient method for most bulk materials is to use a dryer, a machine designed specifically for removing the required amount of moisture in the required length of time without changing the material’s characteristics. A dryer removes water from the material by evaporation. It’s de- signed to handle the evaporated water (which can be a very significant quantity) and to do this in the most com- pact machine using the least amount of energy possible.

0 0 73

ra 5

2. d

0 cn 0

-0 0

Q d % P, 3 Q m r_ 7 i

48

There are lots of different kinds of dryers to choose from, including flash, spray, rotary, fluid bed, and others. Ulti- mately, your choice is going to depend on your material and your process. This article concentrates on the conveyor dryer, a versatile dryer that can effectively handle a wide range of materials, from fragile, heat-sensitive cereal flakes and pet food to rugged coal chunks and heavy briquets. The following sections provide basic information on how the

Powder and Bulk Engineering, May 2000

conveyor dryer works, factors to consider in selecting a conveyor dryer, and tips on how to run one efficiently.

How a conveyor dryer works A conveyor dryer, shown in Figure 1, uses heated, moving air to dry material transported on a belt conveyor inside the dryer housing.

0 0 73

ra 5

2. d

0 cn 0

Powder and Bulk Engineering, May 2000 49

A feeder or spreader feeds the wet material onto the belt conveyor before the housing, which is a large, metal, box- shaped enclosure sided with insulated panels. Access doors are located along the housing ends and sometimes on the sides. More access doors generally provide easier maintenance and, consequently, cleaner, more efficient drying. The dryer’s size varies, depending on the material to be dried. Some are 0.9 meters (3 feet) wide and 6 me- ters (20 feet) long, while others may be as wide as 4.5 me- ters (15 feet) and as long as 60 meters (200 feet) or even more.

The belt conveyor’s surface, or bed, construction may vary, depending on the material to be dried. Some con- veyor beds are plastic or wire mesh belts, while others are more solid perforated metal bedplates, either hinged or overlapping. The mesh and perforations allow air to pass through the materials to be dried.

A power unit moves the bed through the dryer on a track- and-chain system. Gas burners, steam coils, or other heaters heat fresh or recycled air that’s blown into the in- sulated dryer housing by one or more plug fans - that is, fans that are not free-standing, but are directly plugged into the dryer’s side. The air often collects in a plenum above or beside the conveyor before it passes through the bed and collects in another plenum; from here, the air goes back to the heater to be recycled.

As the air becomes loaded with the evaporated moisture, an exhaust fan removes the moisture-laden air from the dryer as a mixture of air and water vapor. The drying air may be reheated and recycled into the dryer through the plug fans. [Editor’s note: For a brief description of the critical properties related to air and water vapor, see the sidebar “Glossary of drying terms” on page 54.1

As the conveyor carries the material through the dryer, the material typically passes through two or more drying zones, each with different temperature and air characteristics. The drying zones are kept separate by internal panels in the housing that help guide the air and by metal curtains, either on hinges or springs, that the conveyor passes through. At the conveyor’s end, the material may be discharged through the dry material discharge to the next process step or may move to a second conveyor within the dryer to be dried fw- ther before passing to the next process step.

Configurations and options. You can choose from many conveyor dryer configurations. The simplest is a single- pass dryer, which has only one conveyor. The multiple- stage dryer has two or more conveyors, configured with one conveyor after another. As the material passes from one conveyor to the next, the particles rotate or change po- sitions, allowing them to be more thoroughly dried. A space-saving alternative is the multiple-pass dryer, which

0 0 73

ra 5

2. d

0 cn 0 77

-0 0

Q d % P, 3 Q m r_ 7 i

50

has two or more conveyors stacked one over the other. The material drops from one conveyor to the next as it pro- gresses through the dryer (see Figure 1).

Powder and Bulk Engineering, May 2000

The hot air in the conveyor dryer can also flow in different patterns. It can flow up or down through the conveyors, through one conveyor before being reheated, or through multiple conveyors in series before being reheated. The choice depends on the material to be dried. The number and location of the drying zones is another variable in the dryer configuration.

Several components can be added or attached to a conveyor dryer. Probably the most important is the feeder or spreader. Many types of feeders and spreaders are available, includ- ing oscillating spout feeders, oscillating belt feeders, and reciprocating spreaders. The feeder or spreader ensures that the material in the dryer is uniformly distributed on the con-

veyor. Figure 2 illustrates what happens with nonuniform material distribution. Air will always take the path of least resistance, flowing preferentially through the shallow parts of the material. This will result in overdried material in the shallow areas and wet material in the deep areas. Thus, it’s important to choose a feeder that will work effectively with your material.

Other components that can be added to a conveyor dryer include rotary breakers or pickers to break clumps of ma- terial, continuous cleaning equipment such as bed brushes that help keep the conveyor surfaces clean and the perfo- rations unplugged, and safety equipment such as fire sup- pression systems.

How convection dries your material. A conveyor dryer uses convection - hot, moving air- to dry the wet mate- rial. If you’ve used a hair dryer after washing your hair or

0 0 73

ra 5

2. d

0 cn 0 L

E

s -. cn

-0 0

Q d % P, 3 Q m r_ 7 i

52

taking a shower, you’ve used a convection dryer. You know that when you need to dry your hair quickly, the best approach is to put the heat level on hot and air speed on high. This works because the heat evaporates the water more rapidly and the air velocity carries the water vapor away more effectively.

Powder and Bulk Engineering, May 2000

The air is like a sponge. The more water there is in the sponge, the less effective it will be at absorbing water. This is why your hair dries more quickly on a dry winter morn- ing than on a humid summer morning. If the sponge (air) is asked to absorb too much water, it will become saturated and not absorb any more water.

To make the water flow from the wet material to the air, a conveyor dryer uses heat to evaporate the water and mov- ing air to carry away the water vapor. The heat transfer from the air to the material will be faster if a higher air ve- locity is used because the higher velocity increases the contact between the hot air and the wet material. Continu-

ously introducing fresh dry air into the dryer and exhaust- ing the humid, moisture-laden air with the exhaust fan dries out the air to improve its absorption characteristics.

Understanding the drying curve. Figure 3 shows a typical drying curve for materials dried in a conveyor dryer. Ini- tially, very little drying occurs as the wet material in- creases in temperature. In some cases, such as food extrusion, the wet material comes to the dryer already at an elevated temperature and the material can progress di- rectly to the next part of the drying curve, known as cun- stunt-rate drying. At this point, the wet material is dried at a relatively high rate as surface moisture and loose water near the material’s surface evaporate. This rapid evapora- tion cools the material, and the material temperature re- mains relatively constant, well below the air temperature.

At a critical point, known as the critical moisture content, the drying rate begins to slow down as the material’s sur- face dries out and drying continues to remove internal

- Material moisture content

0 0 0 Material temperature

Material entering at ambient

0 0 73

ra 5

2. d

0 cn 0 77

-0 0

Q d % P, 3 Q m r_ 7 i rn E. 3 - 6 2. 3 ra

0 0 73

ra 5

2. - dryer can handle larger particles with drying times that span hours or days, but the model needed to do the job may

0 cn 0 -0

E

s ca d %

be excessively long and expensive.

Not all materials, however, dry in the same way or at the S

cn 3

-. same rate in a conveyor dryer. The ease with which mois- ture can difise to the wet material’s surface will affect the drying rate as well as the critical moisture content at which the drying rate begins to slow down. A large particle is more difficult to dry than a small particle because of the in- creased distance the water must travel to get to the mater-

outer casing that seals the moisture inside the material.

8 Q

P, 3 Q m r_

ial’s surface. Some materials such as nuts even have an

7 i rn

54

moisture and even, at low moisture content, chemically bound moisture. This is known as thejazzing rate part of the drying curve. In this part of the curve, the reduced evaporative cooling results in increasing material temper- ature. If the material is dried to almost bone dry, the mater- ial temperature will approach the air temperature.

Powder and Bulk Engineering, May 2000

Applications. In general, the best materials to dry in a con- veyor dryer are those with particles over 1 millimeter (0.04 inches) with drying times of less than 3 hours. A conveyor dryer doesn’t always handle very light powders well be- cause of the perforations in the dryer’s bedplates or mesh belts and the powder’s entrainment in the airstream. The

56

Other material properties limit the temperatures that can be used in the conveyor dryer. Some materials reach a flash point at a high temperature, and some foods can lose nutrients above a certain temperature. However, keep in mind that in constant-rate drying in a conveyor dryer the material is at a much lower temperature than the air, so the air can be much hotter than the material temperature limit without damaging the material.

Powder and Bulk Engineering, May 2000

How to select a conveyor dryer Making your choice from the various conveyor dryer con- figurations and sizes depends on several factors. If your material is fragile and doesn’t clump, you can select a sin- gle-pass dryer. If your material tends to clump together when wet, you should select a multiple-stage or multiple- pass dryer. It will allow the material to be run in a shallow layer on the first conveyor and to be loaded in gradually deeper layers on succeeding conveyors as the material dries and is less likely to clump.

Consider your material properties such as bulk density and particle size to determine the size of conveyor dryer re- quired for a given retention time (the time the material stays in the dryer) and to limit the air velocity to avoid blowing the material around inside the dryer.

Another factor to consider when choosing a conveyor dryer is the amount of energy it will use. Energy in a dryer goes to heating the material and water, evaporating the water, heating the air, and heat losses. The first two items are governed by physics and will be the same in all dryers. Dryer design, however, will determine the last two items. Considering these factors when you select your dryer may help you save money and natural resources when you op- erate your dryer.

You can minimize the energy used to heat air by recircu- lating air in the dryer, minimizing exhaust temperatures, and using makeup air (fresh, dry air introduced into the dryer and controlled by dampers at openings in the dryer’s top or side) that is as hot as possible. Often, a source of heated air, such as spent cooling air, can be used as makeup air to the dryer. You can minimize the heat losses from a conveyor dryer with proper insulation and proper seals around all openings.

Sanitation is critical to some operations, particularly food drying. If your operation requires sanitary conditions, choose a dryer with minimal ledges and crevices where fines can accumulate and with easy access to all internal dryer parts for proper cleaning.

Maintenance issues to consider when you choose a con- veyor dryer are accessibility for cleaning components such as fans, burners, conveyors, track-and-chain assem- blies, and conveyor power units. Also ensure that these

0 0 73

ra 5

2. components are designed for long life under your typical operating conditions. Some corrosive materials or corro- sive washing chemicals will dictate the use of stainless steel for some of the construction. [Editor’s note: The sidebar “Checklist for selecting a conveyor dryer,” on page 58, summarizes some of the critical questions to ask when you’re choosing a dryer.]

d

0 cn 0 -0

E

s ca d %

S

cn 3

-0 0

Q

P, 3 Q m r_

-.

How to run your conveyor dryer efficiently If you keep in mind the basic principles outlined in this ar- ticle, you can make educated decisions regarding your conveyor dryer’s operation. Simple operating adjustments or corrections can provide significant energy savings - 25 percent or more is not uncommon - and increase pro- duction by the same amount.

7 i

Controlling material moisture. The moisture content of a material is typically expressed on a wet weight basis (wwb). The wwb moisture is the weight of moisture in the material expressed as a percentage of the total weight. (Sometimes the material moisture is expressed as a percentage of the dry weight. Be sure you know which basis is being used to ex- press the moisture content for your application.)

Table I shows the amount of water that must be evaporated to produce 1,000 lulograms (2,205 pounds) of dry material at different moisture contents (wwb). Notice that drying material from 30 percent down to 5 percent wwb moisture requires 79 percent more evaporation than drying from 25 percent down to 10 percent wwb. This means 79 percent more latent heat is required to evaporate water.

For this reason, it’s important to operate your dryer to ob- tain the maximum allowable wwb. Overdrying will waste energy and, in most cases, adversely affect your material. For foods, shelf life may be reduced or palatability af- fected. For other materials, color uniformity or texture may be affected.

Another thing you can do to ensure efficient energy use and proper material moisture content is to make sure the material is dried uniformly. If the dryer doesn’t dry uni- formly, some particles will be overdried. This overdrying not only wastes energy but can also represent significant lost production.

Most materials are sold by weight. If you’re overdrying your material to compensate for nonuniform drying, some of your salable production is going out your exhaust stack. For example, drying your material to 7 percent wwb mois- ture rather than 10 percent wwb moisture to compensate for a +_3 percent moisture variation in the dry material means 3 percent of your production is going out the stack. If you’re producing 10,000 kgh (22,050 lbh) of dry ma- terial, a 3 percent loss represents over 2,500 metric tons (2,900 tons) of lost material per year.

58 Powder and Bulk Engineering, May 2000

0 0 73

ra 5

2. d

0 cn 0

60 Powder and Bulk Engineering, May 2000

The simplest way to evaluate a dryer’s ability to dry uni- formly is to imagine what a single particle “sees” as it goes through the dryer. On average, do all particles see the same amount and average temperature of air? If air flows only up through the conveyor, particles at the bottom of the ma- terial layer will be overdried compared with those at the top. If hot and cold air aren’t mixed properly, you will have hot and cold spots in the dryer, hence dry and wet particles. If airflow isn’t uniform across your material, there will be a moisture gradient across it.

Controlling other operation factors. Other key parame- ters you can control for better efficiency include:

Conveyor speed. The conveyor speed determines the material layer depth and retention time. Generally speak- ing, you want to run your material as deep as possible in order to have a long retention time in the dryer and hence a low operating temperature. Run the material too deep, however, and some may clump together or block the air- flow.

=Exhaust airflow. Optimize your exhaust airflow. Pull out too much air and you will waste energy, not enough and your air will be too humid to effectively dry your ma- terial. Your dryer supplier should be able to recommend exhaust fan and damper settings for achieving proper hu- midity in the dryer.

.Air temperature. Once the other parameters are set, control the exit moisture of your material by manipulat- ing the air temperature.

Air bypasses. Finally, make sure you close the internal access doors. Remember that air will always follow the path of least resistance. If you leave the doors open, air will flow through the open doors rather than through the material. PBE

For further reading Find more information on dryers in articles listed under “Drying” in Powder and Bulk Engineering’s comprehen- sive “Index to articles” (in the December 1999 issue and at www.powderbulk.com).

Dan Poirier is manager of applications engineering for the food group, Aeroglide Corp., 100 Aeroglide Drive, Raleigh, NC 27626; 91 9/851-2000, fax 91 9/851-6029 (dpoirier@aeroglide.com). He holds an MASc in mechan- ical engineering from the University of Ottawa.

0 0 73

ra 5

2. d

0 cn 0