How to Achieve Proper Size and Design of a Convection Dryer
To avoid unforeseen costs and poor industrial dryer performance, consider these 10 tips when buying, specifying, operating and maintaining convection dryer.
Convection dryers vary greatly in size, features and delivery method and design of the air and heat distribution. To meet your process requirements efficiently, it is important to select the right type of dryer for your products and processes. It is equally important, however, to properly size and design the dryer.
A processor therefore needs to understand what information is necessary for designing a dryer. Processes and products vary greatly — even within similar product lines. So, thoroughly describing your requirements to the dryer manufacturer — even for “standard” products — is essential. By doing so, a processor can avoid unforeseen costs and poor performance results. Here are five tips that can help when buying a process dryer.
1. Test, Test, Test
Testing is an important early step in the process of selecting and sizing a dryer. The initial test can be a small batch to acquire a time and temperature curve. A five-gallon bucket of wet material is usually sufficient for testing. This pretesting will improve the accuracy of the initial budget estimate and produce samples to help evaluate the product quality under different conditions. Of course, larger and continuous test runs will produce better information, but this is normally performed at a cost.
When conducting tests, be sure that the product being tested is the same as the final product you intend to produce. Changes in the product between testing and designing can produce unwanted consequences.
2. Know Your Dryer Process Specifications Before Buying
There are several product and process specifications you must provide your dryer manufacturer, so they can best understand your needs and provide accurate sizing, utilities and pricing. Key points include the specifications of the product as it enters the dryer, and the required characteristics of the product as it exits the dryer.
Among the product variables you should describe in your RFQ are:
- Density of wet product (pounds per cubic foot bulk density).
- Moisture level of the product when enters the dryer.
- Desired moisture level as product exits the dryer.
- Maximum and minimum heat rise to be used while drying. (This can vary from beginning to end of the process.)
- Temperature of the product when it enters the dryer.
- The product’s heat capacity as related to water’s heat capacity. (Water’s heat capacity is 1 BTU to raise 1 lb of water 1°F.)
- Product size range and the amount of fines contained in the product.
- Flow rate in pounds per hour entering the dryer.
- Any variables in the product daily or seasonally.
Among the process variables you should describe in your RFQ are:
- Maximum and minimum ambient air temperature and humidity delivered to the dryer.
- Physical location considerations. Factors to consider include installation location (overseas, for instance), elevation, local permitting authority and any regulatory authorities having jurisdiction.
- The presence of an existing feed system (continuous, batch, auger, bucket elevator, belt conveyor, etc.) or the need for a new feed system.
Knowing as much about your products and processes upfront will help during all phases of dryer ownership.
3. Allow Enough Time
It takes a set amount of time for the moisture in the product to transfer to the air. Product and process variables influence this variable, called residence time. The dryer must have enough surface area to allow the correct amount of residence time, which is why it is critical to know the volume of product entering the dryer. (The dryer must hold that volume for the required time.) In many cases, depth can allow only a little flexibility in time, otherwise capacity is affected. This is why it is important to know the bulk density of the wet product. Test drying of the product helps determine the correct time needed based on varying conditions.
In a conveyor dryer, time is calculated by volume of product entering the dryer, product depth, dryer belt width and dryer belt length. The simplified equations that follow explain further:
4. Ensure You Have Adequate Energy Input
It takes a set amount of energy for the moisture to transfer from the product to the air. The amount of energy needed is determined by the product variables, the process variables and the energy delivery system of the dryer.
Calculating BTU Required for Drying
The total BTU delivered is calculated by the combination of the air volume and heat rise. For example, consider 20,000 cfm at 170°F heat rise.
Air Temperature – Ambient Air Temperature = Heat Rise
240°F– 70°F = 170°F
A 170°F heat rise is 3,672,000 BTU/hr. A quick way to calculate is:
Cubic Feet per Minute x Adjusting Factor x Heat Rise x Minutes = BTU/hr
20,000 x 0.018 x 170°F x 60 = 3,672,000 BTU/hr, or 3.672 MMBTU/hr
How much water can 3.672 MMBTU/hr remove? Once again, that depends upon the dryer design and the product and process variables. Removing 1020 BTU per pound of water is near 100 percent efficiency. However, we can say in general, a good efficient dryer can evaporate 1 pound of water for 1,500 BTU. In other words:
3,672,000 BTU = 2,448 Pounds of Water per Hour
Descending rate of drying is a part of the drying cycle. It is where the rate of moisture removal begins to fall. During this period of drying, the drying rate is influenced more by residence time within the dryer than heat rise. In fact, in many products, the drying rate slows considerably below 10 percent moisture. The lower you go, the more costly — in terms of energy and area needed to finish the product — it is. As a result, the final moisture level required should only be as low as is needed to properly stabilize the product for storage.
For every product, there is a representative curve that describes the drying characteristics at specific temperature, velocity and pressure conditions. This is known as the drying curve. A typical example is shown.
Energy is normally quantified by BTUs. The total BTU delivered is calculated by the combination of the air volume and heat rise. (See sidebar for example.)
5. Evaluate Dryer Design
The dryer design must deliver the residence time and energy input to accomplish the stated capacity goal. To do so, the dryer manufacturer looks at the drying chamber area based on residence time, and the airflow based on the energy needed to meet moisture level requirements. If the airflow in the given design requires too much velocity, then the drying chamber design must be adjusted.
The maximum and minimum air velocity will vary by dryer type. For conveyor dryers, however, it can be in the range of 50 to 150 feet per minute for the bottom-up airflow pattern. The velocity could be higher for the top-down airflow pattern. To calculate, use this equation:
Other considerations for dryer design include available plant space for the dryer and other dryer features that may be needed.
Available Plant Space. Will stated size fit in area and flow of the plant process? To help determine that, consider:
- Vertical or horizontal flow of material?
- Will the product enter and exit at the same end of the dryer, or at opposite ends of the dryer?
- Where are the dryer air entering and exiting points?
- How much room is needed for maintenance and service access?
- Is product cooling needed? If it is, will it occur right after drying or further in the process?
Dryer Features Required for Your Product. What process- and product-specific dryer features are needed? To help determine that, consider:
- Belt type (mesh-hole sizing).
- Construction materials.
- Fines-handling features.
- Product spreader designs for free or non-flowing materials.
- Services panels and access points for inspection, sampling and maintenance and cleaning.
Finally, consider what utilities are needed, including fire-suppression devices. Where are they located in relationship to the dryer service?
6. Plan for Growth and Expansion
Though you are specifying your dryer for today’s products and processes, think about the dryer design and how to accommodate future growth. Should you prepare to add area to the dryer (making it longer or adding another module) for more capacity in the future? Or will another independent process line produce more flexibility? If possible, size the dryer for extra energy and time from the beginning. Optimal operation is at 80 percent of maximum capacity. It is less efficient to run a dryer at or above its full capacity potential. Designing to operate at 80 percent of maximum capacity also allows for some growth seasonally or for short-term during expansion projects.
Another consideration is whether more power and gas may be needed. How close to capacity is the facility? How will any dryer expansion affect the exhaust air permits?
7. Controls: Essential Managers of the Process
There are several considerations when it comes to control of the dryer.
- Think about how you want to control the dryer. Automated (PLC) vs. manual controls?
- Plan for the controls location. Will they be in a climate-control room, on a prewired panel near the dryer?
Moisture Controls. A dryer is a slave to what you put into it. Sophisticated moisture controls, however, can change the dryer setpoints — varying the operation for the condition of the product — so consistent results can be achieved. If the incoming product varies hourly, daily or weekly, then moisture controls may be helpful. It is best to utilize the exhaust air temperature as your control point. The exhaust air temperature has direct correlation to the product’s moisture content due to evaporative cooling.
Select controls that provide adequate operating reports, taking into account what data will be needed and format.
8. Controlling Emissions
Emissions are a big issue. Do not forget to plan for:
- Dust in the exhaust. Consider cyclones vs. a baghouse vs. just an exhaust fan.
- Odor and noise of the exhaust. The surrounding community will likely be more sensitive than those inside the plant.
- Inside environment. Evaluate how the radiant heat from the dryer will affect the conditions of the plant and ventilation systems.
Finally, take into account any hazardous vapors that may be released during the drying process while planning the dryer design.
Convection dryers vary greatly in size, features and delivery of the air and heat. It is important to select the right type of dryer for your product and process. However, it is equally important to properly size and design the dryer.
9. Do Not Forget Safety and Sanitation
Review your safety guidelines with the dryer safety features to be sure of ease of implementation. Things to consider include fire, OSHA requirements, training and cleaning.
Fire. Have proper fire suppression controls based on your written fire prevention and action plans. It is advisable to have a local authority implement fire-suppression systems in the dryer and integrate them into the building fire system. This authority will also be able to provide regular maintenance.
OSHA. Review the dryer design and safety programs in the context of your written OSHA plans. Involve your OSHA manager in the dryer design and review.
Training. Oversee written training programs on safe dryer maintenance and operation. Be sure dryer designs meet your written lock out/tag out programs and confined space.
Cleaning. Discuss how you plan to clean (wet or dry), and the frequency of cleaning. Will clean-in-place (CIP) systems be a requirement?
10. Validate Your Specifications
After all these considerations are determined, it is important to validate all specifications in the pre-manufacturing drawings and at the factory acceptance testing (FAT) prior to shipment from the manufacturer.
In conclusion, dryers are critical components of most bulk-flow industrial processes. It stands to reason, therefore, that having a properly sized dryer is just as critical as having the proper dryer type. This always starts with the processor knowing his product and process well — and communicating the necessities of the product and process to the dryer manufacturer.