Design Considerations for Dryer Ancillary Equipment

When considering a sewage sludge drying application, most of the focus is on the design, operation, and maintenance of the dryer itself. In reality, sludge drying is a complex process that involves many sub-processes and ancillary components. If any of these systems or processes fail or operate incorrectly, the entire drying system will be affected and may fail.

Additionally, these ancillary systems can be quite costly, require a large space, and be challenging to operate and maintain. Also, the cost, size, and operational requirements of these components vary depending on the type of dryer. Therefore, careful consideration of these components during the project's selection and design phases is essential to create a robust, seamless, efficient, and easy-to-maintain drying system. This article will investigate the major subsystems of a sludge dryer including:

Sludge Feed System
Dried Sludge Conveyance and Storage Systems
Vapor Treatment System
Heating System

Regarding design considerations and comparisons of different designs.

Note, LCI has provided high-level information on various ancillary equipment for reference. For specific designs and specifications, it is recommended to consult the manufacturers of the specific component.

Sludge Feed System

Cake Storage Hopper/Bin

The first item to the sludge feed system of a dryer is the cake hopper, also known as a cake bin, which decouples or buffers dewatering capacity from drying capacity. These hoppers are custom in design, tailored to each application. Therefore, it is important to consider many items in the design of this equipment including:

Size and Capacity

Required storage volume between dewatering and drying
Use of a leveling screw or multiple drops to increase the usable capacity
Understanding that storage volume has a significant impact on cost and footprint

Location

Recommended to be close to the dryer as possible, limiting pumping distances
Climates with consistent freezing temperatures should have the hopper indoors, increasing the building size and cost
Warm Climates can place hoppers outside, reducing building size

Flow

Delivery method to the hopper

Trucks or loaders require a bin which can be dumped into
Conveyors or cake pumps must consider routing and if multiple drops to the hopper are required
Direct feed from dewatering equipment located above the hopper must consider the height available and size restrictions on the hopper

Hopper geometry must consider the bridging of material inside the hopper
Discharge design to feed pump(s)

Feed pump(s) can be located under or to the side the hopper
Recommend installing a gate valve isolating the pump and hopper for servicing
Allowance for duty/standby pumps if required

Allowance for material bypass for emergency situations

Level measurement

Ensures the hopper is not empty or overfilled, examples include:

Lazer
Ultrasonic
Load Cells

The system may need to be designed for combustible gases.
Due to prohibitive cost, minimal maintenance, and high reliability, typically, redundant hoppers are not required

Cake Pump

While a few dryers use a conveyor for feeding, most dryers utilize progressive cavity pumps due to their smooth and consistent flow, as well as the ability to achieve high pumping pressures. Double piston pumps can also be considered but are more complex and have a higher initial capital cost. When designing a pumping system, some of the important items to consider are:

Piping design between the pump and dryer to minimize pressure loss:

Limiting the piping distance and vertical rise
Minimize number of bends, tees, and valves
Use full port components, long radius bends, and large diameter piping
Calculate the additional pressure loss of a specific dryer’s feed system
Have an expert confirm the piping design and anticipated pressure losses

Design for excess pressure and flow capacity
Dry run and pressure protection for the pump
Maintenance

Grit, trash, and high pump pressures create higher wear
Design the layout for ease of maintenance
Consider redundant pumps especially in high operational hour applications due to pump maintenance requirements

The cake storage and pumping systems are critical to the operation of the dryer system. If these systems are not designed properly, sludge cannot be pumped into the dryer limiting or completely stopping the operation.

Dried Sludge Conveyance and Storage System

Cooling Conveyor

After the sludge is dried, it should be cooled to below 120°F (50°C) to avoid the risk of fire and explosion during storage. If a temperature below 120°F (50°C) cannot be achieved naturally, a cooling conveyor should be employed, using a water-cooled jacket and auger to remove heat from the dried sludge. Among the items to be considered in design are:

Cooling Water free from contaminants which foul the heat exchange surface
Design Considerations

Configured to operate with the warmest cooling water supply temperature
Integration of the cooling conveyor as part of the dried sludge conveyance system reducing the length and cost of the additional conveyance system
Temperature Measurement to control the system including optimizing water uses and ensuring the material is cooled properly

Maintenance Considerations

Cleaning of the heat exchange surface may be required from time to time
Typically, redundant cooling conveyor systems are not considered due to infrequent maintenance requirements, complexity, and cost

Rotary Valve

Dryers operate under negative pressure to contain vapor, dust, and odor therefore draw air into the dryer, reducing efficiency, increasing vapor volume, and elevating risk of fire and explosion. Rotary valves (also known as an airlock) are used to minimize leakage air into the dryer. While not all dryer designs include rotary valves, designs which do are more efficient, require less vapor volume to treat and are safer. A few considerations for rotary valve design include:

Design

Design with at least two sealing strips in contact with the frame creating a tight seal
Self-cleaning design to eliminate clogging
Proximity switches to ensure the valve’s rotation avoiding system blockages
Parts in contact with the sludge should be designed for corrosion resistance
Proper rating for fire and explosion hazards

Maintenance

Area around the valve for servicing and cleaning
Replacement of sealing strips to maintain a tight seal
Possible high wear on parts in contact with abrasive materials requiring replacement
Most wear occurs over time and can be avoided with preventative maintenance so typically redundant rotary valves are not considered.

Dried Sludge Conveyors

Once the sludge has been dried and cooled, it is transported for storage. Many factors should be considered when designing a dried sludge conveyance system, such as:

Conveyance distance and direction
Integration with storage systems, such as silos or requirements for multiple drop points
Sizing with excess capacity
Consideration of points where bridging of material may occur
Maintenance requirements
Explosive dust hazards
Cost

Typically, the following conveyors have been used to transport dried sludge:

Screw Conveyors
Tubular Drag (Drag Chain) Conveyors
Bucket Conveyors
Combination of these conveyors

Each conveyor design has its advantages and disadvantages which are listed in the table below:

	Screw Conveyor	Tubular Drag	Chain & Bucket
Conveyance Planes	2D	3D	2D
Maximum Vertical	45°	90°	90°
Length	Short	Long	Short
Tight Footprint	No	Yes	Yes
Compatibility w/ Silos	No	Yes	Yes
Dust Buildup	Low	Low	High
Robust	High	Medium	Medium
Maintenance	Low	Medium	High

Normally due to size, layout and cost, redundant conveying systems are difficult to employ. Therefore, care should be taken to complete preventive maintenance and understand the maintenance requirements of each design.

Finally, as each application has different requirements, there is not a one-size-fits-all conveyor system. Therefore, each conveyance system should be carefully considered during the design process.

Dried Sludge Storage

Dried sludge must be conveyed to storage after drying, so it is important to consider the type of storage system for each drying application. Typical storage systems include:

Silos
Holding Areas
Truck Loadout
Super Sacks

When selecting the correct storage system, some of the items to consider are:

Available Footprint and Height
Available Equipment (Trucks or Loaders for Example)
Disposal/Hauling Method
Complexity
Maintenance and Operator Requirements
Dust Dispersion and Hazard Considerations
Cost

Each type of storage has its advantages and disadvantages which are listed in the table below:

	Silo	Holding Area	Truck	Super Sacks
Cost	High	Low	Medium	Low
Complexity	High	Low	Medium	Medium
Volume	Large	Large	Medium	Small
Horizontal Footprint	Low	High	Medium	Medium
Product Handling	Low	High	Medium	Medium
Cross Contamination	Low	High	Medium	Low
Maintenance	High	Low	Low	Medium
Dust Dispersion	Low	High	Medium	Low
Nitrogen System	Yes	No	No	No
Designed for Explosive Dust	Yes	No	No	No

Typically, due to size, maintenance requirements, and/or cost redundant storage systems are not implemented.

Dried sludge conveyance and storage systems are highly configurable and should be carefully considered for each individual application.

Vapor Treatment System

Condensation System

The drying process creates a substantial amount of vapor which must be treated. Typically, the vapor first flows to a condenser system, which is designed to cool the vapor stream, condensing the evaporated water to be sent for treatment. Various types of condensers are used in drying applications, and the choice of condenser type is influenced by the type of dryer being operated. Direct dryers, which use high volumes of air, typically utilize packed media condensers, crossflow condensers, or rely on heat recovery only with no separate condenser. Indirect dryers, with low volumetric vapor flows, typically employ barometric or shell and tube condensers. Below is a chart comparing the different condensing technologies:

	Barometric	Shell & Tube	Packed Media	Cross Flow	No Condenser / Heat Recovery Only (HR)
Dryer Type	Indirect	Indirect	Direct	Direct	Direct
Condensing Type	Direct	Indirect	Direct	Indirect	None
Heat Recovery	Secondary Plate & Frame	Integrated	Secondary Plate & Frame	Cross Flow	Cross Flow
Complexity	Low	Medium	High	High	Medium (HR)
Fouling	No	Yes	Yes	Yes	Yes (HR)
Maintenance	Low	Medium	High	High	Medium (HR)
Wastewater Volume	High	Low	High	Low	Low (HR)
Emissions Treatment System	Small	Small	Medium	Medium	Large (HR)

Most applications do not consider completely redundant condenser systems due to size and cost constraints. However, it is important to consider the maintenance of each type of condenser system. For example, for a plate and frame heat exchanger, which requires frequent cleaning, a standby unit may be required while the other heat exchanger is cleaned. Also, with packed media condensers, a second set of packing might be advantageous to pull the other set of packing for cleaning.

The operation of the condensing system is often overlooked when selecting a dryer. However, condensing systems are quite different from one another, and the condenser design affects other processes such as emissions control, cooling water volumes, and wastewater volumes.

Emissions (Odor) Control

The exhaust vapor from the condenser system typically requires treatment for odors and pollutants, based on local regulations and the contaminants in the vapor stream. Historically, four types of emissions control systems have been used:

Wet Scrubbers
Biological filters
Carbon filters
Thermal oxidizers.

There are many factors which go into the selection of the emission control system, some of which include:

Volume of vapor to treat
Type and concentration of contaminants
Local regulations
Operation and Maintenance Requirements
Equipment Size
Cost

First, the selected equipment must be able to treat the vapor generated by the drying application. Below is a comparison of the treatment capabilities of the different emissions control systems:

	Wet / Chemical Scrubber	Biological Filter	Carbon Filter	Thermal Oxidizer
H₂S Removal	Yes	Yes	Yes	Yes
NH₃ Removal	Yes	Yes	No	Yes
PFAS Destruction	No	No	No	Yes
High Concentrations	Yes	No	No	Yes
Variable Concentrations	Yes	No	Yes	Yes
Particulate Matter	Yes	No	No	Yes
High Humidity	Yes	Yes	No	Yes

Additionally, each emissions control equipment has advantages and disadvantages which must be considered. Below is a chart comparing the different treatment technologies:

	Wet / Chemical Scrubber	Biological Filter	Carbon Filter	Thermal Oxidizer
Simplicity	Medium	High	High	Low
Maintenance	Medium	Low	Medium	High
Water	Yes	Yes	No	No
Wastewater	Yes	Yes	No	No
Thermal Energy	No	No	No	Yes
Media Exchange	Rare	Rare	Often	Never
Cleaning Required	Yes	Yes	No	No
Chemicals Required	Yes	No	No	No

Due to size and cost, typically, redundant emissions systems are not considered. However, it is important to understand emissions equipment can be complex, so it is important to select a robust system, have recommended spares available and complete preventative maintenance.

The Vapor Treatment System affects many aspects of the drying system including cooling water requirements, wastewater flows, chemical consumption and even air permitting. It is important to consider the vapor system of each dryer as a part of selection criteria to insure the best drying system is chosen for the application.

Heating Systems

All thermal dryers require heat, and each dryer requires specific a type or types of heating media. Historically, four types of heating media have been used:

Air
Water
Steam
Thermal Oil

Air systems are heated by a furnace and air directly fed into the dryer, meaning the combustion air enters the drying area. Water, thermal oil, and steam systems indirectly apply heat either to the drying air through a heat exchanger or directly to the sludge through a heated surface. As different dryers are designed with a specific type or types of heating medium, it is important to understand the advantages and disadvantages of each medium to help select the best dryer for each drying application.

Below are charts of the different heating mediums and their advantages and disadvantages:

	Air	Water	Steam	Thermal Oil
Type	Direct	Indirect	Indirect	Indirect
Efficiency	High	Medium	Medium	High
Exhaust Air for Treatment	High	Low	Low	Low
Flashing	No	Yes	Yes	No
High Pressures	No	Yes	Yes	No
Certified Boiler Operator	No	Maybe	Yes	No
Duckwork / Pipework	Large	Medium	Small	Medium
Duckwork / Pipework Complexity	Low	Medium	High	Low
Maintenance	Low	Medium	Large	Medium
Flammable	Yes	No	No	Yes
Cost of Media	Low	Low	Low	High
Corrosive	Yes	Yes	Yes	No
Blowdown	No	No	Yes	No
Corrosion Inhibitors	No	Yes	Yes	No
Pumping System	Yes	Yes	No	Yes
Heat Transfer Area	Large	Medium	Small	Medium

Heating Systems are one of the more complex aspects of drying requiring a lot of attention and maintenance. Typically, they require a once yearly inspection, but normally redundant systems are not used due to cost and complexity. Again, it is important to complete all preventative maintenance of heating systems to avoid a dryer system shutdown.

Finally, it is important to understand the design and operational requirements of the required heating system to ensure the best dryer system is chosen for a specific application.

Ancillary equipment is an important consideration when selecting and designing a drying system. These systems have a huge effect on cost, operation, maintenance, and system performance. Considering the type of systems each dryer requires and understanding the performance of the complete dryer system is paramount to a successful dryer operation.

If you have questions about drying or dryer ancillary equipment, please contact LCI Corporation, and we would be happy to help.