Jan 26, 2021 | Engineering
Consider a drying system to process 20 tons per hour of combined wet cake and syrup produced in an ethanol plant or distillery. At 66% combined feed moisture dried to 10% moisture content, the mass balance would be as follows:
|Feed Material||Discharge Material|
|Dry Solids||13,600 LB per Hour||13,600 LB per Hour|
|Moisture||26,400 LB per Hour|| 1,511 LB per Hour|
|Total||40,000 LB per Hour||15,111 LB per Hour|
Evaporation: 24,889 LB per Hour
The heat requirement for drying is approximately 27.2 MMBTU.
Direct Heat Dryer
Using a direct heat dryer system for the process above, the system schematic would be similar to the one shown in Figure 1. Note that the dry material recycle loop is not shown since it would be identical for either dryer.
The direct heat dryer in the example is equipped with a combustion system having a dryer inlet gas temperature of 1,000 F. Based on this system, the dryer exhaust gas discharge volume would be 53,000 ACFM. The system is provided with a flue gas recirculation system. This is included for two reasons:
- Since the gas is laden with water vapor and combustion products (from having passed through the burner), it lowers the amount of oxygen present in the drum in order to prevent combustion from occurring inside the dryer.
- It reduces the amount of gas having to go to the thermal oxidizer for destruction of VOC.
This operating cost of the system is not just the drying but also the cost to operate a thermal oxidizer. This type of system also requires two additional fans, which cause increased horsepower requirements and exponential complexity of process control.
Steam Tube Dryer
Using a steam tube dryer system for the process above, the dryer system schematic would be similar to the one shown in Figure 2.
Because the steam tube dryer employs steam to do the work, very little air is required. Typically, the dryer is run under slight negative pressure and a small amount of air is vented into the dryer to offset dust from puffing out. Thus, the primary content in the exhaust stream is water vapor instead of air.
Since water is condensable, the exhaust gas goes through a condensing scrubber where a great deal of the water is collapsed back to liquid phase and removed. The remaining saturated air stream is taken back to the boiler and used as combustion air. The boiler acts as the VOC combustor – potentially negating the need for an RTO.
Employing a steam tube dryer for processing DDGS may initially appear to be a more costly system (because the dryer itself is more expensive, and a boiler is required).
Yet, for true comparison, the following factors of the overall system also need to be considered:
- Steam tube dryers do not require heated air to operate.
- Steam tube dryers do not require recycle gas stream for fire prevention.
- Steam tube dryers are easily controlled by simple modulation of a steam valve.
- Steam tube dryers do not require a thermal oxidizer system for most biomass systems. If an RTO is required, it is 75% smaller than with direct heat.
- Steam tube dryers have an atmospheric environmental impact a fraction of direct heat dryers.
- Considering the cost of an RTO verses a boiler, the steam tube dryer system may have a smaller initial cost.
- Since the boiler for a steam tube dryer system may act as a thermal oxidizer, the operating cost is much lower.
Learn more about When you would need a Steam Tube Dryer vs. a Direct Heat Dryer.
Contact our Application Engineers to discuss your operation’s specific needs and how we can help you achieve The Lowest Cost per Revolution.
Nov 11, 2020 | Engineering, Technology
How Materials are Dried and Processed using Steam Heat transferred through Tubes
A steam tube dryer is like a rotating shell-and-tube heat exchanger in which medium pressure, saturated steam is charged through a rotary steam joint, into the manifold and then into several heat transfer tubes – typically at a nominal 100-150 psig (and up to 450 psig/500F service for reacting granular solids). Concentric rows of these tubes run the length of the dryer; one to six rows depending on the dryer size and duty requirements.
A steam and condensate manifold chamber, referred to as a Steam Chest, is mounted on the product discharge end of the dryer. This Steam Chest distributes steam to each of the steam tubes and collects the condensate formed as the steam transfers its latent heat to the material being processed. The steam condenses and the condensate is removed from the dryer and taken back to the boiler. The dryer utilizes only the latent heat of condensation, making the dryer one of the more efficient drying systems in the world.
The process material is fed into the drum through a screw conveyor and then tumbles over the outside of the tubes. As indirect heat, the thermal load of the saturated steam in the tubes is not in direct contact with the material. The steam remains inside the tubes while the material lies outside of them. The material cascades on the outside of the tubes and condenses the steam on the inside of the tubes, and that conductive and radiant heat is used to dry the material.
The latent heat is efficiently drawn from the steam (about 850 BTUs per pound) and transferred through the tube wall and into the material – which drives off its moisture. Our dryers execute this process at a consistent temperature – providing a very uniform heating of the material particles.
The water condensed out of the steam is continuously removed from the steam chest through a stationary siphon and cycled back to the boiler for regeneration. And the dried product discharges through ports in the shell at the steam chest end of the dryer.
For certain applications, an alternative to the fabricated Steam Chest design is needed. For example, the Louisville Dryer Pipe Manifold design is a available based on customer preference. The Pipe Manifold is comprised of concentric rings of pipe, one for each row of steam tube in the dryer, set on different planes, which are connected to a center steam and condensate distribution chamber with steam supply pipes, and condensate return pipes. Each row of steam tubes extends through the dryer and is welded to its corresponding manifold ring.
- Steam Tube Drying is thermally very efficient and friendly as heat loss through the cylinder wall is minimal since the steam tubes are fully enclosed by the dryer.
- This indirect heat system is also much safer for processing temperature-sensitive organics and volatile chemicals which could catch fire even at low temperatures.
- The exhaust gas from the dryer is about 80 % less than what is required for direct heat dryers, which significantly reduces the size and cost of the air pollution control equipment.
- Using saturated steam enables us to maintain an inert environment – which is safer for your materials and production/maintenance team.
- Since Steam Tube Dryers process under lower temperatures and without gases present in a direct-fired dryer, your material will not get scorched or burnt as much as they might from using Direct Heat Dryers.
- Thus, your material will maintain or receive better coloring, and its bypass proteins will be protected by not being overheated (which is important for digestion of Distillers Dried Grains by livestock).
To view how our Steam Tube Dryers process temperature-sensitive, volatile or corrosive materials, watch this video.
- WHY choose our custom Steam Tube Dryers for processing Distillers Dried Grains (DDGS) and other temperature-sensitive materials?
- HOW do our Steam Tube Dryers process materials?
- WHAT components comprise our Steam Tube Dryers?
- WHERE in a Steam Tube Dryer is the material processed?
- HOW LONG do our Steam Tube Dryers operate efficiently, safely and profitably?
- WHO is Louisville Dryer Company?
Using clean steam rather than products of combustion, Louisville Steam Tube Dryers are one of the most ecologically friendly drying technologies. As indirect heat dryers, they use the latent heat from clean low-temperature saturated steam rather than hot gases from fossil fuel-fired combustion systems. This highly efficient heat-transfer design also makes Louisville Steam Tube Dryers one of the most thermally efficient drying technologies.
The residual non-condensable gases left inside of the steam tubes once the steam has condensed are vented through a flexible connection attached to a common Vent Header that is mounted at the feed end of the dryer. This Vent Header has thermostatically controlled vent valves which allow the cooler non-condensable gases to be efficiently vented as they accumulate while containing valuable steam energy inside the steam tubes.
Since the Steam Tube Dryer utilizes the latent heat of steam to drive the drying process, only a small amount of sweep air is required to remove the water vapor driven off the product generated in the drying process. This is usually less than about 30% of the exhaust gas required for a direct heat dryer for the same process parameters.
The gap between the stationary Feed and Discharge housings at either end of the Steam Tube Dryer is sealed to keep ambient air from leaking into the dryer. Various sealing technologies are used depending on the process parameters. These seals may be as simple as tensioned woven fabric belts and as sophisticated as machined packed and inert gas purged seals (e.g. solvent extraction and other vapor capture applications).
Louisville Dryer Company is an ASME Division I Design and Manufacturing Center
Our Steam Tube Dryers are custom engineered for each specific application. The steam side component is a pressure vessel, designed and manufactured in conformance with the latest edition of ASME Section VIII, Division I. They are stamped and registered with the National Board. Materials of construction range from carbon steel to various nickel alloys and duplex stainless steels.
We have completed thousands of installations processing hundreds of materials around the world and pioneered many of the technologies applied to today’s Direct Heat Dryers and coolers, Indirect Heat Dryers and calciners, Steam Tube Dryers and Water Tube Coolers.
Materials processed include organic and inorganic chemicals, petrochemicals, grains, metals, aggregates, and waste byproducts. If the material is granular solid, it probably has been processed in a Louisville Dryer.
Contact our Applications Engineers today to begin finding your custom solution.