Regenerative thermal oxidizers (RTOs) are used for controlling VOC emissions from a broad spectrum of industries. A regenerative thermal oxidizer uses ceramic media as the heat exchanger medium, providing high heat recoveries.
The purpose of valves, or dampers, in a regenerative thermal oxidizer system is to provide tight shutoff of the process air to maintain destruction and removal efficiency (DRE) and proper heat exchanger operation for thermal energy recovery. Valves serve to regulate the flow of air and to isolate ducting and equipment for maintenance without interrupting other connected units. Valve designs should take into account the maximum system pressure, temperature changes and stresses imposed by the connecting ducting so as to prevent distorting and misaligning the sealing surfaces.
The sealing surfaces should be of such material and design that the valve will remain tight over a reasonable service period. Proper valve design is critical for high VOC destruction efficiency over a long equipment life. Cycling more than 400,000 times per year, regenerative thermal oxidizer valves must operate reliably and must seal to less than 0.25 percent leakage at full system pressure.
Damper TypesGenerally, two kinds of dampers are used in regenerative thermal oxidizers. The dampers connected directly to the oxidizer's heat exchanger canister, which direct the flow of process air into and out of the canisters, are called “high-cycle dampers” because they need to open and close every few minutes on a continuous basis. Most other dampers associated with regenerative thermal oxidizers are called “standard-duty” or “low-cycle dampers” due to their lower cycling frequency. Examples of the low-cycle dampers include isolation and diverter dampers. This article will focus exclusively on high-cycle dampers, which are critical to the overall performance of a regenerative thermal oxidizer, namely that of achieving high destruction efficiencies. Figure 1 depicts a typical three-canister oxidizer with high-cycle inlet, outlet and purge valves.
Regenerative thermal oxidizers designs can be classified into three categories: single-canister, odd-canister and even-canister designs. The style of damper used is determined by the canister design. Some commonly available damper designs used in oxidizers include:
- Butterfly dampers, which are the most common. Styles include single-blade, dual-blade and dual-seat valves.
- Poppet dampers, which includes single poppet, dual poppet and four-way poppet valves.
- Single-can rotary valve.
While the odd- and even-canister designs warrant traditional butterfly or poppet dampers, the single-canister design uses the single-can rotary valve.
Other important issues to consider when selecting dampers for regenerative thermal oxidizers applications include:
- Valve actuation -- hydraulic, pneumatic or electric.
- Valve seats -- metal-to-metal seats or tadpole.
- Maximum allowable leakage rate (0 to 1 percent).
- Materials of construction.
- Resistance to condensable organics and other particulates.
A comparison of these criteria for the various damper types is provided in table 1.
Butterfly Dampers. Butterfly dampers have been applied to regenerative thermal oxidizers for more than 30 years. Consisting of a flat plate inserted into a gas stream, the valve is rotated by means of a motor and linkage (often called an actuator) to control the gas stream flow. When the damper is in the closed position, the flat plate almost completely blocks gas flow. When the damper is in the fully open position, the flat plate is aligned with the direction of gas flow and, therefore, provides very little flow restriction.
Butterfly dampers occupy less space than any other valve style and have broad versatility by the virtue of their design. They are relatively tight sealing without excessive operating torque requirements. They offer a simple and reliable means of gas control for both modulating and on-off type applications.
To actuate (for opening or closing), butterfly dampers employ a center-mounted rotating disc or discs that typically rotate 90˚. The solid rotating disc, which is generally round, must resist thermal and mechanical deformations. The type and size of the damper dictates the torque requirements and, by extension, the actuation requirements. (For example, beyond a certain torque, pneumatic actuators become less desirable than hydraulic dampers due to their size.)
Butterfly dampers used in regenerative thermal oxidizers typically are the on-off type with two designs variations:
- The single-blade damper is the most common type. It consists of a single solid disc (“blade”) that seats against a metal or a compressible-bulb (tadpole) seal.
- The double-blade damper (figure 2) is used when “zero” leakage is required, usually in applications where very high destruction efficiencies are desired. Double-bladed dampers also are supplied with either a metal or a tadpole seat.
Figures 3 and 4 provide schematics of the single-blade and double-blade dampers, respectively. With some valve designs, leakage across the valve occurs when the blade is completely shut off. On one side of the blade is the contaminated VOC stream; on the other is the purified air that is exiting the oxidizer. Leakage from the high VOC stream into the purified stream directly affects the overall destruction efficiency of the oxidizer. Thus, lower leakage results in higher destruction efficiency. With double-blade dampers, the leakage of the VOCs from one side of the valve to the other can be eliminated by purging the gap between the two blades using a fresh air source, such that any leakage across the blades consists of VOC-free air.
Another butterfly damper developed more recently provides zero leakage without requiring two separate discs. This valve, a hybrid of the single- and double-blade damper designs, is called a dual-seat damper because it uses two metal seats (figure 5). It achieves the performance of the double-blade dampers with the single-blade configuration. The gap between the two seats is purged using fresh air, thus achieving zero leakage.
Poppet Dampers. Poppet dampers have been applied to regenerative thermal oxidizers for more than 15 years. They consist of a flat circular plate that is raised or lowered, typically by an electrical or pneumatic actuator. When the flat plate is in the closed position, it provides a gas seal by pressing against a seat shaped like a short cylinder. Gas attempting to pass through the cylinder is blocked. When the damper is open, there is a 1 to 2' gap between the flat plate and the cylinder opening. Poppet dampers are used for on-off control only; they are not appropriate for modulating applications.
Initially, poppet valves were developed for service in fabric filter systems or baghouses; only later were they applied to regenerative thermal oxidizers. Fabric filter systems require two-way service with poppets either open or closed. System outlet poppets are single-disc, low-leak models. System-bypass poppets are zero-leak and employ a double blade and seat with seal air. These valves range in size from 20 to 47" dia. for industrial baghouses and 48 to more than 96" dia. for power generation baghouses.
Poppets for regenerative thermal oxidizer service are more complex in that they seal multiple gas paths while diverting gas in different directions. Figure 6 illustrates a two-way poppet for regenerative thermal oxidizer service. Oxidizer systems that are designed with two-way poppets should have one inlet and one outlet damper to provide fail-safe conditions during power outages and upset operating conditions.
Poppet dampers for oxidizers are driven with pneumatic or hydraulic cylinder actuators for high-cycle service and low-leakage isolation. Hydraulic cylinders provide the most reliable type of drive for service where 400,000 cycles per year are expected. They operate best when oriented vertically. Poppet dampers are available with zero-leak blades and seats as well.
Poppet valves with three-way and four-way configurations also have been used in regenerative thermal oxidizer systems. A three-way poppet has one inlet and two outlets. It cycles between the two seats, diverting flow through one while sealing the other. A four-way poppet has two inlets and two outlets. This style has been used in compact regenerative thermal oxidizer systems.
Single-Canister Rotary Valve. Rotary valve designs have been applied to regenerative thermal oxidizers for more than 10 years. The single-canister rotary valve design eliminates the need for separate inlet, outlet and purge valves and replaces them with a single large valve. The sequence of the bed function as an inlet, outlet or purge is achieved by the rotation of this single valve. The valve is located below the heat recovery chambers and is electrically or pneumatically driven. The rotation of the valve (or distributor) continuously controls the airflow from inlet plenum to one half of the heat exchange media, through the retention chamber, out through the other half of the heat exchange media, and then out through the outlet plenum.
The cylindrical canister holds multiple heat recovery chambers. The air is cycled through an inlet chamber for preheating and later through an outlet chamber to reheat the heat exchanger bed before exiting. Before chambers switch from inlet to outlet flow, they are purged of any residual unoxidized gases. This purging ensures minimal VOC spikes and maximizes destruction efficiency.
When the single valve actuates, it requires several minutes per rotation to ensure a smooth transition from inlet to purge to outlet, thereby reducing upstream pressure fluctuations that are more typical with traditional regenerative thermal oxidizer designs. The single valve also requires less maintenance compared to multiple valve regenerative thermal oxidizer systems.
One disadvantage of using a single-canister rotary valve is that it utilizes a machined metal-to-metal surface to achieve tight sealing. This makes it more susceptible to wear. The seating arrangement also makes single-can regenerative thermal oxidizers more susceptible to particulate contamination, resulting in loss performance over time due to inorganic particulate wearing on the machined metal-to-metal surfaces. The compact single-canister design also makes it more difficult and expensive to maintain.
A reputable regenerative thermal oxidizer supplier can help you evaluate the specifics of your process and determine the best type of oxidizer system and damper valves to optimize your VOC destruction efficiency.
Sidebar: 10 Tips: Valves/Dampers for Oxidizers
- Valve designs should take into account the maximum system pressure, temperature changes and stresses imposed by the connecting ducting so as to prevent distorting and misaligning the sealing surfaces.
- The sealing surfaces should be of such material and design that the valve will remain tight over a reasonable service period.
- The style of damper used is determined by the canister design. While the odd- and even-canister designs warrant traditional butterfly or poppet dampers, the single-canister design uses the single-can rotary valve.
- When selecting dampers for regenerative thermal oxidizers, consider the requirements for valve actuation, valve seat design, leakage rate, materials of construction, and resistance to condensable organics and other particulates.
- The type and size of butterfly damper dictates the torque requirements and, by extension, the actuation requirements.
- Poppet dampers are used for on-off control only. They are not appropriate for modulating applications.
- Oxidizer systems that are designed with two-way poppets should have one inlet and one outlet damper to provide fail-safe conditions during power outages and upset operating conditions.
- Poppet dampers operate best when oriented vertically.
- The single-canister rotary valve design eliminates the need for separate inlet, outlet and purge valves and replaces them with a single large valve.
- The single-canister rotary valve utilizes a machined metal-to-metal surface to achieve tight sealing, which makes it more susceptible to wear.