A Primer on Damper Technologies for Regenerative Thermal Oxidizers

By Anu Vij

Regenerative Thermal Oxidizers (RTOs) are commonly used for controlling VOC emissions from a broad spectrum of industries. A RTO is basically a thermal oxidizer that uses ceramic media as the heat exchanger medium, providing high heat recoveries. Many variations of this technology are offered by RTO vendors, which broadly fall under three categories: odd canister designs; even canister designs; and single canister designs.

Over the past few years, much attention has been given to the pros and cons of the different type of ceramic media used in a RTO system. However, other critical aspects of the RTO system (viz. dampers, burners, etc.) have not been widely covered.

Overview of RTO Damper Technologies

The purpose of valves (or dampers) in a RTO system is to provide tight shut off of process air in order to maintain destruction and removal efficiency (DRE) and proper heat exchanger operation (Thermal Energy Recovery or TER). Valves serve not only to regulate the flow of air, but also to isolate ducting and equipment for maintenance without interrupting other connected units. Valve designs should take into account 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 a year, RTO valves must operate reliably, and must seal to less than 0.25 percent leakage at full system pressure.

Generally two kinds of dampers are used in RTO systems. The dampers directly connected to the RTO heat exchanger canister that direct the flow of process air into and out of the canisters, are called the high cycle dampers because they need to open and close every few minutes on a continuous basis. Most other dampers associated with a RTO system are called standard duty or low cycle dampers due to their lower cycling freq¬uency. Examples of the low cycle dampers are isolation dampers, diverter dampers, etc. The discussion below pertains to the high cycle valves, which are critical to the overall performance of the RTO, namely that of achieving high destruction efficiencies. Figure 1 depicts a typical three-canister RTO with the high cycle inlet, outlet and purge valves.

RTO designs can generally be classified into three categories: single canister, odd canister, and even canister designs. While the odd and even canister designs warrant traditional butterfly or poppet dampers, the single canister design involves an entirely different technology called the rotary single can valve. Some commonly available damper designs include:

• Butterfly Dampers (most common):
  • Single Blade dampers;
  • Dual Blade dampers; and
  • Dual Seat dampers.
• Poppet Dampers:
  • Single poppets;
  • Dual poppets; and
  • Four-way poppets
• Single Can Rotary Valve.

Other important issues to consider in selecting dampers for RTO applications are:

• Valve Actuation:
  • Hydraulic;
  • Pneumatic; and
  • Electric.
• Metal-to-Metal seats v/ tadpole seats;
• Leakage Rates (0 percent to 1 percent);
• Material of construction; and
• Resistance to condensable organics and other particulates.

Butterfly Dampers

Butterfly dampers have been applied to RTO’s for more than 30 years and basically consist of a flat plate inserted in a gas stream. It is rotated by means of a motor and linkage (often called an actuator) in order to control the gas stream flow. When the damper is in the closed position it almost completely blocks gas flow. When it 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 valves, and have broad versatility by the virtue of its design. They are relatively tight sealing without excessive operating torque requirements. They offer simple and reliable means for both modulating and on-off type of applications. A typical single blade butterfly damper is shown in Figure 2.
Butterfly dampers employ a center mounted rotating disc or discs that typically rotate 90 degrees for opening or closing. The solid rotating disc (generally round shaped) must resist thermal and mechanical deformations. The type and size of the damper dictates the torque requirements and by extension, also the actuation requirements (e.g. beyond a certain torque, pneumatic actuators become less desirable than hydraulic dampers due to their size).

Butterfly dampers used in the RTOs are on-off type with two designs variations:

• Single-blade damper (as shown in Figure 2) is the most common type used in the industry. It consists of the single solid disc (“blade”) which seats against a metal or compressible bulb (tadpole) seal; and
• Double-blade damper (shown in Figure 3) is used when “zero” leakage is required, usually in applications where very high destruction efficiencies are desired. Double-bladed dampers are also supplied with either a metal or a tadpole seat. Figure 6 shows butterfly damper seating arrangements.

Figure 4 and 5 provide a schematic of the single blade and double blade dampers, respectively. The leakage across the valve occurs when the blade is completely shut off. On one side of the blade is contaminated VOC stream, while on the other side is the purified air that is exiting the RTO. Thus leakage from high VOC stream into the purified stream directly affects the overall destruction efficiency of the RTO. Lower leakage results in higher destruction efficiency. By employing double-blade dampers, the leakage of the VOC from one side of the valve to the other can be completely eliminated by purging the gap between the two blades using a fresh air, such that any leakage across the blade consists of VOC free air.

Another butterfly damper developed more recently provides zero leakage without requiring two separate discs. This damper, which is kind of a hybrid of the single blade and double dampers, is called a dual seat damper because it uses two metal seats, and 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. Figure 6 shows a dual seat damper seating arrangement.

Poppet Dampers

Poppet dampers have been applied to RTO’s for over 15 years and 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 one to two foot 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.

Poppet valves were initially developed for service in fabric filter systems or bag houses and later applied to Regenerative Thermal Oxidizers (RTO). Fabric filter systems require 2-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 double blade and seat with seal air. These valves range in size from 20 inches to 47 inches diameter for industrial bag houses and 48 inches to more than 96 inches in diameter for power generation bag houses.

Poppets for RTO service are more complex in that they seal multiple gas paths while diverting gas in different directions. Figure 7 illustrates a 2-way poppet for RTO service. RTO systems that are designed with 2-way poppets should have one inlet and one outlet damper providing fail safe conditions during power outages and upset operating conditions.

Poppet dampers of this type 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. Poppets of this type operate best when oriented vertically. They are available with zero-leak blades and seats as well.

Poppet valves with 3-way and 4-way configurations have also been used in RTO systems. A 3-way poppet has one inlet and two outlets. It cycles between two seats and diverts flow through one while sealing the other and visa versa. A 4-way poppet has two inlets and two outlets, and has been used in compact RTO systems. Figure 8 illustrates a small 4-way poppet with two cylinder actuators in a vertical orientation.

Single Canister Rotary Valve

The single canister rotary valve design eliminates the need for separate inlet, outlet and valves, and replaces them with a single large valve. Rotary valve designs have been applied to RTO’s for over 10 years. The sequence of the bed function as an inlet, outlet or purge is achieved by the rotation of this single valve. This valve is located below the heat recovery chambers and is either electrically or pneumatically driven. The rotation of the valve (or the distributor) continuously controls the air flow 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 then an outlet chamber for reheating the heat exchanger bed before exiting. Before chambers switch from inlet to outlet flow, they are purged of any residual unoxidized gas. This purging insures minimal VOC spikes, and maximizes destruction efficiency.

The single valve moves at several minutes per rotation and insures a smooth transition from inlet to purge to outlet, therefore reducing upstream pressure fluctuations which are more typical with traditional RTO designs. The single valve also requires less maintenance compared to the multiple valve RTO system.

However, the single canister rotary valve utilizes machined metal to metal surface to achieve tight sealing. This makes it more susceptible to wear and tear. This seating arrangement also makes the single can RTOs 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.

References:
1. Menardi Filtration Glossary, web page
2. Perry’s Chemical Engineering Handbook, Sixth Edition
3. Ron Maxwell, Senior Flexonics/Pathway, Personal Correspondence, 2006
4. Effox Product Literature and Personal Correspondence, 2006

Anu Vij holds a B.S. in chemical engineering and a M.S. in chemical engineering from the University of Southern California. He has fifteen years experience as an engineer and project manager in the air pollution equipment fields. Anu has been with Pro-Environmental since the company’s inception and now serves as operations director. Pro-Environmental in Rancho Cucamonga specializes in air abatement and plant process emissions control. Anu Vij: 909.989.3010; www.pro-env.com.