Most process engineers are aware of Class A safety ovens used for processing volatile, flammable solvents. The requirements for these ovens are outlined in NFPA 86 Standard for Ovens and Furnaces. Among other requirements, mechanical ventilation is used to dilute flammable vapors to a point where they are too lean to ignite. The process starts with a timed purge, ensuring that four volumes of dilutant are flushed through the system before the heat source is enabled. The dilution airflow rate is calculated to prevent the vapor concentration from exceeding 25 percent of the lower flammable limit (LFL). If the system includes a solvent monitor, then operation at up to 50 percent of the LFL is permitted (figure 1).
As the quantity of solvent per batch increases, so does the dilution air requirement. All of this makeup air must be heated from ambient to the process temperature; then, it is discarded to the outdoors through the exhauster. At some point, the energy (kW) required to heat the makeup air can become unmanageable.
Though less well known, another type of Class A oven also is described in NFPA 86. It is called a low oxygen atmosphere Class A oven with solvent recovery. This variation of Class A employs a different strategy to provide safety from the ignition of the flammable vapors (figure 2). In these ovens, the oxygen concentration is reduced by the introduction of inert gas, most commonly nitrogen. Each type of flammable solvent has an associated limiting oxidant concentration (LOC). Below this oxygen percentage, it is not possible to ignite the vapors. The LOC for the most common solvents has already been determined and is available in NFPA 86.
In a low oxygen atmosphere oven, NFPA 86 requires the use of a pair of oxygen analyzers to control the oxygen concentration in the oven. The first analyzer is used in a closed control loop to supervise the purge and establish the running oxygen concentration to a preset value. The maximum allowable value is 3 percentage points below the LOC. Many organic solvents have a LOC value of around 9 or 10 percent in nitrogen, so a typical maximum O2 operating concentration might be in the 6 to 7 percent range. Lower values are permissible if the process calls for it (such as for oxidation-reduction) because lower values only increase the safety factor.
The second oxygen analyzer provides redundancy. If the preset alarm value is exceeded for any reason, the heating system is disengaged. Operator intervention is required to clear the alarm and restart the process.
To reduce the purge time and also minimize inert gas usage, some systems employ a pair of solenoid valves to provide high-flow and low-flow paths for the introduction of nitrogen. The high-flow (purge) valve is opened to rapidly reduce the oxygen concentration, allowing the heat to be energized sooner. Then the low-flow (run) valve takes over, providing just enough flow to maintain the oxygen concentration at the desired level. A single modulating valve also may be used to achieve the same effect.
These ovens must have a fully welded liner and tight door-seal gaskets. Also, they often include shaft seals to minimize inert gas consumption. NFPA 86 does not require pressure-relief venting on this type of oven. The design should include automatic door locks to prevent opening during operation, however. If the inert atmosphere were suddenly lost, there would be a risk of fire or explosion.
A common method of implementing the door lock is by using a solenoid-locking door switch. Once the door closes, inserting the tongue (key) into the switch, it cannot be released without the solenoid being energized. The control system is designed to prevent releasing of the door unless the cycle is completed, the heating system has been cooled, and a breathable atmosphere has been restored inside the oven.
Because low oxygen ovens do not rely on the dilution of vapors to prevent ignition, they can process an unlimited amount of solvent per batch. They frequently run at higher vapor concentrations than would be possible in a regular Class A oven. This makes them suitable for solvent recovery (figure 3).
Standard Class A ovens operate at a typical vapor concentration of around one-half of one percent by volume. At these low concentrations, the dewpoint of the vapors is low, and very cold temperatures would be required to make them condense. Furthermore, if the dewpoint of the vapors is below the freezing point of water, then moisture present in the dilution makeup air is likely to freeze and accumulate within the condensing coil.
Low oxygen Class A ovens can evaporate large volumes of solvent while requiring only a small flow of nitrogen to maintain the inert condition. This results in vapor concentrations that are suitable for condensing using chilled water or even tower water. If the vapors being evaporated do not include any substantial amount of water, then a direct-expansion refrigeration system may be used with the evaporator coil serving as the solvent-recovery condenser. If coolant is not already available at the customer location, a dedicated recirculating chiller can be provided with the oven system. These chillers are small, self-contained units and they frequently use water with 50 percent glycol as the coolant.
The dewpoint of the solvent vapors must be calculated carefully to allow for proper selection of the condensing temperature (figure 4). If the heat exchanger surfaces are not below the dewpoint, then no solvent will be recovered. The coolant temperature will typically be at least 15°F (8.3°C) below the dewpoint.
Various types of heat exchangers can be used to condense the solvent vapors, but a simple recovery system might employ a tube-and-fin heat exchanger — the same style that is used in air conditioning systems. A fraction of the recirculating oven atmosphere —known as a bleed stream — is extracted and forced through the coil. Vapors condense on the cold surfaces of the heat exchanger and drain off by gravity into a storage reservoir. After the vapors are removed, the balance of the bleed stream is returned to the oven.
Over time, the entire oven atmosphere is passed through the cooling coil many times over, stripping out vapor with each pass. A pressure-relief vent is located on the downstream side of the coil, where the vapor is at its lowest concentration. This is where the inert gas pressure is released from the system. This gas will contain a small amount of residual solvent vapor, so it needs to be piped to a safe area outside the building.
At the end of the drying cycle, a compressed air purge is conducted to flush out the inert gas and restore a breathable atmosphere inside the oven. The oxygen level is monitored by both of the gas monitors. When the level reaches 19.5 percent the oven door lock is released, providing safe access for personnel. The minimum 19.5 percent oxygen level is specified in NFPA 86 and complies with OSHA safety standards.
Standard Class A ovens are fairly simple, with lower capital equipment cost and moderate cost of operation. They can only process a limited quantity of solvent per batch, however, and they do not lend themselves well to solvent recovery.
Low oxygen Class A ovens are more complex, with a higher capital equipment cost. They do require an inert gas supply for operation, but the flow rates are fairly low. These ovens can safely process a nearly unlimited quantity of solvent per batch, and they lend themselves well to solvent recovery.
If your process requires the drying of large volumes of solvent, or if solvent recovery is an attractive option, you may benefit from exploring the “other” Class A oven.
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