The Story of the Stack
Voluminous plumes of exhaust are a familiar sight to anyone who works in an industrial processing plant. Their sheer variety -- short and white, brightly colored, or with trails that go on for miles -- draws attention, and those with flames, or flares, garner even more comment. Occasionally, those with a taste or odor that offends draw complaints.
Far too many different processes generate effluent to address all in one article. That would be an ambitious and formidable task for anyone to take on and would probably fill several volumes. Rather, this article focuses on describing and interpreting the visual emissions from the chimneys of drying processes that evaporate just water.
The process of thermal drying, by definition, evaporates and removes liquid from a solid, suspension, paste or other mass containing moisture. To increase process efficiency and prevent a moisture block, the liquid must be removed from the system in the form of vapor. To achieve this, it essential to create a flow of gas (air) through the system.
Airflow normally is accomplished via a mechanical device such as a fan or eductor. A forced draft system pushes the exhaust gases away from the process while an induced draft system pulls the gas from the process. Most typically, this gas is directed through a dust/air-gas separation or pollution control system, then into a chimney (more commonly referred to as a stack or a flue).
The stack has several important functional aspects. First, it channels and directs flue gases to the atmosphere at safe elevations. Second, the design often is fundamental to inducing the airflow up and out of the process (designated as the
Flue gases -- or exhaust gases, exhaust stream, effluent and emissions, as they also are known -- from the drying process comprise various components. Depending on the process heat source, the exhaust gases may contain products of combustion, process dust, process air and the moisture evaporated from the process (typically, water vapor). Some drying processes will evaporate volatile substances, and the exhaust stream may contain volatile organic compounds (VOCs). Others may use fuels or process materials that have a high sulfur content, and the exhaust stream may contain sulfur dioxide (SO2) or sulfur trioxide (SO3) compounds -- contributors to acid rain. The exhaust gas stream also carries energy: the sensible heat invested into the process and the latent heat carried away by vapor in the flue gas.
The visible plume emanating from the stack can provide useful information about the process's current operating condition. Numerous sophisticated instruments and specialist companies are available to accurately test the exact composition and status of the exhaust gas, and these tests should be carried out periodically on every emission point in all industrial processing facilities. There is, however, valuable information that one can glean simply by looking at the plume and understanding what it represents.
The Dryer StackMost commonly, one would expect the exhaust from a drying process to form a "cloud" (or plume) and quickly dissipate by diffusion into the atmosphere. This would be consistent with condensation theory, which states that "condensation is the passage of a substance from a gaseous form to a liquid (or solid) state."1 As the liquid being evaporated in most drying applications is water, this would probably be the case. However, the drying process is not quite so simple, and the "cloud" is affected by many different conditions.
First, consider the physical phenomena occurring in the stack. It's important to note that there is a difference between a gas and a vapor. A vapor is really a subset, or type of gas. While a vapor can approach the condensing or liquid state, gases remain in the gaseous state at normal atmospheric conditions.2 Vapors may exist in either a gas, liquid or solid state at the same atmospheric conditions. Additionally, vapors, although very much gaseous, do not follow the ideal gas laws.
Warm air can hold more water vapor than cool air. So if one reduces the temperature of an exhaust stream containing water vapor and air without changing the pressure, water condensation will occur as the relative humidity approaches that of saturation. Relative humidity is defined as the ratio of the air's actual vapor pressure to the saturation vapor pressure.3 The ratio of the mass of water vapor to total volume of the mixture (gas) is defined as the absolute humidity.4
Condensation occurs due to the removal of the vapor's latent heat, which will transform the vapor into water droplets referred to as mists, fogs or clouds (the plume). Small particles such as dusts, fumes and smokes will act as catalysts for the condensation process, providing the "seed" onto which the water vapor condenses.
In a typical dryer exhaust gas stream for a process that is evaporating water, the water vapor evolved from the process will be superheated. By definition, superheated vapor is vapor that is at a temperature above the saturation temperature for the ensuing pressure.2 For example, water vapor above 212oF (100oC) at 14.696 psia (1 atmosphere) is superheated. The vapor will change from superheated to saturated as the temperature falls below the dewpoint temperature, as long as the atmospheric pressure remains constant (isobaric). Therefore, for this process, dewpoint is defined as the temperature at which water vapor begins to condense out of a vapor-air mixture.2
Hence, water molecules in the exhaust air stream will remain in a gaseous (vapor) state until the gas stream's temperature reaches the dewpoint temperature. This temperature represents the saturation point of the vapor-air mixture and is well documented in psychrometric charts.
Visual cues can tell you whether the exhaust stream contains saturated or superheated water vapor. Saturated water vapor typically is visible as a mist, fog or cloud while superheated water vapor is transparent. Therefore, the visible plume bellowing from a pure steam line is defined as saturated steam. But, a close observation of the steam relief line would show that directly at the discharge, there is a short transparent haze or shimmering section: a superheated steam section. Once the plume dissipates into the surrounding air due to diffusion, it will once again become superheated because "the water vapor will be above the saturation temperature for its partial pressure."2 In other words, most water vapor in the atmosphere is superheated. (An example of saturated water vapor in the atmosphere is fog or mist.) This phenomenon, as it relates to the drying process, will be further discussed later.
Exhaust Gas TemperatureOne cannot draw any conclusion about the exact temperature of the exhaust gas from simply looking at the plume. For a constant pressure (assuming the stack emission is at atmospheric pressure), the dewpoint is a function of dry bulb temperature and concentration of vapor in the gas stream; that is, the absolute humidity. For a low concentration of water vapor, the dewpoint may be well below the ambient condition dewpoint, and there will be no visible condensation.
So, if a stack emission has no visible plume, one can assume that either there isn't any vapor, smoke or dust in the emission, or the emission has such a relatively small quantity of vapor present that the dewpoint is not reached. In exhaust streams with only a relatively small amount of vapor, the evolved water vapor remains superheated.
If the absence of a plume is due to the latter, you may find that conditions change on a seasonal basis. For example, a plume may be visible in winter but not in summer. In this situation, the low ambient temperature of winter cools the exhaust air to a temperature below the dewpoint, causing a plume to form. But, in summer the higher ambient temperatures do not cool the exhaust sufficiently, so its temperature remains above the dewpoint, and no plume is formed. Similarly, a plume may appear on cool summer nights but disappear with the heating of the day.
If a plume is formed -- indicating that the absolute humidity is relatively high and the ambient or exhaust temperature (dry bulb) is below the dewpoint temperature -- it is interesting to see how far above the stack the plume starts to form. For instance, if it forms on the stack tip, you can assume that exhaust gas temperature is below the dewpoint temperature. This indicates that condensation is occurring inside the stack, and condensate may be forming on the inside walls of the stack. Alternatively, if there appears to be a break between the top of the stack and the formation of the plume you can assume that the gas stream's temperature is above the dewpoint temperature. In this case, the vapor is superheated leaving the stack, so there is little chance for condensate to form inside the flue.
Keep in mind, this discussion relates to water and water vapor only. Certain other vapors -- SO2 or NOX -- may be present in the exhaust gas. Depending on the parts-per-million (ppm) concentration, these vapors could condense out at a higher temperature than water vapor. For exhaust streams that may contain these types of vapors, one must design the system and select the construction materials carefully. The combination of water and other vapors can lead to acids that can destroy the fabric of the dryer, dust/pollution control system and the stack itself.
Smokes, Carryover and Environmental FactorsIf smoke or other products of combustion are present in the flue gases, the plume may extend for a considerable distance. Since this is a by-product of the intended process -- namely drying -- it offers no significant information about the operation's status. But, one could make some observations about the combustion efficiency with regard to true stoichiometric combustion by comparing the color of the smoke to that of the optimal combustion emission.
Also, depending on the fuel used for the heat source, the fuel's emissions may contribute to and influence the plume formation. For example, in processes using coal as the fuel source, the carryover of ash from the combustion process provides a seed for the condensation process and, consequently, promotes plume development. Another heat source -- using steam with a heat exchanger or coil, for example -- would not contaminate the air stream in any way.
Smoke frequently can be confused with dust: As an emission, dust is a far more prevalent discharge in the drying process. Depending on the color of the emission -- specifically as it relates to the product color -- one can easily see if dust is being released into the environment.
And, there are other tell-tale signs. For instance, if a fine secondary plume is visible beyond the primary condensate dissipation, one can presume that it is caused by dust or smoke release. Once again, the color may provide further information.
An uncontaminated plume from a drying process should be white to slightly blue or blue gray, depending on local conditions. If the plume appears darker, or has a slight color tinge matching the product being dried, one can conclude that either the water vapor emission has been contaminated or the dust collection system is not operating effectively.
The length of the plume is affected by environmental conditions. For instance, a strong wind may extend the plume while a still day may cause the plume to sit short, on top of the stack. Stack height also will influence the visible effects caused by wind.
Certain dust collection systems -- wet scrubbers and wet cyclones -- use water to increase the cleaning efficiency. In these systems, the exhaust stream will be brought to saturation, and the plume will be full. Noting whether the condensate has any contamination will indicate carry over. For example, with fully saturated streams, one often feels "rain" falling in areas around the stack. If these "rain drops" leave a stain or a visible mark on materials and equipment in the area, one can conclude that the condensate has been contaminated and stack emissions are not clean.
Overall, the higher the temperature of the exhaust gas, the higher the losses and the less efficient the operation. A visual inspection can indicate a potential problem.
One can see if the process is not operating as required by viewing the exhaust plume. One can not, however, draw any conclusion as to whether the operation is optimal. Obviously, the exhaust gas stream can and should be analyzed to verify that the plume's visual cues are true.