Beat the heat

Many air emission control technologies use high operating temperatures to destroy toxic organic chemicals being released through industrial facilities' stacks. Control devices may have a stack gas temperature up to 1,000 degrees Kelvin (ºK) if there is no heat recovery equipment. Workers on an elevated structure could be exposed to the hot gas plume exiting a short stack, creating an unsafe working environment. Consequently, plume temperatures need to be evaluated for operating and meteorological conditions that could cause a plume to impact an elevated structure.

Plume temperature estimates can be made using dilution calculations based on air dispersion model results. This technique can also address concerns about potential exposure to a hot gas plume.

The advantage of this technique is that a wide range of variables can be evaluated at a lower cost than performing lab experiments or making field measurements. Potential problems may be more readily identified and resolved in the design phase.

Estimating plume temperature
The plume temperature increase above ambient, DT (ºK), can be estimated using the stack temperature and dilution of the plume with air:

DT = (Ts - Ta) / D
where:
Ta = ambient temperature (ºK)
Ts = stack temperature (ºK)
D = plume dilution (unitless)

Plume dilution due to air entrainment during dispersion is calculated by dividing the initial stack concentration by the predicted downwind concentration on plume centerline. The downwind concentration can be predicted using an air dispersion model. The plume temperature is the sum of the temperature increase above ambient, DT, and the ambient temperature.

A case study
Suppose a thermal oxidizer has a stack exit temperature of 900 degree Fahrenheit (ºF) (755ºK), a stack diameter of 0.9 meters, a stack height of 15 meters and a volume flow rate of 2,000 cubic meters per minute at 100-percent operating capacity. What is the maximum predicted plume temperature at downwind distances of 30 meters and 40 meters?

The EPA SCREEN3 dispersion model was developed by the U.S. Environmental Protection Agency (EPA) and is based on the generalized Gaussian plume model. It was used to predict downwind concentrations on the plume centerline for wind speeds ranging from 1 to 15 meters per second (m/s) for class D, the most commonly occurring atmospheric stability class.

Figure 1 shows that the predicted plume temperature increase (DT) ranged from 10.1 to 22.5ºK on the plume centerline. The maximum predicted temperature increase of 22.5ºK occurred at 30 meters downwind for a critical wind speed of 10 m/s. This temperature must be added to the ambient temperature of 298ºK to get a maximum plume temperature of 320.5ºK.

The SCREEN3 model increases the dispersion coefficients to account for buoyancy-induced dispersion. Therefore at a low wind speed, the larger dispersion coefficients result in lower predicted concentrations, greater dilution and a lower plume temperature. The discontinuity at low wind speed in Figure 1 is due to the buoyancy-induced dispersion term.

The SCREEN3 model uses final plume rise in calculations. If a better estimate of plume height is desired near the source, then gradual plume rise should be calculated using the equations in EPA's Industrial Source Complex Model User Guide. For the example above, at the critical wind speed of 10 m/s, the predicted gradual plume height was only 21 meters while the predicted final plume height was 62 meters.

Evaluating plume temperature results
Worker heat stress may be an issue at some locations, even without exposure to a hot plume. The American Conference of Government Industrial Hygienists has developed a heat stress index to predict body temperature based on environmental factors. Heat stress is a complex function of ambient temperature, humidity, solar load, acclimatization, work/rest regimen and clothing type. Therefore, heat stress does not correlate well with ambient or plume temperature alone. These simple limits have been extrapolated for use with a hot plume:

  • Two minutes exposure for an air temperature of 321ºK (118ºF); and
  • Forty-seven minutes exposure for an air temperature of 309ºK (97ºF).

One must determine whether workers will routinely or only occasionally be at the location of concern. The discomfort of a high plume temperature relative to atmospheric temperature will likely drive a worker to move out of a plume. Therefore, in a high plume temperature area, workers must be able to evacuate quickly.

Judgment is required in selecting ambient temperature, applying the simple temperature limits and analyzing plume temperature predictions. If there is concern about exposure to a hot plume, the facility management should consult with an industrial hygienist.

Alternatives
It is important to consider the occurrence frequency of the critical wind speed, wind direction, atmospheric stability class and thermal oxidizer operating load. If the probability of these events occurring simultaneously is low, then more frequently occurring but less extreme conditions should be evaluated.

If a potential worker exposure problem is identified during the design phase, then the distance between the stack and the structure may be increased. Another alternative is to use a tall stack. Access to an elevated structure may be limited during critical conditions - wind speed, ambient temperature or operating load - to prevent worker exposure to a hot plume.

It is noted that complex building source configurations may not be accurately evaluated using a Gaussian model and may require a wind tunnel study.

Summary
Conservative assumptions, such as plume centerline concentrations, may provide an upper boundary to plume temperature estimates.

Engineering judgement, however, is required to identify and evaluate the most important factors: meteorological conditions; ambient temperature; wind direction toward a structure; and critical operating capacity, as well as the probability of all these occurring simultaneously.

This article originally appeared in the 06/01/1999 issue of Environmental Protection.

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