Natural air movement in an industrial plant is like the movement of a small boy. It is never completely still, does not seem to have any definite pattern, and is difficult to stop. If asked why a helium-filled balloon tugs at its string or why smoke floats, almost everyone will answer that the object in question is lighter than air. However, no one seems to acknowledge that air has weight.

THE WEIGHT OF AIR

If various objects are lighter than air, then air must be heavier than those objects. This means that air has weight. In fact, standard 70°F air at sea level weighs 0.075 lb per cubic foot. This weight designation will be used for the purposes of this article. For example, a 10-ft-by-10-ft-by-10 ft room contains 1,000 cu ft of air (1,000 cu ft × 0.075 = 75 lb). The fact that air has weight figures into almost every instance of air movement in an industrial plant.

When air temperature is increased for any reason, the air moves. The lighter, warmer air begins to rise. This results in two definite air motions. First, air rises to a plant's upper levels. From there, it may move in any direction. Second, air begins to flow toward the location at which air is being heated. This air can and will come from any direction, allowing more air to flow near the floor and replace the air flowing upward.

DETERMINING AIRFLOW

Air at different temperatures, or densities, does not mix well. This often results in air flowing in opposite directions at different levels in a plant. If the direction of airflow in a plant needs to be determined, it may require the use of a number of smoke tubes, bombs, or bubbles or tracer testing at different locations to obtain sufficient and accurate airflow patterns. Sometimes, it is necessary to determine airflow at different plant elevations before the cause of a problem can be determined.

The following situation is an example of a problem that was solved by ascertaining airflow: The metal going through a press in a large press plant was turning a light brown, and the customer would not approve the color change. The plant engineering department was notified of the problem. An engineer looked at the operation and, without conducting any tests or studies, concluded the problem was caused by the air that was being delivered in an area by a makeup-air unit. He thought that the air coming from the makeup-air unit was contaminated by the discharge from nearby boiler smoke stacks at the powerhouse. On his recommendation, the air-supply unit was shut off. However, the metal continued to change color.

The engineer then determined, with a smoke tube located about 30 ft north of the press and 5 ft above the floor, that the air was flowing to the north. This convinced him that the air was flowing over the press and could not be causing the problem.

The production people continued to complain to the plant engineering department and demanded that something be done. A request for help was sent to the corporation's offices, which forwarded the request to the industrial-hygiene department. An industrial hygienist and a ventilation engineer were sent to the plant to see if they could solve the problem.

After a meeting in the plant engineer's office during which the situation was explained, the plant engineer showed the industrial hygienist and ventilation engineer the press and surrounding area. Nothing that might be causing the problem was observed at or near the press.

The industrial hygienist and ventilation engineer then began to widen the area under observation to find the cause of the problem. They came to a large heat-treat area about 200 ft directly north of the problem press. This area was well-ventilated with a well-designed canopy hood. However, metal washers were being heat-treated on the south side (the press side) of the heat-treat area outside of the exhaust hood. After the heat treating, the washers were submerged in oil to control the rate of cooling. After the washers were removed from the oil quench, any oil remaining on the washers was removed with a vapor degreaser.

The industrial hygienist and ventilation engineer immediately recognized the potential problem. Whenever the liquid or vapor used in the degreaser was exposed to heat, it would break down into phosgene, a colorless volatile liquid or gas. They believed this gas would have the ability to change the color of the metal going through the press. The question then became whether the gas was flowing south some 200 ft and coming in contact with the metal going through the press.

To help determine the airflow from the operation to the press, the ventilation engineer requested a length of thin wall pipe and a roll of masking tape. He then taped a smoke bomb to one end of the thin wall pipe and then stood a few feet south of what was considered to be the cause of the problem.

Upon the lighting of the smoke bomb, smoke flowed north, toward the washer heat-treating operation. As he approached the operation, the ventilation engineer raised the bomb on the thin wall tubing as high as he could. The smoke began to rise, continuing to do so until it filled the monitor running north and south. The smoke then flowed south until it was over the problem press, at which point it turned downward. The smoke showed that some of the air from the washer heat-treating area flowed over the press. The industrial hygienist and ventilation engineer felt this was the cause of the metal's color change.

When they asked why this operation was not ventilated, the industrial hygienist and ventilation engineer were told the production department felt it was not necessary, even though it was a plant rule that all such operations have exhaust ventilation. That night, a makeshift ventilation system using fireproof canvas was installed. With the makeshift hooding, the problem press was able to operate the following day without metal changing color.

There are several things to learn from this operation:

• The air moved because it had weight.

• The addition of heat caused the air to rise.

• Flowing through the monitor, the air slowly cooled and flowed over the press on the floor.

• Checking airflow at one place seldom tells the complete story.

• If the air checked by the plant engineer was shown to be flowing north from the press, from where did it come? That may have been more important than where it was going.

• More often than not, air near a ceiling or roof will flow in a different direction than air near a floor.