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.
WEIGHT VS. VOLUME
When air is moving, it can have considerable inertia. The following situation is an example of how air's weight can be used to adjust its velocity.
Along the length of one side of the 30-ft long, 12-ft wide, and 10-ft high room, drive engines filled with highly toxic material were torn down, cleaned, and rebuilt on workbenches. Concentrations of the material in the area had been found to be injurious to health. Near the wall on the backsides of the workbenches were local exhaust hoods for contaminant control. Opposite the workbenches were a number of air-supply grilles.
A study disclosed that air from the grilles flowed at a high velocity over operators, creating a low-pressure area at their breathing zone. The air then flowed across the workbenches, disturbing the flow of air as it entered the exhaust hoods. Some of the supply air flowed down between the workbenches and outside wall. Then, it reversed its flow across the floor and moved up over the operators and into their breathing zones.
It was evident that the incoming air had to be supplied at very low velocities if control was to be achieved. A ventilation consultant recommended that the entire wall at which air was supplied be redesigned into a supply plenum, with the entire face consisting of perforated metal. The holes in the metal were to be sized so the desired outside air could be supplied with a velocity of 2,000 fpm through the holes.
The ventilation consultant explained that air would be entrained among the holes while losing velocity. Therefore, within inches of the air leaving the plenum, its velocity would be determined by the total volume of air and its relation to a cross section of the room. The velocity of the air would be low enough that problems would not be caused as the air flowed around operators or entered the exhaust hoods.
If 10,000 cfm were to be supplied to a room, 750 lb of air would have to be handled each minute. Successfully reducing the velocity of a 750-lb mass traveling at several hundred feet per minute to a velocity of about 30 fpm in a space of only a few feet would appear to be impossible. If this volume of air were to be supplied through 10 grilles, there still would be the problem of controlling 10 75-lb items. While this would be easier, it is questionable if it could be done. However, dividing the 750 lb into hundreds, if not thousands, of smaller weights with a perforated plate creates a simple solution.
By considering the weight, not just the volume of air, an impossible problem can become a rather simple one. How far air travels after leaving the open end of a duct or grille before its velocity is reduced to a desired level is a function of the size of the discharge openings and the leaving-air velocity. These two factors also dictate the volume of the air being moved. The total weight of the air depends on the air volume. Therefore, the weight of the air is a determining factor in the change of air velocity.
RESISTANCE
Problems can arise during the operation of an air-handling system when the distribution ductwork is only partly installed or the system's external resistance is much less than calculated.
Basic fan characteristics are important. With forward-curved centrifugal fans, the power curve continually rises toward free delivery. Therefore, reduced resistance can lead to overloading. Backward-inclined or curved fans do not have this problem and, therefore, have “non-overloading” horsepower characteristics.
The following situation is an example of a problem solved by revising air resistance. During the construction of a theater, a building contractor requested that a heating and cooling unit's fan be operated to increase the drying of the theater's plaster. The only ductwork connected to the heating and cooling unit was the main supply duct between the unit and the theater. No branches led away from this main, no registers had been installed, and the return duct system was not in place.
The sheet-metal contractor objected to starting the unit, but the owner insisted. The fan motor started, but only ran for a short time before the breakers “kicked out.” The unit was restarted several times, and each time the same thing happened. The owner's engineer believed that the motor selected was too small and the unit's manufacturer should furnish and install a larger motor. The sheet-metal contractor said that he installed the unit as specified and was not responsible. The manufacturer was contacted and agreed to send someone to determine who was at fault and what should be done.
The following day, a man representing the manufacturer arrived on site. After hearing the problem, he looked at the unit, which had only one supply duct, no return duct, and no filters. (This was evident because the covers that were supposed to be over the filter openings were not in place.) The air-supply plenum was in place, but openings had been cut where return-air ducts should have been connected in the top of the unit casing.
After examining everything, the manufacturer's representative put the covers over the filter-access openings and laid flat sheet metal over part of the return-air openings in the unit casing, which reduced the size of the openings. The owner's engineer objected. He believed the motor could not handle the air against the existing resistance and felt the manufacturer's representative was adding more resistance to the system.
When the manufacturer's representative finished, the fan drive motor was energized. The motor started and continued to run. A number of people did not understand why the motor did not overload because it now was operating against a much higher resistance.
The manufacturer's representative then explained why the motor overloaded in the first place and why it did not after more resistance was added to the system. He said the problem had to do with the weight of the air being handled. The system had been designed to handle a certain volume of air against the resistance of the ductwork external to the unit.
For purposes of the discussion, the manufacturer's representative used an air volume of 2,000 cfm. He said that each cubic foot of air weighed 0.075 lb; therefore, 2,000 cfm would weigh 150 lb. However, because of the lower resistance of the incomplete system, the fan was handling 3,000 cfm. Therefore, the weight of the air was 225 lb. This greatly increased the work the fan was doing, which resulted in the breakers “kicking out.”
CHANGES IN AIRFLOW DIRECTION
Problems can occur when a large volume of air is forced to change directions. An example of this kind of dilemma occurred when a new paint booth was installed in a plant. A painter stood in front of the 12-ft-wide booth and sprayed paint on large sheet-metal parts moving through the booth. The booth had a complete ceiling that extended out over the painter. The front of the booth was open, allowing air to flow in and exhaust on the opposite side. When the booth was put into operation, everything seemed work to as planned. However, dirt particles soon were found on freshly painted parts as they left the booth.
The booth manufacturer was contacted and sent a representative to make minor changes. This did not resolve the problem. Because the booth still was not operating properly, assistance was requested from the plant's corporate central office.
A ventilation engineer from the central office visited the operation. He studied the entire operation, including the supply-air system. The supply air was introduced above the booth's ceiling, flowing from the rear to the front of the booth. The air then had to make a 180-degree turn, allowing it to enter the face of the booth. A series of pipes carrying compressed air and different paints were located across the upper part of the booth's face.
The ventilation engineer said that when the air was forced to turn 180 degrees in front of the booth, it created a low-pressure area. This drew air that was carrying paint pigment from within the booth and over the pipes. Some of the paint pigment was deposited on the pipes, where it dried and fell off into the supply air entering the booth below the pipes. The supply air then carried these particles into the booth, where they were deposited on the freshly painted parts.
The booth manufacturer's representative insisted the solution was to reduce the size of the booth's front opening with sheet-metal panels, increasing the velocity of the air entering the booth.
Some time later, the manufacturer's representative contacted the ventilation engineer, admitting the changes the manufacturer made worsened the problem. Both parties again visited the operation and verified the outward flow of air at the top of the booth using smoke. Turning blades were installed to force supply air to flow into the booth near the underside of the booth's ceiling. When this was done, smoke in the air stream proved all outward flow of air had been stopped. The pipes located at the top of the booth's face were cleaned, and the problem was solved.
This situation was another example of how the weight of air can create negative areas when its mass changes direction. As explained, a negative condition is created if an air mass flows around an obstruction. For an easily visible example of this condition, take a look at the back of a moving van. When an air mass flows around the back of the van, it becomes covered with dirt.
CONCLUSION
The explanations and examples
Kenneth E. Robinson, CIH, began his HVAC career as an engineer in training with the Dailaire Division of Dail Steel Products Co. of Lansing, Mich., in 1935. Following his retirement from General Motors Corp.'s industrial-hygiene department in 1974, he lectured on energy conservation in industrial plants as a representative of the American Society of Heating, Refrigerating and Air-Conditioning Engineers. Over the years, he has written extensively on industrial ventilation and been involved in the making of a number of educational films, videotapes, and slide presentations.
When the manufacturer's representative finished, the fan drive motor was energized. The motor started and continued to run. A number of people did not understand why the motor did not overload because it now was operating against a much higher resistance.
The manufacturer's representative then explained why the motor overloaded in the first place and why it did not after more resistance was added to the system. He said the problem had to do with the weight of the air being handled. The system had been designed to handle a certain volume of air against the resistance of the ductwork external to the unit.
For purposes of the discussion, the manufacturer's representative used an air volume of 2,000 cfm. He said that each cubic foot of air weighed 0.075 lb; therefore, 2,000 cfm would weigh 150 lb. However, because of the lower resistance of the incomplete system, the fan was handling 3,000 cfm. Therefore, the weight of the air was 225 lb. This greatly increased the work the fan was doing, which resulted in the breakers “kicking out.”
CHANGES IN AIRFLOW DIRECTION
Problems can occur when a large volume of air is forced to change directions. An example of this kind of dilemma occurred when a new paint booth was installed in a plant. A painter stood in front of the 12-ft-wide booth and sprayed paint on large sheet-metal parts moving through the booth. The booth had a complete ceiling that extended out over the painter. The front of the booth was open, allowing air to flow in and exhaust on the opposite side. When the booth was put into operation, everything seemed work to as planned. However, dirt particles soon were found on freshly painted parts as they left the booth.
The booth manufacturer was contacted and sent a representative to make minor changes. This did not resolve the problem. Because the booth still was not operating properly, assistance was requested from the plant's corporate central office.
A ventilation engineer from the central office visited the operation. He studied the entire operation, including the supply-air system. The supply air was introduced above the booth's ceiling, flowing from the rear to the front of the booth. The air then had to make a 180-degree turn, allowing it to enter the face of the booth. A series of pipes carrying compressed air and different paints were located across the upper part of the booth's face.
The ventilation engineer said that when the air was forced to turn 180 degrees in front of the booth, it created a low-pressure area. This drew air that was carrying paint pigment from within the booth and over the pipes. Some of the paint pigment was deposited on the pipes, where it dried and fell off into the supply air entering the booth below the pipes. The supply air then carried these particles into the booth, where they were deposited on the freshly painted parts.
The booth manufacturer's representative insisted the solution was to reduce the size of the booth's front opening with sheet-metal panels, increasing the velocity of the air entering the booth.
Some time later, the manufacturer's representative contacted the ventilation engineer, admitting the changes the manufacturer made worsened the problem. Both parties again visited the operation and verified the outward flow of air at the top of the booth using smoke. Turning blades were installed to force supply air to flow into the booth near the underside of the booth's ceiling. When this was done, smoke in the air stream proved all outward flow of air had been stopped. The pipes located at the top of the booth's face were cleaned, and the problem was solved.
This situation was another example of how the weight of air can create negative areas when its mass changes direction. As explained, a negative condition is created if an air mass flows around an obstruction. For an easily visible example of this condition, take a look at the back of a moving van. When an air mass flows around the back of the van, it becomes covered with dirt.
CONCLUSION
The explanations and examples
Kenneth E. Robinson, CIH, began his HVAC career as an engineer in training with the Dailaire Division of Dail Steel Products Co. of Lansing, Mich., in 1935. Following his retirement from General Motors Corp.'s industrial-hygiene department in 1974, he lectured on energy conservation in industrial plants as a representative of the American Society of Heating, Refrigerating and Air-Conditioning Engineers. Over the years, he has written extensively on industrial ventilation and been involved in the making of a number of educational films, videotapes, and slide presentations.
Related Articles:
Underfloor-Air-Distribution Design Concepts
An Inside Look at Air-Filter Selection
High-Plume Dilution Blowers Chosen to Exhaust Medical-Research Labs