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.

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