Ventilator Calculation Examples
Assume a building used as an automobile storage garage 50 feet wide by 150 feet long by 20 feet high to the eaves, 26 feet to the ridge of the roof.
1. Volume of space from the eaves down is 50 X 20 X 150 = 150,000 cubic feet. Volume of space from the eaves up is 50 X 3 X 150 = 22,500 cubic feet. Total volume = 172,500 cubic feet.
2. From the table of air change requirements, you will find that storage garages require four changes per hour for proper ventilation. Total air to be exhausted is then 172,500 X 4 = 690,000 cubic feet per hour, or 690,000 –: 60 = 11,500 cfm.
3. Spacing ventilators along the roof as indicated in Figure 6-29, with the aforementioned rules in mind, the resultant number of ventilators would be seven, spaced 15 feet from each end
and 20 feet apart. Each ventilator would need to exhaust 11,500 –: 7 = 1643 cfm.
4. Suppose we have a mean temperature difference of 10°F, an average wind velocity of 5 mph, and a stack height of 26 feet (floor to roof ridge).
5. Turning to the capacity tables under that of the 30-inch cone damper unit in the proper columns for the factors given in step 4, we find that the capacity is 1750 cfm, which exceeds our requisite of 1643 cfm obtained in step 3. Even though this is a trifle higher than necessary, it should be used, inasmuch as the next size smaller would be too far under the required capacity. This can be seen by referring to the 24-inch unit, the capacity of which is 1165 cfm.
We have therefore determined that to properly ventilate this building, seven 30-inch cone damper ventilators will be necessary, located on the ridge of the roof and spaced 20 feet apart and 15 feet from each end.
Air Leakage
Air leakage is the passage of air in and out of various cracks or openings in buildings. It is also sometimes referred to as infiltration.
Air leaking into a building may be caused by wind pressure or by differences in temperature inside and outside of the building. In the former case, the wind builds up a pressure on one or two sides of a building, causing air to leak into the building. As shown in Figure 6-2 previously, the action of the wind on the opposite side or sides produces a vacuum that draws air out of the building. Thus, as shown in Figure 6-4 previously, a plan view of a single-room building is shown having a window and a door on one side and a window on the opposite side. The details are greatly exaggerated so that you can see the cracks.
Note that when the wind hits the A side of the building, its momentum (dynamic inertia) builds up a pressure higher than inside the building, which causes the air to leak through any cracks present, as indicated by the arrows.
As the wind traverses the length of the building, the air currents as they continue past the side C converge and produce a vacuum along side C by induction. Because the pressure on the outside of C is lower than inside the building, air leaks out as indicated by the arrows.
Air leakage due to temperature difference or thermal effect is usually referred to as stack or chimney effect. Air leakage due to cold air outside and warm air inside takes place when the building contains
cracks or openings at different levels. This results in the cold and heavy air entering at low level and pushing the warm and light air out at high levels, the same as draft taking place in a chimney.
Thus, in Figure 6-4, assume a two-story building having a window open on each floor. Evidently when the temperature inside the building is higher than outside, the heavy cold air from outside will enter the building through window A and push the warm light air through window B, as indicated by arrow a; as it cools, it will increase in weight and circulate downward, as indicated by arrow b.
Although not appreciable in low buildings, this air leakage is considerable in high buildings unless sealing between various floors and rooms is adequate.
A reasonable amount of air leakage is actually beneficial to health. Any attempt to seal a building drum-tight will cause the inside air to become stale and putrid. Emphasis should be placed on the reduction of heat transmission rather than the absolute elimination of air leakage.
The application of storm sash to poorly filtered windows will generally result in a reduction of air leakage of up to 50 percent. An equal effect can be obtained by properly installed weather stripping.
Garage Ventilation
The importance of garage ventilation cannot be overestimated because of the ever-present danger of carbon monoxide poisoning. During warm weather, there is usually adequate ventilation because doors and windows are kept open. In cold weather, however, people close up openings tight as a drum, with considerable danger. Nobody can breathe the resulting carbon monoxide concentration long without being knocked out—hence the importance of proper ventilation in cold weather, regardless of physical comfort.
Where it is impractical to operate an adequate natural ventilation system, a mechanical system should be used that will provide for either the supply of 1 cubic foot of air per minute from out of doors for each square foot of floor area or the removal of the same amount, discharging it to the outside as a means of flushing the garage.
The following points should be carefully reviewed when considering a ventilating system capable of removing carbon monoxide from an enclosed area:
1. Upward ventilation results in a lower concentration of carbon monoxide at the breathing line and a lower temperature above the breathing line than does downward ventilation for the same rate of carbon monoxide production and air change and the same temperature at the 30-inch level.
2. A lower rate of air change and a smaller heating load are required with upward ventilation than with downward ventilation.
3. In the average case, upward ventilation results in a lower concentration of carbon monoxide in the occupied portion of a garage than is had with complete mixing of the exhaust gases and the air supplied. However, the variations in concentration from point to point, together with the possible failure of the advantages of upward ventilation to accrue, suggest the basing of garage ventilation on complete mixing and an air change sufficient to dilute the exhaust gases to the allowable concentration of carbon monoxide.
4. The rate of carbon monoxide production by an idling car is shown to vary from 25 to 50 cubic feet per hour, with an average rate of 35 cubic feet per hour.
5. An air change of 350,000 cubic feet per hour per idling car is required to keep the carbon monoxide concentration down to one part in 10,000 parts of air.