Volume of air required to operate a ventilating and cooling system is a very fundamental requirement. Before air volume is estimated or calculated, careful consideration should be given to the following factors. Be sure that the decision you make will accomplish the most important objectives or will overcome the most important problems. This should be the overriding consideration in the selection of a system of ventilation. Availability of money to finance the system is a factor that must be considered early in the planning stage. The funds available will affect the objectives you set and will influence the system of ventilation selected to meet the objectives. |
The method selected for calculating air volume will relate closely to the objectives to be accomplished by the system. Based on the objectives and funds available, you can now consider the alternatives and select the best method to use in calculating air volume requirements. A description of these methods follows. This is a time-honored approach to a determination of air volume requirements. It is based on the theory that a complete change of air in a room or building should be made at a certain time frequency. The rate selected is frequently an arbitrary decision. It may be based on experience with similar installations or may be established by a health or safety code. Many fan manufacturers have published charts that show recommended rates of air change for typical installations. Although the rate of air change method of calculating air volume has been used for many years, Chicago Blower Canada engineers have found it unsatisfactory except in relatively small buildings or rooms. For jobs that involve personnel comfort. this method is not recommended if the building is over 50,000 cubic feet in content or more than l00' in length.
CFM per square foot of floor area method: this method of calculation is a modern adaptation of the rate of air change formula. Total air volume (cubic feet per minute) is determined by multiplying the total square feet of floor area by an arbitrary CFM per square foot figure. The figure selected may be as low as 2 CFM or as high as 12 CFM per square foot. Four CFM per square foot has been recommended as a minimum for summer ventilation of large assembly type operations. This method of calculation, like the rate of air change method, is likely to produce unsatisfactory results in many cases. Failure to control air distribution and air velocity can be a major weakness in the whole concept. Selection of a CFM per square foot figure should be based on experience and a proven method of air distribution.
Rate of air velocity method: This method of calculating air volume needed for a system is highly recommended by Chicago Blower Canada. A breeze conditioning system can be highly effective in providing personnel comfort in hot weather if the recommendations outlined below are observed. It has been determined from field experience that an average air velocity of 150 feet per minute (FPM) to 200 FPM is usually sufficient for personnel cooling under summertime conditions. The CFM required to do the job is calculated by multiplying the cross section of an area through which the air is to move by the desired velocity. This is expressed in the following formula.
As building size increases, there are factors that will affect the average air velocity through the cross section of the building. The longer the building, the greater the amount of air leakage from windows, doors, elevator shafts, etc. To offset this air leakage, air velocity should be increased. This is done by relating the calculated velocity to the length of the building. The results will provide an effective velocity of approximately 150 ft./min.
In some buildings. it is not possible or practical to install a complete ventilation system. In such situations, zone cooling may be effectively used. The problem is similar to a spot cooling application, but usually involves a relatively larger area. Effective zone cooling may be accomplished by use of air circulators. See Air circulation below. A very satisfactory zone cooling method is the use of supply-type PRVs to flood the problem area with fresh, cooler air. The adjacent drawing and zone cooling table illustrate the capacity of several Chicago Blower Canada PRV models to effectively cool an area. The figures are based on discharge of air approximately l5 feet above floor level.
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Chicago Blower Canada Industrial Process Ventilation Engineers firstname.lastname@example.org Chicago Blower Canada Co. http://www.chicagoblowercanada.com/biblower.html http://www.chicagoblowercanada.com/airfoilblower.html
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