Standards of Ventilation

Wherever possible, ventilation rates by mechanical means should be calculated on the basis of the amount of fresh air to be supplied for each occupant. This should vary from 420 to 1.000 cft per hour or even more. In cases where the occupation cannot be determined, ventilation rates should be based on the number of complete air changes to be provided. Table shows the number of air changes to be provided in various buildings where the number of occupants cannot be ascertained.

 Rates of Air Change with Mechanical Ventilation

The ventilation of factories and workshops is governed by the Factories Act. Much depends upon the size of the factory, both as regards floor area and height, in relation to the number of occupants, and on the nature of the process employed. 

Mechanical Ventilation 

Mechanical ventilation system should invariably be installed when a definite rate of ventilation is necessary or when it is desired to have the conditions within the building under control. 

Mechanical system of ventilation can be classified as follows : 

Extract  Ventilation  

This is cheap both to install and operate but it should be remembered that an exhaust fan is a means of creating a low- pressure area in proximity to it, with consequent depression over a wider area. Air from outside will therefore enter through any opening to counteract this depression, and it is important that inlets be placed at the opposite end of the building with respect to the exhaust (extract ) fan, or at least as far away from it as possible. Any inlet which is positioned near the exhaust fan is likely to cause a short circuit leaving an area adjacent to the inlet unventilated. 

Supply System 

This is the reverse of the extract ventilation method, in that the air is positively introduced into the room or an enclosure by means of fans, thereby creating a pressure within the enclosed space. Naturally, too high an air pressure cannot be tolerated with comfort and suitable outlets must therefore be arranged to allow the air to flow out of the room.

In practice the supply system is frequently employed where fresh air only is to be used, e.g., in operating theatre suites, since full control is maintained over the air introduced including its heating, cooling and filtering. However, where circumstances permit the use of recirculation, it is a relatively simpler matter to arrange for regulated amounts of air, at room temperature, to be returned through the fan system thus achieving economy on heating or cooling loads. 

Balanced Ventilation 

This is the only method which can be relied upon to give satisfactory results under all conditions, as the air movements can be definitely regulated at all points from inlet to extract point. A disadvantage of the balanced system is that it is dependent for successful operation on the closing of all windows and it is liable to cause complaint in offices and other buildings where normally the windows may be opened at the discretion of the occupants.