In the recommendations of the National Institute for Occupational Safety and Health (NIOSH), the American Society for Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the American Conference of Governmental Industrial Hygienists (ACGIH) and the Environmental Protection Agency (EPA) in ventilated facilities, especially with technological processes, general mechanical ventilation should perform such a distribution of air that, in addition to maintaining the required conditions of thermal comfort, should interact with local exhaust ventilation encapsulating the source of air pollution [ACGIH 2019].
According to §148, item 4 of the Announcement of the Minister of Investment and Development of April 8,2019, on the publication of a uniform text of the Regulation of the Minister of Infrastructure on technical conditions to be met by buildings and their location [Dz. U. 2019, poz. 1065],
In a room where a technological process is a source of local emission of harmful substances with an unacceptable concentration or oppressive odour, hoods working w'ith general ventilation should be used as this would enable meeting the quality requirements of the internal environment specified in the occupational health and safety regulations.
In rooms related to the treatment of nanomaterials, in medical, chemical and physical laboratories intended for treatment of nanomaterials, in medical, chemical and physical laboratories, local exhaust ventilation is usually supported by general supply or exhaust-supply ventilation. The organization of air distribution in the room has a significant effect on the quality of the air, including the concentration of its impurities. It is particularly important to properly shape the air flows in large rooms, e.g. large halls. Although the flow' of air supplied to rooms required by law' is often provided, w'rong location of ventilation openings, e.g. a significant number near window's and walls, can lead to draughts in workplaces located close to these openings, while in the middle of a room proper air movement will not be provided and its pollution will accumulate.
Rooms where works with nanomaterials are performed should be equipped with mechanical ventilation ensuring that nano-objects and their aggregates and agglomerates (NOAA) are extracted from potential sources of emission and proper air distribution. It is possible thanks to the installation of properly co-operating general and local mechanical ventilation [Mierzwinski 2015]. Improper general mechanical ventilation may cause improper air distribution in the room and, as a consequence, formation of unventilated areas and accumulation of NOAA of significant concentrations in various room areas. The lack of local ventilation or the use of inefficient ventilation (e.g. fume cupboard) near the NOAA source may result in their spreading in the room air.
Basic parameters used to determine the type of ventilation in a room (negative pressure, balanced, positive pressure) and calculate the air exchange rate are the volume flows of the air entering and leaving the room. In a given room, different air distribution systems can be used, depending on whether the installed ventilation systems have been turned on or off, and the type of interaction of general and local mechanical ventilation.
Due to the air condition in a given room, after switching on/off the general mechanical ventilation system in the room the following can occur:
- • negative pressure ventilation when the volume of air supply is lower than the volume flow of extracted air,
- • balanced ventilation when the volume flows of supply air and exhaust air are similar,
- • positive pressure ventilation when the volume of supply air is higher than the volume flow of extracted air.
The main difficulty in designing indoor ventilation systems is the need to maintain a constant value of negative pressure or positive pressure. The above equilibrium can be obtained by using an appropriate disproportion between supply and extract air volume flows. The automation and control system, whose executive component is the Variable Air Volume (VAV) controllers, is responsible for maintaining the balance of volume air flows and specific proportions between them [Szymanski et al. 2015].