Impact des stratégies de ventilation dans une enceinte de piscine intérieure sur la concentration du trichlorure d’azote, de l’évaporation de l’eau et du confort thermique

Abstract

Nitrogen trichloride (NCl3), also called trichloramine, is a disinfection by-product commonly found in the water and air of indoor swimming pools, that forms in swimming pool water due to a reaction between chlorine and bodily fluids (like sweat and urine) brought in the water by bathers. This gas is associated with chronic diseases such as asthma, and often causes eye and skin irritation. Due to its higher density than air, NCl3 tends to accumulate close to the water surface and above the deck. It easily becomes airborne when water is agitated and can impair lung functions and cause illnesses such as asthma and bronchitis in workers and bathers, when inhaled. Mixing ventilation is the main ventilation strategy used in aquatic centers to maintain the NCl3 concentrations at an acceptable level. This ventilation strategy aims at diluting any contaminant in the air to reduce its concentration before the air is extracted and exhausted. However, swimming pool halls are known to be significantly energy-intensive, thanks for a large part to water evaporation from the basin that needs to be replaced and heated. Consequently, building codes recommend maintaining a low airspeed above the pool and deck to minimize evaporation and to ensure swimmers’ thermal comfort. Since NCl3 tends to accumulate at the water surface and above the deck, ASHRAE’s recommendations are to add lower extraction vents to capture it. However, no guideline is found in literature on the number of vents, their position in the enclosure or their minimum airflow rate. Also, sufficient air movement in the occupied zone and near the water and deck surfaces are required to move these heavier than air molecules. However, this airflow may cause an unwanted increase of water evaporation or unwanted discomfort for bathers. Both parameters can negatively impact the management of an indoor swimming pool enclosure. This thesis carries out a numerical investigation of the impacts of ventilation strategies on NCl3 concentrations in the air of a swimming pool enclosure in Montreal (Canada). More specifically, the outdoor air intake, thus the recirculation of air, the number of air changes per hour and the addition of extraction vents at deck level are investigated for this particular swimming pool enclosure. The modifications (parameters and strategies) that show the most interesting results in terms of NCl3 reduction are then investigated for their impact on the water evaporation rate from the pool surface and on thermal comfort of bathers, as they are standing on the deck. A total of 14 scenarios aiming at reducing NCl3 levels in the breathing zone were simulated and compared to the reference case. They all could potentially be implemented in the swimming pool hall, although some will require more modifications to the existing HVAC system. Then four of these scenarios were chosen, based on their capacity in reducing NCl3, and water evaporation rates and thermal comfort were investigated and compared to the reference case. Results show that the minimum recommendations by ASHRAE are not sufficient to eliminate NCl3 accumulation and stratification in the lower breathing zones. Moreover, air recirculation mostly affects the breathing zones of standing occupants on decks and of lifeguards, but has a smaller impact on the breathing zones of swimmers or people sitting on the deck. Eliminating air recirculation does not guarantee better air quality in terms of NCl3 in these breathing zones. Results also show that the position of these vents affects the air movement above the water surface, causing either unwanted accumulation or reduction of NCl3. The results also demonstrate the importance of a numerical assessment of the airflow patterns in a pool enclosure and especially above the water and deck before planning the position of these deck level extraction vents. The present findings also show that adding deck-level air exhaustion vents allow NCl3 reduction in all breathing zones (lower and higher) without negatively impacting thermal comfort and with only a slight increase in the water evaporation rate. This combined ventilation strategy, mixing ventilation (with upper extraction) and deck-level extraction offers a promising method if these deck level vents are positioned in an optimal way in regard to the airflow pattern in a swimming pool enclosure. The findings for the complex under study may be used to guide similar research in other aquatic centers and for the continuous development of building codes and air quality standards.

Date	30 Oct 2023
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Stéphane Hallé (Supervisor)