3.1. Architectural Considerations, Materials and Methods
The standard four-bed ADB ward (Base Case) has a combined bed area of 56.25 m
2 and a floor to ceiling height of 3.6 m gives it a volume of 202.5 m
3 (
Figure 3a). The en-suite shared bathroom found in this type of ADB ward (with mechanical extract fan) was ignored in order to focus on the natural (buoyancy-induced) airflow regime. Each bed is within an area approximately 3.3 m × 3.9 m (
Figure 3a). The natural ventilation openings of the Base Case were side-hung windows, modelled as five bays of side-hung casement windows. Given the 100 mm maximum restrictions [
5], the total effective area of each opening was found to be 0.15 m
2. This area of opening was modelled as a vertical orifice measuring 0.125 m by 1.2 m on the external wall. The 125 mm opening marginally exceeds the 100 mm requirements because the upper and lower “triangular” portions of the open window were added to the horizontal 100 mm width. The total area of window opening is 0.6 m
2.
For the NPV system in the ADB ward, four ducts (each measuring 0.3 m × 0.6 m in cross section) and two supplementary ducts (each measuring 0.4 m × 0.7 m) were used for fresh air supply. The total area of air inlets was therefore 1.28 m
2 which is 53% more than total area of window openings. Due to the need to ensure fresh air was supplied to the exact location on each of the four beds, two possible designs of the NPV system that were considered and modelled are: the
Side–Side Duct Arrangement; and the
Top–Bottom Duct Arrangement (
Figure 3). The three-dimensional positions of the Side-Side and Top–Bottom NPV ducts along (
Figure 3b) represent the distinct feature of the NPV cases being investigated for the ADB ward. The clear headroom and height of opening for each duct in the Side–Side arrangement was 2.45 m while for the Top–Bottom arrangement, the headroom was 2.15 m. The exhaust stack was proportionately sized in cross section as 1.28 m
2, representing 1% of the net floor area as done in Lomas and Ji [
40] for sizing advanced natural ventilation systems. The central exhaust stack was located internally along the external wall (
Figure 3a) to ensure balanced access of stale to the exterior. The stacks were assumed to rise up to 2.5 m above the roof level. The longest ducts meant for the furthest beds were required to span a distance of 5.4 m (
Figure 3a).
Figure 3.
Layout of activity database (ADB) ward showing: (a) Plan view of Side–Side and Top–Bottom NPV duct locations, central CBNV ducts and two possible locations of exhaust stacks; as well as (b) The NPV duct designs in 3D.
Figure 3.
Layout of activity database (ADB) ward showing: (a) Plan view of Side–Side and Top–Bottom NPV duct locations, central CBNV ducts and two possible locations of exhaust stacks; as well as (b) The NPV duct designs in 3D.
3.2. Results of the ADB Ward
The predictions from CFD are in agreement with findings from literature that with displacement strategy (
i.e., Base Case windows), fresh air from the side-hung openings flood the space at floor level (
Figure 4). As the same orifice serves as air outlet, the mean age of air of at the floor is 2100s, before the currents rise upwards upon reaching the opposite wall (
Figure 4d). Upon reaching the opposite wall, the air currents begin a return sweep with a mean Age of air at bed height of between 2730s and 2940s. Thermal stratification is clear from the predictions of ward temperature (
Figure 4a), PMV (
Figure 4b) and Age of air (
Figure 4c). The temperature at 1 m (bed heights) is generally around 27 °C, indicating a rise of around 9 °C from ambient (outdoor) temperature. The overall mean ward temperature at head height is 29 °C. There would be pockets of stale warm air collecting at the celling with values approaching 36 °C. The relative air change rate predicted from the CFD simulations for the Base Case is 1.8 ACH.
Figure 4.
Airflow characteristics of the base case design showing contours of: (a) Temperature; (b) PMV; (c) Age of air; and (d) Streamlines of displaced air particles as they flood the space.
Figure 4.
Airflow characteristics of the base case design showing contours of: (a) Temperature; (b) PMV; (c) Age of air; and (d) Streamlines of displaced air particles as they flood the space.
When the ADB ward is fitted with the NPV system, the shorter ducts of both Case 1 (Side–Side NPV) and Case 2 (Top–Bottom NPV) bring in fresh air with a horizontal throw as expected (
Figure 5). The Top–Bottom design delivers greater volume of fresh air for the longer ducts, while for the Side–Side design, the elbow (corner) in the longer duct leads to a reduction in flow velocity and subsequently air flow rates (
Figure 5a). In both cases, airflow within the ducts can be described as steady laminar (Rayleigh number,
Ra < 10
5). In the Top–Bottom duct arrangement (
Figure 5b) the presence of canopy is useful re-directing air from the top duct unto the bed, whereas air from the bottom (shorter) duct experiences a horizontal throw.
In all NPV cases, the mean Age of air at bed height is ≈240 s and temperature stratification gradient is much less evident. There is potential for draught to be experienced by the patients, due to the instant discharge of fresh air over the beds with minor trubulence. The predictions of PPDR (for both types of NPV designs) show that for an outdoor simulation temperature of 18 °C, between 11% and 25% of patients may experience draught (
Figure 5c,d). This proportion of dissatisfied persons would expectedly be higher at a lower ambient temperature. The relative air change rates predicted from the CFD simulations are: 4.8 ACH for Side–Side NPV and 5.2 ACH for Top–Bottom NPV.
Figure 5.
Temperature contours for: (a) NPV Case 1; and (b) NPV Case 2 showing horizontal throw of fresh air from shorter ducts; with draught potential for (c) NPV Case 1; and (d) NPV Case 2.
Figure 5.
Temperature contours for: (a) NPV Case 1; and (b) NPV Case 2 showing horizontal throw of fresh air from shorter ducts; with draught potential for (c) NPV Case 1; and (d) NPV Case 2.
The distinct designs of the NPV ducts in Case 1 and Case 2 also have marginal implications on the speed of incoming air within the ducts and hence on airflow rates. From the steady-state CFD results of the Side–Side design (Case 1), the computed mean velocity of air at the discharge orifice is 0.31 m/s for the shorter duct and 0.26 m/s for the longer duct. The reduced velocity in the longer duct is due to the “L” shape bend which leads to pressure loss, and not particularly due to distance. This was confirmed in the Top–Bottom design, where the mean velocity of ≈0.3 m/s is maintained for both long (top) and short (bottom) ducts. Relative to the cross-sectional area of duct, this air speed gives each patient an absolute mean flow rate of 0.0324 m3/s or 32.4 L·s−1.
The airborne contaminants originate from source location (
Figure 3a, Position “
X”) in all Cases. The concentrations were calculated at the assumed (horizontal and vertical) position of a sleeping head for two bed positions (
Figure 6). In all Cases, the airborne contaminants are able to travel further across the ward with a relatively high concentration at a distance of up to 8 m from the source. However, in the Base Case, the concentration across the width of Bed “A” builds up from 7% up to 13.9% before it attenuates to ≈8% across Bed “B” (
Figure 6a). In the NPV Cases, however, the concentration across Bed “A” drops from ≈12% down to ≈3% in Case 1 (Side–Side) and to ≈2% in Case 2 (Top–Bottom). Across Bed “B” the concentration is ≈2% for Case 1 and ≈1% for Case 2 (
Figure 6a). The marked differences in the horizontal spread of the contaminants in the three Cases can be appreciated from the contour plots for the Base Case (
Figure 6b), Case 1 (
Figure 6c) as well as Case 2 (
Figure 6d).
Figure 6.
Predicted contaminant (CT) concentration results for: (a) All three cases at height of 1.05 m along trajectory of release; and 2D horizontal axis contours for (b); Base Case (c) Case 1; and (d) Case 2.
Figure 6.
Predicted contaminant (CT) concentration results for: (a) All three cases at height of 1.05 m along trajectory of release; and 2D horizontal axis contours for (b); Base Case (c) Case 1; and (d) Case 2.
Contaminant removal efficiency (CRE) was estimated at assumed head position on all four beds as well as at standing height in the centre of the ward. The CRE for each position in the Base Case is significantly higher than at similar positions for Case 1 and Case 2 (
Table 1). Although the NPV in Case 2 (Top–Bottom) offers relatively better CRE values than Case 1 (Side–Side), both systems are significantly superior to the CRE of Base Case, by a minimum order of 34.4% (Bed C, in Case 1) and a maximum order of 80% (Bed D in Case 2).
The airflow characteristics the Top–Bottom and Side–Side duct arrangements differ marginally in airflow rates, but the differences in the direction of airflow are significant. The short ducts of both cases are observed to deliver fresh outdoor air into the space with an expected lateral throw in the same direction as the duct. In previous NPV studies [
23], such horizontal throw led to the ducts being offset backwards by 0.3 m from the desired point of discharge. This horizontal throw is also present in the multi-bed use of NPV where the discharged air from shorter ducts partially overshoots the bed (
Figure 7). An improvement was needed to channel the discharged air directly over the bed.
Table 1.
The contaminant removal efficiency (CRE) values computed at five locations for all three Cases in ADB ward.
Table 1.
The contaminant removal efficiency (CRE) values computed at five locations for all three Cases in ADB ward.
Cases | Bed-A | Bed-B | Bed-C | Bed-D | Ward Centre |
---|
Windows (Base Case) | 0.73 | 0.57 | 0.32 | 0.30 | 0.40 |
Side–Side NPV (Case 1) | 0.34 | 0.24 | 0.21 | 0.09 | 0.20 |
Top–Bottom NPV (Case 2) | 0.30 | 0.18 | 0.13 | 0.06 | 0.16 |
Difference between Base Case and Case 1 | 53.4% | 57.9% | 34.4% | 70.0% | 50.0% |
Difference between Base Case and Case 2 | 58.9% | 68.4% | 59.4% | 80.0% | 60.0% |
Figure 7.
Re-design of the Top–Bottom NPV duct to create vertical air discharge channels, shown as: (a) External facade view; and (b) Cross-sectional view.
Figure 7.
Re-design of the Top–Bottom NPV duct to create vertical air discharge channels, shown as: (a) External facade view; and (b) Cross-sectional view.