5 | VENTILATION SIMULATION: PHOENICS

5.1 Natural Ventilation

To reduce energy demand for ventilation, natural ventilation is desirable. In spring and autumn, when the weather conditions are favorable, natural ventilation is used in the office to provide fresh air. The following model and starting points are taken for the calculation:

office floor 28mx41m blockage as core of 15mx15m walls with a surface temperature of 23 °C ceiling with a surface temperature of 25 °C adiabatic floor people and computers with 18 W/m2 heat flux 3 degrees tilted operable windows openings with -0,25 Pa and 0,5 Pa pressure outdoor air temperature of 18 °C

To determine whether the input gives results which are comfortable for users, temperature, draught and air change rate are checked. Windows on the positive pressure side are located at the South-West, the negative pressure on the North-East. The grey area represents the core. The temperature is logically lower at the air inlet sides and higher at the outlet. the average temperature is 23 °C, well within the comfort range. Locally, at the side of the outlets, the temperatures are slightly higher. (Figure 56)

One office level with an occupancy of 50 people needs 25-50 m3/h of fresh air per person. This is equal to a flow rate of 1250-2500m3/h. The volume of the space is 923 m2 * 2.7 = 2492 m3. Hence the air change rate needed equals to 1250-2500/2492 = 0.5-1.0. With a minimum air change rate of 2, the entire office has sufficient fresh air. (Figure 57)

Figure56: Temperature spread across the office floor.

For people to not feel uncomfortable,a draught rate of less than 30% is required and preferable below 20%. With an average draught rate of 3.8%, windows can be opened comfortably. Only in the direct proximity of the windows, a slightly higher draught rate could be experienced logically .

When conditions are less favourable, like in winter and summer, mechanical ventilation will provide fresh air to the users. 14 Convectors of 0.1x3.2 m below the windows blow fresh air and 5 central outlets of 0.2x0.2m will transport stale air out of the space. This is visualized in the model below, where the convectors are marked blue and the outlets red.

Figure58: Draught rate.

5.2 Mechanical Ventilation

The summer situation is simulated. The walls and floor have the same characteristics, however now the ceiling is cooled with water and therefore has a temperature of 18. The air blown into the space by the convectors also has a temperature of 18 °C.

For temperature comfort not only the air temperature affects the result but even more important the radiant temperature T3 °C. (Figure 61)

Figure60: Temperature spread.

The air temperature is roughly equal to 28 °C which would normally be considered as too high to be comfortable. However the radiant temperature is 18°C. Because this means that the human body cools down sufficiently, the air temperature of 28 °C is acceptable. The draught rate has an average value of 4% and therefore the mechanical ventilation causes no discomfort in terms of draught. (Figure 62) Lastly the AGE of the air is checked. The maximum age of the air is 4095 seconds. The minimal air change rate is therefore 3600/4095 = 0.88. The average age of the air is 3557 seconds which equals an air change rate of 1. These values are within the range as described for the natural ventilation. (Figure 63)

Figure62: Draught rate.