Draughts caused by old-style ventilation systems are a common problem in schools built under the Building Schools for the Future (BSF) programme and before. This problem is reflected in the 2015 revision of TM 57 (Integrated School Design) and the forthcoming update to BB101 (Guidelines on Ventilation, Thermal Comfort and Indoor Air Quality in Schools).
Cold Draughts can be an issue in classrooms, offices and many other workplaces, where the ventilation system has not been designed to modern standards.
Addressing this issue whilst maintaining acceptable Indoor Air Quality (IAQ) requires a new generation of Smart Ventilation Units (SVUs), capable of delivering draught-free ventilation.
A draught can be simply described as unwanted local cooling, though quantifying the draught problem is less simple as there are different methods to be considered.
One approach is based in terms of the temperature difference (ΔT) between room air already in the occupied zone (To), and that of inlet air arriving at the occupied zone (Ta). The occupied zone is defined as the point at the centre of the space (e.g. classroom, office etc.), 1.4 metres above floor level.
To – Ta = ΔT
The greater the value of ΔT, the greater the potential for draught.
Clearly, the temperature at which air enters the space is crucial to what happens in the occupied zone. There must be sufficient time for the incoming air to be mixed thoroughly with room air at, say 21°C, to allow its temperature to be raised.
For example, if air were to enter the room at 16°C, it will begin falling as soon as it enters the room as there is no natural buoyancy. Thus, this incoming air plume will arrive at the occupied zone well before it has had a chance to increase its temperature to 21°C – resulting in an uncomfortable draught for the occupants (see Fig. 1).
Figure 1: Open windows
In several countries of similar latitude to the UK, draught is assessed in terms of ISO EN 7730 (Ergonomics of the thermal environment). This method is based on the concept of percentage Draught Rating (DR), which quantifies the proportion of room occupants that would feel discomfort on account of draught.
The allowable maximum value of DR is 15%, with any value greater than this being regarded as unacceptable. Figure 2 shows the curves for a DR of 15%, assuming normal turbulence intensity for classrooms of 40%.
Figure 2: Curves for DR = 15%
Two conclusions can be drawn from these curves:
- That the temperature of incoming air arriving at the occupied zone (x-axis) should be at least 19°C. If it is less than this, then higher values of DR come into play.
- That if the velocity of incoming air at the occupied zone is over 0.15 m/sec (y-axis), then the temperature of this air will need to be increased appreciably if higher DR values are to be avoided. E.g. with an air velocity of just 0.2 m/sec, the air temperature will need to be at least 26°C.
Conventional ventilation systems in the UK have difficulty in meeting either of these challenges when it comes to winter time operation – reinforcing the need for a new generation of ventilation systems – the SVU.
AirMaster Smart Ventilation Units (SVUs) address the draught problem in two stages:
- Providing close control of inlet air temperature.
- Ensuring sufficient mixing of inlet air and room air to make certain of the required temperature increase before the air arrives in the occupied zone.
Whatever value is chosen for inlet temperature (say 19°C) the ‘smart’ control of AirMaster Smart Ventilation Units (SVUs) ensures that this is maintained as a minimum value, regardless of external temperature.
If the inlet temperature rises above the set point, the automatic bypass damper opens sufficiently to allow a proportion of air to enter directly from the outside, without passing through the heat exchanger. Provided the external temperature is low enough to make a difference, the inlet temperature will fall back to set point.
Conversely, should the inlet air temperature fall below the set point, the inlet fan slows down automatically, whilst the exhaust fan speeds up. This results in a lower flow of cold air being warmed by an increased volume flow of warm air, thus producing an increase of inlet temperature.
Consequently, inlet temperature will be returned to its set point of 19°C (see figure 3).
Figure 3: If the target room temperature is 21ºC, then inlet set point would be say, 2º less = 19ºC. Inlet fresh air is tempered by a combination of heat recovery, automatic bypass and fan differential speed.
As temperatures increase in the summer, AirMaster units deploy night time cooling, an effective way to reduce overheating risk.
This smart temperature control mechanism enables close control of air temperature in classrooms and other spaces, removing a major cause of discomfort.
Using the Coanda effect to maximise mixing
To provide sufficient mixing time once the fresh air has entered the room, AirMaster Smart Ventilation Units (SVUs) use the Coanda effect to move the supply air across the ceiling for a significant throw of 6 – 8 metres, before falling away to the back of the space.
As it traverses the ceiling, the inlet air slows down and entrains room air, thus slowly increasing in temperature. By the time it is ready to detach from the ceiling and fall away at the back of the classroom, the incoming air has had the time to be warmed by the 2°C necessary for it to reach room temperature. This means that draught risk is eliminated (see figure 4).
Figure 4: Coanda air distribution
AirMaster SVUs ensure that higher velocity combined with proximity to the ceiling creates a situation where warmer room air is ‘entrained’ from below into the jet of inlet air. This means that the jet is ‘pushed’ towards the ceiling (the Coanda effect) and at the same time more air is set in motion, causing the velocity of the jet to fall simultaneously with rising temperature.
Ideally, the jet velocity falls to approximately 0.15 m/sec on reaching the end of the room. AirMaster SVUs ensure that the whole room is well ventilated and that temperature and air quality are evenly distributed. Draught Rating is kept within the 15% boundary.
Other key features of AirMaster SVUs for classrooms and other spaces: