DRAUGHT (British English) Draft (American English) Indoor Climate -course Marianna Luoma

Discussion with your neighbour What is draught? What is the definition for draught? What causes draught? How could draught be avoided?

Causes for draught Badly air-tightened doors and windows Ventilation, for example fresh air intake devices Location of ventilation devices, wrong location of supply air devices (next slide) Location of furniture Cool, high window surface or wall surface which does not have a radiator under it What other causes?

Wrong location of supply air devices or too cool temperature and too high velocity

Definition Sensation of draught is caused by the local cooling of skin, which happens too suddenly Thus, it is caused by heat transfer – not by the irritation of air movement on the skin

Sensibility to draught Different parts of the body react differently to similar conditions Neck is the most sensible part of the body for draught Usually the air flow coming towards human body is tolerated better than the air flow coming from the back of the human body Things like what we are doing, how we are dressed and how we like the thermal conditions, affect our feeling of draught

Thermal conditions Operative temperature for normal office building is 21-24 ºC (THE FOLLOWING SLIDE) When the temperature is above this temperature or close to 24 ºC, our body tolerates higher air velocities compared to the situation when the temperature is closer to 21 ºC Sensibility to draught increases when the temperature is below the optimal operative temperature It is typical for Finland that indoor air temperature is close to 21 ºC which means that there is a high possibility for draught ….

Optimal thermal conditions for office work Metabolic rate in office work is 1,2 met Winter: Insulation of clothing is 1 clo Optimal operative temperature is 21,5 °C Summer: Insulation of clothing is 0,5 clo Optimal operative temperature is 24,5 °C 16.5.2018

Essential environmental factors Air temperature Air velocity Fluctuation of air velocity In the simpliest draught criteria the percentage of dissatisfied persons depends on air temperature and air velocity

Fluctuation of air velocity It is expressed by the turbulence intensity … which is the standard deviation of air velocity devided by the mean air velocity The greater the turbulence intensity, the greater the possibility for draught

Air velocity in a room, measured by direction-independent velocity sensor Measured by Hannu Koskela, Occupational Health Institute, Finland

Percentage of persons dissatisfied People are different and the sensibility for draught varies from person to person Can be expressed as the percentage of dissatisfied to certain air flow conditions in the occupied zone The amount of dissatisfied persons depends also on the air temperature The air movement towards the neck is felt more uncomfortable than the air movement towards the face or the air movement coming from upwards Feet are not as sensitive to draught

Draft rate DR=(0,37 v Tu+3,14)(34-Ti)(v-0,05) 0,62 DR = Draft rate, % Ti = Air temperature, ºC Tu = Turbulence rate of air flow = the standard deviation of the air movement divided by the mean air velocity, % V = Air velocity, m/s The formula is based on a set of experiments, in which the air jet is blown towards the necks of persons that are in optimal operative temperature. (150 persons, 3 minutes averages used in calculation of air velocities ISO 7730)

The effect of thermal radiation when sitting by a cool window Besides the air temperature and air movement, heat transfer is affected also by thermal radiation If the heat transfer caused by the air movement and by the cool window are appearing at the same side of the body, the likelihood to draught is obvious Especially harmful are the air movements through the window frames and leakages through the walls, that are appearing on the same side of the body

Local draught risk in an office room, cooling load 100 W/m2 Simulation by Hannu Koskela, Occupational Health Institute, Finland

Local thermal discomfort The maintaining of the thermal balance of the body does not guarantee the feeling of thermal comfort Local thermal discomfort can be caused by : Draught Vertical air temperature difference Too warm or too cool floor surface temperature Radiation asymmetry … thus the thermal conditions in an occupied space have to be uniform

Different draught criteria D2: Draught curves Draught rate in this PowerPoint file Third criteria (shown in a separate copy)