What are the limiting climatic conditions of a construction site?
Climatic conditions to be respected
The thermal equilibrium of the human body is ensured by physiological regulation mechanisms that keep the internal temperature constant despite the variation of the main parameters of the heat balance, i.e. :
- Heat exchanged with the ambient air by convection,
- Heat received by radiation from walls and machines,
- Heat evacuated by evaporation of sweat,
- Internal release of heat linked to the power of the work carried out.
As the possibilities of physiological mechanisms are limited, it is necessary to ensure climatic conditions compatible with the work to be carried out.
The parameters that influence climatic conditions and possible policy options to improve them are given below:
- Convective exchanges depend on air temperature and air speed.
- Exchanges by evaporation of sweat depend on the value of the :
- Dry temperature ts in °C,
- Wet bulb temperature th in °C
- Air velocity v in m/s.
Acceptable boundary conditions can be evaluated by the resulting temperature recommended by various standards or recommendations.
tr = 0.7*th + 0.3*ts – v
In any case, the physical parameters necessary to evaluate the climatic conditions of a construction site are :
- ts in °C
- th in °C
- v in m/s
Representation of the parameters characterising the state of the air
The psychrometric diagram
The state of the air can be characterized by :
- Its temperature
- Its humidity
The temperature can be measured by an ordinary thermometer or by the dry bulb thermometer of the psychrometer.
This temperature is called the dry temperature.
Humidity can be assessed in several ways:
- Absolute humidity x or mass of water vapour in kg per kg of dry air,
- Relative humidity F (%) (ratio of the vapour pressure in the mixture to the saturation pressure of water vapour at the same temperature),
- Temperature of the wet bulb of the psychrometer called wet wet temperature th (°C)
It is convenient to use the psychrometric diagram on which one can easily read these different values
The following values correspond to a point in the diagram:
- Dry temperature ts (right in blue)
- Wet bulb temperature th (straight lines in green)
- Relative humidity (%) (black curve)
- Absolute humidity x (g/kg dry air) (black vertical line)
- Enthalpy J (kcal/kg dry air) (straight red)
Knowledge of any two of these values is sufficient to determine the other air state characteristics.
The SUB’ROCA engineers are at your disposal to assist and advise you in the dimensioning of your ventilation installations. Whether your project concerns a meshed network in an underground mine or a complex digging project in another environment, our teams have the technology to calculate the air requirements and performance necessary to ensure ventilation that complies with the health and safety of personnel.