In recent years, fog engineering has been increasingly applied in landscape design, humidity control, and temperature regulation. And it is becoming increasingly close to people's lives and playing an increasingly important role. In view of this, we analyzed and studied the principle and practical application of the spray system.
Firstly, conduct an analysis of human comfort factors
In our daily lives, we may notice that the temperature values in the weather forecast may sometimes feel inconsistent with the actual temperature sensation of the human body. In fact, this is because there is a difference between "perceived temperature" and temperature. The human body's perception of heat and cold (also known as "perceived temperature") is influenced by a combination of many meteorological factors. Among them, temperature has the greatest impact on the human body's sense of warmth and coldness, and it is also related to meteorological factors such as atmospheric humidity and wind.
1、 The influence of meteorological conditions on human thermal sensation
Meteorological conditions affect different parts of the human body through the sensory organs. As the weather changes, the comfort sensation of the human body also changes. The meteorological elements that have a significant impact on human comfort sensation are mainly temperature, humidity, and wind speed.
Ø 1.1 The impact of temperature on human comfort perception
Human beings are homeothermic animals, and in order to maintain a balance between body temperature and external temperature (T), the human body must maintain coordination with the ambient temperature through heat dissipation and production. When T>28 ℃, the human body releases heat through sweating, which in turn causes the loss of salt in the body. The process of heat dissipation can cause peripheral blood vessel dilation, thereby increasing the burden on the heart. High temperatures can also reduce certain functions of advanced neural activity in humans, such as attention, precision, motor coordination, and reaction speed. When T<17 ℃, the human body maintains a constant body temperature by generating heat or reducing heat dissipation. Trembling is a way of generating heat by increasing muscle activity; Adding clothes can reduce the loss of heat.
Ø 1.2 The impact of humidity on human comfort perception
Humidity affects human thermal metabolism and water salt metabolism: as humidity increases, it becomes more difficult for sweat to evaporate, hindering heat dissipation; When the humidity decreases, the lips and upper respiratory tract become dry, and a hard skin layer also forms on the skin, hindering evaporation; In addition, when human tissues become dry, their thermal conductivity greatly decreases.
Ø 1.3 The impact of wind on human comfort perception
Wind affects human thermal metabolism: When the temperature is above 36 ℃, the skin temperature of the human body rises due to the influence of wind, sweat evaporation is strengthened, a large amount of water is lost, and temperature regulation is disrupted; When the temperature is low, wind enhances the conduction and convection of atmospheric heat, thereby accelerating the loss of body heat.
2、 Calculation of perceived temperature
Among the three main meteorological factors that affect human comfort, temperature has the most significant effect, followed by relative humidity and wind. Therefore, when designing the calculation formula for perceived temperature, in addition to focusing on temperature factors, the comprehensive effect of meteorological factors such as humidity and wind is also considered, that is, the human body's comprehensive perception of the surrounding environmental conditions.
Sensory temperature=37- (37- t)/(0.68-0.0014 * h+1/A) -0.29 * t * (1-h/100)
t:  Temperature (℃)
h:  Humidity (%)
v:  Wind speed (m/s)
A: 1.76 + 1.4*(v**0.75)
In summer, people are more sensitive to the highest temperature of the day and the temperature difference between the highest and lowest temperatures. The smaller the temperature difference in a day, the more "stuffy" it feels, while the opposite is more comfortable.
When the air temperature is above 32 degrees Celsius; Humidity exceeding 80%; In environments with wind speeds below level 2, people generally feel stuffy and unbearable, and even experience the phenomenon of being caught off guard.
Conclusion:
1. The main factors affecting human comfort are temperature, humidity, and wind speed.
2. The evaporation of water from mist spray can lower the ambient temperature, making the human body feel comfortable.
3. When the air is relatively dry, mist spraying can increase the humidity of the ambient air and improve human comfort; When the air temperature and humidity are high, using only mist spray is not suitable.
4. Wind can accelerate the exchange of heat, significantly improving the cooling effect of mist spray.
After analyzing the factors affecting human comfort, let's talk about the temperature regulation of the fog system.
The experiment shows that:
When the air humidity is lower than 70%, the extreme maximum temperature difference between the spray area and the direct sunlight area is larger (generally up to 5 ℃~15 ℃), and the spray effect is obvious; The extreme temperature difference between the spray area and the direct sunlight area is not large when the air humidity is higher than 70%, and the spray effect is poor. At this time, adding wind power can improve the cooling effect.
The cooling range of the spray area will be different from far to near, and the humidity will also increase from far to near. The temperature difference in the direct area is mainly reflected in the changes of cloud layers and surface media. Grass and shrubs have an average temperature 2 ℃ lower than paved ground; The average temperature difference between the shaded area and the direct area is 2-3 ℃; The average temperature difference between the detection area near the water surface and the direct radiation area is 3-5 ℃.
For places with high air humidity, mist spray cooling not only has a weak effect but also increases air humidity, which in turn increases human discomfort. It is recommended to increase the wind source appropriately, and the specific implementation should be coordinated with landscape design.
It is suggested that the system should be able to detect air humidity and automatically adjust the operation time and interval of the mist spray system for cooling calculation
Calculation process
1、 Air releases heat (measured in 1 cubic meter of air): Q=mcΔt
Q： quantity of heat
m： The mass of air per cubic meter (1.21 kilograms at 0 degrees Celsius and 1 atmosphere)
C： Specific heat capacity of air (with a value of 1.006)
Δt： Temperature difference (5 degrees Celsius)
Calculated as: 6.0863 kilojoules
That is, a decrease of 5 degrees Celsius per cubic meter of air temperature requires a heat release of 6.0863 kilojoules.
2、 Water mist absorbs heat (calculate the amount of water required for water mist to absorb 6.0863 kilojoules of heat from 40 degrees Celsius to 100 degrees Celsius) Q=mc Δ t
Q： Heat (6.0863 kilojoules)
m： The quality of water
C： Specific heat capacity of water (value 4.19)
Δt： Temperature difference (water mist changes from 40 degrees Celsius to 100 degrees Celsius, which is 60 degrees Celsius)
Calculated as: 0.0242 kg
That is, for every 5 degrees Celsius decrease in air temperature per cubic meter, 0.0242 kilograms (24.2 liters) of water need to be evaporated.
3、 When the temperature drops by 5 degrees Celsius with a unit of 1000 square meters and a volume of 5000 cubic meters (with a spatial height of 5 meters),
The system flow rate is 24.2 milliliters per minute x 5000=121 liters per minute. The general system is set to start every 3 minutes for 60 seconds.
Using precision ceramic nozzles, when the system pressure is 70 kg/cm2, the flow rate of a single nozzle is 57 ml/min,
Therefore, approximately 2100 nozzles should be installed within a 1000 square meter area.
Considering the requirements for system stability and flexibility, the system can be divided into several sets of units for operation based on various environmental conditions. According to different cooling requirements, multiple configurations of the system are calculated. In practical applications, customers can choose based on the actual situation on site, investment budget, and main functional requirements.
After years of designing and constructing mist spraying projects, I have some insights. Is mist spraying suitable for all occasions? No, areas with poor air mobility and severe pollution are not suitable.