Dewpoint

During the warmer months, we can use the “Dew Point” as useful and interesting discomfort index. This particular weather element gives us an idea of how comfortable or uncomfortable we feel within the framework of hot and humid weather of the summer season. This information can be used at any time in our daily lives.

The dew point is the value that should lower the air temperature for the existing water vapor begins to condense.

The dew point can be calculated directly from the data of temperature and relative humidity existing in a given time. These data may come from weather reports issued radially, or registered by instruments. These pages offer readers the table to easily calculate the “dew point”.

Here’s an example. If on a given day, we have our 26th city in temperature and 60% relative humidity, the corresponding dew point (according to the table) is 18. But what are these 18 degrees? Well, if the zone air is rapidly cooled, and the temperature drops to 26 degrees today until the 18th of “dew point” (ie, a decrease of 8 degrees in a few minutes), happen two consecutive weather. First, tiny droplets of liquid water (dew) on all smooth surfaces found outdoors will form. These droplets of dew are those that give the name to “dew point”. Then the condensation will occur also in the air, forming water droplets innumerable suspension, which form a mist.

Therefore, the dew point is the temperature at which a cooling air, mist formation begins, and dew on the objects.

Humans have the physiological ability to maintain constant 36 ° (or 37) to the internal temperature of the body. Simultaneously, the temperature of the skin is usually exposed to the outside of 32 degrees (so long as it is not exposed to sunlight).

We emphasize that a person “feels cold” when your skin temperature drops below 32 degrees. Conversely, the sensation of heat increases when the temperature of the skin begins to overcome the thermal value.

When there is no wind and air temperatures are high, our body has a natural cooling process, tending to prevent heat buildup in the body. The cooling mechanism starts its operation, when the skin begins to perspire. Then, by evaporation of water droplets which constitute the sweat, the skin and the entire body is cooled and the excess heat lost.

Well so what role does our “dew point”?

Meteorologists and doctors have noted that the dew point is very important to measure the ease or difficulty with which the skin sags heat outside the body has too much weather index.

In general, we all say that there is “heavy weather” or “uncomfortable” when we feel suffocated by the heat and humidity. That particular sensation occurs especially in environments with elevated dew point (20 ° or more). These are situations in which, in the absence of wind or air currents, the organism is difficult or impossible to be cooled by the evaporation of perspiration.

On an estimated basis, it has been found that when the air dew point is between 8 and 13th, we experience a high sense of comfort, especially with temperatures of 20 to 26 ° in the shade and in a place without wind .. Son these the most conducive for normal body cooling and this is how our body can best meet their specific functions all weather conditions.

The ideal starting point for human life is the dew 10th. This value exists in the air, for example 30 ° in temperature and 30% humidity, and also with temperature 20 ° and humidity 55% (note Table). Interestingly, when the dew point is 10 °, the actual amount of moisture in the air is 7.5 grams of water vapor per cubic meter of air (at normal atmospheric pressure).

When the room air has a dew point of 20 to 25 °, perspiration is copious people. Then, the discomfort is increasing, and it is said that “time is very heavy.” Discomfort or “heaviness” weather because sweat evaporates with difficulty, or not evaporation and therefore body can not effectively cool a typical value at which starts to become noticeable environmental discomfort is for a value 20 ° in the dew point. This value e (for example) under the following weather conditions.

1) An Evening with 34th temperature and 45% humidity.

2) One night with 26th and 70% humidity.

3) A temperature morning with 22 degrees and 90% humidity.

When our region gets very hot and humid air from the south and center de¡ Brazil and the Argentine North-East, it is in the form of winds North, North East or North West. Under these extreme weather conditions, the dew point can reach exceptional value of 24 °. In this brand up for example in a 36th afternoon sing temperature and 50% humidity; also on a night with 30 and 72% humidity, or in a dawn with 25 and 95 humidity.

Schismatic These conditions often occur in some summer days in the area of Buenos Aires, River Plate and southern coastal waterway.

Once we know the dew point (obtained by means of the table) we can use the box climatic comfort feeling.

Wind chill

As is known, the temperature of the outside air is not always a safe and reliable indicator to determine the cold that a person can feel, if exposed outdoors. There are other meteorological parameters influencing such as wind speed, radiation and humidity. The windchill term is used to describe the lower temperature that a human being feels as a result of the combination of  temperature, humidity and wind in summer.

The wind then removes the layer of air surrounding skin, which serves to reduce the feeling of heat as long as the temperature does not exceed the skin temperature (32 ° C).

The most effective way for the body to lose heat is sweating. Perspiration evaporates consuming gives our body heat. When humidity is high, evaporation is less and therefore increases the wind chill. While, when the humidity is low, evaporation increases and therefore our body loses heat and reduces our windchill.

The dry bulb temperature, or just dry temperature, air temperature measured without considering environmental factors such as radiation, humidity and air movement, which have the potential to significantly affect thermal sensation.

One of the most used instruments for measuring the dry bulb temperature is the mercury thermometer, which consists of a thin glass tube whose base, bulb-shaped, containing a metal deposit of the slurry.The assembly is hermetically sealed to maintain a partial vacuum therein. Thanks to its large capacity expansion mercury rises in the tube as the temperature increases, enabling the measurement of the latter by a graduated scale. The mercury thermometer is exposed directly to the air but protected from moisture and sunlight. Because mercury is bright white on the other hand, is considered reflective enough to avoid almost completely absorbing heat radiated by the elements of the environment. When all these conditions are met, the mercury thermometer accurately indicates relatively dry air temperature.

Due to the toxicity of mercury, the European Union recently banned the manufacture of thermometers with this metal. Instead the use of liquids with properties similar expansion, such as alcohol or ether is encouraged. On the other hand it is important to mention that there are other types of thermometers to measure the dry bulb temperature:

• Resistance thermometer. Uses materials whose electrical resistance changes with temperature, generally transient metal oxides such as magnesium, cobalt, copper or nickel.
• Gas thermometer. It is based on the pressure variations of certain gases, according to its temperature.
• Bimetallic strip thermometer. Rely on differential expansion due to temperature, wrapped with two sheets of very different expansion coefficients.
• Thermocouple. Use the electromotive force generated by increasing the temperature of the welding of two different metals.
• Digital thermometer. It includes a built-in electric circuit, which is sensitive to temperature changes microchip.

Wet-Bulb temperature :

Wet bulb temperature

If we have to study evaporative cooling, psychrometry, thermodynamics, or just work in a place where there are cooling towers, the wet bulb temperature and dry bulb. Often confused and not clearly understand the concept.
The wet bulb temperature is a temperature measurement type physical properties of a system consisting of a mixture of gas and vapor (usually air and water vapor). The wet bulb temperature is the lowest temperature that can be reached using the evaporation of water only

Generally, the wet-bulb temperature is measured by a standard thermometer located under the shade, but with its bulb wrapped in a cotton wick (a porous, absorbent or similar material) whose bottom is submerged in a container of water. Using a system fan is exposed to a constant air flow of approximately 3 m / s, which causes the water rises by capillary action along the wick to evaporate relatively easily.Evaporation processes heat absorption , which causes the bulb to cool down gradually. Bulb temperature is lowered so that the air that surrounds (contained in the pores of the wick) is completely saturated. A value representing the wet-bulb temperature is then obtained in the thermometer.

when You taking it to everyday things, is the temperature that we feel when we have wet skin and expose ourselves to moving air.Temperaturaes Such an indicator the amount of moisture that is present in the air.
Then the wet bulb temperature is the minimum temperature that can be reached by evaporative cooling. For a given air at a known pressure and dry bulb temperature (normal temperature we measure with a thermometer), the wet bulb temperature corresponding to relative humidity, dew point, and other properties.
 When air is below the saturation level, that is, when the relative humidity is below 100%, the wet bulb temperature is always lower than the dry bulb temperature. In this sense, the wet bulb temperature expressed indirectly the humidity , because while the lower the relative humidity the water easily evaporates and more intense are the processes of heat loss. This also means that in dry climates the relative difference between the simultaneous temperature dry bulb and wet bulb is always greater than in humid climates. On the other hand, the difference between the two temperatures is an indication of the potential of evaporative cooling systems: if the difference is big these systems are usually quite efficient if its efficiency is very small decreases drastically.
The human body uses this principle (evaporative cooling) through perspiration. thats why high humidity will cause discomfort for human body .

Wet-bulb Temperature have direct relation with the relative humidity please check Psychrometric Chart

Psychrometric Chart

1. Evaporative Coolers:

There is many names for this coolers (outdoor air coolers, Evaporative air coolers, swamp coolers or desert coolers)

Wikipedia.org

Direct Evaporative coolers (DEC)

How it works: Main stream of air passing throw wet evaporative media

Advantages:  Low-cost, easy to maintain, hug amount of air flow, very low power consumption

Disadvantages: very poor performance in humid climate

Theoretical efficiency: 90%

Practical Efficiency 75-85%

Indirect evaporative coolers (IEC)

How it works: first stream of air passing throw wet media cooling down the water, second  stream (supply air) will be pre-cooled with indirect ouch with the cold water (or heat exchanger)

Advantages:  little tricky maintain, no humidity added to supply air

Disadvantages: very poor performance in humid climate, less efficient than direct evaporative coolers

Theoretical efficiency: 70%

Practical Efficiency 50-60%

*Coolerado Claims Efficiency of 110% using Mass heat exchanger technology

Indirect-direct evaporative coolers (IDEC)

How it works: first stream of air passing throw wet media cooling down the water, second  stream (supply air) will be pre-cooled with indirect touch with the cold water (or heat exchanger) then it will pass throw the second stage evaporative media

Advantages:  very high efficiency, good performance in humid climate

Disadvantages: little tricky to maintain, smaller amount of air flow, expensive.

Theoretical efficiency: 135%

practical Efficiency 112-127%

Hybrid evaporative coolers:

How it works: Main stream of air passing throw wet evaporative media then via DX (or chilled water coil)

Advantages: ability to control the supply air temperature

Disadvantages: power consumption is higher, but still much less than traditional compressor based systems

2. Fan and pad systems:

How it works: widely applied in poultry and green houses , evaporative media will be installed on one side of the greenhouse and fans on the opposite side, this fans will generate suction pressure sucking air via the wet pads

Advantages:  Low-cost, easy to maintain, hug amount of air flow, very low power consumption

Disadvantages: very poor performance in humid climate

Theoretical efficiency: 80%

Practical Efficiency 60-70%

3. High pressure Misting system (water fog/fogging system) :

It’s another type of evaporative cooling, the system is based on very high pressure 70 bar water pump (while normal water tap pressure is 3 bar max) spraying water via very small orifice nozzles (0.1-0.3MM)

this will produce very fine drops of water (5-15 micron) it will look like fog and evaporate very fast while traveling in air at distance of 1-2 meters max

Advantages:  very high efficiency almost achieving the wet-bulb temp

Disadvantages: very expensive, nozzles get blocked easily, need very clean treated water.

Note: some systems on the internet valuable for couple of hindered of dollars promoted as “high pressure misting system” most of them have pressure less than 5-10 bar, they will just spray water and get everything wet , high pressure misting systems with just 10-15 nozzles prices start with minimum 1500$ +

Water pressure 50-70 bar

Theoretical efficiency: 100%*

Practical Efficiency 85-95%*

*Based on system sprays 5 milliliter of water mist for every cubic meter of air

*Water pressure 60 Bar

*nozzle orifice 0.25mm

Cooling towers:

Theoretical efficiency: 80%

Real life Efficiency 50-60%

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How evaporative cooling works:

When warm air moves over water surface some water will start evaporation this action of transforming water to vapor will observe energy (the heat from the warm air) , this heat which taken from remaining amount of water in the liquid state, resulting in a cooler water.

The common mistake people think that the air get cooler, but this is not true, the remaining water after evaporation effect getting cooler,The cooler air is the result of the warm air when its interact with the cold water while passing throw that cold water (simple heat exchange action)

Evaporation of 1 liter of water observes about 2300 BTU  or 674 watt of thermal energy (lower temperature)

So basically the amount of water can be evaporated per hour can be converted to BTUs of cooling

Photo by hydrocool.co, how evaporative outdoor coolers work

Evaporative cooling system capacity:

The cooling capacity of evaporative cooling can’t be calculated as fixed formula, it really all about wet-bulb depression, as we know wet-bulb temperature related current ambient temperature and humidity level which keep changing during day and night hours

Wet-bulb Depression: the difference in temperature between the ambient air temperature (Ta) and the wet-bulb temperature (Tw)

Supply Air temperature = Intake air temperature – (Wet-bulb Depression x evaporative cooling system Efficiency(%)

Generally the cooling capacity can be calculated with following formula:

Cooling capacity [kW] = Air mass rate [kg/s] x  air specific heat capacity [kJ/kg K] x air temperature change [K]

 Evaporative cooling Efficiency:

The evaporative cooling is the most efficient cooling system consumes about 15-25% power compared to traditional compressor based cooling system to provide the same amount of cooling

Evaporative cooling capacity usually calculated as the percentage of wet bulb temperature can be achieved (wet-bulb efficiency), for example if the system able to supply the temperature of the wet-bulb that means the system efficiency is 100%

wet-bulb efficiency =( ( intake air temperature – Supply air temperature ) / ( intake air temperature – wet-bulb air temperature ) ) x 100

EER and COP rating also can be applied into the evaporative cooling system, but can be calculated only according to known design condition the ambient temperature, humidity level, and location elevation

EER= Cooling capacity (Btu/h)/ Total unit electrical power (Watt)

COP= heat removed  / electricity consumed

Evaporative cooling in history:

Civilizations throughout the ages have found ingenious ways to combat the heat in their region.

The Arabs hung wet blankets over doors and windows to benefit from evaporative cooling

also the windcatcher (Malakef) was used in ancient Egypt , and Persian Gulf thousands of years ago in the form of wind shafts on the roof, which caught the wind, passed it over subterranean water in a tunnel and discharged the cooled air into the building.

Examples of evaporative cooling in nature:

When you sweat, this is a very clear example for evaporative cooling. When you do any effort you sweat, your skin will drop in temperature.

A vulture can urinate on itself, thereby increasing the moisture on its legs. The vulture is cooled as the urine evaporates. Birds can flutter their neck region to bring air quickly in and out of the mouth, which increases evaporative cooling. Animals such as elephants can also increase evaporative cooling by getting their bodies wet.

click here for : Evaporative Cooling  Types

Evaporation, the process by which an element or compound transitions from its liquid state to its gaseous state below the temperature at which it boils; in particular, the process by which liquid water enters the atmosphere as water vapor. Evaporation, mostly from the sea and from vegetation, replenishes the humidity of the air. It is an important part of the exchange of energy in the Earth-atmosphere system that produces atmospheric motion, and therefore weather and climate. The rate of evaporation depends on the temperature difference between the evaporating surface and the air, the relative humidity, and wind.

• Evaporation is one kind of vaporization.
• Vaporization is a phase transition from the liquid or solid phase to vapor (Boiling,evaporation,Sublimation).
• Vaporization is the opposite of condensation.

Phase_change, wikipedia.org

We see condensation everywhere around us in life, and you notice that your cold drink glass or can get wet, also when you see your car glass every morning foggy wet

so condensation is typically a cold surface pulling liquid out of vapor this transformation of a vapor (or Gas) to a liquid

• Boiling transforming liquid to vapor (or Gas) will observe energy
•  Evaporation transforming liquid to vapor (or Gas) will observe energy

*Energy definition here explains the heat

To make water transforming from liquid status to vapor you can apply direct heat by boiling water, but in here is another way to evaporate the water by using high-velocity air flow on the water surface

This process we call it “Evaporative cooling”, and this is how evaporative outdoor air coolers work

” The process to convert one liter of water to vapor you need about 2300 BTU “

References

1.  Britannica: Evaporation
2.  PBS American Experience “Mike” Test
3. Wikipedia