Air Properties. Need to Know It, Before You Can Control It!
|Air properties? What is air in the first place? Let me see. Air is the most common gas in the atmosphere. But on our beloved earth, this most common gas is what we breathe in and out every day. That is our air.|
What’s it composed of?
|Since you and I, and your pet needs Oxygen for our correct body function – and not to say, staying alive, Oxygen is needed. However, Oxygen in 100% composition is toxic to our body. Therefore, a huge part of Nitrogen is present in our air.|
The percentage of Nitrogen gas in air is about 78%, whilst Oxygen takes approximately 21%. The 1% left, is shared by Carbon Dioxide, Argon, dust, water vapour, and a mixture of various gases.
That is the most important part of a few more air properties, that we need to keep in mind.
No air, no nothing. Even typing this is page is not possible.
Air properties and temperature:
“Why are you talking about temperatures now. Aren’t we supposed to be talking about air?”
Yes, we are supposed to be talking about air, but temperature has a big part to play in air properties.
Imagine your thermometer is dry
At this stage, the thermometer’s bulb, will only measure the air temperature, proportional to the kinetic energy of the air molecules.
If the kinetic energy of air molecules is high, then the temperature will be high. Temperature page explains in detail, about relationship between the kinetic energy, and the corresponding temperature.
However, this measurement has to be carried out in the absence of radiation and conduction from any other equipment within the space. Simply put, the temperature reading has to be influenced by air temperature only.
You must have got the clue already. Yes, this is the measurement needed for dry bulb temperature.
You can call it as, the actual air temperature.
However, our discussion in heat transfer section has stated that every matter above 0 Kelvin will have heat and this heat can be transferred to another body, with a lower temperature.
Therefore air, at specific temperatures above 0 Kelvin, will also contain heat, proportional to its mass and its specific heat capacity (c kJ/kg K-1).
Specific heat capacity of any matter varies with temperature, and pressure. The value is not constant.
This varying value of specific heat capacity will be multiplied with the temperature at the corresponding point,
To give us enthalpy
Enthalpy (h kJ/kg or Btu/pound), is the specific heat of a given matter, at a given temperature and pressure. This is one of the many air properties, which can give us the value of heat, per kilogram (or per pound) of subject matter.
In our case, the enthalpy is per kilogram, or per pound of dry air.
Please don’t be confused with specific heat capacity. Recapping,
- specific heat capacity is the heat, per mass, per absolute temperature,
- whereas enthalpy, is heat per mass only. Temperature has already been included in it
Air properties and its mass and volume:
Oh, my. Now we have another problem. How to calculate the mass?
Bring a scale. No, I’m just pulling some legs here.
Valid question you have asked me there? Almost caught me before I could get away.
Now, thanks to previous scientists, we have what we call as specific volume.
Specific volume is the volume of a given matter, at a given mass. The unit for v (specific volume) is m3/kg (or ft3/pound).
Essentially, it is the measure of matter’s density. The higher the value of specific volume, the higher the volume required, per mass of matter.
In other words, it is less dense. The opposite will happen for low specific volumes.
This particular property is normally used to measure the specific volumes of gases. Applications in liquids are quite limited. But, again, that’s a different story to this one.
Now, this part of few more air properties to come, can already be measured. We just have to measure the volume of air in a space, and voila, we’ll have mass already.
So, let’s see what air properties have we encountered. The dry bulb temperature, the enthalpy, and the specific volume.
Values for temperatures, enthalpy and specific volume, is enough for us to calculate one part of heat energy it contains. Keep reading.
Air properties and humidity:
Hang in there. Did you say temperatures? I thought there’s only one temperature
|In fact, there is another property of air that we need to consider, since we are on earth, and water is plenty. Thus, the wet bulb temperature exists. It is defined as the temperature of evaporation.|
We have to keep in our mind that, the air has heat and it is capable to be transferred to another body, at a lower temperature, especially to water molecules on the surface of river, sea, your drink, rain drops, and your perspiration.
Evaporation rings the bell.
Heat from air, will excite water molecules on the surface, and evaporate it. The evaporated water will be carried together with air. However, water vapour and air will still maintain its property.
The relationship goes like this. Warmer air is capable to carry more water vapour per mass of dry air, compared to cooler air. Its just common sense to us now, as higher heat may evaporate higher quantities of liquid.
Measurement of humidity (that is the amount of water vapour in dry air), can be done in several ways. We’ll begin with,
- Absolute humidity. This is defined as, the total mass of water vapour, per unit volume of air. The unit for this measurement is kg/m3 (or pounds/ft3) of air. This is simple.
- Specific humidity (w), however, is quite different. This is measured as the total mass of water vapour per unit mass of dry air.
This time, there is no unit, as the numerator and the denominator have the same unit. Divide kg, by another kg, you’ll get 1.
However, there are times when the units are represented as grams/kg of dry air. No difference really. It only makes chart and table reading easier, as the amount of water vapour contained in a given mass of air, is quite small.
If absolute humidity is measured as the mass per volume of air, this measures the mass per mass of dry air. The other name for specific humidity is, humidity ratio.
Specific humidity is more useful in calculating heat content of a given air, as mass has already been included in the unit.
We can absolutely, use values for absolute humidity for heat calculations, but we have to multiply it with our specific volume. It's possible, but time consuming.
- Continuing our discussion on Air properties. Next in line is the Relative humidity. Wow, a whole load of things in air properties.
Yes indeed. Relative humidity is the ratio of water vapour content in a given dry air volume, to the maximum allowable water vapour in that same dry air volume, at a given temperature. In other words, it is the ratio of absolute humidity, to the maximum humidity that the volume of dry air can handle.
I’ll explain further.
Each volume of air, at a given temperature has a specific heat. This specific heat is able to evaporate a specific amount of water vapour. The larger the temperature, and hence the heat, the more water vapour it can handle.
What if, you put that warm air, in a place where water is very scarce? Yes, it's capable of carrying an amount of water vapour, but the water is not there to be carried, in the first place.
This particular air property, is analogous to two person, with different strengths.
One is capable of carrying 10 stones, another is only 3 stones. But, you provide three stones each. Both will carry three stones indeed, but the stronger one is under utilised, whereas the weaker one, carries as much as it can carry. The utilisation is 30% and 100% respectively.
Similar to our case, the measurement is in percentage, no units. If relative humidity is close to 0%, then it is capable of carrying a lot more water vapour. If it is close to 100%, then it is capable of carrying just a few more water vapour.
- I did mention about relative humidity. Did I mention about the maximum relative humidity?. Not yet. Behold.
It is 100%. Nothing more.
Reason being, it is limited by the maximum heat of dry air. Dry air may only carry the amount of water vapour as much as the amount it can possibly evaporate.
What happens if we add in more water vapour?
Let me ask you, what happens if a London’s double decker is already full with passengers? It can accommodate no more passengers.
Similarly, air will not be able to accommodate more water vapour beyond 100% relative humidity, or saturation point.
It will ‘unload’ the excess water vapour, in form of condensation.
This is the air property, that we know as dew point, or saturation, and this happens at the saturation temperature.
I have not explained about how to measure wet bulb temperature, have I?
It is all, for a reason. We have to understand the concept of relative humidity.
So, measuring wet bulb temperature would be as easy as eating a pie now.
You just have to modify the normal thermometer and cover it with a wick pre-soaked in water.
Next, you can either whirl the thermometer (with suitable string attached), or shake the thermometer.
Why so? Doing so, you will encourage evaporation. If the air is nearly saturated, then the amount of water evaporated from the wick, will be low. Hence, the heat removed from the wick is low. Thus, the temperature will drop slightly.
Yes, wet bulb temperature is indeed dependent on the relative humidity. The higher the relative humidity or air, the higher the wet bulb temperature.
Wet bulb and dry bulb temperatures will have the same number if, the relative humidity is 100%. This means, the air could no longer support any more evaporation.
In other words, difference between wet bulb temperature and dry bulb temperature is the measure of evaporative ability of air.
This is why, you’ll feel cooler at low humidity places, even if the dry bulb temperature is high, as your perspiration will quickly evaporate, into thin air…
That’s a whole load on Air properties section.
Air properties and heat:
However, we still have to talk about a few more issues, to make this discussion, a fruitful one, and your deeper understanding into air conditioner operation.
We’ll talk about sensible and latent heat of air.
- Sensible heat, is defined as the heat of dry air. It is the "dry heat" from anything that has the temperature above 0 Kelvin
- Whereas latent heat is the heat contained in the water vapour, carried by the air. The higher the water vapour content, the higher the latent heat
Combine those two, and you’ll have total heat of air, at a given temperature, for a given air mass.
Of course, evaporation temperature, enthalpy, and specific water volume of and within air, is dependent on the pressure at which the air is subjected.
So, where am I getting at with all those explanation?
And that somewhere is to say that, the study of air property is also known as psychrometry. The values of different temperatures, enthalpies, humidity, and specific volumes, are used to form a chart, known as the psychrometric chart.
Study of air properties, could not be done with theoretical calculations alone.
We will need empirical data, in tabular and graphical form, to design and control air conditioner operations, that suits your comfort!
Enough on air properties?
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