Phase and Temperature Change

Phase change of substance can be obtained by heating or cooling. When we heat the substance the temperature will rise until melting point or boil temperature. For example, when we heat ice at below 0^oC (let’s say -5^oC), it will not melt immediately, but temperature will rise until melting point temperature (0^oC) first. Then, ice will melt while the temperature stays at  0^oC. After all of water melt, then temperature will rise again until boiled temperature (100 ^oC). Water temperature will stays at 100 ^oC, when water is boiled. Then water phase will change to vapor. After all become vapor the temperature will increase again.

For simplifying these phenomena:

•           Rising temperature until melting point temperature. The phase is solid
•           Phase change from solid to liquid. This phenomenon doesn’t change temperature
•           Rising temperature from melting point temperature to boiling point temperature. The phase is liquid
•           Phase change from liquid to gas. This phenomenon doesn’t change temperature.
•           Rising temperature of gas.

The explanation above is heating process. Cooling process is similar but the opposite.

For calculate heat needed to removed, the general formula are:

•           Every Rising temperature: Q = m.c.∆T
•       Where Q: heat, m: specific heat of the substance, ∆T: temperature differences
•          Change phase from solid to liquid: Q=m.L
•       Where L: heat fusion of the substance
•          Change phase from liquid to gas: Q = m.U 
•       Where U: heat of vaporization

These conditions are approach, in actual we may find that water will evaporate although the temperature is not at boiling temperature. Also the other phenomena such as melting.

Why is air in air-conditioned room dry?

When we are in the room or car which is air conditioned, we feel air surrounding us is dry. How come?

Air contains several kinds of gases such as nitrogen, oxygen, carbon dioxide, etc. Besides that air also contains moisture. If moisture in the air is high, then it makes air becomes more humid. The amount of moister in the air is called humidity. Humidity can be quantified as relative humidity and humidity ratio.

 

Latent heat is heat which doesn’t change the temperature. This heat only change phase. Moister inside air causes latent heat becomes high. More moisture contained in the air will cause higher latent heat because heat removed from air is not only used to decrease temperature, but also condense moisture.

Air-conditioner cool the air and it causes air temperature decrease until dew point temperature, if the air is still cooled, temperature will decrease followed condensation of moisture. Dew point is the temperature when moisture is saturated.

Room air is sucked to the air conditioner and blown again to the room. Before it is blown again, air is cooled by air-conditioner. In other words air is circulated and cooled by air-conditioner. The component of air-conditioner which cool the air is evaporator, thus air which is close to evaporator is colder than other and achieve dew point temperature earlier, then start to produce condensate. That is why water is often (condensate) found at air conditioner.

Since moisture in the air condenses at evaporator, the amount moisture is being reduced and resulting dry air. 

Cooling Load Estimation

Design and selection HVAC System based on determination cooling load and heating load. There are several methods to determine cooling load and heating load. The most common method is cooling load temperature differences/Solar cooling load/Cooling load factor (CLTD/SCL/CLF) method. This method developed by ASHRAE along ago.

Cooling load is heat that must be removed from space or room or zone want to be cooled. There are several components must be considered.

1. Conduction from building envelope. Heat is transferred from higher temperature to lower temperature. If outside temperature is higher than inside, so spontaneously heat enter inside space. Heat can enter the space through wall, roof, skylight, or windows.
2. Radiation from sun through window and sky light
3. Conduction through ceiling, partition walls and floor.
4. Internal heat gain such as people, light, and equipment in the room.
5.  Heat gain from infiltrating from outside through door, window, etc. 
6. Cooling load calculation also needs to consider the ventilation heat gain and heat generated by fans in system.

There are latent heat and sensible. Sensible heat is heat which causes changes temperature. Besides that, Latent heat is heat which is not makes temperature difference. This heat cannot be measured by thermometer because no difference temperature occurs. This heat only causes phase change. The simple explanation about latent heat is when water is being heated and boiled, its temperature is stay at 100 ^oC although it is still being heated. This heat is used to change phase not to increase temperature.  Inside surrounding air contains moisture, latent in this case is related to phase change of the moisture.

Conduction from building envelope, radiation from sun, are the example of sensible heat and internal heat from people, infiltration is the example of both sensible and latent heat because moisture is involved.