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.

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.