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Difference Between Sensible And Latent Heat

Two types of heat are important in air conditioning. These include:

  1. Sensible Heat
  2. Latent Heat

What is Sensible Heat?

Sensible heat is heat exchanged by a thermodynamic system that changes the temperature of the system without changing some variables such as volume or pressure. As the name implies, sensible heat is the heat that you can feel. The sensible heat possessed by an object is evidenced by its temperature. As temperature increases, the sensible heat content also increases. However, for a given change in sensible heat content, all objects do not change temperature by the same amount. Each substance has its own characteristics relationship between heat content and temperature. The proportionality constant between temperature rise and change in heat content is called the specific heat, measured in calories per gram per degree Celsius or joules per kilogram per kelvin. Water for example has a specific heat of 1 Cal/g/oC. In general, the gain in heat is accompanied by either a change in volume or a change in pressure (e.g. the water in the pot swells somewhat as you heat it; if you heat gas in a fixed volume, its pressure goes up).

What is the difference between sensible and latent heat?
Why is latent heat important?
What is an example of sensible heat?

What is Latent Heat?

This is the energy absorbed or released by a thermodynamic system during a constant temperature process. Examples include ice melting or water boiling. When a solid turns into a liquid (melts) or a liquid turns into a gas (evaporates), the loosening of attraction among the molecules requires energy. If you raise ice from -20oC to 0 oC, you put in sensible heat. If you keep adding heat to the ice, it melts but its temperature is constant, the sensible heat of ice/water system is not increasing but you continue to add heat energy to it. Energy is conserved, such that the extra heat tears apart the frozen ice molecules and sets them loose as a liquid. The liquid is therefore storing this energy in a form that you cannot sense. This energy is call latent heat.

To melt all the ice, you have to pump in quite a bit of heat, but you cannot sense any change in the heat content because ice/water system remains at 0oC. Only after all the ice has melted does the temperature of the water rise. At this point, the heat you put in is once again creating a change in sensible heat. However, although you cannot feel it, the liquid has stored all that latent heat. The only way you will observe the latent heat is if you try to transform the water back to ice. If you take the temperature down to 0oC, that alone will not freeze water; you must keep pulling out heat until you have removed every joule of latent heat. Only then will all the water freeze and you can begin to remove more sensible heat and lower the temperature of the system below 0oC.

Heat energy is conserved no matter how the phase change occurs. If you put heat into water, it can evaporate. It evaporates on its own even if you do not add heat. The water will cool off; i.e. some of the sensible heat is lost and converted to latent heat. Conversely, if you cool off some water vapor, it can condense into liquid. If it condenses on its own, it will give off (sensible) heat and get warmer. Therefore sensible heat can be felt while latent heat is the type of heat that cannot be felt.

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