Measuring heat
Temperature measures the average velocity (speed) of the molecules that make up a substance, that is, measures sensible heat. It cannot be measured, the latent heat energy necessary for the change of state of matter.
This fact quickly seen by comparing the temperature of 212F water with a temperature of 212F pairs. Although the distance and mutual arrangement of the molecules are different, thermometer reads the same for both liquid and gas. There is often more latent heat required to change the state, than sensible heat required to change the temperature between the two States. In fact, it takes five times more heat to change water into vapor (latent heat), as it does to heat the water of 32to 212 FF (sensible heat). The same thing happens with refrigerants, but at a much lower temperature.
Since most of the heat transfer in mechanical systems of cooling is carried out by the latent heat transfer, we must go to accurately measure the latent heat, as well as sensible heat content.
BTU measures:
- The Enthalpy Heat Transfer
- Heating and Cooling
- Heating and Cooling Load
- In Enthalpy Of The Refrigerant (Enthalpy)
British Thermal Units, or BTU, to satisfy this need. Therefore BTU is the unit of measurement of heat capacity and heat transfer in mechanical refrigeration work. It is used to Express the machine thermal power, as well as its sensible, latent, and total power in cooling mode. It is also used to Express the enthalpy, or enthalpy of the refrigerant. A BTU is the amount of heat needed to raise the temperature of 1 pound of water 10 Fahrenheit.
If we heat for 1 pound of water which has just melted from the ice at 32F we will cause its temperature to rise. It will start on melting temperature (32F) and, ultimately, to go to 212F. This is a reasonable process heating. We can measure in BTU of heat transferred into the water with this sensible heat transfer process.
Since there is a 1 pound of water, and it takes 1 BTU to warm each pound 1 F, and as we warmed up water 212F, it will take 180 BTU for this task. In other words, 180 BTU heat must move from the flames through the surface of the pan, into the water. If there were 2 pounds of water instead of 1, you will need twice the heat transfer (360 BTU) to achieve the same temperature.
This shows that the BTU can be used to measure reasonable processes of heat exchange and, more pounds of fluid is heated, the more heat must be transferred. This is true regardless of fluid; water, air, or refrigerant. The more work, the greater the number of pounds of air flow necessary for heating or cooling coil. Similarly, the more work, the greater the number of pounds of refrigerant, which will be distributed by the refrigeration system to move in the necessary heat from the air.
Specific heat capacity, allows us to compare how easy various substances heated. It also allows us to calculate the amount of heat transferred into or out of, a substance within a reasonable heat transfer process...
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Refrigeration 
