Lesson 6. HVAC Science Laws of Thermodynamics

HVAC Science, The Laws of Thermodynamics

You won’t have to do lengthy mathematical calculations or analyze complex chemical formulas in your daily routine as an HVAC Technician, but you will advance much faster with significantly less frustration if you understand the basic science behind HVAC.

Theory of Heat
The first law of thermodynamics states that energy can neither be created nor destroyed.  When you think about this, it is profound.  When you burn gasoline in your car you don’t destroy it, you just change it to heat or work.  Heat or work is the only two ways to change energy.

Heat is a process of energy transfer.  This transfer occurs when there is a difference in temperature.  Temperature is a measurement of the energy (or enthalpy, a word you need to study and understand).  Temperature is the result of the molecular motion.

The laws of thermodynamics state that heat can only flow from a higher temperature to a lower temperature.

Heat is heat, but we measure heat using different scales:
• Fahrenheit
• Celsius
• Rankin
• Kelvin.

When you calculate the size Air Conditioner needed for a building, the system you use will take into account the three different ways heat can be transferred into the space or building.
• Radiation:  Heat radiates from the hotter material to the cooler material.  The heat from the sun is a good example.  Radiation can travel through space or a vacuum.  Radiation will decline over a distance.
• Conduction:  Heat travels through a material by conduction.  Heat is conducted through steel much faster than through ice, but both conduct heat.
• Convection:  Heat can transfer from a hot object to air and then to a cooler object.  An example is the heat from a furnace heats the air by convection and the air warms your body.

Remembering that heat or energy can’t be destroyed, we can convert heat measurements to energy measurements.  As an example a unit of heat is the BTU (British thermal unit).  We can measure electrical energy in watts.  We can use a formula to convert Heat BTU’s to electric watts.

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