HVACR is harder if you don’t understand the concepts behind the ideal gas law. First, let’s define some words.

• Matter – Matter is “stuff,” anything that has weight and takes up space.
• Mass – You can think of it as weight for most purposes; it is a measurement of how much “stuff” there is.
• Volume – How much space the stuff takes up.
• Temperature – The average intensity of heat energy at that point. It is literally the average molecular velocity of the stuff you are measuring.
• Pressure – The force exerted on, in, or by matter.

In HVACR we are constantly dealing with concepts contained in the ideal gas law which informs us of how matter and contained environments respond to changes in temperature, volume, pressure, and mass.

What is an ideal gas?

An ideal gas is a gas that obeys the ideal gas law. It behaves in a predictable way with changes in volume, pressure, temperature, and mass.

The ideal gas law:  PV: nRT

While many gases behave close to ideal at normal temperatures, there is no gas that obeys the ideal gas law in all conditions.

• P = Absolute pressure (gauge pressure + atmospheric pressure)
• V = Volume
• n = Mass measured in “moles” (the number of molecules)
• R = The universal gas constant (varies depending on the units of measure being used. Example: [lbf ft/(lb mol oR)]= 8.3145 )
• T = Absolute temperature (temperature in a scale that starts at absolute zero like Kelvin or Rankine)

The result is that many gases that we work with behave in about the same way with changes in mass, volume, temperature, and pressure.

Why Does it “Matter” (Pun Intended)

Understanding the relationships between temperature, pressure, volume, and mass in contained (closed) environments like an HVAC refrigerant circuit and uncontained (open) environments like the air outside is really helpful.

The most practical thing to know is this: A decrease in temperature causes a decrease in pressure or a decrease in pressure causes a drop in temperature. An increase in either results in the opposite effect.

In HVACR it is important that we understand the impact changes in temperature, pressure, volume, and mass will have inside a system.

When we add or remove refrigerant we are changing the refrigerant mass, which impacts the pressures and temperatures. When one system has a smaller condenser coil than another, the decrease in internal volume will increase the pressure and temperature if the refrigerant mass is the same.

When the air passing over an evaporator coil is colder, the pressure in the coil will be lower because the gas law teaches us that a decrease in temperature equals a decrease in pressure.

All of these factors are interconnected and if you don’t understand the ideal gas law, you won’t understand how they impact one another.

Publication date: 7/8/2019

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