Lesson 7. HVAC Science, Latent Heat and Sensible Heat

HVAC Science, Latent Heat and Sensible Heat

Sensible Heat
Most of us understand Sensible Heat.  This type of heat is measured by a common thermometer and represents a transfer of heat without a change in the state (Latent Heat) such as ice melting or water evaporating.  If we heat water from 45F degrees to 80F degrees, the sensible heat changes 35F degrees.

Latent Heat is the heat that makes the magic of Air Conditioning possible.  Latent heat represents the energy transferred when a substance changes state.  When water freezes, to form ice, a change of state called latent heat of fusion occurs.  When water is boiled and turned to a gas, this change of state is called the Latent heat of vaporization.

A significant amount of energy in the form of latent heat will occur without a change in sensible heat.  As an example a pound of 33F degree water will require 1 BTU to lower the sensible heat to 32F.  (1 BTU is required to change the temperature of 1 pound of water 1 degree)  By comparison you must remove 144 BTU’s of Latent heat to change 32F water to 32F ice.

The magic in Air Conditioning occurs when the refrigerant in the system is changed from a liquid to a gas.  This change of state makes the transfer of massive amounts of BTU’s possible.  The change of the liquid to a gas where energy is absorbed is called endothermic and the release of energy when the gas changes back to a liquid is called exothermic.

Another term that you will use daily is Saturation Temperature.  The saturation temperature/pressure combination is the point where a change of state takes place.  Remember that the saturation temperature is the boiling point and that this boiling point changes, with a change in pressure.

Pressure of gas is measured in PSI or pounds per square inch.  Be aware of two different measures:
• PSIA:  Pounds per Square Inch Atmospheric (Absolute).   This is a measure of pressure, including the pressure of the atmosphere, taken at sea level and 70F.
• PSIG:  Pounds per Square Inch Gauge.  This is the pressure in a closed system sealed from and not subject to atmospheric pressure.

You will learn much more when you prepare for your EPA Certification with our EPA Study Guide

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