Heat transfers in three ways
The H in HVAC stands for heating, which is a major part of our business. Heat is transferred in three ways, namely conduction, convection and radiation. You experience heat differently with each method: how this works and why it’s important for your HVAC system is the subject of this blog.
We could start with an example of a heat exchanger or an evaporator in a cooling machine but, to keep things simple, let’s use a bonfire to explain the three forms in which heat manifests itself. Gather round, enjoy feeling the heat of the flames on your skin as someone pokes the logs in the fire with a steel rod… We’re all familiar with a bonfire like the one below.
The heat or thermal energy in this fire is transferred in three different ways, namely:
- Thermal conduction
- Thermal convection
- Thermal radiation.
Before we go further, set your mind to a molecular level. Everything is made from these minuscule particles. Solid items like the steel rod have their molecules and atoms in a tight grid, while the air between the bonfire and the people sitting around it consists of freely flowing molecules.
Three forms of heat transfer
Conductive heat transfer occurs when a temperature difference exists between two materials in contact with each other. Putting the tip of a steel rod into the fire will cause this kind of heat transfer. The molecules of the rod which are in the fire will heat up and, as they get more excited, bounce against the molecules beside them, transferring the energy from one end of the rod to the other.
The air in the direct vicinity of the fire gets heated up through conduction. Heated air rises as cold air takes its place. The atmosphere around us is a mixture of different gases. Hot gas molecules occupy more space than cold air molecules, making them less dense and lighter. As the convection stream rises it heats up the surrounding air. Cold air which drops down will eventually be heated up again and also rise into the sky.
All objects radiate. Molecules of every object send out electromagnetic radiation as they are accelerated. Every molecule consists of particles which are charged, like the neutrons and electrons in an atom. Look at what happens when someone pokes the fire or adds a flammable substance like alcohol or turpentine. As the substance ignites, the fire is set ablaze and the particles accelerate. This suddenly increases the radiation and everyone around the bonfire feels the heat on their faces.
Our engineers use this information when calculating the capacity of a HVAC installation. To keep a room or cabin in the right condition they have to know which dynamic physical processes are occurring. In summertime, heat from the outside seeps into the cabins in various ways. How much depends on the temperature difference and the conductivity of the materials from which the vessel is made.
But it does not stop there: the transfer of heat is intertwined in many different processes of the HVAC system. From a cooling machine condenser and evaporator to the heat exchanger in a hydraulic system and the cooling coil of the air handling unit.
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