How does thermal radiation affect HVAC systems?
– by Jeroen Fijan – 15/01/19
Have you ever wondered how sunbathing on your skiing holiday can be so comfortable, even when the ambient temperature is below freezing?
In this blog we explore the form of heat called ‘thermal radiation’: What exactly is thermal radiation, what causes it, and what impact does it have on HVAC systems?
What is thermal radiation?
Thermal radiation is a form of heat transfer (the other two forms are thermal conduction and thermal convection) that is emitted by every object with a temperature above absolute zero (‑273.15 °C).
We humans feel it whenever we are near a source of heat such as a panel radiator or a fire. If there is a straight unobstructed line from you to a hot object, thermal radiation is what warms up your skin and clothes.
Is radiation visible to the naked eye?
Thermal radiation consists of electromagnetic radiation emitted by charged particles that are being accelerated. You can actually see this: the light emitted from a fire, for example, is electromagnetic radiation. It is emitted in a wavelength which your eye perceives as light.
The same happens when a welder heats up a piece of metal. As the metal gets hotter, its molecules vibrate faster and faster. Thermal radiation is produced, and the metal emits light ranging from a deep red to a bright yellow glow.
How does radiation effect HVAC systems?
The curious thing about thermal radiation is that it doesn’t warm up the air in the direct vicinity of the object as thermal conduction does. Thermal radiation moves from one object directly to another: for example, a sofa, carpet or bed next to a sunlit window will tend to get hotter than the intervening air.
One of the biggest heat loads caused by thermal radiation we encounter is sunlight. While it can easily be avoided in commercial accommodations simply by closing a sunscreen, things are more complicated on a stylish yacht with lots of windows.
Our capacity calculations always include the impact of solar radiation. A smart positioning of supply and exhaust grills is key to a comfortable environment, allowing heat emitted from warmed-up furniture to be immediately conducted away.
Thermal radiation is everywhere. All objects with moving molecules emit it. In outer space, the only form of heat is actually thermal radiation as there can be no convection or conduction in a vacuum. In other words, all the heat we get from the sun is caused by thermal radiation.
Like to find out about thermal conduction? Read Peter Bos’s blog about heat transfer.
Jeroen Fijan | R&D Manager
Jeroen Fijan has been working at Heinen & Hopman since 2001. He started as a draughtsman and, over the years, worked his way up to the top of the R&D division. Sustainability is a top priority in the quest to improve H&H’s products and processes.