Have you ever wondered why, after a winter night, some of your car windows are iced over whilst others are not? Happens after a clear night with no clouds. To understand the reason one must appreciate the fact that something loses heat, and thus gets cold, for two reasons; losing heat to the local air by convection and losing heat to any colder surrounding object due to thermal radiation. On a cold clear night those windows of your car that have a clear line of sight to the cold night sky will lose heat quickly due to radiative exchange with something big that is effectively at about -270 degC – space. Windows that only see your (relatively warm) house will not lose as much heat, stay warmer and thus not freeze. If you can, park under a tree. If you can afford one, park in a garage.
Let’s turn that cooling example around into a heating one. There’s nothing so British as sitting in a leather wing back chair, in front of a fire, in ones smoking jacket, dressing gown or maybe simply your suit. You will of course be in your rather drafty country house, with a fine brandy or port in hand, trying to keep warm and not worry about the soaring costs of keeping the family estate running as it has done so for the last 350 years. The air around you will be chilly and it will only be the radiative heat loss from the fire that’s keeping your nose and other extremities warm. The high wing back chair will help ensure that the cold air that’s rushing towards the fire doesn’t cool you back down in the process.
The temperature that you feel that causes comfort, let’s call it a ‘comfort temperature’, is a combination of the local air temperature, the local air speed and a temperature that is indicative of how hot the things around you are (check out MRT for more details). ‘Wind chill’ might be another example of such a composite temperature.
So, considering the flower pot heater is designed to make you feel warmer, what does the local comfort temperature look like compared to just the air temperature?
In the space surrounding the pots the comfort temperature is higher. In other words if you were to put your hand that close to the pots you would feel an increase in temperature. If you put the pots far away in say a corner of the room then you wouldn’t.
In the next blog I’ll try to put it all together and come up with a new design that hopefully will squeeze as much value out of the 140W as possible.
In the mean time, if you want to learn more about what it is to be British and to sit in a wing back chair, watch this fascinating story from Rowley Birkin QC.
18th December 2013, Ross-on-Wye