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AskPablo: Brrr, it’s cold outside…

| Monday January 15th, 2007 | 1 Comment

This week’s AskPablo is inspired by the cold weather that we are experiencing here in California right now. Backyard pools have frozen over, the orange crop is seriously threatened, and the homeless population is at risk of hypothermia. All of this is occurring just over one week after I returned from Boston, where it was 71°F! Whether you are inside your house or venturing outside you might benefit from knowing about the different modes of heat loss and what you can do to stay warm and conserve energy.


Everything in the universe wants to be in equilibrium. Therefore it should be no surprise that, when you place a warm substance next to a cold substance, they eventually reach the same temperature. In much the same way, the heat in your house or in your body wants to escape and reach equilibrium with the outside air. When this outside air is as cold as it is now, that desire is that much stronger. So in order to maintain a constant indoor temperature or body temperature you need to produce more heat and prevent more heat loss.
There are several modes of heat transfer that I am going to discuss this week. While we all know them from experience we may not have thought about them in theoretical terms. The ways in which a warm body loses heat are: 1) Conduction, 2) Convection (natural and forced), and 3) Radiation
Conduction
Conduction heat transfer occurs when two masses of differing temperatures are in direct contact. For example, if you sit down on a cold rock you may become hypothermic in a hurry. To avoid this problem while hiking or watching your favorite sports team simply place an insulating barrier between yourself and the cold surface, like a foam pad. In a home you may experience conduction heat transfer if your floor consists of a concrete slab poured directly onto the ground. This is one reason why many homes are raised off of the ground with a crawl space underneath (other reasons include termites and radon that leaks out of the ground). Conduction heat transfer uses the following equation where “It is defined as the quantity of heat, Q, transmitted in time t through a thickness L, in a direction normal to a surface of area A, due to a temperature difference ΔT, under steady state conditions and when the heat transfer is dependent only on the temperature gradient.” ( www.Wikipedia.org):
Conductivity.png
Convection
Convection heat transfer occurs when the thin film of air surrounding a warm body becomes heated. Warmer air is more buoyant than colder air and so it wants to rise. As this warm air rises away from the surface colder air takes its place and the cycle continues. This cycle is called natural convection because it occurs on its own. When airflow is directed across a warm surface (as in a furnace) it is called forced convection. You can feel convection at work when you go outside on a cold day. Most of the cooling that you experience is due to convection from exposed skin. When there is wind you are feeling forced convection. Convection plays a role in the loss of heat through windows. You may think that a curtain will prevent heat loss, and to some degree it will, but unless there is a relatively tight seal between the curtain and the window convection heat transfer can occur. Convection is governed by the following equation (from www.Wikipedia.org):
convection.png
where:
q’ – local heat flux
h – local convection coefficient
Ts – surface temperature
T∞ – ambient temperature
Radiation
Radiation heat transfer is the only form of heat transfer that occurs in a perfect vacuum. Thermos bottles use this to their advantage by holding a liquid warm by isolating it from the outside world with a vacuum. In addition, they often coat the glass with a low emissivity coating that inhibits radiation heat transfer as well, keeping liquids warm for an incredibly long time. This sort of coated vacuum tube is also often used in solar water heating. Around the house you will see radiation heat transfer most frequently in electric heaters, which emit a great deal of infrared radiation. Since the infrared end of the spectrum is right next to the visible red wavelengths the electric heater glows reddish-orange. We use the following equation with radiation heat transfer to determine the amount of energy emitted (in Watts, from www.Wikipedia.org):
Radiation.png
where:
the funny looking symbol is the Stefan-Boltzman constant (5.670 400(40)×10−8 W·m-2·K-4)
A is the area
and T is the surface temperature in Kelvin
I hope that you can find ways to use this information to keep yourself and your house warm. Seal drafty doors, windows, and power outlets, apply plastic film over your window frames (ask at your local hardware store) or install tight-fitting curtains or baffled fabric blinds, and keep your thermostat a few degrees lower to save a few bucks and prevent a bit of carbon dioxide from entering the atmosphere.
Pablo Päster, MBA
Sustainability Engineer
www.AskPablo.org
Pablo(dot)Paster(at)gmail(dot)com
View Pablo Paster's profile on LinkedIn


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  • Nitin

    At what temperature radiation heat transfer should be taken into account ?. The fluid is air flowing in microtube downwards with constant heat flux at wall.Pl reply