8.6 Convection  (Page 2/4)

 Page 2 / 4

Strategy

Heat is used to raise the temperature of air so that $Q=\text{mc}\text{Δ}T$ . The rate of heat transfer is then $Q/t$ , where $t$ is the time for air turnover. We are given that $\text{Δ}T$ is $\text{10}\text{.}0º\text{C}$ , but we must still find values for the mass of air and its specific heat before we can calculate $Q$ . The specific heat of air is a weighted average of the specific heats of nitrogen and oxygen, which gives from [link] (note that the specific heat at constant pressure must be used for this process).

Solution

1. Determine the mass of air from its density and the given volume of the house. The density is given from the density $\rho$ and the volume
$m=\text{ρV}=\left(1\text{.}\text{29}\phantom{\rule{0.25em}{0ex}}{\text{kg/m}}^{3}\right)\left(\text{12}\text{.}0\phantom{\rule{0.25em}{0ex}}\text{m}×\text{18}\text{.}0\phantom{\rule{0.25em}{0ex}}\text{m}×3.00\phantom{\rule{0.25em}{0ex}}\text{m}\right)=\text{836}\phantom{\rule{0.25em}{0ex}}\text{kg.}$
2. Calculate the heat transferred from the change in air temperature: $Q=\text{mc}\text{Δ}T$ so that
$Q=\left(\text{836}\phantom{\rule{0.25em}{0ex}}\text{kg}\right)\left(\text{1000 J/kg}\cdot º\text{C}\right)\left(\text{10.0º}\text{C}\right)\text{= 8}\text{.}\text{36}×{\text{10}}^{6}\phantom{\rule{0.25em}{0ex}}\text{J.}$
3. Calculate the heat transfer from the heat $Q$ and the turnover time $t$ . Since air is turned over in $t=0\text{.}500\phantom{\rule{0.25em}{0ex}}\text{h}=\text{1800}\phantom{\rule{0.25em}{0ex}}\text{s}$ , the heat transferred per unit time is

Discussion

This rate of heat transfer is equal to the power consumed by about forty-six 100-W light bulbs. Newly constructed homes are designed for a turnover time of 2 hours or more, rather than 30 minutes for the house of this example. Weather stripping, caulking, and improved window seals are commonly employed. More extreme measures are sometimes taken in very cold (or hot) climates to achieve a tight standard of more than 6 hours for one air turnover. Still longer turnover times are unhealthy, because a minimum amount of fresh air is necessary to supply oxygen for breathing and to dilute household pollutants. The term used for the process by which outside air leaks into the house from cracks around windows, doors, and the foundation is called “air infiltration.”

A cold wind is much more chilling than still cold air, because convection combines with conduction in the body to increase the rate at which energy is transferred away from the body. The table below gives approximate wind-chill factors, which are the temperatures of still air that produce the same rate of cooling as air of a given temperature and speed. Wind-chill factors are a dramatic reminder of convection’s ability to transfer heat faster than conduction. For example, a 15.0 m/s wind at $0º\text{C}$ has the chilling equivalent of still air at about $-\text{18º}\text{C}$ .

Wind-chill factors
Moving air temperature Wind speed (m/s)
$\left(\mathbf{\text{ºC}}\right)$ $\mathbf{2}$ $\mathbf{5}$ $\mathbf{\text{10}}$ $\mathbf{\text{15}}$ $\mathbf{\text{20}}$
$\mathbf{5}$ $3$ $-1$ $-8$ $-\text{10}$ $-\text{12}$
$\mathbf{2}$ $0$ $-7$ $-\text{12}$ $-\text{16}$ $-\text{18}$
$\mathbf{0}$ $-2$ $-9$ $-\text{15}$ $-\text{18}$ $-\text{20}$
$\mathbf{-}\mathbf{5}$ $-7$ $-\text{15}$ $-\text{22}$ $-\text{26}$ $-\text{29}$
$-\text{10}$ $-\text{12}$ $-\text{21}$ $-\text{29}$ $-\text{34}$ $-\text{36}$
$-\text{20}$ $-\text{23}$ $-\text{34}$ $-\text{44}$ $-\text{50}$ $-\text{52}$
$-\text{40}$ $-\text{44}$ $-\text{59}$ $-\text{73}$ $-\text{82}$ $-\text{84}$

Although air can transfer heat rapidly by convection, it is a poor conductor and thus a good insulator. The amount of available space for airflow determines whether air acts as an insulator or conductor. The space between the inside and outside walls of a house, for example, is about 9 cm (3.5 in) —large enough for convection to work effectively. The addition of wall insulation prevents airflow, so heat loss (or gain) is decreased. Similarly, the gap between the two panes of a double-paned window is about 1 cm, which prevents convection and takes advantage of air’s low conductivity to prevent greater loss. Fur, fiber, and fiberglass also take advantage of the low conductivity of air by trapping it in spaces too small to support convection, as shown in the figure. Fur and feathers are lightweight and thus ideal for the protection of animals.

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Joseph
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no can't
Lohitha
where we get a research paper on Nano chemistry....?
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
what are the products of Nano chemistry?
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
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da
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Bhagvanji
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Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
revolt
da
Application of nanotechnology in medicine
has a lot of application modern world
Kamaluddeen
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narayan
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ya I also want to know the raman spectra
Bhagvanji
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what about nanotechnology for water purification
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
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Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
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Alexandre
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Rafiq
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Damian
How we are making nano material?
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What is meant by 'nano scale'?
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LITNING
scanning tunneling microscope
Sahil
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Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
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Mahi
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Rafiq
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Anam
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Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
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write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
what king of growth are you checking .?
Renato
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