Environmental Engineering Reference
In-Depth Information
electricity to drive the fan motor. Air-based collectors are a particularly interesting
alternative as providers of support heating. However, heat storage is more diffi cult
with systems that use air - based collectors.
6.2.4 Vacuum-Tube Collectors
With fl at-plate collectors the air between the absorber and the front glass panel is
the source of most of the heat loss. It causes convective heat loss and continuously
transports the heat from the absorber to the glass panel. This then emits the heat
unused back into the environment.
How a Blown-Up Plastic Bag Evacuates Its Air
The term vacuum originates from the Latin word 'vacus', which means
'empty' or 'free'. A vacuum is generally thought of as space empty of matter.
However, it is practically impossible to produce a totally air-less space on earth.
In technology and in physics a vacuum is interpreted merely as air pressure that is con-
siderably lower than normal air pressure. To produce a vacuum, one uses a vacuum
pump to pump the air out of a stable space. In principle, it is also possible to use one's
mouth to produce a rough vacuum - for example, by sucking the air out of a glass bottle.
However, a plastic bag cannot be evacuated - at least not in any normal environment
on earth. On the other hand, an almost perfect vacuum exists in space. If one opens an
empty plastic bag in space and then closes it again, there will also be a vacuum in the
blown-up bag. If one returns to earth with the bag, the ambient air pressure compresses
it again.
Ambient air pressure originates through the power of the weight of air columns above
the earth's surface. The atmospheric air weighs around 10 tons per square metre of earth
surface. The question is, why doesn't this enormous pressure simply smash the glass of
a collector? The answer is simple: the space under the glass is also fi lled with air, which
produces the necessary counterpressure. On the other hand, if the air in the space behind
the glass is evacuated, the glass will normally bend and shatter.
Heat losses can be reduced considerably if a vacuum exists between the absorber
and the front glass because of the air movement in the collector. This is the principle
used by vacuum fl at-plate collectors. As the outer air pressure would press the front
covering against the absorber, spacers are needed between the underside of the col-
lector and the glass covering. The vacuum cannot be kept stable for a long period
because penetration of air where the glass and collector housing intersect cannot be
completely prevented. Therefore, vacuum fl at-plate connectors must be evacuated
again at certain intervals. This is done by attaching a vacuum pump to a special
valve on the collector. These drawbacks were the reason why vacuum fl at - plate
collectors never became popular.
Another type of collector available is the vacuum-tube collector, which does not
have the same disadvantages as the vacuum fl at - plate collector. With vacuum - tube
collectors, a high level of vacuum is completely enclosed in a glass tube.
