Thermosiphon is a physical effect and refers to a method of passive heat
exchange based on natural convection, which circulates a fluid without the
necessity of a mechanical pump. Thermosiphoning is used for circulation
of liquids and volatile gases in heating and cooling applications, such as heat
pumps, water heaters, boilers and furnaces. Thermosiphoning also occurs
across air temperature gradients such as those utilized in a wood fire chimney,
or solar chimney. This circulation can either be
open-loop, as when the substance in a holding tank is passed in one direction
via a heated transfer tube mounted at the bottom of the tank to a distribution
point—even one mounted above the originating tank—or it can be a vertical
closed-loop circuit with return to the original container. Its purpose is to
simplify the transfer of liquid or gas while avoiding the cost and complexity
of a conventional pump. Simple thermosiphon
Convective movement of the liquid starts when liquid in the loop is heated,
causing it to expand and become less dense, and thus more buoyant than the
cooler liquid in the bottom of the loop. Convection moves the heated liquid
upwards in the system as it is simultaneously replaced by cooler liquid
returning by gravity. Ideally, the liquid flows easily because a good
thermosiphon should have very little hydraulic resistance.
= Heat pipes = In some situations the flow of liquid
may be reduced further, or stopped, perhaps because the loop is not entirely
full of liquid. In this case, the system no longer convects, so it is not a usual
"thermosiphon". Heat can still be transferred in this
system by the evaporation and condensation of vapor; however, the
system is properly classified as a heat pipe thermosyphon. If the system also
contains other fluids, such as air, then the heat flux density will be less than
in a real heat pipe, which only contains a single fluid.
The thermosiphon has been sometimes incorrectly described as a 'gravity
return heat pipe'. Heat pipes usually have a wick to return the condensate to
the evaporator via capillary action. A wick is not needed in a thermosiphon
because gravity moves the liquid. The wick allows heat pipes to transfer heat
when there is no gravity, which is useful in space. A thermosiphon is
"simpler" than a heat pipe. (Single-phase) thermosiphons can only
transfer heat "upward", or away from the acceleration vector. Thus, orientation
is much more important for thermosiphons than for heatpipes. Also, thermosiphons
can fail because of a bubble in the loop, and require a circulating loop of
pipes. = Reboilers and calandria =
If the piping of a thermosiphon resists flow, or excessive heat is applied, the
liquid may boil. Since the gas is more buoyant than the liquid, the convective
pressure is greater. This is a well known invention called a reboiler. A
group of reboilers attached to a pair of plena is called a calandria.
The term "phase change thermosiphon" is a misnomer and should be avoided. When
phase change occurs in a thermosiphon, it means that the system either does not
have enough fluid, or it is too small to transfer all of the heat by convection
alone. To improve the performance, either more fluid is needed, or all
other fluids should be pumped out of the loop.
Solar energy Thermosiphons are used in some
liquid-based solar heating systems to heat a liquid such as water. The water
is heated passively by solar energy and relies on heat energy being transferred
from the sun to a solar collector. The heat from the collector can be
transferred to water in two ways: directly where water circulates through
the collector, or indirectly where an anti-freeze solution carries the heat
from the collector and transfers it to water in the tank via a heat exchanger.
Convection allows for the movement of the heated liquid out of the solar
collector to be replaced by colder liquid which is in turn heated. Due to
this principle, it is necessary for the water to be stored in a tank above the
collector Computing
Thermosiphons are used in computing to describe a system for watercooling the
internal computer components, most commonly referring to the processor.
While any suitable liquid can be used, water is the easiest liquid to use in
thermosiphon systems. Unlike traditional watercooling systems, thermosiphon
systems do not rely on a water pump but rely on convection for the movement of
heated water from the components upwards to a heat exchanger. There the water is
cooled and is ready to be recirculated. The most commonly used heat exchanger is
a radiator where air is blown actively through a fan system to condense the
vapour to a liquid. The liquid is recirculated through the system, thus
repeating the process. No pump is required—the vaporization and
condensation cycle is self-sustaining. = Uses =
Modern processors get relatively hot. Even with a common heat sink and fan
cooling the processor, operating temperatures may still reach up to 70
°C. A thermosiphon can handle heat output at a much wider temperature range
than any heat sink and fan, and can maintain the processor 10–20 °C cooler.
In some cases a thermosiphon may also be less bulky than a normal heat sink and
fan. = Drawbacks =
Thermosiphons must be mounted such that vapor rises up and liquid flows down to
the boiler with no bends in the tubing for liquid to pool. Also, the
thermosiphon’s fan that cools the gas needs cool air to operate.
Engine cooling Early cars and motor vehicles used
thermosiphon circulation to move cooling water between their cylinder block and
radiator. As engine power increased, increased flow was required and so
engine-driven pumps were added to assist circulation. More compact engines then
used smaller radiators and required more convoluted flow patterns, so the
circulation became entirely dependent on the pump and might even be reversed
against the natural circulation. An engine cooled only by thermosiphon is
also very sensitive to low coolant level, i.e., losing only a small amount
of coolant stops the circulation; a pump driven system is much more robust and
can typically handle a lower coolant level.
See also Convection
Geothermal heat pump Heat pipe and Loop heat pipe
Passive solar Reboiler
Siphon Solar heating
Thermic siphon Vapor-compression refrigeration
Watercooling Thomas Fowler
References ^ "Thermosyphon technology for
Artificial Ground Freezing". simmakers.com.
^ Holubec, I.. "Flat Loop Thermosyphon Foundations in Warm Permafrost.
^ btfsolar.com ^ "Thermosiphon Heat Exchangers".
apogee.net. ^ "What is a Heat Pipe? - Other Topics -
Articles - Chemical Engineering - Frontpage - Cheresources.com".
Cheresources.com Community. ^ Brian Norton Solar Water Heaters: A
Review of Systems Research and Design Innovation, Green. 1, 189–207, ISSN
1869-8778 ^ Kuemel, Bernhard. "CPU Vapor Cooling
Thermosyphon". http:www.overclockers.com/. Retrieved 26
August 2012. External links
Definition of thermosiphon Another definition of thermosiphon
HP Labs report on thermosiphons for computer cooling
Antique thermosyphon engine cooling