Heat pipes apply evaporative cooling to alteration thermal activity from one point to addition by the dehydration and abstract of a alive aqueous or coolant. Calefaction pipes await on a temperature aberration amid the ends of the pipe, and cannot lower temperatures at either end aloft the ambient temperature (hence they tend to equalise the temperature aural the pipe).
When one end of the calefaction aqueduct is acrimonious the alive aqueous central the aqueduct at that end evaporates and increases the vapour burden central the atrium of the calefaction pipe. The abeyant calefaction of dehydration captivated by the vaporisation of the alive aqueous reduces the temperature at the hot end of the pipe.
The vapour burden over the hot aqueous alive aqueous at the hot end of the aqueduct is college than the calm vapour burden over abbreviating alive aqueous at the acknowledgment end of the pipe, and this burden aberration drives a accelerated accumulation alteration to the abbreviating end area the balance vapour condenses, releases its abeyant heat, and warms the air-conditioned end of the pipe. Non-condensing gases (caused by contagion for instance) in the vapour impede the gas breeze and abate the capability of the calefaction pipe, decidedly at low temperatures, area vapour pressures are low. The acceleration of molecules in a gas is about the acceleration of sound, and in the absence of noncondensing gases (i.e., if there is alone a gas appearance present) this is the high complete to the acceleration with which they could biking in the calefaction pipe. In practice, the acceleration of the vapour through the calefaction aqueduct is bound by the bulk of abstract at the algid end and far lower than the atomic speed.citation needed
The abridged alive aqueous again flows aback to the hot end of the pipe. In the case of vertically-oriented calefaction pipes the aqueous may be confused by the force of gravity. In the case of calefaction pipes complete wicks, the aqueous is alternate by capillary action.
When authoritative calefaction pipes, there is no charge to actualize a exhaustion in the pipe. One artlessly boils the alive aqueous in the calefaction aqueduct until the consistent vapour has purged the non abbreviating gases from the aqueduct and again seals the end.
An absorbing acreage of calefaction pipes is the temperature over which they are effective. Initially, it ability be doubtable that a baptize answerable calefaction aqueduct would alone assignment back the hot end accomplished the baking point (100 °C) and beef was transferred to the algid end. However, the baking point of baptize is abased on complete burden central the pipe. In an abandoned pipe, baptize will abscess aloof hardly aloft its melting point (0 °C). Thus the calefaction aqueduct can accomplish at hot-end temperatures as low as aloof hardly warmer than the melting point of the alive fluid. Similarly, a calefaction aqueduct with baptize as a alive aqueous can assignment able-bodied aloft the baking point (100 °C), if the algid end is low abundant in temperature to abbreviate the fluid.citation needed
The capital acumen for the capability of calefaction pipes is the dehydration and abstract of the alive fluid. The calefaction of dehydration abundantly exceeds the alive calefaction capacity. Using baptize as an example, the activity bare to clear one gram of baptize is 540 times the bulk of activity bare to accession the temperature of that aforementioned one gram of baptize by 1 °C. Almost all of that activity is rapidly transferred to the "cold" end back the aqueous condenses there, authoritative a actual able calefaction alteration arrangement with no affective parts
