Thermosiphon Heat Exchangers


These heat exchangers are sometimes referred to as heat pipes, which is a misnomer. While they may resemble heat pipes in that they appear as sealed finned coils, tubes are connected but they do not include any wicking.

Thermosiphon heat exchangers are sealed systems that consist of an evaporator, a condenser, interconnecting piping, and an intermediate working fluid (refrigerant) that is present in both liquid and vapor phases. Two types of thermosiphon are usedócoil type and sealed tube type. In the sealed tube thermosiphon, the evaporator and the condenser are usually at opposite ends of a bundle of straight, individual thermosiphon tubes, and the exhaust and supply ducts are adjacent to each other (this arrangement is similar to the that in a heat pipe system). In coil-type thermosiphons, evaporator and condenser coils are installed independently in the ducts and are interconnected by short lengths of working fluid piping (this configuration is somewhat similar to that of a coil loop system).

In thermosiphon systems, a temperature difference and gravity cause the refrigerant to circulate between the evaporator and condenser. As a result, thermosiphons may be designed to transfer heat equally in either direction (bidirectional), in one direction only (unidirectional), or in both directions unequally.


  • passive heat exchange with no moving parts,
  • relatively space efficient,
  • the cooling or heating equipment size can be reduced in some cases,
  • the moisture removal capacity of existing cooling equipment can be improved,
  • no cross-contamination between air streams with coil type.


  • adds to the first cost and to the fan power to overcome its resistance,
  • requires that the two parts be placed so the liquid condensate can return to the evaporator part by gravity,
  • requires that the two air streams be adjacent to each other with the sealed tube type,
  • may require a significant temperature difference to initiate boiling,
  • requires that the air streams must be relatively clean and may require filtration.

Links to more detail

Heat Pipes
Coil Loop System


Thermosiphon units offer an alternative that can improve the dehumidification capability of most air conditioning systems without adding cooling capacity and can replace dehumidification reheat with less energy use. With the evaporator part across the entering air face of the chilled water or direct expansion cooling coil and the condenser part across the leaving air face, moisture removal can be improved. Entering air is precooled, reducing the sensible load and allowing more moisture removal, and the leaving air is reheated without the use of external energy.

Best Applications

Mold and mildew can form when relative humidity levels are consistently over 60%.Where lower humidity is desired and the existing equipment is unable to maintain the desired humidity levels, or where increased ventilation air is needed, a thermosiphon system is an alternative.

Possible Applications

Two-phase thermosiphon loops can be used for solar water heating.

Applications to Avoid

If conventional equipment is capable of delivering the required latent capacity without overcooling the supply air and requiring external reheat energy, thermosiphon systems are unlikely to be cost effective.

Technology Types (Resource)

A thermosiphon is a two-part sealed system containing a two-phase working fluid (refrigerant). Because part of the system contains vapor and part contains liquid, the pressure in a thermosiphon is governed by the liquid temperature at the liquid-vapor interface. If the surroundings cause a temperature difference between the two parts where liquid and vapor interfaces are present, the resulting vapor pressure difference makes vapor flow from the warmer to the colder region. The flow is sustained by condensation in the cooler region and by evaporation in the warmer region. The condenser and evaporator must be oriented so that the condensate can return to the evaporator by gravity.

Although similar in form and operation to heat pipes, thermosiphon tubes are different in two ways: (1) they have no wicks and hence rely only on gravity to return the condensate to the evaporator, whereas heat pipes use capillary forces; and (2) thermosiphon tubes depend, at least initially, on nucleate boiling, whereas heat pipes vaporize the fluid from a large, ever-present liquid vapor interface. As a result, thermosiphon heat exchangers may require a significant temperature difference to initiate boiling. Thermosiphon tubes require no pump to circulate the working fluid. However, the geometric configuration must be such that liquid working fluid is always present in the evaporator section of the heat exchanger.

Thermosiphon loops differ from other coil energy recovery loop systems in that they require no pumps and hence no external power supply, and the coils must be appropriate for evaporation and condensation.


Performance of a thermosiphon loop indicates an effectiveness of between 40% and 60% when the overall temperature difference is over 40°F and with face velocities of 250 to 600 feet per minute. For example, if entering air at 80°F is cooled by the thermosiphon evaporator to 70°F and the air off the cooling coil is reheated from 55°F to 65°F by the condenser section, the effectiveness is 40 % [=(65-55)/(80-55) = 40%].

Other Information

Heat Pipes
Coil Loop System