Thermosyphon (or thermosiphon) is a method of heat exchange based on natural circulation. It circulates liquid refrigerant over a heat exchanger without the use of a mechanical pump. In this article, Jeroen Schröer, director of Wijbenga, explains the ins and outs.
Text: Jeroen Schröer (Wijbenga)
Opening image: Thermosyphon evaporator with separator.
In refrigeration plants, this natural circulation almost always takes place in a closed circuit. Applications are found mainly in (semi-)industrial systems that work with ammonia as a refrigerant. Thermosyphon systems can operate with a small temperature difference, are controllable over a large capacity range, do not require complex controls and fit well into a concept with low refrigerant charge.
Evaporator and liquid separator
A natural circulation system consists of an evaporator and a liquid separator. The evaporator can be designed as a spiral (air cooler), tube (shell & tube) or plates (plate exchanger, falling film). Liquid separators come in many forms today (traditional gravity, HAM, cyclone, demistorer, uniflow, etc.), but all aim to separate the droplets from the vaporizer and absorb the liquid fluctuations due to varying capacities. Practical examples of common applications are oil cooling, water or glycol cooling and NH₃/CO-cascade capacitors.
The driving force behind the operation of a thermosyphon system is gravity and the ratio and specific gravity of the inlet fluid at the inlet and the (wet) suction gas at the outlet.
P Static Drop Line – ΔP Liquid Line
ΔP heat exchanger + ΔP wet return line
P static height (fall line) = ρ * g * h
Where ρ is the density of the liquid (kg/m³), and h is the height difference between the liquid level in the separator and the inlet of the heat exchanger (m).
ΔP liquid line consists of pipe resistance and fittings such as a manual valve or control valve.
ΔP heat exchanger is the resistance on the refrigerant side of the heat exchanger. The calculation of resistance in phase change heat exchangers is extremely complicated and must therefore always be specified by the manufacturer. It is also important to know whether the manufacturer only states the friction pressure losses or whether the static pressure losses are also added here.
ΔP wet return line consists of pipe resistance and fittings such as a manual valve or control valve.
When designing a thermosiphon system, the design of the piping must always be taken into account. If the pipe diameter of the return pipe is chosen too large, the flow may become irregular and cause the evaporator not to function. The two-phase return line should preferably have an annular flow. Turbulent flow with large gas bubbles, also known as slug or chern flow, should be avoided. In extreme cases, circulation can stop or even create a reverse flow with the evaporator boiling ‘dry’.
With too small a diameter or too many bends and valves, the total pressure drop will be so high that the natural circulation cannot be maintained. It can even lead to a choking of the flow.
Measure the circulation rate
A thermosyphon system works as a ‘flooded’ system; more liquid circulates over the heat exchanger than evaporates. This ensures a good distribution of the refrigerant in the evaporator and works optimally. The higher the circulation rate will be, the higher the resistance across the heat exchanger and pipes will be. At a certain point, a balance is created here. It is difficult to predict where this point will be and how high the rate of circulation will be. It can be measured with a flow meter and can even be influenced if desired by placing a (manual) control valve in the liquid line. Check valves must not be installed in the wet return line between the evaporator and the separator.
Causes of malfunctions
If a thermosyphon system is not working properly, it can be for several reasons. Often a combination of factors can also play a role. Contamination can prevent proper heat transfer. This contamination can be located on the coolant side, in the form of, for example, an excess of oil. But it can also be present on the secondary side, such as a water or glycol circuit. There must therefore always be sufficient service connections for draining oil or coolant. Another reason can be a static height of the liquid column that is too low, so that the natural circulation cannot get started properly. The circulation can also be negatively affected if the resistance across the cooling circuit is greater than expected. This resistance can come from the heat exchanger, contamination, fittings or piping.
Thermosyphon with CO₂
Natural refrigerants are the future. Also in this perspective
installations built where CO used in thermosiphon systems. When using CO₂ in thermosyphon systems, the high density of the gas and the higher pressure drops that will occur must be taken into account. It may therefore be necessary to work with higher static heights to bridge these resistances.