Free-Cooling by the ‘Thermosyphon’ Method

Free-cooling is the term used to describe a method of achieving a temperature differential in a working medium such as water, in a chiller.  It is a way of removing heat from a system under a particular set of conditions without incurring the expense of using mechanical means i.e.  running the compressors.  Thermosyphon is one method of achieving this and is discussed below.


The physical effect behind a so-called thermosyphon has been well known for a long time.  Several chiller manufacturers have offered their products with this enhancement which is an alternative to conventional free-cooling by means of direct application of water/glycol to air heat exchangers.

Manufacturers of Turbocor based oil-free chillers have re-discovered the thermosyphon effect as a supplementary function for high efficiency chillers to further improve energy efficiency.

Detailed information about the physical principles behind the thermosyphon effect and related products can be obtained from the websites of most European manufacturers in this field.  However, also other manufacturers of high efficiency chillers such as Star Refrigeration (see Youtube video Thermosyphon Indigochiller build-up), Smardt Chiller Group, and Engie Refrigeration for example, offer thermosyphon enhancements for their respective products.

Today it has become almost a commodity in the high efficiency chiller segment of the market although some manufacturers use different names and refer to an economizer instead of thermosyphon.

We will explain the physical background of the thermosyphon effect.


Basic Principles

The thermosyphon effect is also referred to as natural circulation. To explain the meaning of natural we will at first recall the principle of the forced circulation cycle of the classic vapour compression refrigeration process [see figure right].

Refrigerant circulates in the system and its direction is driven by pressure differences.  In order to create such pressure differences required to become the driving force of the circulation, a compressor is required.  The compressor raises the low pressure of the vapour from the evaporator, to a high pressure vapour which is then released to the condenser.

The condenser and evaporator are devices which perform phase transitions between refrigerant in its vapour and liquid states, and absorb or reject latent heat.  While the condenser transforms vapour to liquid and rejects heat to some media like cooling water or ambient air, the evaporator transforms liquid into vapour by absorbing necessary heat from the media which is to be chilled.

The flow direction of the liquid refrigerant from condenser to evaporator through the expansion valve is determined by the pressure difference between these two devices.

 It is common practice to represent a refrigeration process in a log(p)-h diagram (p: pressure, h: specific enthalpy), which is shown in figure below:

Condensation and evaporation occur at two different pressure levels which are created through a compressor.  Each pressure level corresponds to a specific temperature, either the evaporation temperature or the condensation temperature.  In order to ensure the desired direction of the heat flow in the heat exchangers, the evaporation temperature has to be below the required outlet chilled media temperature, and the condensation temperature has to be above the required outlet cooling water or ambient air temperature.  Since chilled and cooling media temperatures are given for the process, the required pressure difference Δp is unambiguously determined by them.

The existence of a pressure difference Δp is essential for the process and created by the compressor which thereby enforces the circulation of the refrigerant. Provision of electrical energy to the compressor is necessary to create the pressure difference Δp. Hence the efficiency of the vapor compression process is determined and limited by the amount of electrical energy used to run the process.

Now the explanation of the ideal thermosyphon process becomes very easy from the description above and shown in the figure below:

The difference between the vapor compression process of figure 2 and the thermosyphon process of this figure is obviously the disappearance of compression and expansion in the thermosyphon process.

Ideally, the refrigerant never changes its pressure and its temperature (ignoring super-heating or sub-cooling at the moment); it changes between its two phases vapor and liquid only. During evaporation the refrigerant picks up heat from the chilled medium, during condensation it rejects heat to a cooling medium like water or air. The resulting high efficiency of the thermosyphon process is now due to the fact that no electrical energy for a compressor is required at all to compress vapor. This feature of a thermosyphon chiller is common with free cooling devices of other types.

Natural circulation now refers to the specific design of a thermosyphon chiller which – in an ideal manner – is driven by flow induced by gravity. Vapor is produced in the evaporator and can escape from there only through a pipe into the condenser where it is transformed into liquid. It returns to the evaporator through pipes as well which must be directed downwards from the condenser to the evaporator. No fan or pump is needed to enforce these flows.

The drawing below decribes the design of a typical thermpsyphon chiller:

A thermosyphon chiller can operate in two modes. If bypass valves 1 and 2 are closed, then the chiller operates in vapour compression refrigeration cycle using an additional flash tank separator for the compressor’s economizer and the refrigerant flow follows the red, cyan, and light green piping.

In thermosyphon mode, opening of valve 1 and shutting off the compressor connects the evaporator directly to the condenser.

Opening of valve 2 and closing the expansion valve to the flash tank separator connects the condenser directly to the evaporator and allows the condensate to flow to the evaporator due to gravity along the light and dark green piping lines.

The only devices still driven by electricity in the thermosyphon mode are the fans of the condenser.  Again, thermosyphon chillers have this property in common with other conventional free-cooling devices.


First Impressions

The cooling capacity of a chiller in thermosyphon mode depends on the achievable mass flow of refrigerant due to natural circulation.  Every bypass pipe between condenser and evaporator has to be properly dimensioned to allow unrestricted flow.  All bypass valves have to have lowest possible resistances and no leakages when closed.  Valves have to open and close without the need of any differential pressure.  These are necessary prerequisites for a thermosyphon chiller and are confirmed in the publications of manufacturers who offer such chillers.  Even if the refrigerant flow would be unrestricted and no noticeable pressure drops would occur within the loop, the mass flow would not be as high as in a vapor compression cycle.

Therefore the cooling capacity in thermosyphon mode is much smaller than in a vapor compression refrigeration mode. This is acceptable for most applications since required cooling capacities at low ambient air temperature conditions are also much smaller than at high ambient air temperature conditions.  Again, this does not make a difference to conventional free cooling devices.

It is a benefit of thermosyphon chillers that this kind of free cooling requires only a minimum of additional hardware comprising piping and two valves.  By far the most expensive components, which are coils and fans, are shared between the two different operating modes. Sharing of components normally used only in a vapor compression refrigeration mode is one of the benefits of a chiller with a thermosyphon feature and when compared to conventional free cooling devices.

However, sharing of components may also be regarded as a weakness.  Common components correlate cooling capacities in vapor compression refrigeration mode with cooling capacities in thermosyphon mode.  These two capacities are no longer independent of each other.  Conventional free cooling devices offer much more flexibility to arrange conventional and additional free cooling capacities.

Sharing of components also inhibits parallel operation of free cooling devices with conventional chillers.  At some ambient temperature condition free cooling devices are enabled and operate in parallel with the chiller.  If ambient temperature decreases furthermore, free cooling devices provide more and more cooling capacity and the vapor compression refrigeration cycle may be switched off eventually.

A thermosyphon chiller, however, can operate in one of two modes only.  If the required cooling capacity is still high, switching to the more energy efficient thermosyphon mode may not provide sufficient cooling capacity.  Manufacturers try to circumvent this obvious weakness by introduction of a second refrigeration circuit.  One circuit would then still be operated in vapour compression refrigeration mode, the other circuit in thermosyphon mode.  However, this solution is applicable to very large chillers beyond 1 MW only.  The sweet spot of the market for such chillers is around 500 kW for Turbocor based air-cooled chillers.  Such chillers are single circuit machines due to cost reasons.



Free cooling by a conventional or a thermosyphon device requires specific relations between chilled medium outlet temperature and ambient air temperature.

We use the following symbols for temperatures:

  • Evaporation temperature : To
  • Condensing temperature : Tc
  • Chilled media inlet temperature : Tin
  • Chilled media outlet temperature : Tout
  • Ambient air temperature : Ta

The ideal situation in thermosyphon mode is given in case of To = Tc. Tout is above To and Ta must be below To.

The figure below shows temperatures of chilled media, refrigerant, and air while they flow through the heat exchangers from inlet to outlet.

Air is heated up from ambient air temperature to some higher temperature absorbing heat from the condensation process.

Refrigerant temperature will not change during condensation and evaporation processes.

The temperature of the chilled medium is lowered since heat is transferred to the air by the process.

As an example, Cofely Refrigeration has published data which are shown here, see reference [1].

(Image shown above Cooling capacity of QUANTUM-Ecoloop A030-P1C-L2)

(curves correspond to different chilled media outlet temperatures between 6°C and 14°C.)

At Tout = 14°C and Ta = 4°C for an air-cooled chiller of type A030 with typically 270 kW nominal cooling capacity, the thermosyphon process shall provide about 42 kW cooling capacity or 15% of the chillers nominal cooling capacity.




Chillers with a thermosyphon feature are an alternative to conventional free coolers. Benefits are low costs for the additional hardware comprising pipes and valves and the shared use of expensive chiller components like coils and fans. These chillers also avoid a more complex hydraulic piping as well as additional pumps and heat exchangers for closed glycol loops.

The weakness of thermosyphon chillers is a lack of flexibility with respect to available free cooling capacity and the need to select one of two operating modes. For these reasons thermosyphon chillers cannot replace conventional free cooling devices in most cases but they may be an alternative in specific cases where they can match exactly the customer’s requirements.

We expect conventional free cooling devices continuing to have a much higher market share and thermosyphon chillers to remain the exceptional solution – but it is good to know about this feature.




[1]         QUANTUM-Ecoloop Brochure, ©2010 COFELY Refrigeration GmbH

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