So the patriot in me wants to entertain her readers with the story of a milestone in the history of engineering thermodynamics – set by an Austrian!
Rittinger’s challenge was to find a method for economic production of salt from brine in the 1850s in Ebensee, Upper Austria. To extract salt from the brine, water needs to be evaporated. However, forests had already been cut down and the transport of coal was prohibited by the lack of railroad tracks.
These were the times when progress in technology was not yet fuelled by cheap fossil fuel. Innovation was triggered by scarcity.
(Edit: 3 years later a wrote a more detailed article about Rittinger and his invention.)
Rittinger’s Steam Pump
Brine was heated in a vessel, but the vapor was not simply vented to the environment: The steam vessel was closed, and the vapor fed into a mechanical compressor. This compression was driven by hydro power – a water wheel!
The compressed vapor was fed into a cavity beneath a bottom of the vessel, Rittinger has called this a double bottom . After compression, the temperature and the pressure of the vapor in the double bottom increased. Therefore heat could be transferred to the cold brine in the upper part of the vessel when the high pressure vapor in the cavity below to condensed.
The system operated in batch mode: In order to get the system going, it needed to be heated up to about 100°C. Then it was closed, brine was heated (more), salt precipitated in the upper part of the vessel, and water accumulated in the double bottom.
This principle of mechanical vapor compression is still used in today’s systems for salt production . Salt is continuously collected at the bottom of the boiler.
Why does this work? Why does the system resemble a heat pump?
1) How does a heat pump work?
A heat pump used for domestic heating energy allows for extracting energy from the (rather) cold environment to heat the (rather) hot building. Since this would violate the 2nd Law of Thermodynamics, you need to pay a toll – electrical energy needs to be provided to drive the process. As pointed out before ‘hot’ and ‘cold’ need to be stated in terms of absolute temperatures. Then temperature differences are not that impressive. The heating energy delivered to the building is equal to the sum of the energy extracted from the environment and the electrical energy, and the electrical energy is only a fraction of the total heating energy (about a factor of 3 to 4 in domestic heating).
The refrigerant is flowing in a closed cycle: It gains energy when it evaporates at low pressures and low temperatures – the ambient / source temperature – and it dumps that energy when it condenses at high pressures and high temperatures. A compressor is needed to increase the pressure, and after condensation the pressure is lowered again by means of an expansion valve.
2) Steam Pump versus Heat Pump
In Rittinger’s steam pump energy is retrieved from the vapor, after (another part of the) vapor has been put into a state by compression that allows for condensation at rather high temperatures. So the vapor is both ‘the environment’ as well as the ‘refrigerant’ here.
Here is an illustration of Rittinger’s system I crafted for our German blog: Brine is fed in to the upper part of the vessel (from the left, blue). Vapor in the upper part of the closed vessel for brine (dark red) is compressed (light red) and this heated and fed into the lower part of the vessel. An auxillary heating system (right) adds more hot steam.
Modern systems using mechanical vapor compression are open and deliver salt continuously (Rittinger’s system used a closed vessel opened regularly to collect the salt – so overall it was also open in a sense): Brine is fed in – salt and water leaves the system. However, it is not uncommon in engineering thermodynamics to compare on open system with an equivalent closed system. E.g. a gas turbine (jet engine) is open, but nonetheless similar to a theoretical model system with connected inlet and outlet. The open steam pump can be converted into a closed system by inserting the missing expansion valve theoretically.
You need a minimum temperature difference to transfer heat from the vapor to the liquid brine – this is a heat exchanger. The boiling point of a solvent increases with the concentration of a solution, and the concentration of salt is higher in the liquid (that’s the point of the whole system after all). Thus the vapor needs to be compressed as much as to overcome at least this temperature difference.
A heat pump system is characterized and benchmarked by its COP – Coefficient of Performance, ‘useful output over input’, that is: Heating energy over electrical (mechanical) energy put in. The COP increases with decreasing the temperature difference between the heat target and heat source.
In a steam pump these differences were small compared to typical values used in domestic heating; this might explain why the ‘heat pump principle’ had been applied to salt production long before it had been considered for heating of buildings .
 History of Heat Pumps by M. Zogg, Report by the Swiss Federal Office of Energy, 2008. Rittinger’s ‘steam pump’ is mentioned on page 13.
 An anonymous report, quoting the overview part of Rittinger’s original paper, 1855 (German)
 Drawing of Rittinger’s steam pump (reconstructed), German captions. Update 2015: Link broken.
 Modern solutions for mechanical vapor compression, e.g. used in the salt industry. Mechanical vapor compression is described on page 4.
 PPT Presention on how salt production is done today in Ebensee. In German, but the essential figure in slide 8 has English captions. The heat pump (Wärmepumpe) / vapor compressor is depicted in slides 3 and 8.