Water is actually commonly used as a thermodynamic fluid in various applications, especially in power generation and heating systems. However, there are certain conditions and limitations that may make water unsuitable for specific thermodynamic processes. Here are a few reasons why water may not be used in certain scenarios:
Temperature limitations: Water has a limited temperature range in which it remains in a liquid state under normal atmospheric conditions. Water boils at 100 degrees Celsius (212 degrees Fahrenheit) at sea level, which imposes an upper limit on its use in high-temperature applications. For processes requiring higher temperatures, alternative working fluids with higher boiling points, such as specialized oils or refrigerants, may be preferred.
Pressure limitations: Water has a relatively low critical point, which is the temperature and pressure above which it cannot exist in a liquid phase. The critical point of water is at 374 degrees Celsius (705 degrees Fahrenheit) and 22.1 megapascals (MPa). Beyond these critical conditions, water becomes a supercritical fluid with different properties. In certain thermodynamic processes that involve extreme pressures or temperatures, other fluids that have higher critical points or better stability under those conditions might be used.
Corrosion and scaling: Water has the potential for corrosion and scaling, particularly when exposed to certain metals or minerals. Corrosion can damage equipment and reduce system efficiency, while scaling can lead to the formation of deposits that impede heat transfer. To mitigate these issues, water treatment processes are often employed, such as adding corrosion inhibitors and using water softeners. However, in some cases, alternative fluids may be preferred to avoid these challenges altogether.
Specific performance requirements: Certain thermodynamic processes have specific performance requirements that may necessitate the use of alternative fluids. For example, in refrigeration or air conditioning systems, refrigerants with desirable properties like low environmental impact, high energy efficiency, and appropriate thermodynamic characteristics are preferred. Different refrigerants, not necessarily water-based, may be used to meet these requirements.
Despite these limitations, water remains an incredibly versatile and widely used thermodynamic fluid, especially in steam power plants, hydropower generation, and heating systems. Its abundance, relatively low cost, and excellent heat transfer properties make it a preferred choice in many applications where its limitations are not a hindrance.