Читать книгу Hydraulic Fluid Power - Andrea Vacca - Страница 26

Hydraulic control technology began and progressed with water as a working fluid until the beginning of the twentieth century, when mineral oil fluids became available. After that, the use of water for hydraulic control systems remained confined to very specific and limited applications, and basically all applications have been “oil hydraulics.”

In the 1990s, the fluid power community renewed the interest for the use of water hydraulics for both mobile and industrial applications. This was mainly pushed by a couple of major players in the fluid power industry [12]. The main motivations for this renewed interest were driven by the following properties of water with respect to mineral oil:

 lower viscosity (about 30 times lower), which means less energy losses due to fluid shear;

 lower temperature dependency of viscosity;

 higher bulk modulus (about 50% higher);

 lower air release (up to 20 times); and

 higher specific heat (more than double) and higher thermal conductivity (about five times), which promote the cooling ability of the working fluid.

Additionally, one should not forget that water is inexpensive, and it does not pollute the environment. Despite these advantages, the following limitations have so far prevented the success of water hydraulics:

1 – Lower viscosity. Lower viscosity is a merit when it comes to frictions but is a limit for the lubrication ability of the fluid. Fluid lubricity is an essential property for guaranteeing the correct operation of fluid power components, particularly volumetric pumps and motors. These components present lubricating gaps in which the fluid has to generate a film able to avoid direct contact between the parts in relative motion.

2 – Corrosion and oxidation. While normal steel and iron can operate without durability problems with mineral oils, metals and other materials compatible with water tend to be more expensive in general.

3 – Vapor pressure and freezing point. The typical values of vapor pressure and freezing point limit the operating range of water hydraulic systems, particularly with respect to the fluid temperature. Normally, water hydraulic systems need to operate in the range of 4–50 °C.

4 – Bacteria and fungi. Water is the essence of life. This is not good news for a hydraulic machine. When the system is at rest, bacteria and fungi can grow in the different parts of the system, causing problems related to fluid contamination.

Because of these factors, components for water hydraulics applications need to have a special design, and they usually end up with high manufacturing costs. Consequently, water hydraulics is still considered to be a niche field, which is now almost limited to high‐pressure washing systems, reverse osmosis water production systems, and some other marine applications.