How Does a Fluid Pump Work?

Fluid transmission and control uses fluid as the transmission medium and performs transmission and control according to the basic principles of fluid mechanics. It is a practical engineering technology. Its research scope is limited to the flow of working fluid in closed pipelines or vessels. The commonly used working media of fluid transmission and control systems are liquids and gases (compressed air). ^[1]

The selection of the heat pipe working fluid depends on some physical properties of the fluid and the chemical compatibility of the fluid with the container and wick material. The requirements for working fluids for heat pipes are:

(1) High latent heat of vaporization;

Under standard conditions, the composition of dry air is shown in Table 2-6, and its molecular weight is 28.962. Impurities such as water vapor and other gases are also contained at sea level.

Basic functions and practical performance of working fluid

Liquid zone oil must fulfill three basic functions: transmit power (force and motion); lubricate all important moving surfaces and transfer heat generated in the system to the appropriate cooler. In addition, there are some secondary functions, that is, to prevent all exposed rust and corrosion; as a pressure sealing medium, to prevent friction particles from damaging the friction surface: to help remove moisture without emulsifying and to promote exhaust without permanent foaming.

Hydraulic oil must have certain practical properties, the most important of which are: viscosity, viscosity index, bulk elastic modulus, friction resistance, pour point, combustible, stability, and rust and corrosion resistance. Viscosity index refers to the ability of a liquid to prevent viscosity change caused by temperature. The higher the viscosity index, the smaller the change in viscosity of the oil with temperature. Generally, oil viscosity index is required to be above 90 in precision machinery. The pour point refers to the temperature at which the oil stops flowing under a certain drop. It is an important factor for working systems in low temperature environments; this value is at least about 14 ° C lower than the minimum working temperature required by the system. Flammability refers to the flash point of the oil (that is, the minimum temperature of the flammable gas), and the flammability, which is the temperature required for the combustion of the oil. It should be paid special attention in high temperature working environments. The service life of a hydraulic oil is characterized by its stability. When oxygen reacts with various components in the oil: it will degrade the Shan liquid. The oxidized product contains acidic macromolecules that corrode metal parts, which destroys oil lubricity. When the temperature rises, the chemical reaction of the oil is strengthened, and solid substances such as resin and asphalt may be generated, which will block filters, valves, pumps and motors. Therefore, the working temperature of hydraulic oil is generally limited to 65 ° C or lower.

Working fluid commonly used hydraulic oil

Hydraulic devices have a wide range of applications and different conditions of use, so there are many types of hydraulic oils. The following classification methods are generally used:

Petroleum-based hydraulic oil has the advantages of meeting various viscosity requirements, good viscosity-temperature characteristics, good lubricating properties, good oxidation resistance, and good rust and corrosion resistance, so it is widely used. Table 2-3 lists the performance of domestic hydraulic oil. Because hydraulic oil is more expensive, high water-based hydraulic oil was developed at the beginning of the 21st century. For example, oil-in-water emulsified hydraulic oil is one of them. It is made by mixing and emulsifying 90-95% water with additives such as mineral oil The physical properties of this oil, such as viscosity and compressibility, are not much different from water. It has better rust resistance and lubricity than water, and has the advantages of flame resistance and low price. However, due to its low viscosity and poor lubricity, it can only be used in a certain range.

Recently, in addition to the study of cheap high-water-based hydraulic oils, foreign countries have also studied the application of electro-viscous oils whose viscosity varies with electric field strength in hydraulic devices, under electric field strengths of tens of KV / cm. The viscosity of a single-phase liquid can increase by several tens percent; the viscosity of a colloidal solution can increase by several tens of times. By using this effect, a flow control valve without a movable solid part can be made to realize flow and direction control. ^[1]