Could you please tell me what these letters represent?

The Combined Water Treatment (abbreviated as CWT) technology is a new water treatment method that was officially developed in Germany in 1982; it evolved from Neutral Water Treatment (abbreviated as NWT). Neutral water treatment involves adding hydrogen peroxide or gaseous oxygen to neutral, high-purity feed water, and it is suitable for once-through boilers. Later, due to the insufficient buffering capacity of neutral pure water – the corrosion rate of steel increases sharply when the pH is below 6.4 – a treatment method combining ammonia and oxygen with a trace amount of ammonia was introduced, namely CWT, to maintain the feedwater pH at 8.0–8.5. The advantages of this approach are becoming increasingly evident. It has now been recognized and promoted by many **electricity sector entities, and the suitable range for pure water pH has been extended to 7.0–9.0; inexpensive gaseous oxygen is commonly used in practical applications.

1. If the major boiler overhaul takes a long time, it is necessary to check the corrosion and thickness of the boiler’s water side. Before maintenance, the temperature should be reduced to room temperature in accordance with the operating procedures; all water must be drained and the area dried using an inert gas, otherwise the metal on the water side will suffer from electrochemical corrosion due to the presence of air. After drying, air can be introduced to inspect the metal surface on the water side of the boiler: The color of the metal surface on the water side of the boiler is determined by the components of the protective film on that surface, and these components in turn depend on the treatment method used for the feedwater. When an alkaline method is used for water treatment, the film layer formed by methods such as conventional alkaline treatment and fully volatile alkaline treatment (AVT) is primarily composed of ferric tetraoxide, which is black in color ； The film layers formed by neutral oxygen treatment (NWT) and combined ammonia-oxygen treatment (CWT) have a dual-layer structure; iron tetraoxide is present right at the surface of the steel, while the outer layer is primarily composed of spinel-type iron oxide, which is reddish-brown in color. If that’s not the case, it means something is abnormal. Furthermore, to determine whether the protective film is in good condition, one should consider not only its color but also factors such as its density, continuity, bonding strength with the metal substrate, and thickness. 2. Definition of supercritical fluid: A liquid whose temperature and pressure are both above the critical values is called a supercritical fluid. Properties of supercritical fluids: It is essentially still in a gaseous state, but it differs from ordinary gases as it is a dense gaseous form. Its density is two orders of magnitude greater than that of ordinary gases, and is similar to that of liquids. Its viscosity is lower than that of a liquid, but its diffusion rate is faster than that of a liquid (by about two orders of magnitude), so it has good fluidity and transport properties. Its dielectric constant changes sharply with pressure (for example, an increase in the dielectric constant facilitates the dissolution of some highly polar substances). Principles of application of supercritical fluids: The solubility of substances in supercritical fluids is greatly influenced by pressure and temperature. By increasing the temperature or reducing the pressure (or both), it is possible to separate the substances dissolved in the supercritical fluid, thereby achieving separation and purification (it functions simultaneously as distillation and extraction). For example, under high pressure, the supercritical fluid is brought into contact with the material; the active components in the material dissolve in the supercritical fluid (i.e., extraction occurs). After separation, the pressure of the supercritical fluid containing the dissolved substances is reduced, causing the substances to precipitate out. If there is more than one active ingredient (solute), gradually reducing the pressure allows multiple solutes to precipitate step by step. There is no phase change during the separation process, resulting in low energy consumption. The application prospects of supercritical fluids include supercritical fluid extraction, supercritical fluid chromatography, and chemical reactions in supercritical fluids, among others; however, supercritical fluid extraction is the most widely used application. Many substances have a supercritical fluid region, but due to CO2’s relatively low critical temperature (364.2 K), low critical pressure (728 MPa), as well as its non-toxic, odorless, and environmentally friendly properties, CO2 supercritical fluid is often used in practical applications. Such as using supercritical CO2 to remove caffeine from coffee beans, nicotine from tobacco, and glycerides from soybean or corn germ, as well as for deodorizing peanut oil, palm oil, and soybean oil. Another example is the extraction of carthamin and carthamidin from safflower (which are effective components in treating hypertension and liver diseases), as well as the extraction of evening primrose oil from evening primrose (which has good therapeutic effects on cardiovascular diseases), among others. The only drawback of using supercritical technology is the involvement of high-pressure systems; when used on a large scale, it requires high standards in terms of process and technology, and the equipment costs are also high. However, due to its many advantages, it remains highly valued.