December 2016 Archives

(5) Air ratio control in combustion

To increase the thermal efficiency of a boiler, it is necessary to improve the combustion efficiency. In general, liquid fuel and gas fuel are used for combustion. In both cases, all of their energetic materials, such as carbon (C) and hydrogen (H), must be converted into thermal energy where required.

Combustion is a phenomenon that combustible substances chemically combine with oxygen in the air to produce thermal energy. Complete combustion is a combustion reaction that consumes all of the combustible substances to produce heat at 100% efficiency. The burn-up equation of carbon and hydrogen is as follows:

From this formula, we can see that 12 kg of carbon requires 32 kg of oxygen to achieve complete combustion. Fuel analysis allows you to estimate how much oxygen is required for a certain fuel type to combust completely based on the combustible components.
In addition, oxygen required for combustion is supplied from the air. From the atmospheric constituent listed in [Table 1], you can also estimate the air volume required for complete combustion.

[Table 1] Atmospheric constituent (%)

However, complete combustion is not possible at the theoretical air volume in actual combustion processes. Unburned fuel will be emitted as black smoke and discharged from a chimney. This is due to an inefficiency of combustion burners. Burners currently available are not capable of achieving complete combustion.

To eliminate the emission of black smoke, it is necessary to feed more volume of air than the theoretical requirements for combustion. However, excess air will lower temperature inside the furnace and will increase the exhaust gas temperature and heat loss. The combustion management required here is called 'air-ratio control'.

From [Table 1], fresh air fed in the boiler contains 21% of oxygen. The concentration decreases because this oxygen will be consumed in the combustion process. The concentration of consumed oxygen can be determined by measuring oxygen concentration in the exhaust gas. ([Fig.1])

Calculate the air ratio by the following formula:

The air ratio is generally managed at 1.3. Besides the oxygen concentration, the air ratio is also controlled by the CO₂ concentration increased by combustion.

We often experience transparent materials like windows or glasses become foggy due to humidity or steam. When this happens, it is not only inconvenient but also dangerous at the same time as it may lead to a serious accident.
This fogging occurs when the hydrophobic glass surface forms water droplets, which cause the light to diffuse, refract, and reflect. On the other hand, the contact angle of water droplets becomes smaller on hydrophilic surfaces. This causes moisture to spread into an even layer on the surface as the continuous phase and does not generate fogging.
Glasses or plastics become fogged up by the diffused light reflection when the surface temperature falls below the dew point and moisture in the air forms tiny water droplets. Methods listed in [Table 1] are adopted as the means to prevent fogging.

[Table 1] Anti-fogging method

For example, a formula made primarily of waterproof cross-linked polymer and polyether-polyol mixed with surfactants is available as anti-fog coatings. The hydrophilic characteristics added by polyether-polyol prevent the film from fogging by absorbing moisture. Even at the moisture-absorbing saturation point, the surfactants spread water droplets to form a wet surface and keep the surface fog-free.
Because surfactants are generally water-soluble, the anti-fog performance may become degraded as they evaporate with water. To prevent this drawback, several types of different surfactants are mixed together.
To evaluate the anti-fog effects, perform the followings: [1] Move the sample from the thermo-hygrostat bath at -10℃ to another bath kept at 30℃ and 65%RH to check if the sample becomes fogged up; [2] Move the sample from the thermo-hygrostat bath at 10℃ and 65%RH to another bath kept at 40℃ and 65%RH to check if the sample becomes fogged up; [3] Expose the sample to 90℃ steam and check to see if an interference pattern of water appears.

This volume introduces the other materials used for electroless plating besides nickel.

(1) Electroless gold plating

Gold-boron alloy plating is used for tiny electrical components.

(2) Electroless silver plating

Since early times, this method has been used as a silver mirror reaction in various fields. Examples of this plating are conductive treatment for giving an electroforming mother die the electrical conduction property and a mirror finish performed on the inner surface of thermos bottles.

(3) Electroless chrome plating

This displacement plating from trivalent chromium is not designed for depositing a thick layer.

(4) Electroless cobalt alloy plating

This metal works as magnetic plating and is used for memory apparatus. This type of plating includes cobalt-iron-phosphorus alloy, cobalt-tungsten-phosphorus alloy, cobalt-nickel-manganese alloy and more. Electroless plating composed of only cobalt is also available and used for high-density memory apparatus.

(5) Electroless tin plating

This method has long been used as immersion tin. This is also classified as displacement plating and is not designed for forming a thick layer. It is used for preventing rust temporarily or improving lubrication and solderability.

(6) Electroless copper plating

There are two methods available. With the first method, a layer of up to 1 µm can be deposited under normal temperature. In the latter method, a layer of between 20 and 30 µm can be deposited in a high-temperature bath. The first method is used for through-hole plating on printed circuit boards or conductive treatment for general plastic plating. The latter is used for circuit formation of printed circuit boards, etc.

(7) Electroless palladium plating

This low cost material with excellent electrical properties is used for electrical contact points and connectors as an alternative of gold plating.

(8) Electroless solder plating

The metal is deposited over copper on electrical components and printed circuit boards by displacement plating.

* What is electroless plating
Electroless plating is a technique used to deposit a layer of metal plating without using electrical energy provided to the plating bath from outside.
The reaction can be generally classified as follows:
Electroless plating = Chemical plating = Displacement plating + Autocatalytic plating
Only a thin layer can be deposited by displacement plating where the plated metal (workpiece) is dissolved into plating solution and the metal in the solution is deposited on the workpiece surface in return. The electroless plating generally refers to an autocatalytic process where the metal is deposited only on the workpiece surfaces with catalytic actions. This method allows you to deposit a thick layer.

Ultraviolet rays degrade coatings of painted products and fade their colors.
The phenomena of coating degradation include the followings: [1] Loss of gloss or image definition; [2] Cracking, checking, and loss of perfection; [3] Chalking; [4] Flaking and loss of adhesion; [5] Yellow discoloration; [6] Fade-out of pigment colors; [7] Biological degradation. All of these degradation events are closely related to ultraviolet rays.
It has been discovered that ultraviolet rays contribute significantly to the degradation of the coating ingredients, such as acrylic polymer, polystyrene copolymer, aromatic polyester, and aromatic polyurethane.

Traditional methods of protecting polymers from ultraviolet rays are the use of UV-absorbing pigments or opaque pigments. Pigments like carbon black or titanium dioxide absorb ultraviolet rays and prevent degradation of coating films. Such pigments that act as the UV stability enhancers may be referred to as UV shielding agents.

However, using such pigments may result in undesirable colors, loss of transparency, poor ability to protect the surface and various disadvantages in product design. As an extreme example, color selection is no longer available if you use carbon black.

Because clear coating is becoming more and more important in automobile painting, using opaque pigments that act as UV shielding agents is not an option. The shielding agents only protect the polymers directly underneath. Thus, they cannot protect high-gloss coatings.
Since UV-shielding pigments do not solve all the problems, more UV shielding agents have been developed to this day, such as molecular UV absorbent, light-stable antioxidant, hindered phenol, and hindered amine light stabilizer, for example.

Just like the moisturizer with sunscreen popular among women nowadays, the ultraviolet absorber has improved the performance of protection clear coating. This effect is achieved by combining light stabilizer and ultraviolet absorber.
In this double-block protection, only about one percent of ultraviolet absorber converts unfavorable solar radiation of short wavelength into thermal energy. Hindered amine light stabilizer will act as free radical scavengers to prevent coatings from being degraded. This mechanism has made the base coat plus clear coat application possible in automobile painting systems, which resolved most of the troubles caused by ultraviolet rays.

Chinese white (zinc oxide) absorbs and dissipates ultraviolet rays. Optical transmittance of Chinese white changes drastically at the wavelength range of 380 to 400 nanometers. It transmits visible lights but does not transmit ultraviolet rays. Zinc oxide also absorbs infrared radiation at 1,000 nanometers or less and starts transmitting it at the higher range.
The reflectance is lower for ultraviolet rays, but it increases in the blue range. It becomes the highest at 480 nanometers and slightly decreases in the infrared territory.

(2) Properties of plating film

(1) Film composition

The plating film consists of the aluminum layer composed of the same materials as the plating bath over the Fe-Al alloy layer generated by a reaction of molten aluminum and a steel substrate. Additional processing is required for post-plating treatment since the alloy layer Fe₂AL₅ (52 to 55% of aluminum) is extremely hard but fragile.
For steel plates requiring drawability, Al-Si alloy bath with 2 to 3% silicon (Si) is used for inhibiting the development of Fe-Al alloy layer.

(2) Corrosion resistance

Molten aluminum plating has superior resistance to climate conditions than hot dip galvanizing, especially in coastal areas or in the atmosphere with high content of sulfur dioxide. If they are exposed for a long period, they may be discolored. However, the corrosion will not progress further because the corrosion products are not soluble.

(3) Heat resistance

[Fig.] shows the increasing amount of oxidization measured at certain temperatures after heating the various types of steel plates.

Up to the relatively low temperature of around 500°C, the dense alumina layer formed by oxidation occurred on an aluminum surface layer inhibits the diffusive intrusion of oxygen. For temperature higher than this, the Fe-Al alloy exerts the superior resistance to oxidation when the aluminum diffuses into the steel substrate.

(3) Major applications

[Table] Major applications of molten aluminum plating