Start and heat the glass tempering furnace to the rated operating temperature. Monitor and measure the temperature in the furnace using a thermometer, while recording the heating time with a stopwatch. The heating time refers to the time it takes for the average temperature of each temperature controlled area to reach the rated operating temperature after the furnace door is closed.
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2020年3月17日星期二
What effects do the ceramic rollers in the furnace have on the flatness of tempered glass?
The repeated use and cleaning of the ceramic rollers over time inevitably lead to wear, especially when sanding is required to remove some tough deposits on the rollers. To heat the glass on the rollers with an uneven surface or out of roundness to the transition temperature will certainly result in deformation.
Although the ceramic rollers used in the glass tempering furnace have superior thermal stability and high resistance to thermal shock, the possible unevenness of its internal structure may also lead to deformation under high temperatures. The thermal deformation of rollers will cause the ceramic roller bed to bend, leading to the deformation of glass conveyed on the roller surface.
2020年1月13日星期一
Classification of Vacuum Level
The so-called “vacuum” refers to a gaseous state where the pressure in a given space is less than 101325 Pascal (i.e. a standard atmospheric pressure around 101KPa).
The standard or unit of vacuum measurement is called vacuum level. In the international general standards and China’s current standard in force, the most commonly used unit in vacuum measurements is Pascal ( Pa.). 1 Pa is equal to 1N/㎡. The unit used in technical atmosphere is Bar.1Bar=1kgf/cm2. The unit now used less often is torr. 1Torr is defined as 1/760atm.
International general classification of vacuum levels
The international standard usually divides the vacuum into four levels: low vacuum, medium vacuum, high vacuum, and ultra-high vacuum. In absence of strict criteria, the vacuum levels are roughly divided as follows:
Low Vacuum Level （< 105~102Pa） —— The pressure difference obtained by low vacuum is used to clamp, lift and transport materials, as well as to vacuum and filtrate (e.g. vacuum cleaner, vacuum suction cups, etc.)
Medium Vacuum Level （< 102~10-1Pa） —— Medium vacuum is generally used to remove the gas or moisture that is retained and dissolved in the material, as well as for vacuum heat insulation and isolation (e.g. food vacuum freezing, vacuum drying, vacuum packaging, etc.)
High Vacuum Level （< 10-1~10-5Pa） ——vacuum smelting, vacuum coating, and manufacturing of vacuum devices can be done by using the characteristics of low density of residual gas and weak chemical action of any substance in high vacuum. A typical product is vacuum insulated glass. Vacuum insulated glass refers to the glass consisting of two or more pieces of flat glass with micro supporting spacers inside and edges sealed to create vacuum chamber in between. In principle, the vacuum level inside the vacuum insulated glass is lower than 10-1pa, so that the factors such as heat transfer by gas and sound transmission can be ignored.
Ultra-high Vacuum Level （< 10-5Pa） —— In the ultra-high vacuum state, there is almost no atoms or molecules. It can be used for simulation of a space environment, as well as researches on surface physics and surface chemistry, etc.
The international standard usually divides the vacuum into four levels: low vacuum, medium vacuum, high vacuum, and ultra-high vacuum. In absence of strict criteria, the vacuum levels are roughly divided as follows:
Low Vacuum Level （< 105~102Pa） —— The pressure difference obtained by low vacuum is used to clamp, lift and transport materials, as well as to vacuum and filtrate (e.g. vacuum cleaner, vacuum suction cups, etc.)
Medium Vacuum Level （< 102~10-1Pa） —— Medium vacuum is generally used to remove the gas or moisture that is retained and dissolved in the material, as well as for vacuum heat insulation and isolation (e.g. food vacuum freezing, vacuum drying, vacuum packaging, etc.)
High Vacuum Level （< 10-1~10-5Pa） ——vacuum smelting, vacuum coating, and manufacturing of vacuum devices can be done by using the characteristics of low density of residual gas and weak chemical action of any substance in high vacuum. A typical product is vacuum insulated glass. Vacuum insulated glass refers to the glass consisting of two or more pieces of flat glass with micro supporting spacers inside and edges sealed to create vacuum chamber in between. In principle, the vacuum level inside the vacuum insulated glass is lower than 10-1pa, so that the factors such as heat transfer by gas and sound transmission can be ignored.
Ultra-high Vacuum Level （< 10-5Pa） —— In the ultra-high vacuum state, there is almost no atoms or molecules. It can be used for simulation of a space environment, as well as researches on surface physics and surface chemistry, etc.
2020年1月10日星期五
Thermal Resistance, Thermal Conduction, Thermal Conductance, Heat Transfer Resistance
Thermal Resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow.
The law of heat transfer can be compared to the “Ohm’s Law” that describes the law related to the electricity transfer in electrical science.
Ohm’s Law: I=U/R
where I is the electrical current flowing through the circuit; △V is the voltage drop between the two ends of the circuit and the electromotive force that drives the flow of electricity; R is the resistance of the resistor, representing the resistance in electricity transfer.
A similar formula in the process of plane heat transfer is: Φ=T/R where Φ is the amount of heat flow transferred; T is the temperature difference between the two ends and the force that drives the heat flow; R is the thermal resistance representing the resistance in heat transfer.
An object with better thermal conductivity generally has lower thermal resistance, measured in ℃/W or K/W.
Thermal Conduction is the reciprocal of thermal resistance, i.e. C=1/R，in W·m-2·K-1.
Thermal Conductance, also known as heat transfer coefficient, refers to the quantity of heat that passes in unit time through an object of 1㎡ area and 1 m thickness when its opposite surfaces differ in temperature by 1K under steady state conditions. The legal measuring unit in China is W·m-1·K-1.
Heat Transfer Resistance is the inverse of heat transfer coefficient, measured in W-1·㎡·K. The greater the heat transfer resistance, the better the thermal insulation performance.
2020年1月8日星期三
Definition, Harms, and Grading Standard of Noise
By physical definition, noise is the sound that delivers completely irregular oscillations in amplitude and frequency. From the perspective of environmental protection, noise is the unwanted sound.
The distinctive characteristic of noise is that: there is no visible pollutant; it does not generate energy accumulation; it lasts for limited time and is transmitted in limited distance; noise disappears along with the vibration source and cannot be controlled through centralized systems. Noise could come from transportation, machinery and equipment in factories, construction work as well as the social and family activities of ordinary people.
Noise can harm human beings in many aspects, including hearing loss, sleep disturbances, physical and psychological disorders. Working in an environment with a noise level of 100 dB could make people feel unpleasant, uncomfortable, and even suffer from temporary hearing loss.Noise louder than 140 dB will cause eyeballs to vibrate, vision blurring， and fluctuation of respiration, pulse, and blood pressure. It may even cause blood vessels to contract throughout your body, reduction in blood supply, and adverse effect on speaking ability.
dB is the standard unit of measurement for noise, called decibel. It is the unit for expressing the ratio of power.
dB Levels
| |
0-20 dB
| Very quiet, hard to hear |
20-40 dB
| Quiet, whisper |
40-60 dB
| Normal indoor conversation |
60-70 dB
| Loud, harmful to nerve |
70-90 dB
| Very loud, could cause damage to nerve cells |
90-100 dB
| Intensified noise, hearing impaired |
100-120 dB
| Unbearable, temporary hearing loss with only one minute of exposure. |
| Greater than 120 dB | Itching or pain in the ear, extremely or totally deaf |
| About 300 dB or higher | Permanent hearing loss |
The national noise emission standard for social environment are as follows:
Applicable areas | |||
0
|
50
|
40
| |
1
|
55
|
45
| |
2
|
60
|
50
| |
3
|
65
|
55
| |
4
|
70
|
55
| |
Hertz is also a unit of frequency in the international system of units. It measures the repetitive times of cyclical variation per second. Its symbol is Hz. Noise can be divided into low frequency noise, medium frequency noise and high frequency noise according to the sound frequency.
Low frequency noise refers to sounds with frequencies below 500 Hz.
There are five main types of low-frequency noise sources in China’s residential communities: elevators, transformers, water pumps in high-rise buildings, central air conditioning systems, and traffic.
Medium frequency noise refers to sounds with frequencies between 500～1000 Hz. Normal conversation in daily life and the sounds of most musical instruments are in the range of medium frequency.
High frequency noise refers to sounds with a frequency greater than 1000 Hz. High-frequency noises come from modern means of transport that we are familiar with, such as cars, trains, motorcycles, tractors, and airplanes, as well as the noises generated by tweeters, construction sites, shopping malls, sports and recreational venues, etc.
2020年1月7日星期二
Vacuum-degassing
When a variety of materials that makes up the vacuum insulated glass, such as glass, stainless steel, getter, and glass solder, are exposed to air, the surface of the material will absorb and dissolve a certain amount of gas. It later becomes the source of gas released by the materials sealed between the layers of the vacuum insulated glass. Vacuum insulated glass is the type of small volume vacuum unit that has a large surface area. A piece of 1 m2 vacuum insulated glass has an inner cavity with a volume equivalent only to the size of a ping-pong ball. Therefore, the gas released in the chamber of the vacuum insulated glass will have a great impact on the level of vacuum. For vacuum insulated glass degassed and sealed under room temperature, its level of vacuum will degrade rapidly within hours. The degassing process of vacuum insulated glass requires not only to fully discharge the gas inside the chamber, but also to bake the entire piece of glass to a certain temperature to release the gas absorbed by the surfaces and dissolved in the materials to the cavity for degassing.
As glass is the main material that makes up the vacuum insulated glass, the degassing temperature must first consider the degassing requirements of the glass material. The gas sources of glass during baking come primarily from its surface, surface layer, and inner layer.
1. Glass surface
There is a large amount of OH- existing on the surface of the glass, which has a very strong affinity to water. As such, the glass surface attracts a large number of water molecules and a small amount of CO2. Some gas is bonded with the glass surface only through physical attraction or weak chemical attraction. Heating it in vacuum to approximately 150℃ will release the gas molecules in a couple of minutes.
2. Weathered surface
The sodium calcium glass used in vacuum insulated glass contains a considerable amount of alkaline oxide which has poor chemical stability and is susceptible to moisture corrosion and weathering. The weathered layer is generally a few microns thick. The gas contained is mainly H2O. When the glass is heated to a certain temperature in a vacuum environment, the water will be released smoothly from the Si-OH-OH-Si structure to form Si-O-Si+H2O. The weathered layer of the glass is then restored. The gas in the glass surface can be removed by baking in a vacuum state for about 1 hour. The weathered layer can also be removed by baking in dry air with a humidity level less than 60% for about 1 hour. The effect is comparable to that of vacuum degassing.
3. Inside of glass body
The body of the glass contains a large amount of gas, mainly H2O and a small amount of CO2, O2 and SO2. It takes certain amount of time to bake and degas.
After the degassing process, as long as the working temperature of the glass does not exceed 300℃, the discharge of gas in the glass has minimal effect on the level of vacuum of the device and can therefore be neglected.
2020年1月6日星期一
Chemical Stability of Glass
Chemical stability: the ability of a substance to maintain its original physical and chemical properties under the influence of chemical factors.
The chemical stability of glass refers to the resistance of glass towards erosion by chemical solutions during the process of use such as water, acid, alkali, salt, gas, etc. It depends on the resistance of the glass against corrosion and the type and characteristics of the erosion medium (water, acid, alkali, and atmospheric agents, etc.).
Main factors affecting the chemical stability of glass:
1. Chemical components
（1） The water resistance and acid resistance of silicate glass mainly depend on content of the silicon, oxygen and alkali metal oxide.
（2） When the glass contains two kinds of alkali metal oxides at the same time, some extreme values will emerge in the chemical stability of the glass due to the “mixed alkali effect”.
（3）The chemical stability of the glass will decrease when the silicon oxide in silicate glass is replaced with an alkaline earth metal or other metal dioxides. But the reducing effect is less than that of the alkali metal.
2. Heat treatment
（1） Generally speaking, annealed glass enjoys the higher chemical stability than tempered glass.
（2） When glass is processed with open flame in the annealing process, its chemical stability will be increased. On the contrary, if glass is annealed in flameless fire, its chemical stability will decrease.
（3） Phase separation will occur during the annealing process of borosilicate glass. Its chemical stability is related to the resulting structure after phase separation.
3. Surface state
（1） The corrosion of glass by medium occurs first on the surface, and the glass surface state has great effect on chemical corrosion resistance.
（2） Therefore, the glass surface state can be improved with the help of various surface treatments.
4. Temperature and pressure
（1）The chemical stability of glass varies greatly with an increase in temperature.
（2）Pressure also has a great effect.
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