steam

[zhēng qì]

Gas

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Steam is also called“ steam ”。 According to pressure and temperature, various kinds of steam are classified as saturated steam and superheated steam. Steam is mainly used for heating/humidification; It can also generate power; As machine drive, etc.

Chinese name: steam
Foreign name: steam
Alias: steam

Application: Temperature and humidity control, power, entertainment, cleaning
Glorious period: The first industrial revolution
Classification: saturated Steam, superheated steam
Nature: The Inevitable Result of Capitalist Political and Economic Development

catalog

seven heating process
eight Improve quality

Background

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industrial revolution Also known as the industrial revolution, it is a leap from the workshop handicraft stage to the big machine production stage in the capitalist period. It is a change in the production field and a revolution in social relations. It is the inevitable result of capitalist political and economic development. The completion of the industrial revolution capitalist country , which has had a profound impact on the world. Britain was the first country to carry out the industrial revolution, which began in the 1860s. At the beginning of the 19th century, it expanded to France and the United States, followed by Germany, Russia and Japan. The industrial revolution in various countries was mainly carried out by introducing machines from Britain. Britain's special position in the industrial revolution made it the "world factory" for a long time.^[1]

classification

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Classification of steam according to pressure and temperature

according to pressure And temperature are classified as: saturated Steam, superheated steam 。 See the figure "Classification of various steam according to pressure and temperature".

Saturated steam

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When the liquid Confined space During evaporation, liquid molecules enter the upper space through the liquid surface and become vapor molecules. Because steam molecules are in disorder Thermal movement They collide with each other and with the container wall and the liquid surface. When they collide with the liquid surface, some molecules are attracted by the liquid molecules and return to the liquid to become liquid molecules. At the beginning of evaporation, the number of molecules entering the space is more than the number of molecules in the returned liquid. As the evaporation continues, the density of the vapor molecules in the space increases, so the number of molecules in the returned liquid also increases. When the number of molecules entering the space per unit time is equal to the number of molecules in the returned liquid, evaporation and coagulation Active balance At this time, although evaporation and condensation are still going on, the density of vapor molecules in space is no longer increasing, and the state at this time is called saturation state. The liquid in saturated state is called Saturated liquid The steam is called dry Saturated steam (also called saturated steam).

Advantages and disadvantages of saturated steam heating

Saturated steam has many advantages, especially at temperatures of 100 ℃ (212 ℉) and above, which makes it an excellent heat source. Some of its advantages are as follows:

characteristic	advantage
Rapid and uniform heating with latent heat	Improve product quality and production efficiency
The temperature can be controlled by controlling the pressure	Can quickly determine and control the temperature
High heat transfer coefficient	The heat transfer area is required to be relatively small, which can effectively reduce the initial equipment investment.
Raw material is water	Safe, clean and low cost

The following points must be kept in mind when using saturated steam to heat products:

1. If dry saturated steam is not used in the heating process, the heating efficiency of the product may be reduced. Contrary to common sense, the steam produced by the boiler is often not dry steam, but wet steam containing some water molecules that are not completely evaporated.

2. The heat loss of thermal radiation makes some steam condense, so the wet steam will be more humid and form condensate. Therefore, a drain valve must be installed at an appropriate part of the pipeline to drain the condensate.

3. Heavy condensed water will fall from the steam and be Drain valve Discharge, but the wet steam with small water droplets will still reduce the heat transfer efficiency of the product. Therefore, it must be installed in some parts of the pipeline Steam water separator 。

4. Since the friction of steam in the pipeline will cause pressure loss, it will also cause the corresponding loss of steam temperature.

superheated steam

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If the saturated steam is heated continuously, its temperature will rise and exceed the Saturation temperature 。 The steam that exceeds the saturation temperature is called superheated steam. The superheated steam has its own application field, such as the turbine used in the generator set, which can drive the motor through the nozzle to the motor turn 。 However, superheated steam is rarely used for heat in industrial processes Transfer process , this is because superheated steam condensation Before releasing the enthalpy of evaporation, it must be cooled to the saturation temperature. Obviously, compared with the enthalpy of evaporation of saturated steam, the heat released by cooling superheated steam to the saturation temperature is very small, which will reduce the performance of process equipment.

Advantages and disadvantages of superheated steam heating

In order to ensure that the drive equipment will not be damaged by condensate, superheated steam is generally used.

In order to improve the thermal efficiency and working capacity, for example, the superheat degree under low-pressure steam reaches a large specific volume, or even vacuum.

The obvious advantage of using superheated steam for both supply and discharge is that no condensate will be generated when steam drives equipment, which can effectively avoid the risk of corrosion and corrosion of equipment due to carbonic acid. In addition, theoretical thermal efficiency of the steam turbine is related to enthalpy values at its outlet and inlet. Therefore, increasing the pressure of superheated steam is equal to increasing the enthalpy value in the inlet direction of the turbine, thus effectively providing thermal efficiency.

Disadvantages of using superheated steam for heating:

characteristic	shortcoming
Low heat transfer coefficient	Inefficient production
Low heat transfer coefficient	Large heat transfer area is required
The steam temperature cannot be regulated by pressure control	The superheated steam shall be transported at a higher speed, otherwise the heat will be lost and the temperature will drop
Use sensible heat to transfer heat	The drop in temperature may have an adverse effect on the product
Temperature may be very high	Solid equipment needs to be built, so a high initial investment is required

For these reasons, saturated steam is more suitable as a heat source in heat exchangers than superheated steam. On the other hand, it has another advantage. High temperature steam can be used as a direct heating source in oxygen free environment. The application of superheated steam in food processing industry, such as cooking and drying, is also in progress.

purpose

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Typical applications of steam

The main uses of steam are: heating/humidification; Generating power; As a driver.

Steam for heating or humidification

1. Positive pressure steam

Positive pressure steam refers to steam with pressure of 0.1 - 5 MPa and temperature of 110-250 ℃. This is the most representative use of steam in industry. It is widely used in heat exchangers, steam boxes and other equipment for heating and humidification.

In most cases, the steam used is saturated, that is, saturated steam. because pressure The corresponding relationship with temperature is fixed and can be heated rapidly with the latent heat of steam. In the food processing industry, superheated steam is sometimes used for cooking, drying and dehydration. Superheated steam Atmospheric pressure It is very easy to control after being heated to 200-800 ℃, so it is used in today's domestic steam oven market.^[2]

2. Negative pressure steam

Negative pressure steam refers to air pressure less than Atmospheric pressure , steam with temperature lower than 100 ℃. The application of this steam has developed rapidly.

When the negative pressure saturated steam is used like the positive pressure steam, the temperature of the steam can be rapidly changed by adjusting the air pressure, making it possible to precisely control the temperature, which is not possible for hot water. However, the negative pressure steam equipment must be equipped with Vacuum pump , because just lowering the air pressure will not make it lower than the atmospheric pressure.

Steam for power or drive

Steam can also be used Work (e.g. as power), for steam turbine, etc. An example that most people are familiar with is the steam locomotive and steam train many years ago^[3] The application of steam as driving power is quite rare. However, the technology of using steam as driving power continues to develop and improve.

Steam turbine is an indispensable equipment for thermal power plant. In order to improve efficiency, the technology of using high-temperature and high-pressure steam has been developed. Some thermal power plants use 25MPa (abs), 610 ℃ Supercritical pressure steam 。 In order to prevent turbine equipment from being damaged by condensed water, saturated steam should be avoided, so superheated steam is used in most cases. On the contrary, it is prohibited to use high-temperature steam in the field of nuclear power plants, because it can cause problems in the materials of turbine equipment, so high-pressure saturated steam is commonly used.

In the age of steam trains, passengers marveled at the bigger and more powerful engines. With the development of the times and the progress of technology, scientists are still developing steam engines, but modern steam engines are not the bigger the better, but the smaller the better. German scientists have recently developed a micro steam engine. Measured, it is only three thousandths of one millimeter.^[4]

Steam for cooling

Water vapor is the gas phase material with the largest heat capacity among gas substances. It has a wide range of applications in cooling high-temperature objects, such as cooling at medium speed during quenching of metal materials and cooling of metal surfaces and tools during metal material processing. It has the advantages of visibility, prolonging tool life, reducing oxidation, reducing chemical emissions, etc. It is an ideal material for cooling during metal cutting.

In the process of metal stretching, there are also advantages such as lubrication, reduction of metal surface oxidation, rapid cooling, etc.

Steam for cleaning

Due to its high temperature, purity and impact effect, water vapor can be quickly decontaminated under the condition of steam cleaning oil stains, viscoses, etc., and at the same time, there is no emission of chemicals that pollute the environment. Saturated steam is widely used in cleaning industry.

Generation mode

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evaporation

Evaporation is carried out on the liquid surface vaporization Phenomenon, which can occur at any temperature, requires heat absorption during evaporation.

The larger the liquid surface area, the faster the evaporation; The higher the liquid temperature, the faster the evaporation; Near the liquid surface The faster the air flows, the faster the evaporation;

boiling

With the heating of water, the temperature of water increases continuously. When a certain temperature is reached, the water begins to boil. After that, although the water is heated, the water temperature no longer rises. The temperature at which water boils is called the boiling point of water. Under normal pressure, the boiling point of water is 100 ℃.

heating process

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Those steam ovens and the usual "steaming" are heated directly by steam, which is in direct contact with the heated items. Similar to these household items but larger in size, many industrial equipment also widely use direct steam heating, such as disinfection cabinets, distillers and vulcanizers for rubber products.

But there is another method, which is more widely used in industry than direct heating. This method is indirect heating.

What is indirect heating?

Indirect heating is achieved through a process called“ Heat exchanger ”Method of heating the equipment.

When the steam flows through the wall surface of the heat exchanger, the heat of the steam is transferred to the side of the heated material. In this way, the steam does not come into contact with the heated material. Typical heat exchangers using steam heating, such as jacketed pots, tube heating Finned heat exchanger (thin fin, air fin).

This method of using heat exchanger for heating is not limited to using steam as hot coal, other hot coal also includes hot water or hot oil.

Advantages of steam heating

What is the difference between steam heating and hot water or hot oil heating?

Heating with steam

The steam supplied to the heat exchanger is in the gaseous state, becomes liquid (condensate) inside and leaves the heat exchanger.

The heat transferred by steam through condensation is obviously more than that by hot water or hot oil through convection. It also has the following advantages:

If the heat exchange area of the heat exchanger is the same

Using steam can shorten the heating time

If the workload is the same

With steam heating, the heat exchange area of the heat exchanger is smaller

These are two important characteristics of steam - in addition, steam has other advantages. Therefore, steam plays an important role in industrial heat sources.