Carbon Dioxide

Carbon dioxide CO 2 (carbon dioxide, carbon dioxide, carbon dioxide, carbon dioxide) as a function of pressure and temperature can be in gaseous, liquid or solid state.

In the gaseous carbon dioxide is a colorless gas with a slightly sour taste and smell. In the Earth's atmosphere contains about 0.04% carbon dioxide. Under normal conditions, its density is 1.98 g / liter - about 1.5 times greater than the density of air.

Phase equilibria of carbon dioxide

Liquid carbon dioxide (carbonic acid) is a colorless, odorless liquid. At room temperature, it exists only at pressures above 5850 kPa. The density of liquid carbon dioxide is strongly dependent on temperature. For example, at temperatures below +11 ° C liquid carbon dioxide is heavier than water, at temperatures above +11 ° C - easier. As a result of evaporation of 1 kg of liquid carbon dioxide under normal conditions is formed by about 509 liters of gas.

At a temperature of approximately -56,6 ° C and a pressure of about 519 kPa liquid carbon dioxide into a solid - the "dry ice" .

In industry, the most common ways to get 3 of carbon dioxide:

• from flue gases of chemical plants, especially of synthetic ammonia and methanol, in the exhaust gas contains approximately 90% carbon dioxide;
• from the flue gases of industrial boilers that burn natural gas, coal and other fuels, the flue gas contains 12-20% carbon dioxide;
• from flue gases produced during fermentation during the production of beer, alcohol, the splitting of fats, waste gas is nearly pure carbon dioxide.

According to GOST 8050-85 gaseous and liquid carbon dioxide is supplied in three forms: the higher, the first and second grades. Welding is recommended to use carbon dioxide higher and first grade. The use of carbon dioxide the second grade for welding is permitted, however, desirable gas dryers. Allowable concentration of carbon dioxide and certain impurities in various grades of carbon dioxide is given in the table below.

Features brands of carbon dioxide

Mark carbon dioxide	Carbon dioxide is the highest quality welding	Carbon dioxide Welding Class	Carbon dioxide is the second grade
The volume fraction of carbon dioxide,%, not less	99.8	99.5	98.8
Water content,%, not more	no	no	0,1
The content of water vapor, g / m 3 , not more	0.037	0.184	Not rated

Safety measures when working with carbon dioxide:

• Carbon dioxide is not toxic and explosive, but its concentration in air of more than 5% (92g / m 3 ), the proportion of oxygen, which can lead to oxygen deficiency, and suffocation. It should therefore be wary of its accumulation in poorly ventilated areas. To register, the concentration of carbon dioxide in the air of industrial premises used gas analyzers - automatic stationary or portable.
• When reducing the pressure to atmospheric pressure liquid carbon dioxide is converted into gas and snow with a temperature of -78,5 ° C, and can cause eye and mucous membrane of frostbite of the skin. Therefore, when sampling liquid carbon dioxide is necessary to use protective goggles and gloves.
• Inspection of the inner vessel previously operated tank for storage and transportation of liquid carbon dioxide should be carried out in a gas mask hose. Tank to warm up to ambient temperature, and the inner capacity to blow air or ventilate. Mask is not allowed to use only after the volume fraction of carbon dioxide inside the equipment will be less than 0.5%.

The use of carbon dioxide in welding
Carbon dioxide is used as an active shielding gas for arc welding (usually semi-automatic welding) consumable electrode (wire), including in the composition of the gas mixture (oxygen, argon).

Supply of welding stations with carbon dioxide can be carried out in the following ways:

• directly from the battery plant for the production of carbon dioxide;
• from a stationary vessel drive - with significant amounts of consumption of carbon dioxide and the absence of an enterprise's own stand-alone stations;
• carbon dioxide from transport capacity - at a lower volume of consumption of carbon dioxide;
• of the cylinders - for small amounts of carbon dioxide pipelines or inability to welding position.

Stand-alone for the production of carbon dioxide - a separate company dedicated plant, which produces carbon dioxide for their own needs and supply of other organizations. Carbon dioxide is fed to the welding positions of pipelines laid in welding shops.

For large volumes of consumption of carbon dioxide and lack of enterprise autonomous station carbon dioxide is stored in fixed containers drives, in that it comes from the transport container (see figure below).

supply of welding stations with carbon dioxide from the stationary vessel drive

At lower feed consumption volumes of carbon dioxide by pipeline can be performed directly from the transport container. Characteristics of some stationary and transport containers are shown in the table below.

TABLE. Characteristics of reservoirs for storage and transportation of carbon dioxide (carbon dioxide)

Mark	The mass of carbon dioxide, kg	Appointment	Time of storage of carbon dioxide per day	Mark gasifier
TSZHU-3 ,0-2, 0	2 950	Transport automobile ZIL-130	6-20	EGU-100
NZHU-4-1, 6	4 050	Stationary storage	6-20	EGU-100
TSZHU ,0-9-1, 8	9 000	Vehicle MAZ 5245	6-20	SU-400
NZHU-12 ,5-1, 6	12 800	Stationary storage	6-20	SU-400
UDH-12, 5	12 300	Stationary storage	Unlimited, is equipped with a refrigeration unit	UGM-200М
TSZHU-40-2	39 350	Railway Transport	40	SU-400
RDH-25-2	25 500	Stationary storage	Unlimited, is equipped with a refrigeration unit	SU-400
NZHU-50D	50 000	Stationary storage	Unlimited, is equipped with a refrigeration unit	SU-400

For small volumes of consumption of carbon dioxide, or the inability of the pipe to welding positions for the supply of carbon dioxide cylinders used. A standard black tank capacity of 40 liters is poured 25 kg of liquid carbon dioxide, which is usually stored at a pressure of 5-6 MPa. As a result of evaporation of 25 kg of liquid carbon dioxide is formed approximately 12 600 liters of gas. Process of carbon dioxide in the storage tank is shown in the figure below.

process of storing the carbon dioxide (carbon dioxide) in the tank

For the selection of the gas cylinder must be equipped with gear, heater and gas dryer gas. When the output of carbon dioxide from a cylinder as a result of the expansion is adiabatic cooling of the gas. At high gas flow rate (more than 18 l / min), it can lead to freezing in the gas contained water vapor and plugging gear. In this connection between the gearbox and the valve cylinder is desirable to place the gas heater. With the passage of gas through the coil it is heated by an electric heating element included in a network with a voltage 24 or 36V.

To remove the moisture from the carbon dioxide gas dryer is used. It is a building filled with material (usually silica, copper sulfate, or alumogels), well absorbent. Heaters are a high-pressure regulator set up, and low pressure mounted after the gearbox.

See also:
semi-automatic welding
welding machines

Welding materials:

welding electrodes welding wire welding flux for arc welding carbon dioxide helium

argon protective gas mixture acetylene propane-butane IAF