The DIRECT ARC FURNACE ( ELECTRIC PROCESS IN STEEL MAKING )

The direct arc furnace consists of a steel shell lined with refractory bricks and a removable roof through which carbon or graphite electrodes ( about 2m long ) pass. Graphite electrodes offer less resistance to current and are considered to be more durable at high temperature but they are more expensive than the carbon electrodes. The number of electrodes correspond to the number of phases.

The electrodes are lowered into the furnace and the current is switched on. The heat generated by a powerful spark between the electrodes and the metallic charge on the hearth melts the charge. The charge usually consists of steel scrap and iron oxide in the form of iron oxide ore. Pig iron is not directly treated in electric furnaces, though sometimes it is partly purified in an open hearth furnace and then transferred to electric furnaces for final treatment and alloying. When the furnace is in operation, the electrodes are consumed and they become shorter. So a definite distance must be maintained between the electrodes and the charge by raising or lowering them by automatic controls. whenever necessary, a new electrode id built up (screwed) on top of the old one.

Modern arc furnaces are built in 0.5 to 80 tons capacity but most favored are those with capacities of up to 80 tons. A melt is produced in 6 to 8 hour for an arc furnace 30 to 35 tons in capacity.

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THROTTLE GOVERNING OF STEAM TURBINES

Steam Pressure at inlet to a steam turbine is reduced by throttling process to maintain the speed of the turbine constant at part load and hence this method of governing is called "Throttle Governing". Construction : Throttle governing system consists of a centrifugal governor, a lever , an oil pump , a pilot piston , control valve , a relay piston and a throttle valve. The throttle valve is moved by a relay piston. The relay piston is actuated by pilot piston control valve. There are two piston valves covering ports in the pilot piston control valve without any overlap. These piston valves are operated by lubricating oil supplied by a gear pump at 2 to 4 bar. The oil returns to the drain from this chamber. Working : When the turbine works on full rated load, the throttle valve will remain open. When the load is decreased, the energy output of the turbine becomes in excess and the turbine shaft speed increases. Hence, governor sleeve will lift. The upward movement of the

ELECTRICAL COMPARATORS AND ITS WORKING

Electrical Comparators are used as a means of detecting and amplifying small movements of a work contacting elements. An electrical comparator consists essentially of a pick-up head or transducer for converting a displacement into a corresponding change in current and a meter or recorder connected in the circuit to indicate the electrical change, calibrated to show in terms of displacement. Generally, an amplifier is needed to provide the requisite sensitivity and to match the characteristics of different parts of the circuit. Electrical comparators can be classified according to the electrical principle used in the pick-up head. Most of the comparators use either a differential transformer, an inductance bridge, a strain gauge or a capacitor as a means of detecting movement of the gauging element. There are different types of electrical comparators. One kind called an electrolimit gauge is used to check or measure the outside diameter of a roll. The object to be checked is place

Advantages And Disadvantages Of Air Refrigeration System :

Advantages Of Air Refrigeration : 1. The refrigerant used namely air is cheap and easily available. 2. There is no danger of fire or toxic effects due to leakages. 3. The weight to ton of refrigeration ratio is less as compared to other systems. Disadvantages Of Air Refrigeration : 1. The quantity of Refrigerant used per ton of refrigeration is high as compared to other systems. 2. The COP of the system is very low. Therefore running cost is high. 3. The danger of frosting at the expander valves is more as the air contains moisture content.

Mechinopedia For Mechanical Engineers

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