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Science in common objects we use Part II

Kedar Kulkarni

“Today let us discuss the cooking gas stove that we have in our kitchens at home,” said Sir Prabhu as he began the lecture.

“It is called LPG – Liquefied Petroleum Gas,” said Meena.

“ Does it mean it is liquid inside the cylinder?” asked Mohan.

Sir Prabhu replied: “Yes, it is liquid,” and added “We know that gases can be compressed and at higher pressures they get converted into liquid form. The boiling point of water is 100 degrees Celsius, likewise the boiling point of propane which is usually the gas in the LPG cylinder is -42 degrees Celsius.” “A boiling at -42 degrees Celsius? Such a low temperature?” asked Raju.  “Yes. This pressurised gas is filled in cylinders of various sizes. The cylinders are connected to the burners by means of a regulator which is mounted on the cylinder. It is designed to lock itself on the cylinder. The regulator has a valve which is operated manually. The tube comes after the valve. The other end of the tube is connected to the stove pipe which is connected to the burners. Each burner has an on/off switch which is also a special type of a valve. Usually you must have observed that when we replace the empty cylinder with a new one, we only remove the regulator from the cylinder and put it on the new cylinder. As soon as we open the valve on the regulator, the liquid gas from the cylinder expands due to pressure difference and fills the tube in gaseous form. This gas is controlled by the valve on the burner stove. The valve does not allow the gas in the tube to escape. It is when we open this valve on the stove that the gas is directed into the burners. It can also control the volume of gas escaping through it and so the flame,” explaine Sir Prabhu.

“Is the round part with many holes the burner?” asked Meera.

“Yes, it is a hollow part which has a disc on top with a number of small holes from where the gas escapes. We generally use a spark lighter or match stick to ignite the gas flame. Once the flame is on it will continue to burn till the valve is turned off, thus cutting the gas supply. The design of the burner tube is such that it sucks the air by venturi effect as the gas is delivered to the burner,” said Sir Prabhu. Since a very tiny spark is sufficient to ignite this gas we have make sure that the gas burners are always switched off properly so there is no leak. In case one is going to be away from for few days, the regulator valve must be off.

(Writer is a mechanical engineer and runs a hands-on science activity centre at Margao)

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