10

BOMB CALORIMETER

AIM

To determine the calorific value of solid and liquid fuels using a bomb calorimeter.

APPARATUS

Bomb calorimeter, Thermometer.

PRINCIPLE

Calorific value of the fuel is the total quantity of heat liberated, when a unit mass (or volume) of the fuel is burnt completely. Calorific value of solid and liquid fuels can be determine with bomb calorimeter.

Gross (or) Higher Calorific Value (HCV) of the fuel is the total amount of heat produced, when unit mass/volume of the fuel has been burnt completely and the products of combustion have been cooled to room temperature.

Net (or) Lower Calorific Value (LCV) of the fuel is the net heat produced, when unit mass/volume of the fuel is burnt completely and the products are permitted to escape.

DESCRIPTION OF EQUIPMENT

Bomb calorimeter consists of a strong cylindrical stainless steel bomb in which the combustion of fuel is made to take place. The bomb has a lid, which can be screwed to the body of bomb so as to make a perfect gas-tight seal. The lid is provided with two stainless steel electrodes and an oxygen inlet valve. To one of the electrodes, a small ring is attached. In this ring, a nickel or stainless steel crucible can be supported. The bomb is placed in a copper calorimeter, which is surrounded by an air-jacket and water jacket to prevent heat losses due to radiation. The calorimeter is provided with an electrically operated stirrer and thermometer.

PROCEDURE

Take a known mass (1.0 g) of the given fuel in clean crucible. The crucible then supported over the ring. A fine magnesium wire, touching the fuel sample, is then stretched across the electrodes. The bomb lid is tightly screwed and bomb filled with oxygen to 25 atmospheric pressure. The bomb is then lowered into copper calorimeter, containing a known mass of water. Note down the initial temperature of the water. Connect the electrodes to 6-volt battery. When the sample is burns and heat is liberated, continue the stirring of water for maintaining uniform temperature and record the maximum temperature attained.

CALCULATIONS

HCV − 0.09H × 587 cal/g

Mass of H₂O from 1 g of fuel = 0.09H

Latent heat of stream = 587 cal/g.

PRECAUTIONS

1. Oxygen should be filled very slowly in the bomb. Afterwards the pressure of O₂ may flow the fuel in the crucible.
2. The fuse wire should remain in contact with the fuel.

RESULT

High calorific value of the given fuel = _____ cal/g.

Low calorific value of the given fuel = _____ cal/g.

VIVA QUESTIONS

1. What is the principle of a bomb calorimeter?
2. Which state of fuel calorific value can be determined with bomb calorimeter?
3. What is combustion?
4. What is the formula for calculating high calorific value?
5. Which substances are liberated in combustion?
6. What is the relation between high calorific value and low calorific value?