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Marcet Boiler Experiment

Objective

To study the relationship between the saturation pressure and temperature of water/steam in the range 0 – 14 bar (gauge) and to study the change in temperature of a body when being heated or cooled.

Safety

The apparatus is a pressure vessel. The pressure must not exceed 14 Bar (gauge)!

Background

1)In order to carry out a heat transfer experiment simultaneously with measurement of vapour pressure, it is required that the rates of heating and cooling of the pressure vessel, the rate of energy addition by the heater and the ambient temperature are recorded.

2)Study the following derivation of a simplified 1st Law (energy) equation relating the temperature of the boiler to time as it is heated and cooled.

Assumptions

i.The temperature is uniform throughout the boiler. Thus the outside surface temperature of the boiler is the same as the steam temperature, T.

ii.Newton’s Law of cooling applies – the rate of heat transfer from the surface is proportional to the surface area, As, and to the temperature difference between the boiler surface and the surroundings ( T- T).

Thus,

where  (kW/m2K) is called the heat transfer coefficient for heat transfer between the boiler surface and the surroundings. By Newton’s law of cooling it is assumed constant.

The 1st law balance for the boiler at time t (secs) is

+ Rate of change of internal energy of the boiler and contents.

Therefore,

Where MC (kJ/ K) is the heat capacity of the boiler which in this simplified development is assumed constant.

So, when being heated,

(1)

and when cooling,

(2)

At a value of T1 on the lot of the measured heating curve and at a value T2 on the cooling curve, the respective slopes,

and

are determined by construction. The value of dQ/dt may be calculated from tabulated measurements of electrical energy consumption (kWh) versus time (hrs), or from a...
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