Thermodynamic Basics

Thermodynamic basics

The concept of heat




All subjects are made up of molecules (molecule = a group of atoms).
If a drop of water, for example, is disturbed, it is possible to see how the molecules move rapidly about inside the drop.
The higher the speed of the molecules, the greater the movement energy they have.
What is called heat is a result of this kinetic energy that the molecules have temporarily.
The extent of the kinetic energy can be gauged directly with a thermometer.
Heat is a form of kinetic energy.
Little kinetic energy ( the molecules move slowly) = low temperature.
High kinetic energy (the molecules move rapidly) = high temperature.
There are a number of different ways (different scales) to list the temperature.

Different temperature scales




The temperature scales have different starting points and are named after their inventors.

Celsius and Fahrenheit
The scales have a zero point and are graded in both plus and minus degrees.
The thermometer reading is a measurement of the kinetic energy of the molecules (= the heat).
Because there is a thermometer reading also at minus grades, the molecules cannot have already stopped moving (heat be zero) at 0°
In terms of pure physics, it is also incorrect to talk about warm and cold grades. Put simply, different temperatures correspond to different levels of heat.
Celsius took water as a starting point when he made his scale.
The boiling point of water was set at 100° and the freezing point at 0° on his scale.
Fahrenheit cut his finger and used both his blood temperature and a refrigerated mixture of ice and salt as his starting points.
The temperature of his blood was set at 100° and the frozen mixture's temperature was set at 0° on his scale.
Conversion between Celsius and Fahrenheit:
C = ((F-32) x 5) / 9
F = (C x 9) / 5 + 32

Kelvin
This scale is based on the kinetic energy of the molecules.
0° K = absolute zero = the molecules have stopped moving completely.

Heat exchange





Heat exchange can take place in three ways
- Through radiation.
- Heat radiation does not depend on a material to transfer the heat. The heat of the sun for example reaches us through radiation.
- By conduction.
- The heat is transferred via contact between different bodies.
- By convection.
- The heat is transferred via a liquid or a gas.
Heat exchange often takes place through a combination of these methods.
For example as in the illustration:
- the heat from the sun is transferred via radiation to a metal post
- from the metal post to the water by conduction
- from the water to the container by convection

Heat transfer can only take place in one direction
Heat is always transferred from an object with a higher temperature to an object with a lower temperature.
The greater the temperature difference, the quicker the transfer of heat.
If a beaker with cold water is placed in a beaker with heated water, the heat will be transferred from the beaker with hot water until the temperature is the same in both beakers.
As the temperature difference between the two beakers reduces, the heat transfer slows down.

Changes in state





Evaporation
As the heat is transferred to the water, the temperature of the water increases successively. However water cannot receive and store infinite quantities of heat.
At approximately 100°C (212°F) and normal air pressure, the water cannot consume any more heat.
If the supply of heat to the water continues, the water will give off as much heat as it is supplied with.
The water disposes of the excess heat by converting to steam. As long as there is still water, the temperature remains at 100°C (212°F) in both the boiling water and in the steam.
Eventually all the water will evaporate.
If further heat is applied to the steam, its temperature will rise (= super heated steam).

Note! Do not confuse the amount of heat and the temperature. For example a burning match has a temperature of approximately 600°C (1112°F), but gives off little heat; possibly enough to heat the water in a thimble a couple of degrees.

Condensation
If the supply of heat is removed, the steam will condense into water after a while.
This is because the heat is transferred from the hot steam to the cooler surrounding.
During condensation, as much heat is released as was consumed during evaporation.

Quantity of heat
Different amounts of heat are required to raise the temperature of the water, to evaporate the water, to super heat the steam and to condense the steam into water.
If, for example, we have a certain amount of heat which we will call 1 unit it would be as follows:
- 1 unit to increase the temperature of the water from room temperature (20°C (68°F)) to boiling point.
- 7 units to convert the boiling water to steam.
- 0.45 units to super heat the steam.
- 7 units to condense the steam to water.
Most heat can also be transferred when the water changes from one state to another.
This applies to all liquids, including refrigerants.
This is one of the reasons that it is so important that the amount of refrigerant in the air conditioning correct.
- Too much refrigerant in the evaporator ⇒ the refrigerant heats up but evaporates only partially ⇒ a smaller amount of heat is taken from the air leading to reduced capacity.
- Too little refrigerant in the evaporator ⇒ the refrigerant evaporates and the steam is super heated ⇒ a smaller amount of heat is taken from the air ⇒ reduced capacity.

Temperature - pressure - volume





Evaporation temperature, condensation temperature
The temperature at which a liquid begins to boil depends not only on which liquid it is, but also on the pressure.
Water for example, begins to boil and evaporate at 100°C (212°F) at normal air pressure (÷ 1 bar).
In order for the water to be able to boil and evaporate, the pressure of the steam must be higher than the air pressure.
This means that if the air pressure is increased, the boiling point also increases. Conversely of the pressure is reduced, the boiling point is lowered.
For example water boils at:
- 180 °C (356°F) if the pressure is 10 bar.
- 20 °C (68°F) if there is a negative pressure of 0.97 bar.

Changes in volume
The following applies to all gases:
- when the volume is reduced, pressure and temperature increases.
- when the volume is increases, pressure and temperature is reduced.
A change in volume also effects the evaporation and condensation temperatures.

Contained system (summary of thermodynamics)




If two beakers, a compressor and a choke valve are connected together as illustrated, it is a contained system.
The compressor and the choke valve divide the system in to a low pressure section and a high pressure section.
Without the choke valve the compressor would only function as a pump.

Function
The choke valve governs the quantity of liquid that is released up to the right beaker. The quantity of liquid must be so much that the final remains of the liquid evaporate precisely before the beaker spout. Too little or too much liquid means that the amount of heat that can be taken up reduces.
The compressor lowers the pressure and the temperature in the right beaker. The liquid in the beaker then boils and evaporates at low temperature. This results in the temperature difference between the flame and the liquid becoming greater and the heat transfer from the flame to the liquid becomes quicker and more efficient.
The compressor extracts the gas from the right beaker, increases the pressure of the gas and temperature and presses out the gas in the left beaker. As a result the temperature difference between the gas and the ambient temperature becomes great. The gas then condenses at a high temperature and the heat transfer from the gas to the air becomes rapid and efficient.