Thermodynamics

From your previous knowledge of science, the words ‘heat’, ‘work’ and ‘energy’ should be familiar to you. These words are in quotes because although they are familiar everyday words, they have special meanings and significance in the study we are about to undertake.

In Frame 10 of the previous programme, we defined a process and state path, and you saw how a process between two equilibrium end states can be shown graphically. A process is caused by a heat or work interaction, or by both simultaneously, and a system could undergo a series of processes as a result of such interactions.

In the previous two programmes we have developed the ideas of work, heat and energy. We have also seen how the energy of a system changes with work and heat interactions at the boundary. So far in the problems you have met, the state of a system has been completely specified; this means all properties have been given. In this programme you will learn how to calculate, or look up in tables or charts, those properties which need to be found. This may be done from formulae or from tables of data acquired and refined over many years. It will enable you to tackle a much wider range of problems on systems and the First Law, which will follow in Programme 5.

We shall begin this programme by looking at problems involving a vapour, usually steam, in closed systems. The system is initially in a known equilibrium state, and as a result of heat and work interactions at the boundary, proceeds to a second equilibrium state. We need to use the equation of the First Law applied to a closed system to solve such a problem. This was given in Frame 11 of Programme 3 as: where the energy term E was shown in Frame 20 of Programme 3 to consist of internal energy, kinetic energy and potential energy. So what would be the First Law equation for problems with steam in closed systems?

In Programme 1 we said that in studying Thermodynamics we have to take a macroscopic view. This is because some of the forms of energy we have to consider can only be treated in terms of macroscopic ideas. Internal energy of a fluid depends on the translational and vibrational energy of countless myriads of tiny molecules, and we can only get a picture of the state of that fluid by using the overall concepts of temperature and pressure. Since internal energy is that mode of energy directly affected by a heat interaction, heat itself is an energy transfer that depends on random molecular activity, which again can only be viewed with macroscopic ideas. Even though similar macroscopic ideas, such as force (being pressure × area) are used describe a work transfer, heat and work, as interactions at a system boundary, are fundamentally different.

In this programme we will continue to explore how work may be obtained from heat. We have seen how the Second Law places a restriction on what may be achieved, and now we will see how the property entropy will help us in our calculations. First we will look again at the steam cycle considered in Frames 69 and 71 in the previous programme. The p—v diagram of the cycle is shown again below. State 1 is saturated liquid at 1.2 MN/m², or 12 bar, and 188°C, and state 2 is saturated vapour at the same temperature and pressure. The lower pressure is atmospheric, 101.325 kN/m², or 1.013 25 bar and 100°C, and the dryness fractions at states 3 and 4 are 0.91 and 0.14 respectively.

In the previous programme we have seen how we may calculate the amount of work that may be obtained from steam delivered to a turbine and expanded to a given pressure. This is one example of a work interaction between a fluid in motion and a rotating wheel with blades. Other examples are a gas turbine, and a gas or air compressor. We now need to get an insight into how this work interaction is achieved in practice.

Our foremost concern in studying Thermodynamics is to find out how we may produce work, and what is the efficiency of the process. In Programme 6 we met the idea of a heat engine, a device within a closed system with heat interactions at high and low temperatures at the boundary, and from which work is obtained. In the previous two programmes we have studied the steam power plant at some length. This is the classical example of the heat engine concept, which is now finding new applications with alternative vapours. A heat source is produced by the combustion of fuel external to the closed system in which the working fluid circulates, and similarly an external heat sink is provided via the condenser. Work is obtained where a rotating shaft passes through the closed system boundary. These are the characteristic features of a heat engine. We now need to think about other prime movers which use fuel and produce work. How many different types can you name?

We shall start this programme by thinking about the reciprocating internal combustion engine.