# Thermodynamic Process Fundamentals

## Introduction

A thermodynamic process is defined by thermal heat transfer between bodies according to the principles of physics. Thermodynamic processes initiate energy changes and transfer within systems. The basic elements of thermodynamic processes include internal temperature changes, volume changes, and pressure variations. Thermodynamic processes fall into specific categories. In an adiabatic process, there is no movement of heat from a system or into a system, thus temperature remains constant. On the other hand, volume remains constant in isochoric processes when other factors vary accordingly. This thermodynamic process is ideal but does hard work. Isobaric processes do not undergo pressure changes while isothermal processes are characterized by constant temperatures.

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Several thermodynamic laws define the equilibrium condition of heat transfer within and outside of a system. Ideally, there are four thermodynamic laws namely, the zeroth law, the first law, the second law, and the third law. Heat transformations take place according to the first law of thermodynamics but can neither be created nor destroyed. Other thermodynamic processes such as thermal nuclear processes are defined by Einstein’s equation which relates the speed of light and the mass of a body in the equation, E = mc2 (where E is the amount of energy released from a body whose mass is m. c is the speed of light) to the internal energy in a system.

Considering the above overview, the following discussion will focus on thermodynamic processes in a specific state characterized by specific parameters including volume, pressure, and temperature. Other factors related to heat transfer in a thermodynamic process include enthalpy and entropy of heat and the corresponding work done through a system and the relationship with its surrounding. In addition to that, the case study illustrates how internal energy works in a thermodynamic process. In particular, the study will concentrate on heat transfer from one point in the system to another.

The above illustration shows how heat is transferred from fuel in a thermodynamic process in a thermal reactor. More particularly, the following discussion will focus on thermodynamic processes encountered in the boiler.

## The Thermal Process

The second law of thermodynamics provides that when two bodies make a thermodynamic contact, a heat gradient causes the flow of heat from a higher to a lower temperature. In the above case study, heat flows from the coolant in the reactor to the boiler water. There is a corresponding decrease in the enthalpy of the coolant while the enthalpy of the boiler water increases correspondingly. As a matter of interest, U bundles in the reactor have to be completely submerged to ensure efficient heat transfer and minimal heat losses. The boiler’s water level must be maintained to achieve the latter.

Once boiling has commenced, vapour bubbles rise to the top of the boiler when they are formed in the process. Pressure and volume changes occur according to the laws of thermodynamics. At this point, steam occupies 40 times more space than the same volume of water in the liquid phase. The boiler level increases increasing the thermal efficiency of energy transfer. This is commonly referred to as boiler swell, though there is boiler shrink. Boiler shrink occurs when the boiler level drops significantly.

According to the laws of thermodynamics, the internal energy is neither created nor destroyed but can be transferred when thermal contact is established between two bodies creating an equilibrium condition in the process. Thus, a steady state is established in a thermal reactor creating thermal equilibrium. Thermal equilibrium is a state reached when the rate of heat transfer and heat generation are equal. However, when the load range is constant and boiling starts, bubbles are created from below the boiler displacing water to the top increasing the water levels. A constant load range is a situation where the same amount or volume of water with the same weight is used in the thermodynamic process. Full load condition is achieved when the water level is at a maximum. A steady swell state is achieved when a full reactor’s thermal power is achieved. On the other hand, if a decrease is experienced below the reactor’s thermal state, then a steady state shrink is achieved in the process. As the water level is increased or decreased, a corresponding state is achieved according to the laws of thermodynamics. Thermal transfers sometimes lead to transient shrinks and thermal swells.

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## Transient Shrink and Swell

Transient shrinks and swells are thermodynamic conditions experienced in a boiler during loading and unloading conditions. During a steady-state, certain thermal equilibrium conditions prevail, but when they temporarily increase, then a condition defined as thermal swell occurs. Alternatively, when a temporary thermal drop is experienced in a boiler, the condition defined as thermal shrink occurs. Volume is one of the elements considered in the study of thermodynamics. These conditions occur due to temporary and rapid changes in the volume of steam bubbles in the boiler. Drastic changes in reactor power also influence transient shrink and swell within a thermal reactor. Thus, the dynamics of both transient shrink and swell are vital in controlling the thermal behavior of a thermal reactor.

## Pressure

Another thermodynamic aspect is boiler pressure. Factors such as gravitational force, steam pressure, steam quality and liquid water help regulate the flow of water within the boiler. Thus, re-circulation is achieved in the process and the re-circulation ratio can be determined by the rate at which water is transformed into steam. As pressure increases less and less amount of heat is required to change liquid water into steam. Thermal transfer in this process is defined in the following equation, Q = m⋅ lv. Q signifies the heat transfer rate, m is the mass of water circulating, and lv is the latent heat of vaporization for steam. Recirculation level reaches its normal value when the rate of flow of feed water and steam are constant, and pressure has reached its normal value (Science and Reactor Fundamentals, 85).

The above discussion briefly focused on the laws of thermodynamics as applied in a thermal reactor for the transfer of heat from the reactor fuel to the boiler. A state of equilibrium is achieved in the process. The laws of thermodynamics with the basic elements of heat transfer, volume, pressure, and internal energy are critical in analyzing the case study. Internal energy is a vital component in thermodynamic processes although the complete transformation of internal energy cannot be attained under normal circumstances. One assumption made in the case study is that heat transfer is uniform. Other issues that affect heat transfer in the above case include the fuel profile and coolant boiling among others.

## Work Cited

Science and Reactor Fundamentals. Heat &Thermodynamics 1 CNSC Policy Planning and Learning Revision 1. 2003. Web.

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