Moving forward in to the higher level concepts within chemical engineering, a transition is made from fluids to heat. There are many similarities between the flow of fluid and heat. Additionally, heat transfer problems typically involve the flow of fluids through pipes or over surfaces. For heat transfer purposes, it is necessary to classify flow as either internal, through a pipe, or external, over a surface. For the purpose of this class, the discussion of heat transfer and exchangers will be kept to a high level understanding instead of all the minor details and calculations.
As stated in lesson 6, heat is a form of energy. It is also commonly used to describe the temperature of an object. However, this is not the technical meaning of heat, but simply a reference to an object’s physical temperature. Temperature and heat are related but not interchangeable.
Consider the example of a hot cup of coffee being placed on the table of a room at room temperature. Some minutes later, one can observe that the coffee has now cooled down. The room doesn’t feel any hotter, so where did the heat go? This is true that one may not perceive the change in temperature of the room, but this does not mean that heat was not transferred. Energy is always conserved. Therefore, if the coffee feels colder, i.e. lost heat and temperature, then the room must have gained heat. The room, however, is a large control volume so the heat can be distributed throughout the whole room instead of just within the small coffee cup. A lower temperature in the coffee is a noticeable effect of the average kinetic energy of the coffee particles decreasing (loosing heat).
Heat can also be defined as the transfer of energy from a hotter object to a colder one. Heat will always flow hot to cold. Temperature gradients are always the driving force of heat transfer. All other forms of energy can be converted into heat, just as heat can be converted into other forms of energy. For example, kinetic energy can be converted into heat via friction that gives off thermal energy as a result of decreasing velocity. Electrical energy is converted to heat when in space heaters or even simple lights in one’s home. Think about a standard light bulb that has been on for a while. It is very hot to the touch. Finally, chemical energy from food is converted within our bodies into heat to keep us warm. The unit for heat is joules or btu in the SI and English system respectively. The standard unit for the rate of heat transfer is the Watt or joule per second.
Heat can be transferred in 3 methods. The first is via convection. This is the type of heat transfer that occurs via the direct contact of colder molecules or particles with hotter ones. For convection to occur, the objects must be touching. Conduction occurs due to the collision of molecules. The faster moving molecules contact the slower moving ones and give
them some of their energy. When the fast moving particles collide with a boundary or wall they cause the wall’s particles to vibrate as well, thus transferring energy. There is no transfer of matter between the touching objects, only thermal energy. This type of heat transfer typically occurs within solids. Some substances conduct heat better than others. For example, metals are great conductors whereas plastics are poor conductors. This deals with the degree of thermal conductivity, a unique property of each substance. The equation for conductive heat transfer is:
𝑄̇ = 𝜅𝐴(𝑇ℎ−𝑇𝑐)
𝑑
Where 𝑄̇ is the heat transfer rate, κ is the heat transfer coefficient, A is the area of heat transfer,
Th is the hotter temperature, Tc is the colder temperature, and d is the distance or thickness of the
boarder.
The second method of heat transfer is convection. This type of transfer takes place when warmer areas of a fluid rise to colder ones. This is known as the mass movement of the fluid. When the hot fluid moves, the colder fluid can take its place. Convection is accomplished via a circulation pattern where the hot and cold fluids are constantly moving around within the media. The equation for heat of convection originates from Newton’s law of cooling, it states:
𝑄̇ = ℎ𝐴(𝑇𝑤−𝑇∞)
Where h is the convective heat transfer coefficient, Tw is the temperature of the fluid and T∞ is
the temperature of the surroundings.
The third and final method if heat transfer is radiation. This method, unlike the previous 2, does not require a medium or any contact between the heat source and the object being heated. Heat can be transferred through empty space via radiation and electromagnetic
waves. The most common form of radiation is the sun’s rays traveling across space to heat the earth. There is obviously no contact between the sun and the earth and a large distance separates the two, yet via radiation the sun is able to heat the earth. The radiative hear transfer equation is
𝑄̇ = 𝜖𝜎(𝑇𝑠4− 𝑇
𝑠𝑢𝑟4 )𝐴
Where σ is the Stefan-Boltzmann constant, 5.6703 * 10-8, T
s is the absolute temperature in kelvin
of the surface, Tsur is the absolute temperature of the surroundings, ϵ is the power emitted by the
object, and A is the area of the emitting body.
Heat exchangers allow two process streams to exchange energy, thermal energy to be exact. The hot flow gets cooled down by heat transfer to the colder flow. Liquid-air heat exchangers are often called radiators. Liquid-Liquid heat exchangers are often run in counter current fashion. Counter current means that the inlet streams enter from opposite sides of the exchanger in order to maximize heat transfer. Exchangers can also run co-currently if need be. This means that both inlets enter from the same side of the exchanger. The most common type of exchanger for a plant is a shell in tube or floating head exchanger. Exchangers typically have an input steam of pure water or steam that is used to either heat or cool the system depending on what is required. The water can enter as high pressure steam, medium pressure steam, low pressure steam, boiler feed water, or cooling water. The different between these inputs is the pressure and phase of the water. The heat transfer equation for a heat exchanger is:
𝑄̇ = 𝑈𝐴(𝛥𝑇)𝐿𝑀𝑇𝐷
Where U is the heat transfer coefficient, A is the heat transfer area, and ΔTLMTD is defined as
(𝛥𝑇)𝐿𝑀𝑇𝐷 = (𝛥𝑇)𝑖𝑛− (𝛥𝑇)𝑜𝑢𝑡
𝑙𝑛((𝛥𝑇)𝑖𝑛
Where (𝛥𝑇)𝑖𝑛 is the temperature difference between the 2 inlet fluids and (𝛥𝑇)𝑜𝑢𝑡 is the temperature difference in the 2 outlet fluids. Ln in the natural log operation.