Heat Transfer and modes of heat transfer. what is Fouling Factor or Resistance. what is log mean temperature difference

 What is Heat Transfer?

According to thermodynamic systems, heat transfer is defined as:

The movement of heat across the border of the system is due to a difference in temperature between the system and its surroundings.

or "it’s simply known as conveying energy and entropy from one location to another."

Types of Heat Transfer:

Heat can travel from one place to another in several ways. The different modes of heat transfer include:

• Conduction

• Convection

• Radiation
  
  

Conduction:

The heat flows from objects with higher temperatures to objects with lower temperatures.

The coefficient of thermal conductivity shows that a metal body conducts heat better when it comes to conduction. The rate of conduction can be calculated by the following equation:

Q = [K.A.(Thot- Tcold)] / d

Convection:

The movement of fluid molecules from higher temperature regions to lower temperature regions.

As the temperature of the liquid increases, the liquid’s volume also has to increase by the same factor and this effect is known as displacement. The equation to calculate the rate of convection is as follows:

Q = Hc .A .(Ts- T)

Radiation:

Radiation refers to the transmission of energy as electromagnetic radiation from its emission at a heated surface to its absorption on another surface, a process requiring no medium to convey the energy.

 

As temperature rises, the wavelengths in the spectra of the radiation emitted decreases and shorter wavelengths radiation are emitted. Thermal radiation can be calculated by Stefan-Boltzmann law:

Q = e ∙ σ ∙ A· (Tr – Tc)^4

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What is Heat Exchanger?

The devices which transfer heat from one fluid to another while keeping them apart or without mixing of the fluids. They are used in Industry, Aerospace etc.

• Fouling Factor or Fouling Resistance (Rf)

The fouling factor represents the theoretical resistance to heat flow due to a build-up of a layer of dirt or other fouling substance on the tube surfaces of the heat exchanger, but they are often overstated by the end-user in an attempt to minimize the frequency of cleaning. In reality, if the wrong fouling factor is used, cleaning may actually be required more frequently.

Overall heat transfer coefficient for Clear H.E:

What is Log Mean Temperature Difference (LMTD)

In order to solve certain heat exchanger problems, engineers often use a logarithmic mean temperature difference (LMTD), which is used to determine the temperature driving force for heat transfer in heat exchangers. LMTD is introduced due to the fact, the temperature change that takes place across the heat exchanger from the entrance to the exit is not linear.

q =UA ΔTm

Where:

ΔTm = log mean temperature difference