Types of Convection Heat Transfer, Examples and Rate of Heat Transfer

A large majority of practical applications of heat transfer (convection) in the chemical process industries involve either heat transfer to a fluid or heat transfer from a fluid. 

The heat flow mechanism in solids is by conduction wherever the heat flow mechanism in fluids is due to convection. Convection is the transfer of heat from one point to another point within a fluid by mixing hot and cold portions of fluid. 

Heat transfer by convection occurs as a result of the movement of the fluid on a macroscopic scale in the form of circulating currents. The circulating currents may be set up either by the heat transfer process itself or by some external agency. It is restricted to the heat flow in fluids. 

In the case of convection heat transfer, the physical mixing of the hot and cold portions of fluid is responsible for the flow of heat from one place to another within the fluid. 

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Types of Convection of Heat Transfer 

i). Free or Natural Convection

When the circulating currents arise from the heat transfer process itself, e.g., from density difference arising in turn due to temperature difference/gradients in a fluid mass, it is called free or natural convection.

Examples

1. Heating of a vessel containing liquid using a gas flame situated underneath. The liquid at the bottom of the vessel gets heated, expands, and rises because its density has become less than that of the remaining liquid. Cold liquid of higher density takes its place and a circulating current is set up. 

2. The flow of air across a heated radiator/heat of a room using a steam radiator. 

ii). Forced Convection

When the circulating is produced by an external agency such as an agitator in a reaction vessel, pump, fan, or blower, the action is called forced convection. Here fluid motion is independent of density gradients. 

Examples

1. Heat flow to a fluid pumped through a heated pipe. 

2. In general, higher rates of heat transfer are obtained in forced convection as compared to natural convection owing to the greater magnitude of circulation in the forced circulation. 

Rate of Heat Transfer of Convection

In the case of convection heat transfer placed from a surface to a fluid, the circulating currents die out near the surface, and the film of the fluid, free of turbulence covers the surface. Through this film heat transfer is taken by thermal conduction and as the thermal conductivity of most fluids is low, the fluid over the surface resistance to heat transfer lies in the film. Therefore an increase in the velocity of the fluid over the surface results in improved heat transfer mainly because of a reduction in the thickness of the film. 

If the resistance to heat transfer is considered as lying within the film covering the surface, the rate of heat transfer Q is given by

\[Q \ = kA (∆T)/x\]

Their effective thickness x is not generally known and therefore this equation is usually rewritten in the form:

\[Q\ = hA∆T\]

This is the basic equation for the rate of heat transfer by convection under steady-state conditions. 

Where h is called the film heat transfer coefficient or surface coefficient or simply film coefficient. The value of h depends upon the properties of the fluid within the film region, hence it is called the film heat transfer coefficient. It depends upon the various properties of the fluid, the linear dimension of the surface, and fluid velocity. 

Numerically, the heat transfer coefficient (h) is the quantity of heat transferred in a unit of time through a unit area at a temperature difference of one degree between the surface and surroundings. h has the units of W/(m2.K) in the SI system. The term 1/h is called thermal resistance. 

The process of transfer of heat from a hot fluid to a clod fluid through the metal wall is very common in the chemical and process industry. The heat transferred may be latent heat of a phase change, e.g., condensation, vaporization, etc., or maybe sensible heat. In all process equipment, e.g., heater, cooler, condenser, reboiler, etc. heat is transferred by conduction and convection.

Read more 

>Thermal Conductivity Measurement Instrument Working Principle and Advantages

>Fourier's Law of Heat Transfer Conduction

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane