Forced Convection

Forced convection is a process or form of heat transfer where fluid movement is produced by an additional supply such as suction machine, forced draft fan, induced draft fan, pump, and others.  Force convection can occurs in our daily life like air conditioning, heat exchanger, steam boiler and other devices.

Heat transfer process in steam boiler occurs in air heater and economizer. This heat transfer type is forced convection because convection occurs due to flue gases are blown by force draft fan / primary air fan to air heater and economizer.

Forced convection heat transfer can be determined by the following equation (use Newton’s Law of Cooling):

Q = h A (Ts - T∞) or Q = h A ΔT

Q         = forced convection heat transfer rate (W)
h          = heat transfer coefficient (W/m2 K)
A         = exposed area of metal
Ts        = temperature of surface heating transfer element (K)
T∞       = free stream temperature (K)

Forced convection is different with natural convection. But sometimes in some cases forced convection is mixed with natural convection. The problem is how to distinguish whether this heat transfer is categorized as forced convection or natural convection. Most engineers use Archimedes number (Ar) parameter to determine dominant convection heat transfer type. The following formula is Archimedes number equation:

Ar = Gr / Re2

Ar = Archimedes number
Gr = Grashof number
Re = Reynolds number

If the value of Ar more than 1, then natural convection is dominant, if the value of Ar less than 1, then forced convection is dominant.

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