Calculation of the heat transfer coefficient

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Chart of nomograms

Reference information

System of nomograms for calculation of heat transfer coefficient

graphic-analytical calculation of the heat transfer coefficient

The heat transfer coefficient characterizes the intensity of heat exchange between the body surface and the environment, i.e. is the ratio of the heat flux density [W/m²] coming from the body into the medium in contact with it, at a temperature difference between the body under consideration and the medium of one degree [K].

The heat transfer coefficient is calculated by the formula

α = Nu · λ / L

Scheme of the system of nomograms for calculating the heat transfer coefficient

Designations of parameters in the formula for calculating the heat transfer coefficient

Nu - Nusselt number;

l - coefficient of thermal conductivity, Watt/(m·K) or kkal/(m²·h·ºС);

L - the characteristic size, mm.

For example, for the pipeline is diameter.

The process of heat exchange between the surface of a solid body and a liquid is called heat transfer.
The surface of the body through which heat is transferred is called the heat exchange surface or the heat-releasing surface.

Thermal conductivity is the process of heat distribution by direct contact between particles with different temperatures.
Thermal conductivity coefficient l characterizes the ability of a substance to conduct heat.

The Nusselt number Nu is a dimensionless heat transfer coefficient. It is determined by the ratio of the intensity of convective heat transfer and the intensity of heat transfer by thermal conductivity for a stationary medium, i.e. is expressed as the ratio of the convective heat flux to the heat flux due to thermal conductivity.

✅ Formulas for calculating the Nusselt number Nu and the system of nomograms for determining the Nusselt number Nu are given on the page "Graph-analytical calculation of the Nusselt number Nu".

Table 1 - Ranges of change of heat transfer coefficient α and сoefficient of thermal conductivity λ under different conditions

Heat transfer coefficient a, kWatt/(m²·K)		Coefficient of thermal conductivity l, Watt/(m·K)
Free convection in gases	5 ... 30	Hydrogen under room conditions	0,2
Free convection waters	10² ... 10³	Air	0,025
The compelled convection of gases	10 ... 500	Dioxide of carbon	0,02
The compelled convection of waters	500 ... 2·10⁴	Pure silver and copper	400
Boiling of water	2·10³ ... 4·10⁴	Carbon steel	50
Liquid metals	10² ... 3·10⁴	Liquids	< 1
Film-type condensation of water pair	4·10³ ... 10⁴	Water	0,6
Dropwise condensation of water pair	4·10⁴ ... 10⁵	Nonmetallic firm materials	< 10
		Porous materials (a fuse, cotton wool etc.)	< 0,25

      What values can the Nusselt number take?
      For example, when water flows inside a heated pipe at a flow rate of 1 m/s, a wall temperature of 100°C, an average water temperature of 40°C, and a pipe inner diameter of 16 mm, the Nusselt number is 150.
      For graphic-analytical calculation of the heat transfer coefficient using a system of nomograms, we accept the range of variation of the Nusselt number: Nu = 0 ... 300..

Table 2 - Systems of nomograms for graphic-analytical calculation of the heat transfer coefficient

Nom-s #1

L=(1.2...8.5)mm

Nu=10...70

l=(0.35...0.85)Watt/(K·m)

a=(1...7)kWatt/(K·m^2)

Nom-s #2

L=(0.9...8.5)mm

Nu=2...14

l=(0.09...0.85)Watt/(K·m)

a=(0.2...1.4)kWatt/(K·m^2)

Nom-s #3