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Electrical calculations

All the calculation factors given below are informations without commitment of any kind on our part. You can make your own estimates, ask us for advice (note: we will only engage in a study if it is part of a precise study contract signed between the 2 parties), or ask a design office.

Heating of liquids, gases or solids

(for the calculation of a mass to heat)

where :

= power in Watt
= mass to heat in kg
= specific heat in kcal/kg.K
= temperature rise in °C ou K

0,86 = coeff. of conversion kcal/h to Watt
t = temperature rise duration in hours

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Heating loss of a surface (natural convection)

(for calculations of insulation or temperature containment)

where :

= power in Watt
h   = Coeff. of convection in W/m²
  = surface area in m²
 = temperature difference between the surface and the air in °C or K

Heat loss by natural convection

Heating surface chauffante in ambiant air following the simplified formula according to Mac Adams.
= temperature difference between the heating surface chauffante and ambiant air
h1 = Coefficient of convection of a horizontal plate, heating face facing up
h2 = Coefficient of convection of a horizontal plate, heating face facing down
h3 =  Coefficient of convection of a vertical plate more than 0.3m high
h4 = Coefficient of convection of a vertical plate less than 0.3m high

Table of dissipated power by convection in Watts/m² in function of the DT and the type of surface: :

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Heat transfer by conduction

(for example for the calculation of heat losses through a wall)

Plane wall

Cylindrical wall

where :

= power in Watt
 = Coeff. of conduction of materials in W/m.K
S  = surface area of materials in m²
 = temperature difference between the 2 walls in °C ou K
= length of cylindrical wall in m
R and r = interiot and exterior radius and the wall in m.

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Heat transfer by radiating

(For the calculation of power losses or entry)

Generalized Kirchhoff law

where :

= power in Watt
 = absorption or emission factor of emitting surface
= 5,675 .  10-8  W/m².K4
T = absolute temperature in K.

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Heat exchange by radiating between 2 surfaces

(Heating by radiating)

where :

= power in Watt
 = mutual coeff. of radiating between the 2 surfaces W/m².K4
T1 and T2 = absolute temperature of the 2 surfaces in K
= angle factor d’angle or surfaces’ shape factor

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Heating by convection

There are many cases of convective heat transfer.
It may be natural (as in radiators or loss of a wall) see above, or forced (case heaters, unit heaters, etc …).
In each case we must consider the shape and size as well as laminar or turbulent flow.
Our study service can help you define your needs by providing us with a precise specification.

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State change

(fusion or evaporation)

where :

 = power in Watt
M = Weight in kg
= masse density in kg/dm3
= Volume in dm3 (or liter)
= latent heat of fusion or evaporation in kcal/kg.
t = time in hour

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