Gravity Heating Systems

The density difference between hot and cold water is the circulating force in self circulating heating systems

A self circulation heating system operates by the force created by the density difference between hot and cold fluid.

The head forcing circulation through a gravity system is proportional to the elevation - he - of the heating elements (or radiators) above the boiler, and the temperature difference between the flow and return pipe lines.

The head available for circulation can be calculated

hl = hrr - ρf) / [(ρr + ρf) / 2]                                            (1)

where

hl = head available for circulation (m, ft)

hr = height of radiator or heating element above boiler (m, ft)

ρf = density of water in flow (hot) pipe (lb/ft3, kg/m3)

ρrdensity of water in return (cold) pipe (lb/ft3, kg/m3)

The thermal expansion of water is approximately 4.2% from 4 oC to 100 oC.

Head units can be converted to pressure units as

p = hl ρ g                                             (2)

where

p = pressure (Pa, N/m2) - Other units?

ρ = density (kg/m3) (using hot or cold density has little impact in this calculation)

g = acceleration of gravity (9.81 m/s2)

Circulating Pressure - Flow and Return Temperature

The forcing pressures in self circulation system with operating temperatures between 50 to 95oC are indicated in the diagram and table below.

Circulating Pressure in Pa (N/m2) per m circulating elevation - he
Return Temperature
(oC)
Flow Temperature (oC)
405060708090
90           0
80         0 64
70       0 59 123
60     0 54 113 177
50   0 48 101 161 225
40 0 41 89 143 203 267

Related Topics

• Heating - Heating systems - capacity and design of boilers, pipelines, heat exchangers, expansion systems and more

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