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Circulating Force in Gravity Heating Systems
Differential pressure due to difference in density between hot and cold water is the circulating force in a self circulating heating system
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A self circulation heating system operates by the force created by the density difference between the hot and cold fluid.
Gravity Head
The head available forcing circulation through a radiator in a gravity system is proportional to the elevation - he - of the radiator or heating element above the boiler, and the temperature difference between the flow and return pipes.
The head available for circulation can be expressed as:
hl = hr ( ρr - 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)
ρr = density of water in return (cold) pipe (lb/ft3, kg/m3)
The thermal expansion of water is 4.2% from 4 oC to 100 oC.
Converting Head to Pressure
Head can be converted to pressure units by the expression:
p = hl ρ g (2)
where
p = pressure (Pa, N/m2) - Other units?
ρ = density (kg/m3). Using hot or cold density have very little impact on this calculation.
g = gravitation 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) | |||||
| 40 | 50 | 60 | 70 | 80 | 90 | |
| 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 |
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