# Heat Transfer

Heat exchanger surface area calculations

David Wells says:

My "Dave's Heat Transfer Rules of Thumbs" are:

For natural air circulation, figure around 10 Watts per square meter [of surface area] per degree C heat transfer, and for forced air, f gure up to about three times this value.

For water natural circulation, figure around 200 Watts per square meter per degree C heat transfer, and again for forced circulation, figure up to three times this.

For oil or organic materials such as non-oxygenated oils, like pure hydrocarbons, for non-boiling heat transfer, figure about one fifth the water values. For pure oxygenated materials for stuff like alcohols or glycols, figur e about one third of the water circulation values.

For boiling or condensation heat transfer of organics, figure around 1000 Watts per square meter per degree C. Assumes vapor is at atmospheric pressure.

For boiling heat transfer of polar oxygenated materials like water or methanol, figure about triple the value or about 3000 Watts per square meter per degree C. Again, assumes fluids condense or evaporate at near atmospheri c pressure.

For calculating the effect of the pressure of the boiler or condenser, figure the heat transfer will go as roughly the cube of the absolute pressure. So, for example, if your boiler operates at sub-atmospheric pressure of , say, 1/8 atmospheres, the heat transfer will be reduced by the cube root of one eigth, which is about 0.5. So the heat transfer rate is reduced by half.

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