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Thermal Transmittance and Thermal Resistance

Thermal transmittance U and thermal resistance R

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Thermal transmittance related to thermal resistance can be expressed as

U = 1 / R                                  (1)

where

U = overall thermal transmittance (W/m2 oC)

R = overall thermal resistance (m2oC/W)

Thermal resistance can be expressed by transforming (1) to

R = 1 / U                                 (1b)

The overall thermal transmittance from a layered construction with individual thermal resistances - like a wall with insulation and outside and inside cladding - can be expressed as

U = 1 / Σ Ri                                (2)

Example - Thermal Transmittance in Heat Exchanger

The thermal transmittance in an heat exchanger can be calculated as

U = 1 / (Rair film + Rcondensate film + Rmetal wall + Rproduct film)

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