Difference between the surface temperature of a cable or duct in air and the ambient temperature.
This equation is solved by iteration with an initial value for $\Delta\theta_s$ of 16°C.
| $\frac{\Delta \theta_s+T_{4iii} n_{cc} W_t+\Delta \theta_{sun}}{2}$ | cables in air |
| $\frac{\Delta \theta_c+\Delta \theta_d}{1+K_A {\Delta \theta_s}^{0.25}}$ | cables in air-filled trough |
| $\theta_{at}-\theta_{de}$ | cables in channel (Heinhold) |
| $\theta_{de}-\theta_{air}$ | cables in riser/J-tube |
| $\theta_e-\theta_{at}$ | heat source in air-filled trough |
| $\theta_e-\frac{T_{sa} \left(T_{sa}+T_{at}\right)}{T_{sa}+T_{at}+T_{st}} W_{hs}$ | heat source in channel |
| $\frac{\Delta \theta_s+T_{4iii} W_{tot}+\Delta \theta_{sun}}{2}$ | PAC/GIL in air |
| $\frac{\Delta \theta_c}{1+K_A {\Delta \theta_s}^{0.25}}$ | PAC/GIL in air-filled trough |
| $\theta_{de}-\frac{T_{sa} \left(T_{sa}+T_{at}\right)}{T_{sa}+T_{at}+T_{st}} n_{cc} W_{tot}$ | PAC/GIL in channel (Heinhold) |
