# Thermal Losses by Transmission

9.7.3.1 Circular Ducts

1 — 1 . 1 I — Di + 2dk, 1

In which

<Pi is linear heat flux, Wm-1 U* is linear U-value, Wm-1 K_1 0ad is air temperature in duct, °C 0a is surrounding air temperature, °C is heat transfer coefficient inside, Wm-2 K_l D, is internal diameter, m

A, s is heat conduction coefficient of insulation, Wm’1 K’1

<4, is insulation thickness, m

Hu is heat transfer coefficient outside, Wm-2 K-1

Ua 77hia 277A»!

This formula is valid for square ducts. For the parallel extension part, one could calculate

-4- = t — + ^ + r-. (9.1.53)

Ub h‘ A’* h»

The total U* is calculated as

U*2AUA + (b-a)UB/P ^ (9.1.54)

In which b is the internal height of the duct and P = 2a + 2b is the perimeter

Length, m.

The energy loss calculation for air temperature losses is

Oend= ea + (estart-ea)eB, (9.155)

In which

9end is air temperature at the end of the duct system, °С 0start is air temperature at the start of the duct system, °С вл is surrounding air temperature, °С

В is calculated as

В (9,156)

Я uPCp

Where

U* is U value of duct, W m-1 K-1

L is length of duct, m

Qv is volumetric airflow, m3 s_i

P is density of air in duct, kg m-3

Cp is specific heat of air in duct, J kg-1 K-1