Overall Heat Transfer Coefficient

\[U=\frac{Q}{A\Delta T_{lm}}\]

Variables

Uoverall heat transfer coefficient (W/(m²·K))

Qheat transfer rate (W)

Aheat transfer area (m²)

LMTDlog mean temperature difference (K)

Description

What is this formula?

The overall heat transfer coefficient quantifies the combined ability of a heat exchanger to transfer heat through convection, conduction, fouling layers, and wall resistance.

When to use it

Use this formula when designing heat exchangers, evaluating thermal performance, estimating heat-transfer area requirements, or analyzing process equipment.

Example

A shell-and-tube heat exchanger transfers 1.2 MW of heat through an area of 75 m². The log mean temperature difference is 40 K.

Formula:

U = Q / (A·LMTD)

Substitution:

U = 1,200,000 / (75 × 40)

U = 1,200,000 / 3000

Result:

U = 400 W/(m²·K)

The overall heat transfer coefficient is 400 W/(m²·K).

Applications

- Shell-and-tube heat exchangers

- Plate heat exchangers

- Condensers

- Evaporators

- Process thermal design

Note

This equation provides an effective overall coefficient that incorporates all thermal resistances between the hot and cold fluids. The value of U depends on fluid properties, flow regime, fouling, exchanger geometry, and wall material.