Number of Transfer Units

\[NTU=\frac{UA}{C_{min}}\]

Variables

NTUnumber of transfer units

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

Aheat transfer area (m²)

Cminminimum heat capacity rate (W/K)

Description

What is this formula?

The number of transfer units (NTU) is a dimensionless parameter that quantifies the thermal size and effectiveness potential of a heat exchanger. Higher NTU values generally indicate greater heat transfer capability.

When to use it

Use this formula when applying the effectiveness-NTU method, designing heat exchangers, evaluating exchanger performance, or estimating required heat transfer area.

Example

A heat exchanger has an overall heat transfer coefficient of 450 W/(m²·K), a heat transfer area of 120 m², and a minimum heat capacity rate of 18,000 W/K.

Formula:

NTU = UA / Cmin

Substitution:

NTU = (450 × 120) / 18000

NTU = 54000 / 18000

Result:

NTU = 3.0

The exchanger has an NTU value of 3.0.

Applications

- Effectiveness-NTU calculations

- Heat exchanger design

- Thermal performance evaluation

- Process optimization

- HVAC and energy systems

Note

NTU is a dimensionless design parameter. Its relationship with effectiveness depends on exchanger configuration, including parallel flow, counterflow, shell-and-tube, and crossflow arrangements. The same NTU can produce different effectiveness values depending on the flow arrangement.