Heat Exchanger Calculator (LMTD Method)

Unit Conversion for Heat Exchanger Duty

**Heat duty units**

- 1 kW = 3412 BTU/hr = 0.003412 MMBtu/hr

- 1 MMBtu/hr = 293.1 kW

- 1 W = 3.412 BTU/hr

**Temperature units**

- °F = °C × 9/5 + 32

- ΔT in °C = ΔT in K

**Area units**

- 1 m² = 10.764 ft²

Exchanger duty Q = m × cp × ΔT or Q = U × A × LMTD. Keep mass flow, specific heat, and temperature difference in one consistent unit system before computing.

Worked Example: LMTD Step by Step

Size the duty of a shell-and-tube exchanger with hot fluid 100 C in, 60 C out, and cold fluid 20 C in, 50 C out (counter-current), U = 500 W/m2K, area = 50 m2.

Step 1 - temperature differences at each end: Delta-T1 = T_hot_in - T_cold_out = 100 - 50 = 50 K. Delta-T2 = T_hot_out - T_cold_in = 60 - 20 = 40 K.

Step 2 - LMTD = (50 - 40) / ln(50/40) = 10 / 0.2231 = 44.8 K.

Step 3 - duty Q = U x A x LMTD = 500 x 50 x 44.8 = 1,120,000 W = 1120 kW.

Check the unit conversion section to express this as BTU/hr (1120 kW = 3.82 MMBtu/hr).

Typical Overall Heat Transfer Coefficients

Selecting a realistic U value is the most uncertain part of an LMTD design. Typical ranges:

| Service | U (W/m2K) |

|---|---|

| Water to water | 800 - 1500 |

| Water to oil | 100 - 350 |

| Steam to water | 1000 - 4000 |

| Steam to oil | 100 - 300 |

| Gas to gas | 10 - 60 |

| Gas to water | 30 - 100 |

| Condensing refrigerant to water | 400 - 1000 |

Use the low end of the range when fouling is expected or the service is dirty; the design U must include an allowance for fouling (typical 0.0002 - 0.0006 m2K/W fouling resistance). Verify the U against manufacturer data or Perry's Handbook for the final design.