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Heat Exchanger Sizing Calculator

Size a counterflow heat exchanger using the LMTD method. Calculate heat duty, log mean temperature difference, and required surface area.

Inputs

°F
°F
°F
°F
GPM
BTU/lb·°F
BTU/hr·ft²·°F

Results

Heat Duty

3,998,400 BTU/hr

Required Surface Area

340.4 ft²

≈ 8 king-size beds

LMTD78.3 °F
Effectiveness57.1%
Mass Flow Rate49,980 lb/hr
How to Use This Calculator
  1. Enter the Hot Fluid Inlet and Outlet temperatures in °F to define the hot-side duty.
  2. Enter the Cold Fluid Inlet and Outlet temperatures in °F — verify the cold outlet is lower than the hot inlet for a valid counterflow arrangement.
  3. Enter the Hot Side Flow Rate in GPM and the Specific Heat of the fluid (water = 1.0 BTU/lb·°F).
  4. Set the Overall U-Value in BTU/hr·ft²·°F: water-to-water ≈ 150–300, steam-to-water ≈ 200–500.
  5. Review the Heat Duty in BTU/hr and the LMTD in °F — the LMTD drives the required surface area.
  6. Use the Required Surface Area in ft² to select or specify a heat exchanger model from vendor catalogs.

How the result changes with Hot Fluid Inlet Temp

Hot Fluid Inlet TempHeat DutyRequired Surface Area
129449,820 BTU/hr70.3 ft²
37112,544,980 BTU/hr597.6 ft²
66127,039,180 BTU/hr804.3 ft²
90339,134,340 BTU/hr910.9 ft²

What each input means

Hot Fluid Inlet Temp
Temperature of the hot fluid entering the heat exchanger.
Hot Fluid Outlet Temp
Desired temperature of the hot fluid leaving the heat exchanger.
Cold Fluid Inlet Temp
Temperature of the cold fluid entering the heat exchanger.
Cold Fluid Outlet Temp
Expected or desired temperature of the cold fluid leaving the heat exchanger.
Hot Side Flow Rate
Volumetric flow rate of the hot fluid (water assumed). For other fluids, adjust specific heat.
Specific Heat (Cp)
Specific heat of the fluid. Water = 1.0; ethylene glycol (50%) ≈ 0.81; oil ≈ 0.45-0.55.
Overall U-Value
Overall heat transfer coefficient. Water-to-water ≈ 150-300; water-to-oil ≈ 20-60; steam-to-water ≈ 200-500.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Hot Fluid Inlet Temp = 200, Hot Fluid Outlet Temp = 120, Cold Fluid Inlet Temp = 60, Cold Fluid Outlet Temp = 100 = 7 input(s) provided
  2. Calculate Heat Duty
    Heat Duty
    3998400 = 3998400
  3. Calculate Required Surface Area
    Required Surface Area
    340.4 = 340.4
  4. Calculate LMTD
    LMTD
    78.3 = 78.3
  5. Calculate Effectiveness
    Effectiveness
    57.1 = 57.1

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