# How to calculate the heat duty for heat Exchangers ?

### Calculation method

The heat load of a heat exchanger can be derived from the following two formulas: 1. Heat load, Theta and LMTD calculation

Where: P = heat load (btu/h) m = mass flow rate (lb/h) cp = specific heat (btu/lb °F) δt = temperature difference between inlet and outlet on one side (°F) k = heat transfer coefficient (btu/ft2 h °F) A = heat transfer area (ft2) LMTD = log mean temperature difference

T1 = Inlet temperature - hot side T2 = Outlet temperature - hot side T3 = Inlet temperature - cold side T4 = Outlet temperature - cold side

The heat load of a heat exchanger can be derived from the following two formulas: 1. Heat load, Theta and LMTD calculation

2. Heat transfer coefficient and design margin The total overall heat transfer coefficient k is defined as:

α1 = The heat transfer coefficient between the warm medium and the heat transfer surface (btu/ft2 h °F) α2 = The heat transfer coefficient between the heat transfer surface and the cold medium (btu/ft2 h °F) δ = The thickness of the heat transfer surface (ft) Rf = The fouling factor (ft2 h °F/btu) λ = The thermal conductivity of the material separating the medias (btu/ft h °F) kc = Clean heat transfer coefficient (Rf=0) (btu/ft2 h °F) k = Design heat transfer coefficient (btu/ft2 h °F) M = Design Margin (%) Combination of these two formulas gives: M = kc · Rf i.e the higher kc value, the lower Rf-value to achieve the same design margin.