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# Plate heat exchanger calculation method

To solve a thermal problem, we need to know several parameters. Further data can then be determined.

The six most important parameters include:

- The amount of heat to be transferred (heat load)
- The inlet and outlet temperatures on the primary and secondary sides
- The maximum allowable pressure drop on the primary and secondary sides
- The maximum operating temperature
- The maximum operating pressure
- The flowrate on the primary and secondary sides

If the flow rate, specific heat and temperature difference on one side are known, the heat load can be calculated.

## 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)

c_{p} = specific heat (btu/lb °F)

δt = temperature difference between inlet and outlet on one side (°F)

k = heat transfer coefficient (btu/ft^{2} h °F)

A = heat transfer area (ft^{2})

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

#### LMTD can be calculated by using the following formula, where ∆T1 = T1–T4 and ∆T2 = T2–T3

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### 2. Heat transfer coefficient and design margin

The total overall heat transfer coefficient k is defined as:

α_{1} = The heat transfer coefficient between the warm medium and the heat transfer surface (btu/ft^{2} h °F)

α_{2} = The heat transfer coefficient between the heat transfer surface and the cold medium (btu/ft^{2} h °F)

δ = The thickness of the heat transfer surface (ft)

R_{f} = The fouling factor (ft^{2} h °F/btu)

λ = The thermal conductivity of the material separating the medias (btu/ft h °F)

k_{c} = Clean heat transfer coefficient (Rf=0) (btu/ft^{2} h °F)

k = Design heat transfer coefficient (btu/ft^{2} h °F)

M = Design Margin (%)

Combination of these two formulas gives: M = k_{c} · R_{f}

i.e the higher k_{c} value, the lower R_{f}-value to achieve the same design margin.

For a more complete explanation of heat transfer theory and calculations, download the following brochure:

The theory behind heat transfer

Contact us and we'll connect you with a plate heat exchanger engineer that can help you with your calculations.

## Quick links:

**How GPHEs work**

**Selection guide**

**Features that matter**

**Plate technology**

**GPHE vs shell-and-tube**

**Calculation method**

**Types of GPHEs**

**Servicing a GPHE**

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