Stainless steel heating circuit distributor - 4 heating circuits

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The mean temperature difference of a plate heat exchanger can be calculated by several methods, depending on the type of heat exchanger and the specific conditions. A common method is the Logarithmic Mean Temperature Difference. Here is how you can calculate it:

Formula for the logarithmic mean temperature difference

LMTD = (ΔT1 - ΔT2) / ln(ΔT1 / ΔT2)

variables

• LMTD: logarithmic mean temperature difference
• ΔT1: Temperature difference at one end of the heat exchanger
• ΔT2: Temperature difference at the other end of the heat exchanger

Example

• Temperature of the hot liquid at the inlet (Th): 150 °C
• Temperature of the cold liquid at the inlet (Tc): 30 °C
• Temperature of the hot liquid at the outlet (Th'): 80 °C
• Temperature of the cold liquid at the outlet (Tc'): 60 °C

1. Calculation of ΔT1 & ΔT2

ΔT1 = Th - Tc = 150 °C - 30 °C = 120 °C

ΔT2 = Th' - Tc' = 80 °C - 60 °C = 20 °C

2. Calculation of the mean temperature difference

LMTD = (120°C - 20°C) / ln(120°C / 20°C)
LMTD ≈ 55.8 °C

Q = U * A * LMTD

• Q: heat flow (W)
• U: total heat transfer coefficient (W/m²·K)
• A: Heat transfer area (m²)
• LMTD: logarithmic mean temperature difference (K)

or

Q = ṁ * cp * LMTD

• Q: heat flow (W)
• ṁ: mass flow rate (kg/s)
• cp: specific heat capacity (J/kg·K)
• LMTD: logarithmic mean temperature difference (K)

cocurrent and countercurrent principle

The heat energy that can be transferred in a plate heat exchanger is not only determined by the size of the exchange surface and the heat transfer coefficient (k), but is also significantly influenced by the flow directions of the media involved. If both media flow in the plate heat exchanger in the same direction, this is referred to as the cocurrent principle. If they flow in opposite directions, however, this is the countercurrent principle.

The flow direction has a significant influence on the average temperature difference between the media, which in turn determines the efficiency of heat transfer.

Temperature curve with the direct current principle | Temperature curve with the countercurrent principle

Theoretical consideration of the connection options of a plate heat exchanger:

In this example we assume two media with the same heat capacity and the same mass flow.

• Medium 1: 80 °C
• Medium 2: 20 °C

With the cocurrent principle, the maximum temperature that can be reached for both media would be 50 °C, which corresponds to the mixing temperature. With the countercurrent principle, however, medium 2 could reach a temperature of up to 80 °C, while medium 1 can be cooled to 20 °C.

This means that the countercurrent principle allows a larger amount of heat to be transferred using the same exchange surface area.

Finding the right plate heat exchanger doesn't have to be complicated. If you have any questions or need support, don't hesitate to contact us. We'll be happy to help you choose the right model for your needs.