Plate Heat Exchanger Experiment: LMTD, U & Effectiveness

The plate heat exchanger experiment determines the heat duty, log mean temperature difference (LMTD), overall heat-transfer coefficient, and effectiveness of a plate-type exchanger in both parallel- and counter-flow. Plate heat exchangers deliver very high heat transfer in a compact size, which is why they dominate dairy, HVAC, and process cooling.

Aim of the experiment

To determine the LMTD, overall heat-transfer coefficient (U), and effectiveness of a plate heat exchanger in parallel and counter flow.

Theory and formulas

The heat lost by the hot fluid equals the heat gained by the cold fluid:

• Q = m_h c_h (T_h,in − T_h,out) = m_c c_c (T_c,out − T_c,in)
• LMTD: ΔT_lm = (ΔT₁ − ΔT₂) / ln(ΔT₁/ΔT₂)
• Overall coefficient: U = Q / (A · ΔT_lm)
• Effectiveness: ε = Q_actual / Q_max, where Q_max = C_min(T_h,in − T_c,in)

For the same terminal temperatures, counter-flow gives a larger LMTD than parallel-flow, so it transfers more heat — the key practical takeaway.

Apparatus required

• Plate heat exchanger test rig with a corrugated plate pack
• Hot-water tank with heater and a cold-water supply
• Flow-control valves to switch between parallel and counter flow
• Flow meters and inlet/outlet thermometers on both streams

Procedure

1. Set the hot and cold flow rates and switch the valves to parallel flow.
2. Allow temperatures to stabilise; record all four inlet/outlet temperatures and both flow rates.
3. Switch to counter flow and repeat the readings.
4. Compute Q, LMTD, U, and effectiveness for each arrangement and compare.

Result

Counter-flow shows a higher LMTD, overall coefficient, and effectiveness than parallel-flow — demonstrating why industrial exchangers are run counter-flow.

Applications

Plate heat exchangers are used in dairy pasteurisation, HVAC, and chemical process heating and cooling. Compare them with the double-pipe heat exchanger and shell-and-tube heat exchanger experiments; LMTD and effectiveness are defined in the engineering lab glossary.

Frequently asked questions

Why is a plate heat exchanger more efficient than a shell-and-tube?

The corrugated plates create turbulence and provide a very large surface area per unit volume, giving a higher heat-transfer coefficient in a compact unit.

Why does counter-flow transfer more heat than parallel-flow?

Counter-flow maintains a larger and more uniform temperature difference along the exchanger, which gives a higher LMTD and more heat transfer.

What is the overall heat-transfer coefficient U?

U combines the convective and conductive resistances of the exchanger and is found from U = Q / (A · LMTD).

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