Why Small Temperature Difference Applications Require Long ( LB SERIES PHE) Plate Heat Exchangers

Why Small Temperature Difference Applications

Require Long ( LB SERIES PHE) or Multi-Pass Plate Heat Exchangers

(And Why Shell & Tube Becomes Inefficient)

1. The Thermodynamic Challenge of Small ΔT

In heat exchanger design, the basic equation is:

$Q = U cdot A cdot Delta T_{lm}$

When the logarithmic mean temperature difference (LMTD) becomes very small, the heat transfer driving force nearly disappears.

For example:

Hot Side	Cold Side
30 → 10°C	8 → 28°C

$Delta T_1 = 30 - 28 = 2K,quad Delta T_2 = 10 - 8 = 2K$ $Delta T_{lm} = 2K$

At this level, the system is approaching the thermodynamic limit.
Any heat exchanger must compensate for the lack of temperature driving force by increasing either:

U – overall heat transfer coefficient
A – effective heat transfer area

2. Why Shell & Tube Becomes Extremely Large

Shell & tube heat exchangers typically have:

Parameter	Typical Value
U value	300–800 W/m²·K
Minimum approach ΔT	8–15 K

When LMTD falls below 5 K, shell & tube units must rely almost entirely on surface area.
The result is exponentially increasing size, weight, and cost.

For the same duty at ΔTlm = 2K:

$A_{S&T} pprox 5sim8 imes A_{Plate}$

This is why shell & tube units become impractical for low-grade heat recovery.

3. Why Plate Heat Exchangers Can Still Operate

Plate heat exchangers (PHE) achieve:

Parameter	Typical Value
U value	2000–6000 W/m²·K
Minimum approach ΔT	1–3 K

The secret lies in:

Corrugated plates
High shear, turbulent micro-channels
Very thin thermal boundary layers

This allows PHEs to maintain high U values even when ΔT is extremely small.

4. Why Long and Narrow Designs Are Required

If a PHE is simply made wider:

Flow velocity decreases
Turbulence weakens
Heat transfer coefficient drops

So instead, engineers design PHEs to be:

Long and narrow, not wide and short

This ensures:

High velocity
Strong turbulence
Stable U value

The long flow path also increases the NTU (Number of Transfer Units), which is essential when ΔT is small.

5. Why Multi-Pass Flow Is Used

Multi-pass configurations force the fluid to:

Repeatedly change direction
Increase internal velocity
Extend effective contact time

This dramatically increases the convective heat transfer coefficient and allows the exchanger to operate closer to the thermodynamic limit.

6. Engineering Summary

Feature	Engineering Purpose
Long plate design	Increase NTU and effective heat path
Narrow flow channels	Maintain velocity and turbulence
Multi-pass arrangement	Increase U value under low ΔT
High pressure drop	Acceptable trade-off for heat recovery

Final Conclusion

Small temperature difference is not a design problem – it is a thermodynamic limitation.
Plate heat exchangers overcome this limitation by using fluid dynamics to amplify heat transfer.

Long, narrow, multi-pass plate heat exchangers are therefore not a design choice, but an engineering necessity for low-grade heat recovery.