Counterflow Heat Exchanger Design

Hot Fluid
Hot fluid temperature entering the exchanger.
Hot fluid temperature leaving the exchanger.
Mass flow rate of the hot fluid.
Specific heat capacity of the hot fluid. Water ≈ 4.18 kJ/kg·°C.
Cold Fluid
Cold fluid temperature entering the exchanger.
Cold fluid temperature leaving the exchanger.
Specific heat capacity of the cold fluid. Water ≈ 4.18 kJ/kg·°C.
Overall
Overall heat transfer coefficient, accounting for resistances. Units: W/m²·°C.

Calculation Explanation

This calculator designs a counterflow heat exchanger by determining the required surface area (A) based on the specified operating conditions.

1. Calculate Heat Duty (Q):
The heat transferred from the hot fluid (or gained by the cold fluid) is calculated. Using the hot fluid:
Q = ṁhot ⋅ cp,hot ⋅ (Thot,in - Thot,out)
Units: kW (kJ/s).

2. Calculate Log Mean Temperature Difference (LMTD):
LMTD represents the average temperature driving force for counterflow:
ΔT₁ = Thot,in - Tcold,out
ΔT₂ = Thot,out - Tcold,in
LMTD = (ΔT₁ - ΔT₂) / ln(ΔT₁ / ΔT₂)
If ΔT₁ = ΔT₂, then LMTD = ΔT₁.
Units: °C.

3. Calculate Required Heat Transfer Area (A):
The fundamental heat exchanger design equation is rearranged to solve for area:
Q = U ⋅ A ⋅ LMTD => A = Q / (U ⋅ LMTD)
Note: Q must be converted to Watts (kW * 1000) for consistency with U (W/m²·°C).
Units: m².

4. Calculate Cold Fluid Flow Rate (ṁcold):
As a check, the cold fluid flow rate required to absorb the heat duty Q is calculated:
Q = ṁcold ⋅ cp,cold ⋅ (Tcold,out - Tcold,in)
ṁcold = Q / [ cp,cold ⋅ (Tcold,out - Tcold,in) ]
Units: kg/s.