Heat Transfer

Sizing Sulfuric Acid Cooling Heat Exchangers

Preliminary sizing considerations for sulfuric acid cooling heat exchangers — covering concentration-dependent corrosion, materials boundaries, U-value caution, temperature approach, and the need for specialist review.

TypeEngineering guide — concept explainer

Definition

Sizing sulfuric acid cooling heat exchangers is the process of estimating the heat transfer area required to cool sulfuric acid from a higher temperature to a target outlet temperature. The fundamental equation A = Q / (U × ΔTₘ) still applies, but the corrosive and concentration-dependent nature of sulfuric acid introduces materials constraints, U-value uncertainty, and safety boundaries that do not apply in clean-water or hydrocarbon service. Concentration, temperature, dilution risk, and materials of construction dominate the sizing and selection process.

Why it matters

Sulfuric acid is one of the most widely produced industrial chemicals, and acid cooling is a common heat exchanger duty in acid plants, fertiliser production, metallurgical processing, and chemical manufacturing. The corrosion behaviour of sulfuric acid is strongly concentration- and temperature-dependent — a material that resists 98% acid at 40 °C may fail rapidly at 70% acid or at higher temperatures. Preliminary sizing must account for these boundaries because an error in materials or temperature approach can cause rapid exchanger failure, acid leaks, and serious safety consequences.

Formula

Heat transfer area

A = Q / (U × F × ΔTₘ)

Heat duty (sensible)

Q = ṁ × Cp × ΔT

Fouled U-value

1/U_dirty = 1/U_clean + Rd_hot + Rd_cold

Units involved

•A — heat transfer area in m² or ft²
•Q — heat duty in kW, W, or BTU/h
•U — overall heat transfer coefficient in W/(m²·K) or BTU/(h·ft²·°F)
•ΔTₘ — log mean temperature difference in K, °C, or °F
•F — LMTD correction factor (dimensionless)
•Rd — fouling resistance in m²·K/W or h·ft²·°F/BTU
•ṁ — mass flow rate in kg/s, kg/h, or lb/h
•Cp — specific heat capacity in J/(kg·K) or BTU/(lb·°F)

Worked example

Preliminary sizing for cooling 98% sulfuric acid from 80 °C to 45 °C using cooling water entering at 30 °C and leaving at 40 °C. Acid flow rate is 5 kg/s. Assume Cp ≈ 1.4 kJ/(kg·K) for 98% H₂SO₄, a conservative preliminary U-value of 300 W/(m²·K) (acid in carbon steel tubes, water on shell side), and total fouling resistance of 0.0004 m²·K/W. Apply 15% design margin.

01Q = 5 × 1400 × (80 − 45) = 245,000 W = 245 kW
02ΔT₁ = 80 − 40 = 40 °C (counter-current)
03ΔT₂ = 45 − 30 = 15 °C
04LMTD = (40 − 15) / ln(40/15) = 25 / 0.9808 = 25.5 °C
05U_dirty = 1 / (1/300 + 0.0004) = 1 / (0.003333 + 0.0004) = 1 / 0.003733 = 268 W/(m²·K)
06A = 245,000 / (268 × 25.5) = 35.8 m²
07A_design = 35.8 × 1.15 = 41.2 m²

Result

Required area ≈ 41 m² (with 15% design margin). Materials selection, corrosion allowance, and compliance with acid plant standards must be confirmed by a specialist.

Common mistakes

•Using generic U-values without accounting for the low thermal conductivity of acid-resistant materials — alloy tubes (e.g., Hastelloy, high-silicon cast iron) have lower conductivity than carbon steel, reducing U.
•Ignoring the concentration–temperature corrosion map — sulfuric acid corrosion behaviour changes dramatically with both concentration and temperature. A 5% change in acid concentration or a 10 °C shift can move from acceptable to aggressive corrosion.
•Cooling acid below the dew point of the cooling medium — if the cooling water side operates near or below the acid dew point, external corrosion or condensation issues can arise on the wrong surfaces.
•Assuming carbon steel is adequate at all concentrations — carbon steel works for concentrated (>93%) sulfuric acid below ~40 °C but corrodes rapidly in dilute or intermediate concentrations.
•Neglecting dilution risk — water leaking into the acid side (e.g., through a tube failure) causes violent exothermic dilution, rapid temperature rise, and accelerated corrosion.
•Forgetting corrosion allowance in the wall thickness — acid service exchangers need corrosion allowance added to the mechanical design pressure calculation.

When to use the calculator

Use the Heat Duty Calculator to estimate Q from the acid flow rate, Cp, and temperature change. Use the LMTD Calculator for ΔTₘ. The Heat Exchanger Area Calculator estimates the required area with design margin. Start with a conservative U-value below clean-fluid ranges — the Typical U-Values Reference gives general ranges, but acid service values are typically at the lower end due to material constraints. Consult the Fouling Factors Reference and Design Margin Reference for supporting assumptions.

Heat Exchanger Area Calculator →Heat Duty Calculator →LMTD Calculator →Fouling Factor Selector →

FAQ

What materials are typically used for sulfuric acid heat exchangers?

It depends on concentration and temperature. Carbon steel is common for >93% H₂SO₄ below ~40 °C. For dilute or intermediate concentrations, or higher temperatures, alloys such as Hastelloy C-276, Alloy 20, high-silicon cast iron (Duriron), or tantalum may be required. Glass-lined or PTFE-lined exchangers are used in some services. Materials selection is outside the scope of preliminary thermal sizing and must be confirmed by a corrosion or materials engineer.

What U-value range is typical for sulfuric acid cooling?

For acid on the tube side and cooling water on the shell side in a shell-and-tube exchanger: roughly 200–500 W/(m²·K), depending on tube material, acid concentration, and flow velocities. Alloy tubes reduce U compared to carbon steel due to lower thermal conductivity. Use the lower end of the range for preliminary estimates.

Why is the cooling water outlet temperature important?

A tight temperature approach (acid outlet close to cooling water inlet) increases the required area. But for acid coolers, the cooling water outlet temperature also affects fouling on the water side — higher water outlet temperatures increase scaling and biological fouling. Keeping the water outlet below ~45 °C is common practice where water quality permits.

What design margin should I apply for acid service?

Typically 10–20% over the calculated fouled area, similar to clean-fluid services but sometimes higher if the acid concentration or operating temperature may vary. The additional margin covers uncertainty in U-value, fouling, and the possibility of operating outside the design acid concentration. See the Design Margin Reference for general guidance.

Can I use a plate heat exchanger for sulfuric acid cooling?

Yes, in some services — plate exchangers with appropriate gasket and plate materials (e.g., PVDF plates, Viton or PTFE gaskets) are used for dilute acid cooling. For concentrated acid at elevated temperatures, shell-and-tube or block-type exchangers are more common. Exchanger type selection must consider acid concentration, temperature, pressure, and leak consequences.