Cooling Water Heat Exchanger Sizing

Definition

Cooling water heat exchanger sizing is the preliminary sizing of a heat exchanger that uses cooling water as the cold-side utility to remove heat from a process stream. The thermal calculation is the same as any other liquid–liquid sensible-heat exchanger — Q = ṁ × Cp × ΔT for the duty, A = Q / (U × F × LMTD) for the area — but the design decisions that surround it are dominated by the cooling-water system: how much temperature rise the cooling-water circuit can absorb, what supply temperature is realistic across the year, what fouling and scaling the water carries, and what tube-side velocity and material the water-side metallurgy will tolerate.

Why it matters

Cooling-water heat exchangers are one of the most common services on any industrial site, and they are also one of the most common places where a thermal calculation that looks fine on paper is undone by the cooling-water system itself. Pick the wrong supply temperature for the worst case of the year and the exchanger does not meet duty in summer. Pick too tight an approach and the area swells, and the exchanger may still be limited by the water-side velocity or pressure drop. Ignore the water quality and the fouling resistance you assumed at design time is wrong within months. Preliminary sizing for cooling-water service is mostly about making the right assumptions explicit before any detailed thermal-hydraulic design or vendor rating begins.

Formula

Units involved

• •Q — heat duty in kW, W, or BTU/h
• •ṁ — mass flow rate in kg/s, kg/h, or lb/h
• •Cp — specific heat capacity in J/(kg·K) or BTU/(lb·°F); cooling water ≈ 4.18 kJ/(kg·K)
• •ΔT_cw — cooling-water temperature rise in K, °C, or °F
• •U — overall heat transfer coefficient in W/(m²·K) or BTU/(h·ft²·°F)
• •ΔTₘ — log mean temperature difference in K, °C, or °F
• •A — heat transfer area in m² or ft²
• •Approach — minimum temperature difference between streams, in K, °C, or °F

Worked example

Common mistakes

• •Designing for an annual-average cooling-water supply temperature instead of the worst-case (summer) supply — the exchanger needs to meet duty when the water is hottest.
• •Picking a cooling-water temperature rise that the site cooling tower or once-through system cannot deliver — typical closed-loop ΔT_cw is 5–10 °C, but the actual limit depends on the site utility design.
• •Specifying an approach temperature that drives the area to be very large — typical closed-loop cooling-water approaches are 5–10 °C; tighter approaches dramatically increase area.
• •Using a clean U-value for cooling water — fouling resistance for cooling water depends strongly on water quality and treatment. Use a published fouling resistance and confirm with the water-treatment scope.
• •Ignoring tube-side velocity limits — cooling water on the tube side typically needs to be 1.0–2.5 m/s to limit fouling without erosion; too low encourages deposition, too high erodes the tubes.
• •Forgetting metallurgy — copper alloys, stainless steels, and titanium have different cost, corrosion, and U-value implications. Cooling-water chemistry (chlorides, pH, biological load) drives the metallurgy choice.
• •Treating the preliminary calculation as a final design — water-side fouling, scaling, biological control, vibration, and pressure drop all need detailed review before procurement.

When to use the calculator

Use the Heat Duty Calculator for Q and the cooling-water flow check (ṁ_cw = Q / (Cp_cw × ΔT_cw)). Use the LMTD Calculator for ΔTₘ — counter-current is the usual configuration but check F if the exchanger is multi-pass. The Heat Exchanger Area Calculator handles A = Q / (U_dirty × F × LMTD) with design margin. Pull U-values from the Typical U-Values Reference, fouling resistances from the Fouling Factors Reference, and a sensible approach from the Minimum Approach Temperature Reference.

FAQ