What is the acid dew point and why does it matter for gas cooler design?

The acid dew point (ADP) is the temperature at which H₂SO₄, HCl or HNO₃ vapour begins to condense from flue gas onto metal surfaces. Below the ADP, condensate is a highly corrosive dilute acid that corrodes carbon steel in days and attacks standard stainless steel within months. Key values: **H₂SO₄ ADP** = 120–160°C for SO₃ concentrations of 5–50 ppm (Nelson or Verhoff-Banchero correlation); **HCl ADP** = 55–75°C (much lower than sulphuric acid ADP). Design rule: gas cooler outlet temperature must be maintained at least 20–30°C above the calculated ADP to provide safety margin for fluctuating gas composition and local cold spots. Options when cooling to below ADP is unavoidable: (1) acid-resistant construction — Hastelloy C-276, titanium, Teflon-lined tubes; (2) glass-fibre reinforced polyester (FRP) shell-and-tube for T < 100°C; (3) wet scrubber instead of dry gas cooler — the scrubber is designed to operate below the dew point and the wet contact environment is its normal operating condition. Lozzar calculates ADP for every gas cooler application and specifies materials accordingly.

Shell-and-tube or plate heat exchanger — how do I choose for my liquid-liquid application?

The decision matrix for liquid–liquid service (no phase change, T < 160°C, P < 25 bar): **Choose PHE when**: both streams are clean (no particles, no fouling), temperature approach ≥ 3°C is acceptable, footprint is constrained, and future flow expansion is likely (plates can be added). PHE achieves NTU = 5–8 in a single unit, enabling temperature approaches of 1–3°C impossible in S&T. PHE capital cost is 20–50% lower than S&T for equivalent duty. **Choose S&T when**: one or both streams contain solids/particles (shellside can be rodded clean), temperature cross is required (S&T multi-pass can achieve true counter-current with temperature cross; PHE cannot), pressure exceeds 25 bar on gasketed PHE, temperature exceeds 160°C on gasketed PHE, or the process fluid is incompatible with available gasket materials. **Mixed strategy**: use PHE for clean scrubbing liquid cooling (water-to-water circuit), S&T for steam condensation, solvent condensation and high-fouling streams. Cost-wise: PHE is cheaper to buy but slightly harder to maintain when fouled (individual plate cleaning); S&T is more expensive but easier to hot-rod and replace tubes.

What data do you need to design and quote a heat exchanger?

For each stream (hot and cold side): (1) **Flow**: mass flow (kg/h) or volumetric flow (Nm³/h for gases, m³/h for liquids) with density; (2) **Temperatures**: inlet temperature and target outlet temperature (or heat duty Q in kW if outlet T is not fixed); (3) **Composition**: fluid name or composition, moisture content, dust/particle content and d50, pH (if aqueous), solvent type if applicable; (4) **Physical properties**: density, specific heat cp, viscosity µ, thermal conductivity λ at operating temperature — or Lozzar will derive these from composition if provided; (5) **Pressure**: operating pressure on each side; maximum allowable pressure drop; (6) **Phase**: liquid, gas or two-phase (if condensing: dew point, latent heat); (7) **Fouling**: expected fouling service (clean, moderate, heavy) and access for cleaning (can tubes be rodded? Can plates be dismounted?); (8) **Materials**: any corrosion concern, existing material specification preference; (9) **Standards**: PED category, ASME VIII, or local pressure vessel code. With this data, Lozzar issues a thermal design (HTRI or hand calculation), a mechanical datasheet and a budget quotation. Lead time: 8–20 weeks for engineered heat exchangers.

process equipment

Heat Exchanger

Recover heat you are already paying for — before it leaves the stack.

In every drying, calcining or combustion process, 20–40% of the energy input leaves as hot exhaust gas. A well-engineered heat exchanger converts this waste heat into combustion air pre-heat, dryer inlet air, hot water, steam or solvent condensate — directly reducing fuel consumption and operating cost. Lozzar designs, supplies and integrates shell-and-tube, plate-and-frame, rotary recuperator, bare-tube and finned-tube heat exchangers as part of complete process systems: gas-to-air pre-heaters for rotary and flash dryers, exhaust gas coolers before bag filters, inter-stage coolers for multi-pass flash dryers, and solvent condensers for closed-loop nitrogen drying circuits.

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Heat Exchanger — Recover heat you are already paying for — before it leaves the stack.

Heat Exchanger Types and Selection Logic

Heat exchangers transfer thermal energy between two fluid streams without mixing them. The fundamental sizing equation is Q = U × A × LMTD, where Q is heat duty (kW), U is overall heat transfer coefficient (W/m²·K), A is heat transfer area (m²) and LMTD is the log mean temperature difference (°C). The choice of exchanger type is driven by temperature level, fouling tendency, phase change, pressure and maintenance access requirements.

**Shell-and-tube heat exchangers (S&T)** are the process industry workhorse. One fluid flows inside the tubes (tube-side) and the other over the outside of the tubes (shell-side). The TEMA standard defines construction types: type BEM for fixed tubesheet (cheapest, no thermal expansion provision — use when ΔT < 50°C between shells); type BEU/AEU for U-tubes (thermal expansion accommodated, easy tube bundle removal, but no individual tube replacement); type BEW/AEW for floating head (full mechanical cleaning of both sides, premium cost, for fouling service). Heat transfer coefficients: gas–gas 20–80 W/m²·K; gas–liquid 50–200 W/m²·K; liquid–liquid 300–1,500 W/m²·K; condensing steam–liquid 2,000–8,000 W/m²·K.

**Plate-and-frame heat exchangers (PHE)** consist of thin corrugated metal plates clamped in a frame. The large surface area per unit volume (200–500 m²/m³ vs 50–150 m²/m³ for S&T) gives high U values (2,000–6,000 W/m²·K for liquid–liquid) in a compact unit. PHEs are gasketed (easily cleanable, dismountable, T < 160°C) or welded/brazed (T up to 350°C, no gasket, not dismountable without cutting). Not suitable for gas–gas duty, high fouling or large temperature cross, or pressures > 25 bar on gasketed units.

**Rotary recuperators (heat wheels)** use a slowly rotating porous or corrugated metal wheel that alternately absorbs heat from hot exhaust gas (half rotation) and releases it to incoming cold air (half rotation). Heat recovery efficiency 70–85%; no cross-contamination risk with sealed sector plates; suitable for large air–air or gas–air pre-heating duty (dryer FD air pre-heat, furnace combustion air). Not suitable for condensing or corrosive gas.

**Bare-tube or finned-tube gas coolers** place bare or finned tubes in the exhaust gas duct. Low U (20–60 W/m²·K bare; 30–100 W/m²·K finned) requires large area but construction is simple, low fouling risk, easily cleanable. Used for pre-cooling hot exhaust gas before a bag filter, condensate recovery from moist exhaust, and economiser duty.

Quick Reference

Shell-and-tube: temperature range-200°C to +600°C

Shell-and-tube: pressure rangeFull vacuum to 100 bar (both sides)

Plate heat exchanger: U value (liquid–liquid)2,000 – 6,000 W/m²·K

Gas-gas exchanger: U value (bare tube)20 – 80 W/m²·K

Rotary recuperator: heat recovery efficiency70 – 85%

Energy saving potential (dryer air pre-heat)15 – 35% fuel reduction

Fouling resistance (design margin)Rf = 0.0001 – 0.001 m²·K/W (per TEMA)

Full specifications ↓

Technical Specifications

All parameters are indicative ranges. Final sizing is determined by process simulation based on your specific material and throughput requirements.

Operating Parameters by Exchanger Type

Parameter	Value / Range	Note
Shell-and-tube: temperature range	-200°C to +600°C	CS to 400°C; SS 304/316L to 550°C; Inconel/Hastelloy to 1,000°C; cryogenic with austenitic SS
Shell-and-tube: pressure range	Full vacuum to 100 bar (both sides)	PED 2014/68/EU compliance; ASME VIII option; hydrostatic test at 1.5× design pressure
Plate heat exchanger: U value (liquid–liquid)	2,000 – 6,000 W/m²·K	10–20× higher than S&T for same fluid pair; 80–90% smaller footprint; gasketed PHE max 160°C / 25 bar; brazed PHE to 350°C
Gas-gas exchanger: U value (bare tube)	20 – 80 W/m²·K	Low U requires large area — offset by low cost per m²; finned tubes increase U to 30–120 W/m²·K
Rotary recuperator: heat recovery efficiency	70 – 85%	Cross-contamination (leakage): 1–5%; not suitable for corrosive or condensing exhaust gas
Energy saving potential (dryer air pre-heat)	15 – 35% fuel reduction	Pre-heating combustion air from 20°C to 200°C reduces gas consumption by ~20% on a rotary dryer; payback typically 12–24 months
Fouling resistance (design margin)	Rf = 0.0001 – 0.001 m²·K/W (per TEMA)	Clean water: Rf 0.0001; cooling water: 0.0002; steam: 0.0001; hydrocarbon liquid: 0.0002–0.0004; exhaust gas with dust: 0.0005–0.001
Heat duty range	10 kW – 50 MW per unit	Multi-unit banks for larger duties; modular skid-mounted units for easy installation
Construction standards	TEMA B/C/R, PED 2014/68/EU, ASME VIII Div.1, EN 13445	Material certificates 3.1 per EN 10204; NDE per ASME or EN 13480; pressure test witnessed

Exchanger Type Selection Matrix

Parameter	Value / Range	Note
Gas–gas (hot exhaust / cold air pre-heat)	Rotary recuperator (>100,000 m³/h); shell-and-tube or bare-tube (<100,000 m³/h)	PHE not suitable (gas-side U too low); rotary gives best ε at large flows
Steam condensation (process steam → condensate)	Shell-and-tube (BEM/BEU): U 2,000–6,000 W/m²·K	Steam on shell side, condensate gravity-drained; vent non-condensables; vacuum operation possible
Liquid–liquid (process cooling / heat recovery)	PHE (clean service, T < 160°C); S&T (fouling, high T/P)	PHE: 80% smaller, 10× higher U; S&T: for higher T/P, fouling or cleaning access requirement
Solvent condensation (closed-loop N₂ dryer circuit)	Shell-and-tube (BEM/BEW): SS 316L; coolant on tube side	N₂ on shell side; solvent condenses on cold tube bundles; condensate collected in sump; ATEX Zone 1 classification inside vessel
Exhaust gas cooler (hot gas to bag filter pre-cooling)	Bare-tube or finned-tube dilution cooler; T_in up to 600°C → T_out 120–180°C	Alternative: dilution air quench (no heat recovery); heat recovery with bare-tube saves fuel, adds capital

Need a technical pre-sizing? Send us your material data sheet, moisture content, required throughput and energy source — we return a technical sizing with drum dimensions and energy balance within 2 business days.

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Application Examples

Reference data from industrial installations. Actual values depend on feed consistency, particle size distribution and required product quality.

Material	Inlet moisture	Outlet moisture	Particle size	Gas temp.	Industry
Rotary dryer combustion air pre-heater (gas–air)	10–20% v/v H₂O in exhaust (hot side)	Dry ambient air → pre-heated to 150–250°C (cold side)	5–30 g/Nm³ in exhaust gas (hot side)	Hot side in: 300–500°C → out: 180–250°C	Minerals / Aggregates / Sand / Fertiliser
Flash dryer exhaust gas cooler (to bag filter)	15–30% v/v H₂O	Cooled to 120–150°C (above dew point)	1–20 µm fine powder at 5–30 g/Nm³	Hot side in: 180–350°C → out: 120–150°C	Chemicals / Pigments / Starch / Pharma
Closed-loop N₂ dryer solvent condenser (gas–liquid)	N₂ + solvent vapour (IPA, EtOH, acetone) at saturation	N₂ stripped of >98% solvent; condensate collected	Vapour-phase only (no particles)	N₂ in: 80–130°C; condenser coolant water 10–20°C	Pharmaceuticals / Fine Chemicals / Pigments
Spray dryer hot air supply heat exchanger (steam–air)	Dry combustion air or steam (heating medium)	Heated process air 150–350°C	Clean air (no particulate)	Steam in: 5–20 bar (152–212°C); air out: 150–350°C indirect	Dairy / Food / Detergents / Ceramics
Scrubber liquid cooler (water–water, closed loop)	Scrubbing liquid (pH 1–13 acidic/alkaline brine)	Cooled recirculation liquid returned to scrubber	Suspended solids 0.1–2 g/L	Hot side in: 40–70°C; cooled to 20–35°C with cooling water	Chemical / Glass / Waste Incineration / Metal
Waste heat boiler / HRSG (flue gas–steam generation)	10–25% v/v H₂O in flue gas	Flue gas cooled from 400–600°C to 160–200°C; steam generated at 5–40 bar	0.5–20 g/Nm³ fly ash in flue gas	Hot side: 400–600°C; steam drum: 152–250°C (5–40 bar sat.)	Cement / Lime / Glass / Waste / Pyrolysis
Fluidized bed dryer inter-stage cooler (product cooling)	Fluidisation air (dry)	Dry product cooled from 80–120°C to 30–45°C	Product in fluidised bed: 100 µm–5 mm	Cooling medium: chilled water 7–12°C or cooling water 20–30°C	Fertiliser / Salt / Sugar / Plastics
Kiln gas economiser (pre-heat feed water with flue gas)	5–15% v/v H₂O in flue gas	Feed water from 60°C to 110–130°C; flue gas cooled by 80–150°C	1–10 g/Nm³ fly ash	Flue gas in: 250–400°C; water in: 60°C; water out: 110–130°C	Cement / Lime / Ceramics / Waste

Don't see your material? Send us your process data and we'll provide material-specific sizing.

Heat Exchanger Configurations

Shell-and-Tube (S&T)

Cylindrical shell housing a bundle of straight or U-tubes. Tube-side and shell-side fluids exchange heat across the tube wall. TEMA types: BEM (fixed tubesheet, ΔT < 50°C), BEU (U-tube, removable bundle), BEW/AEW (floating head, full cleaning access). Tube materials: CS, SS 304/316L, duplex, titanium, Hastelloy C-276, copper-nickel. Shell diameters 100–3,000 mm; tube count 4–10,000; heat area 0.5–5,000 m². Multi-pass tube-side configuration (2, 4, 6 passes) increases tube-side velocity and U without increasing shell size. Designed to TEMA Class B (chemical) or C (general) or R (refinery).

Best for:High temperature, high pressure, fouling service, solvent condensation, steam generation — the universal process exchanger

Plate Heat Exchanger (PHE)

Stack of thin corrugated metal plates (0.5–1 mm thick) clamped between fixed and moveable frames. Alternating channels carry hot and cold fluid in counter-current flow. Gasketed PHE: NBR, EPDM or Viton gaskets; plates SS 316L, titanium or Hastelloy; T ≤ 160°C, P ≤ 25 bar; fully dismountable for inspection and plate addition. Brazed PHE: copper-brazed SS plates; T ≤ 225°C, P ≤ 45 bar; compact, no gaskets, not dismountable. Welded PHE: laser-welded plates; T ≤ 350°C, P ≤ 40 bar; for aggressive chemicals. U values: 2,000–6,000 W/m²·K (water–water), 500–3,000 W/m²·K (steam–water). NTU up to 8 in a single unit (vs 2–3 in S&T), achieving temperature approaches of 1–3°C.

Best for:Liquid–liquid heat recovery, scrubber liquid cooling, process water heating — highest efficiency and smallest footprint for clean service

Rotary Recuperator (Heat Wheel)

Slowly rotating (5–20 rpm) segmented wheel of corrugated metal or ceramic honeycomb. The wheel alternately passes through the hot exhaust gas stream and the cold inlet air stream, absorbing and releasing heat. Heat transfer area: 1,000–5,000 m²/m³ of wheel volume — extremely compact. Recovery efficiency ε = 70–85% with no moving parts other than the wheel and drive. Sector plate seals limit cross-contamination to 1–5% leakage. Materials: aluminium (T < 200°C), stainless steel (T < 400°C), ceramic (T < 900°C). Wheel diameter: 0.5–16 m; flow capacity: 1,000–2,000,000 m³/h. Not suitable for corrosive or condensing exhaust gas or where any cross-contamination is unacceptable.

Best for:Large-scale combustion air pre-heating for rotary dryers and kilns — best recovery efficiency at very large gas flows (>50,000 m³/h)

When to Integrate a Heat Exchanger into Your System

Dryer or kiln exhaust temperature exceeds 250°C and fuel cost is a significant operating cost

Air pre-heater or rotary recuperator recovers 15–35% of fuel cost. Simple payback is almost always under 3 years at current European gas prices.

Closed-loop nitrogen drying circuit evaporates valuable organic solvent (IPA, ethanol, acetone)

Shell-and-tube condenser is mandatory in this circuit — it recovers >98% of solvent, which is both the environmental compliance requirement and the primary payback driver.

Exhaust gas temperature exceeds the maximum rating of your downstream bag filter fabric

Bare-tube gas cooler upstream of bag filter cools exhaust to <130°C for polyester fabric, eliminating the need for P84/PTFE bags — saving €5,000–50,000 per year in bag replacement cost depending on filter size.

Scrubbing liquid temperature rises above 40°C in recirculation and reduces acid gas absorption efficiency

Plate heat exchanger on the scrubbing liquid recirculation loop maintains liquid temperature at 20–35°C — keeping Henry's law equilibrium favourable for HCl, SO₂ and NH₃ absorption without increasing reagent dose.

When NOT to Add a Heat Exchanger

Exhaust gas is below 150°C — the available temperature driving force is too small to justify capital cost

Consider instead:Insulation improvement or process optimisation instead →

Gas stream contains sticky, hygroscopic or condensing contaminants — tubes will foul rapidly and the heat exchanger becomes a maintenance liability

Consider instead:Direct gas quench or dilution cooler instead →

Very high dust load (>30 g/Nm³) makes tube-side fouling unmanageable without frequent cleaning — plant availability suffers

Consider instead:Cyclone pre-separator to reduce dust load before heat exchanger →

Acid gases (HCl, SO₂) are present and gas temperature will drop below dew point in the exchanger — bare metal tubes will corrode within months

Consider instead:Wet scrubber or acid-resistant alloy exchanger (Hastelloy / FRP / PTFE-lined) →

Not sure which dryer is right for your process? We'll review your specifications and recommend the optimal solution.

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Frequently Asked Questions — Heat Exchangers

The fuel saving can be estimated from the sensible heat recovered. For a rotary dryer burning natural gas (LHV ≈ 9.5 kWh/Nm³, stoichiometric air 9.5 Nm³/Nm³ gas, actual air 10–12 Nm³/Nm³ at λ = 1.05–1.25): **Heat recovered** Q = ṁ_air × cp_air × (T_hot_out − T_cold_in), where cp_air ≈ 1.0 kJ/kg·K. If exhaust gas at 400°C pre-heats combustion air from 20°C to 200°C: Q = ṁ_air × 1.0 × (200 − 20) = 180 kJ/kg of air. For a dryer consuming 100 Nm³/h gas with air factor λ = 1.1: air flow = 100 × 9.5 × 1.1 = 1,045 Nm³/h ≈ 1,350 kg/h. Heat recovered = 1,350 × 180 / 3,600 = 67.5 kW. At 100 Nm³/h gas consumption (950 kW thermal), this represents a 7.1% saving — roughly proportional to the air pre-heat temperature divided by flame temperature (1,800°C adiabatic). For precise calculation, Lozzar uses combustion simulation software (FGLNG, CanTherm) to account for variable specific heats, dissociation losses and excess air.

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Request a Quote for This Equipment

Include in your enquiry:

→Hot side: fluid name/composition, mass flow (kg/h) or volumetric flow, inlet temperature (°C), target outlet temperature or heat duty (kW)
→Cold side: same data as hot side
→Operating pressure on each side (bar g) and maximum allowable pressure drop (Pa or bar)
→Gas composition if applicable: moisture (%v/v), dust concentration (g/Nm³), acid gases (ppm), pH
→Fouling tendency: clean / moderate / heavy; is tube-side rodding possible?
→Phase change: condensing or evaporating on either side? Provide dew point and latent heat if known
→Required construction standard: PED 2014/68/EU, ASME VIII, or local code
→Material preference: CS, SS 316L, duplex, titanium, Hastelloy, FRP
→Installation context: new project or retrofit? Is it part of a dryer / scrubber / filter system?
→Annual operating hours and any seasonal flow variation

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