Design principles and parameters of plate-type indirect-fired air heaters
As a key component of industrial hot air systems, the plate-type indirect combustion air heater utilizes special alloy baffles to ensure safe heat transfer. During operation, the high-temperature exhaust gases generated by fuel combustion and the target airflow flow in opposite directions within separate chambers, achieving contactless heat exchange through the metal interface. This design completely eliminates the mixing of flue gas impurities into the clean hot air, while also eliminating the safety hazards associated with direct contact. It provides a stable heat source for drying, heating, and material heat treatment applications.
1. Core Design Principles
①Heat Transfer and Media Isolation Mechanism:
The fuel (gas/oil, etc.) completes its combustion reaction within a sealed combustion unit, generating high-temperature exhaust gases. This exhaust gas flows through a dedicated heat removal channel, while the cool air to be heated circulates in an adjacent independent air duct. The heat exchange core is constructed from multiple stacked and welded alloy sheets (mostly stainless steel or heat-resistant steel) with a specific corrugated topology, forming a meshed yet completely isolated dual-channel exhaust-air network. Heat is transferred from the high-temperature exhaust gas side to the low-temperature air side through the metal interface, achieving zero media contact throughout the process.
②Thermodynamic Optimization of Flow Patterns:
Counterflow: Exhaust gas and air flow in opposite directions through the channel, maximizing the thermodynamic potential difference and significantly improving heat recovery (plate-type architecture is naturally suited to this high-efficiency pattern).
Perpendicular Flow: The media flow direction is perpendicular to the space, simplifying the system structure and making the flow resistance controllable. However, the heat transfer intensity is reduced compared to the counterflow pattern (some designs use mixed flow patterns to enhance adaptability).
③Enhanced Heat Transfer Structure Design:
Corrugated Topological Effect: The precise corrugated structure of the plate surface (such as staggered peaks and valleys, sawtooth arrays, etc.) simultaneously achieves:
✓ Geometric expansion of the heat transfer interface
✓ Inducing strong fluid turbulence in low-velocity areas
✓ Disrupting the formation of thermal boundary layers
→ Synchronous increase in the convection coefficients of both media
④Microscale Heat Transfer: Submillimeter-level metal interlayers (typically 0.5-1.2mm) significantly reduce thermal conductivity resistance, ensuring extremely fast heat transfer across the interface.
2. Key Design Parameters
To design an efficient and reliable plate-type indirect-fired air heater, it is necessary to comprehensively consider the following key parameters and their interrelationships:
| Parameter Category | Parameter Name | Symbol | Units | Design Considerations & Critical Insights |
|---|---|---|---|---|
| Thermal Demand | Process Air Volume Flow | Qa | m³/h, Nm³/h | Primary sizing input (note STP/operating conditions) |
| Air Inlet Temperature | Ta_in | °C | Temperature of untreated air stream | |
| Target Air Outlet Temperature | Ta_out | °C | Required heated air temperature (critical process constraint) | |
| Permissible Air Pressure Loss | ΔPa | Pa | Impacts blower selection & energy consumption | |
| Flue Gas Side | Fuel Type & Calorific Value | - | - | Dictates consumption rate (LHV essential) |
| Fuel Consumption Rate | Qf | m³/h, kg/h | Derived from thermal load calculations | |
| Combustion Gas Inlet Temperature | Tg_in | °C | Critical thermal zone (700-1000°C+ typical) | |
| Target Exhaust Gas Temperature | Tg_out | °C | Must exceed dew point + safety margin to prevent corrosion | |
| Permissible Flue Gas Pressure Loss | ΔPg | Pa | Affects burner backpressure/exhauster specs | |
| Core Performance | Total Thermal Output | Q | kW | Q = Qa × ρa × Cpa × (Ta_out - Ta_out) |
| Mean Temperature Driving Force | MTDF | °C | Primary design driver (maximized in counter-flow arrangement) | |
| Overall Heat Transmission Coefficient | U | W/(m²·K) | U = [1/(1/hg + δ/λ + 1/ha + Rf)] - Target optimization | |
| Required Heat Transfer Surface | A | m² | Core design outcome (A = Q/(U×MTDF)) - Dictates cost/size | |
| Thermal Conversion Efficiency | η | % | η = [Q/(Qf×LHV)]×100% (Benchmark: >85% in modern systems) | |
| Geometry & Flow | Heat Transfer Plate Material | - | - | Corrosion-resistant alloys (AISI 304/316L/310S) |
| Conduction Barrier Thickness | δ | mm | 0.5-1.2mm typical (balances conduction/strength) | |
| Surface Corrugation Profile | - | - | Determines convection coefficients & pressure loss | |
| Flow Passage Hydraulic Diameter | - | mm | Influences cross-section & velocity profiles | |
| Media Flow Velocity | Va/Vg | m/s | Optimized for heat transfer vs. pressure drop | |
| Operational Limits | Maximum Operating Pressure | - | Pa | Structural integrity baseline |
| Peak Material Temperature | - | °C | Critical for flue gas side material selection | |
| Acid Dew Point | T_dp | °C | Dictates minimum exhaust temperature (Tg_out > T_dp + buffer) | |
| Cross-Contamination Prevention | - | - | Zero-leakage mandatory (safety-critical requirement) |
Core Thermal Calculation Formulas:
Heat Load Calculation:Q = ṁ_air * Cp_air * (Ta_out - Ta_in)
(ṁ_air is the air mass flow rate, Cp_air is the specific heat capacity of air at constant pressure, ≈ 1.005 kJ/(kg·K))
Fuel Consumption Estimation:ṁ_fuel = Q / (LHV_fuel * η)
(ṁ_fuel is the fuel mass flow rate, LHV_fuel is the fuel lower heating value, η is the estimated design efficiency)
Required Heat Exchange Area Calculation:A = Q / (U * LMTD)
(U should be estimated based on empirical formulas or simulations, LMTD is calculated based on temperature)
Thermal Efficiency Calculation:η = [ṁ_air * Cp_air * (Ta_out - Ta_in)] / [ṁ_fuel * LHV_fuel] * 100%
plate-type indirect-fired air heaters
The plate-type indirect-fired air heater, with its unique corrugated plate structure and physical isolation design, achieves efficient, safe, and compact hot air production. Its design is a complex systems engineering effort, centered on accurately calculating the heat load (Q), optimizing and maximizing the total heat transfer coefficient (U) and mean mean temperature difference (LMTD) to achieve the required minimum heat exchange area (A), while strictly meeting the safety requirements of zero flue gas-to-air leakage, pressure drop limits (ΔPa, ΔPg), and material requirements for temperature and corrosion resistance, as well as dew point corrosion protection. Careful selection and optimization of the design parameters listed in the table are key to creating high-performance, long-life, safe, and reliable equipment.