Gas-Fired Infrared Systems

Gas-Fired Infrared Systems

Gas-fired infrared heaters convert natural gas or propane combustion energy directly into radiant heat through high-temperature radiating surfaces. These systems achieve radiant fractions of 40-70% compared to 10-20% for conventional furnaces, delivering superior performance in high-ceiling facilities, high-infiltration spaces, and outdoor applications. Gas infrared systems divide into two fundamental categories based on burner surface temperature: high-intensity (luminous) and low-intensity (tube) heaters.

Combustion-to-Radiation Conversion

Energy Balance

Total heat input from fuel combustion distributes among multiple pathways:

$$Q_{input} = Q_{radiant} + Q_{convective} + Q_{flue} + Q_{loss}$$

Radiant fraction: Proportion of input energy emitted as infrared radiation:

$$f_{rad} = \frac{Q_{radiant}}{Q_{input}}$$

High-intensity heaters achieve $f_{rad}$ = 0.60-0.70 through direct combustion at radiating surface. Low-intensity systems achieve $f_{rad}$ = 0.40-0.55 due to heat exchanger losses and lower surface temperatures.

Temperature-Radiant Power Relationship

Stefan-Boltzmann law governs radiant emission:

$$Q_{radiant} = \varepsilon \sigma A T^4$$

Doubling burner surface temperature from 700 K to 1400 K increases radiant power by 16× per unit area, explaining the dramatic performance difference between high-intensity and low-intensity systems.

Practical implications:

• High-intensity (1800°F): Compact size, high mounting, spot heating
• Low-intensity (800°F): Larger area, uniform distribution, moderate mounting

System Categories

High-Intensity (Luminous) Infrared Heaters

Operating characteristics:

• Burner surface temperature: 1400-1800°F (760-980°C)
• Visual glow: Bright orange-red (hence “luminous”)
• Direct flame impingement on ceramic or metal matrix
• Vented combustion (products exhausted outdoors)
• Input capacity: 20,000-150,000 Btu/h per burner head

Burner technologies:

1. Ceramic tile burners: Refractory ceramic tiles with drilled ports
2. Porous matrix burners: Metal or ceramic foam allowing flame stabilization within matrix
3. Metal fiber burners: Woven metal mesh (stainless steel) with surface combustion

Applications:

• Loading docks and outdoor areas
• Spot heating in warehouses
• Aircraft door areas
• Construction site heating
• Athletic field sidelines

Low-Intensity (Tube) Infrared Heaters

Operating characteristics:

• Tube surface temperature: 600-1000°F (315-540°C)
• No visible glow (infrared emission only)
• Burner separated from radiating tube
• Vented or unvented configurations
• Input capacity: 40,000-400,000 Btu/h per tube assembly

Tube configurations:

1. U-tube (vacuum/negative pressure): Burner at one end, exhaust at other, induced draft
2. Straight tube (positive pressure): Burner pushes products through straight run
3. Multiple tube arrays: Connected tubes for large area coverage

Applications:

• Warehouses and distribution centers
• Manufacturing facilities
• Aircraft hangars
• Agricultural buildings
• Gymnasiums and recreation centers

Burner Design and Combustion

Premix Combustion Systems

Gas and air mix before combustion (most infrared heaters):

$$\phi = \frac{(F/A){actual}}{(F/A){stoichiometric}}$$

Where $\phi$ = equivalence ratio

Design targets:

• $\phi$ = 0.95-1.00 (slightly lean to stoichiometric)
• Uniform gas-air distribution prevents flashback/liftoff
• Combustion stabilization at high-temperature surface

Burner Matrix Materials

Ceramic materials:

• Silicon carbide (SiC): Excellent temperature resistance
• Alumina-silicate: Lower cost, adequate durability
• Zirconia-stabilized ceramics: Premium applications
• Operating limit: 2000-2200°F

Metal materials:

• Stainless steel 304/316: Common for fiber burners
• Inconel alloys: Superior oxidation resistance
• FeCrAl alloys: Balance of cost and performance
• Operating limit: 1800-2000°F

Degradation mechanisms:

• Thermal stress cracking (ceramic)
• Oxidation and scaling (metal)
• Particulate deposits from combustion products
• Expected life: 7-12 years with proper maintenance

Radiant Tube Heat Exchangers

Heat Transfer Analysis

Burner heat release transfers to tube wall through convection and radiation:

$$\frac{1}{U} = \frac{1}{h_{gas}} + \frac{t_{wall}}{k_{wall}} + \frac{1}{h_{outside}}$$

Where:

• $h_{gas}$ = Combined convective and radiant heat transfer inside tube (25-50 Btu/h·ft²·°F)
• $t_{wall}$ = Tube wall thickness (typically 0.040-0.080 in)
• $k_{wall}$ = Thermal conductivity (8-10 Btu/h·ft·°F for aluminized steel)
• $h_{outside}$ = External convection and radiation (3-6 Btu/h·ft²·°F)

Temperature drop: Gas stream enters tube at 1800-2000°F, exits at 600-800°F. Tube surface averages 700-900°F.

Tube Materials

Aluminized steel:

• Type 1 (aluminum-silicon coating): 1250°F continuous use
• Type 2 (pure aluminum coating): 1100°F continuous use
• Cost-effective for low-to-medium intensity
• Typical life: 15-20 years

Stainless steel:

• 409 stainless: Economy grade, 1400°F maximum
• 430 stainless: Better oxidation resistance
• 304/316 stainless: Premium applications, high-intensity zones
• Typical life: 20-30 years

U-Tube Configuration

Vacuum (negative pressure) operation:

• Induced draft fan pulls combustion products through tube
• Burner located at tube entrance
• Negative pressure prevents leakage into space
• Required for unvented applications
• Draft: -0.05 to -0.15 in w.c.

Advantages:

• No combustion products in occupied space (if unvented)
• Lower tube wall temperature (longer life)
• Quiet operation

Disadvantages:

• Induced draft fan wear and maintenance
• Limited to shorter tube runs (40-60 ft)

Straight Tube Configuration

Positive pressure operation:

• Burner blower pushes products through tube
• Higher pressure allows longer runs
• Typically vented to outdoors
• Pressure: +0.10 to +0.25 in w.c.

Advantages:

• Simple construction, fewer components
• Longer tube runs possible (60-100 ft)
• Lower maintenance (no induced draft fan)

Disadvantages:

• Must be vented (combustion products under pressure)
• Slightly higher tube temperature
• Potential leak concerns at connections

Reflector Systems

Reflectors redirect otherwise wasted upward radiation toward occupied zone:

$$\eta_{system} = \eta_{heater} \times (1 + R_{eff})$$

Where:

• $\eta_{heater}$ = Heater combustion efficiency (75-85%)
• $R_{eff}$ = Effective reflector gain (0.40-0.70)

Typical improvement: Reflectors increase effective radiant output by 40-70%, equivalent to adding 40-70% more heaters without reflectors.

Reflector Geometry

High-intensity heaters:

• Parabolic reflectors for focused beam
• Focus point 12-18 in below reflector
• Beam angle: 60-90°
• Reflector area: 2-4× burner area

Low-intensity tube heaters:

• Semi-cylindrical or “V” shaped reflectors
• Positioned 2-6 in above tube
• Continuous along tube length
• Reflector width: 18-30 in for 4 in tube

Material Selection

Polished aluminum (most common):

• Reflectivity: 85-92% in infrared spectrum
• Anodized surface resists oxidation
• Moderate cost
• Periodic cleaning maintains performance

Stainless steel:

• Reflectivity: 70-80%
• Excellent durability in harsh environments
• Higher cost
• Lower maintenance

Painted steel (economy):

• Reflectivity: 60-70%
• Requires high-temperature paint
• Lower cost, higher degradation
• Not recommended for premium applications

Venting and Air Supply

Vented Systems

All high-intensity and some low-intensity heaters require venting:

Vent sizing: $$A_{vent} = \frac{Q_{input}}{D \times V}$$

Where:

• $A_{vent}$ = Vent area (in²)
• $Q_{input}$ = Input rate (Btu/h)
• $D$ = Draft factor (typically 0.02-0.04 in w.c./ft)
• $V$ = Velocity limit (1500-2500 fpm)

Vent materials:

• Category I (draft hood): Type B vent
• Category III (forced draft): AL29-4C stainless
• Category IV (condensing): AL29-4C or PVC (per manufacturer)

Unvented Low-Intensity Systems

Some low-intensity tube heaters operate unvented (products of combustion released into space):

Code requirements (NFPA 54/NFGC):

1. Maximum input: 400,000 Btu/h total in space
2. Space volume requirement: 20 ft³ per 1000 Btu/h input
3. Combustion air openings: Two openings, each 1 in²/4000 Btu/h
4. CO detection recommended

Advantages:

• 95-98% effective efficiency (no flue loss)
• Lower installation cost
• Simplified mechanical system

Disadvantages:

• Adds moisture to space (combustion byproduct)
• Potential for CO exposure if improperly maintained
• Not allowed in all jurisdictions
• Unsuitable for tightly sealed buildings

Combustion Air Requirements

Natural draft systems: $$A_{air} = \frac{Q_{total}}{N \times C}$$

Where:

• $A_{air}$ = Free air opening area (in²)
• $Q_{total}$ = Total input all appliances (Btu/h)
• $N$ = Number of openings (typically 2)
• $C$ = Capacity factor (4000 Btu/h per in² for outdoor air)

Forced draft systems: Manufacturer-specified intake sizing, typically direct-piped outdoor air.

Performance Metrics

Combustion Efficiency

Steady-state combustion efficiency from flue gas analysis:

$$\eta_c = 100% - \frac{(T_{flue} - T_{air}) \times K}{CO_2%}$$

Where $K$ = fuel-specific constant (natural gas: 0.62)

Typical values:

• High-intensity vented: 75-82%
• Low-intensity vented: 78-85%
• Low-intensity unvented: 95-98% (no flue loss)

Radiant Efficiency

Fraction of total heat output emitted as radiation:

$$\eta_{rad} = \frac{Q_{radiant}}{Q_{output}} = \frac{Q_{radiant}}{\eta_c \times Q_{input}}$$

Measurement: Radiant heat flux sensors at specified distance, integrated over hemisphere.

Typical values:

• High-intensity: 60-70%
• Low-intensity: 40-55%

Energy Effectiveness

Delivered useful heat to occupied zone:

$$E_{eff} = \eta_c \times \eta_{rad} \times \alpha_{surface} \times (1 - f_{stratification})$$

Where:

• $\alpha_{surface}$ = Surface absorptivity (0.85-0.95 typical)
• $f_{stratification}$ = Fraction lost to stratification (0.10-0.25)

System comparison:

Safety Considerations

Surface Temperature Hazards

Contact burn risk:

• High-intensity burner face: 1800°F - severe burn on contact
• Low-intensity tube: 800°F - severe burn on contact
• Reflector surfaces: 300-600°F - burn hazard
• Guard screens required below 10 ft mounting

Combustion Product Exposure

Carbon monoxide:

• Properly adjusted: < 50 ppm air-free
• Acceptable limit: < 100 ppm air-free
• Symptom threshold: > 400 ppm air-free
• Regular combustion analysis mandatory

Ignition of Combustibles

Clearances to combustibles:

• High-intensity: 36 in minimum (manufacturer specific)
• Low-intensity: 18 in minimum (manufacturer specific)
• Reflector to ceiling: 12-24 in
• Never mount above stored combustible materials

Earthquake Restraint

In seismic zones (SDC C or higher):

$$F_{seismic} = S_{DS} \times W \times I_p$$

Where:

• $S_{DS}$ = Design spectral acceleration
• $W$ = Equipment weight
• $I_p$ = Component importance factor (1.0-1.5)

Suspension systems require lateral and longitudinal bracing per ASCE 7.

Installation Best Practices

Mounting and Support

Suspension systems:

• Rigid pipe/rod hangers for high-intensity (minimize swing)
• Cable/chain acceptable for low-intensity (limited swing)
• Support spacing: 8-12 ft for tube heaters
• Hanger size based on weight plus seismic loads

Roof/structure penetrations:

• Maintain clearance per code (typically 18 in to combustibles)
• Firestopping around penetrations
• Weatherproof jack for outdoor exposure
• Thermal expansion consideration for long tube runs

Gas Piping

Sizing: NFPA 54 longest length method or pressure drop calculation:

$$\Delta P = \frac{0.0001306 \times Q^{1.82} \times L}{C^{1.82} \times D^{4.82}}$$

Where:

• $\Delta P$ = Pressure drop (in w.c.)
• $Q$ = Flow rate (ft³/h)
• $L$ = Length (ft)
• $C$ = Factor for specific gravity
• $D$ = Inside diameter (in)

Typical practice: Size for < 0.5 in w.c. drop at maximum firing rate.

Controls Integration

Minimum control components:

1. Manual gas valve (shutoff)
2. Automatic gas valve (two-stage redundancy preferred)
3. Ignition system (electronic or standing pilot)
4. Flame safeguard (flame rod or thermocouple)
5. Temperature limit control (high-temperature shutoff)
6. Low-voltage thermostat circuit

Advanced controls:

• Modulating gas valve for capacity control
• Outdoor reset for automatic setpoint adjustment
• Occupancy sensors for unoccupied setback
• Building automation system integration
• Remote monitoring and diagnostics

Maintenance Requirements

Scheduled Inspection Tasks

Pre-season (annually):

• Visual inspection of burner surface (cracks, deposits)
• Combustion analysis (CO, CO₂, O₂, efficiency)
• Ignition system operation
• Gas pressure verification (manifold and inlet)
• Reflector cleaning and condition assessment
• Control system functional test

Mid-season (as needed):

• Burner cleaning if performance degradation
• Flame appearance check
• Reflector cleaning in dusty environments

Component Replacement

Expected service life:

• Burner matrix: 7-12 years (high-intensity), 15-20 years (low-intensity tube)
• Ignition system: 3-5 years
• Gas valve: 10-15 years
• Reflectors: 10-20 years (cleaning extends life)
• Induced draft fan (tube heaters): 8-12 years

Performance indicators for replacement:

• Burner surface cracking or erosion
• Inability to achieve combustion targets
• Excessive CO production despite adjustment
• Tube oxidation/scaling reducing output
• Reflector degradation below 70% original performance

Browse Subtopics

• High-Intensity Infrared Heaters - Luminous ceramic burners, porous matrix technology, spot heating applications
• Low-Intensity Infrared Heaters - Radiant tube systems, U-tube and straight configurations, warehouse heating

Reference Standards

• ANSI Z83.20/CSA 2.34: Gas-Fired Infrared Heaters
• ANSI Z83.6/CSA 2.32: Gas-Fired Infrared Patio Heaters
• NFPA 54 (National Fuel Gas Code): Installation requirements
• UL 574: Electric Oil Heaters (includes some gas infrared)
• ASHRAE Handbook - HVAC Systems and Equipment, Chapter 15: Infrared heating design

Gas-fired infrared heaters provide efficient radiant heating for high-ceiling and high-infiltration applications through direct conversion of combustion energy to infrared radiation, achieving energy effectiveness 2-3 times higher than conventional forced-air systems in challenging environments.

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