Electric Infrared Heaters

Electric Infrared Heaters

Electric infrared heaters convert electrical energy directly into infrared radiation through resistance heating elements operating at controlled surface temperatures from 500-2000°F (260-1090°C). Unlike gas-fired systems requiring combustion, venting, and fuel supply infrastructure, electric infrared provides clean, precise, instantly controllable radiant heating suitable for process applications, spot comfort heating, paint curing, plastic forming, and semiconductor manufacturing. Element temperature determines wavelength spectrum from near-infrared (short-wave, 0.7-1.4 μm) through far-infrared (long-wave, 3.0-10 μm), enabling optimization for specific material absorption characteristics.

Operating Principles

Resistance Heating Fundamentals

Electrical power converts to heat through resistive dissipation:

$$P = I^2 R = \frac{V^2}{R}$$

Where:

• $P$ = Power (W)
• $I$ = Current (A)
• $V$ = Voltage (V)
• $R$ = Resistance (Ω)

Heat generation rate per unit volume:

$$\dot{q}{gen} = \frac{P}{V{element}} = \frac{I^2 \rho}{A_{cross}}$$

Where:

• $\rho$ = Resistivity (Ω·m)
• $A_{cross}$ = Element cross-sectional area (m²)

Temperature-dependent resistivity:

For most alloys (Ni-Cr, Fe-Cr-Al):

$$\rho(T) = \rho_0 [1 + \alpha(T - T_0)]$$

Where $\alpha$ = temperature coefficient (0.0001-0.0004 per °C)

Result: Positive temperature coefficient provides self-limiting behavior (resistance increases with temperature, reducing current and preventing thermal runaway).

Radiant Energy Emission

Stefan-Boltzmann law governs total radiant emission:

$$E = \varepsilon \sigma T^4$$

Element efficiency (electrical to radiant conversion):

$$\eta_{rad} = \frac{P_{radiant}}{P_{electrical}} = \frac{\varepsilon \sigma A T^4}{P}$$

Temperature dependence:

Conclusion: Higher element temperature dramatically improves radiant efficiency while reducing convective losses.

Wavelength Distribution

Wien’s displacement law determines peak emission wavelength:

$$\lambda_{max} = \frac{2898}{T} \text{ (μm, K)}$$

Spectral classification:

Material Absorption Characteristics

Absorptivity varies dramatically with wavelength and material:

Water (and water-containing materials):

• Peak absorption: 3.0 μm, 6.0 μm
• Food drying, paint curing (water-based): MIR/FIR optimal

Plastics (PE, PP, PVC):

• Peak absorption: 3.4 μm
• Thermoforming, heat shrinking: MIR optimal

Glass:

• Low absorption: 0.7-2.5 μm (transparent to NIR)
• High absorption: > 2.8 μm
• Glass heating: MIR/FIR required

Metals:

• Absorption increases with wavelength
• Polished: α = 0.05-0.15 (NIR), 0.20-0.40 (MIR)
• Oxidized: α = 0.40-0.70 (NIR), 0.60-0.85 (MIR)

Textiles and paper:

• Broad absorption: 2-10 μm
• All infrared types effective

Electric Infrared Technologies

Quartz Lamp (High-Intensity NIR)

Construction:

• Tungsten wire filament (0.5-2.0 mm diameter)
• Quartz glass envelope (fused silica)
• Inert gas fill (argon, nitrogen, or halogen)
• Linear or U-shaped configuration
• Power: 500-5000W per lamp

Operating characteristics:

• Filament temperature: 3500-4500°F (2200-2750 K)
• Envelope temperature: 1800-2200°F (1250-1480 K)
• Emission spectrum: 70% in 0.7-2.0 μm (near-infrared)
• Radiant efficiency: 85-92%
• Response time: 1-3 seconds to full output

Tungsten filament resistance:

Cold resistance: $R_{cold}$ = 10-30 Ω typical Hot resistance: $R_{hot}$ = 100-300 Ω typical Resistance ratio: $R_{hot}/R_{cold}$ = 10-15

Inrush current: Initial surge 10-15× operating current due to low cold resistance. Requires slow-start controllers or current-limiting devices.

Halogen cycle (if halogen-filled): Tungsten evaporates from filament → combines with halogen gas → halogen-tungsten compound deposits back on filament → extends life to 5000-10,000 hours vs. 2000-3000 for standard.

Advantages:

• Highest radiant efficiency (85-92%)
• Instant on/off (1-3 second response)
• Precise control via dimming
• Compact size, high power density
• Deep penetration (NIR spectrum)

Disadvantages:

• High surface temperature (fragile, hot)
• Relatively short life (5000-10,000 h)
• High inrush current (control complexity)
• Visible glow (bright white-yellow light)
• Limited to NIR spectrum only

Applications:

• Paint and coating curing (rapid)
• Plastic thermoforming (rapid heating)
• Paper drying (surface drying)
• Semiconductor wafer processing
• Food processing (surface cooking)

Quartz Tube (Medium-Intensity MIR)

Construction:

• Resistance coil (Ni-Cr or Fe-Cr-Al alloy)
• Quartz tube envelope (12-25 mm diameter)
• Air or inert gas atmosphere
• Coil supported by ceramic end caps
• Power: 250-2500W per tube

Operating characteristics:

• Coil temperature: 1400-1800°F (1030-1260 K)
• Tube surface temperature: 1200-1600°F (920-1140 K)
• Emission spectrum: Peak 2.5-3.5 μm (medium-infrared)
• Radiant efficiency: 70-80%
• Response time: 30-90 seconds to 90% output

Heat transfer mechanism:

1. Resistance coil generates heat (I²R)
2. Radiation from coil to quartz tube inner surface
3. Conduction through quartz wall (minimal, thin wall)
4. Radiation from tube outer surface to target

Quartz properties:

• High infrared transmittance in 0.7-3.5 μm range
• Excellent thermal shock resistance
• Maximum continuous: 1800-2000°F
• Emissivity (oxidized surface): ε = 0.85-0.92

Tube wall temperature control:

$$T_{tube} = \left[\frac{P_{input}}{\pi D L (\varepsilon \sigma + h_c)}\right]^{1/4}$$

Where:

• $D$ = Tube diameter
• $L$ = Tube length
• $h_c$ = Convective heat transfer coefficient

Advantages:

• Moderate temperature (safer than quartz lamp)
• Longer life (8,000-15,000 hours)
• Good radiant efficiency (70-80%)
• Medium-wave spectrum (versatile applications)
• Lower inrush current vs. quartz lamp
• Reasonable cost

Disadvantages:

• Slower response vs. quartz lamp (30-90 sec)
• Visible glow (red-orange)
• Fragile quartz tube
• Moderate power density

Applications:

• Industrial drying (coatings, inks)
• Plastic heating (sheet forming)
• Powder coating curing
• Textile processing
• Food processing (through-heating)

Metal Sheath (Low-Intensity FIR)

Construction:

• Resistance wire coil (Ni-Cr alloy)
• Magnesium oxide (MgO) insulation
• Metal sheath (stainless steel, Incoloy)
• Compacted construction (wire embedded in MgO)
• Power: 100-2000W per element

Operating characteristics:

• Sheath temperature: 700-1100°F (640-870 K)
• Emission spectrum: Peak 3.5-5.0 μm (far-infrared)
• Radiant efficiency: 50-65%
• Response time: 3-8 minutes to 90% output
• Life: 15,000-25,000 hours

Thermal mass:

High thermal mass (metal + MgO) provides:

• Slow heating (3-8 min to steady state)
• Slow cooling (maintains heat during short cycling)
• Thermal averaging (smooth output despite voltage fluctuations)
• Durability (resistant to mechanical shock)

Sheath materials:

Stainless steel 304/316:

• Maximum continuous: 1200°F
• Good corrosion resistance
• Emissivity: ε = 0.70-0.85 (oxidized)
• Most common

Incoloy 800/825:

• Maximum continuous: 1400°F
• Excellent oxidation resistance
• Higher cost
• High-temperature applications

Carbon steel:

• Maximum continuous: 900°F
• Economy applications
• Limited corrosion resistance
• Requires protective atmosphere

Advantages:

• Rugged, durable construction
• Long life (15,000-25,000 hours)
• Can be shaped (bent into custom forms)
• Weatherproof (outdoor capable)
• Safe (lower surface temperature)
• Low inrush current

Disadvantages:

• Lower radiant efficiency (50-65%)
• Slow response time (3-8 minutes)
• Lower power density
• Limited to far-infrared only

Applications:

• Comfort heating (outdoor patios, warehouses)
• Snow melting (pavement, walkways)
• Process warming (moderate temperature)
• Agricultural (livestock, greenhouses)
• Freeze protection (pipes, tanks)

Ceramic Panel Heaters

Construction:

• Resistance wire embedded in ceramic matrix
• Ceramic composition: alumina-silicate or cordierite
• Flat panel or shaped geometry
• Thickness: 0.5-2.0 in
• Power: 50-500W per panel (lower density)

Operating characteristics:

• Surface temperature: 500-900°F (530-760 K)
• Emission spectrum: Peak 4.0-6.0 μm (far-infrared)
• Radiant efficiency: 40-55%
• Response time: 8-15 minutes to 90% output
• Life: 20,000-40,000 hours

Ceramic properties:

• High emissivity: ε = 0.90-0.95
• Excellent thermal mass (smooth output)
• Fragile (susceptible to thermal shock if poorly designed)
• Maximum continuous: 1200-1400°F

Energy distribution:

• Radiant: 40-55%
• Convective: 40-50% (significant due to large area, low temperature)
• Conductive: 5-10% (through mounting structure)

Advantages:

• Highest emissivity (0.90-0.95)
• Very long life (20,000-40,000 hours)
• Uniform surface temperature
• Soft, comfortable radiant heat
• Silent operation

Disadvantages:

• Lowest radiant efficiency (40-55%)
• Very slow response (8-15 minutes)
• Low power density
• Fragile ceramic (careful handling required)
• Higher cost per watt

Applications:

• Comfort heating (saunas, residential)
• Low-temperature process heating
• Food warming (holding cabinets)
• Reptile/animal enclosures
• Slow drying processes

Reflector Design

Reflector Geometry

Parabolic reflectors:

For point or linear source, parabolic cross-section focuses radiation:

$$y^2 = 4fx$$

Depth of focus:

$$d = \frac{D^2}{16f}$$

Where:

• $D$ = Reflector diameter/width
• $f$ = Focal length
• Element positioned at focal point for collimated beam

Elliptical reflectors:

Two focal points: element at one, target at other. Concentrates maximum radiation on target.

V-groove reflectors:

For quartz lamps, V-groove with polished surfaces:

• Angle: 90-120°
• Redirects radiation from back and sides
• Simpler than parabolic, lower cost

Reflector Materials

Reflectivity in infrared spectrum:

Gold reflectors:

• Highest reflectivity (96-98%)
• Chemically inert
• Expensive (only for premium applications)
• Process heating, semiconductor manufacturing

Polished aluminum (anodized):

• Excellent performance (90-94%)
• Moderate cost
• Most common industrial choice
• Periodic cleaning maintains performance

Reflector Efficiency

Net radiation reaching target:

$$q_{target} = q_{direct} + q_{reflected}$$

Where:

$$q_{reflected} = q_{element} \times F_{view} \times \rho \times F_{target}$$

Example calculation:

Quartz lamp, 2000W, parabolic aluminum reflector:

• Direct radiation to target: 60% (1200W)
• Backward radiation: 40% (800W)
• View factor (element to reflector): 0.85
• Reflectivity: 0.92
• Reflector to target: 0.95

Reflected component: 800W × 0.85 × 0.92 × 0.95 = 594W

Total to target: 1200W + 594W = 1794W = 90% efficiency

Without reflector: Only 1200W = 60% efficiency

Gain: 50% increase in delivered radiation through reflector use.

Control Methods

Phase-Angle Control

SCR (silicon-controlled rectifier) varies power by delaying firing angle:

$$P = P_{max} \times \frac{1}{\pi} \left[\pi - \alpha + \frac{\sin(2\alpha)}{2}\right]$$

Where:

• $\alpha$ = Firing delay angle (0-180°)
• $P_{max}$ = Full power output

Characteristics:

• Smooth power variation: 0-100%
• Fast response (cycle-by-cycle control)
• Generates harmonic distortion
• Suitable for resistive loads

Harmonic mitigation: Use at > 50% power to minimize THD (total harmonic distortion).

Zero-Crossing Control

SCR switches at voltage zero-crossing, varies power by cycle skipping:

$$P = P_{max} \times \frac{N_{on}}{N_{total}}$$

Characteristics:

• No harmonic distortion
• Stepwise power control
• Longer thermal time constant (averaging effect)
• Preferred for sensitive electrical environments

Pulse Width Modulation (PWM)

Cycles element on/off at fixed frequency:

$$P = P_{max} \times \frac{t_{on}}{t_{on} + t_{off}}$$

Typical frequency: 0.1-10 Hz (thermal mass provides averaging)

Characteristics:

• Digital control (simple microcontroller implementation)
• No power quality concerns (full cycles)
• Requires thermal mass for smooth output
• Suitable for all element types

Temperature Feedback Control

PID control:

$$P(t) = K_p e(t) + K_i \int e(t)dt + K_d \frac{de(t)}{dt}$$

Where $e(t)$ = setpoint - measured temperature

Tuning parameters:

• $K_p$ (proportional): 2-10 typical (depends on thermal mass)
• $K_i$ (integral): 0.01-0.1 (eliminates steady-state error)
• $K_d$ (derivative): 0.1-1.0 (anticipates changes)

Temperature sensing:

• Thermocouples (process temperature)
• RTDs (precise control, ±0.1°C)
• Infrared sensors (non-contact, fast response)

Application Engineering

Power Density Calculation

Required power density depends on application:

$$P_{density} = \frac{q_{load} + q_{loss}}{A_{heated} \times \eta_{system}}$$

Where:

• $q_{load}$ = Heating requirement (W/m²)
• $q_{loss}$ = Environmental loss (convection, conduction)
• $A_{heated}$ = Target area (m²)
• $\eta_{system}$ = System efficiency (0.60-0.90)

Typical power densities:

Mounting Distance

Intensity at target surface:

$$I = \frac{P \times \eta_{rad} \times \cos^3(\theta)}{r^2}$$

Optimization: Balance intensity (close mounting) with uniformity (distant mounting).

Rule of thumb: Mount at distance = 1.5-3.0× heater width for uniform coverage.

Example:

• 500mm wide quartz lamp array
• Mounting distance: 750-1500mm
• Provides pattern factor < 1.5

Array Design

Spacing for uniform coverage:

$$S = 2L \times \tan(\theta/2)$$

Where:

• $S$ = Center-to-center spacing
• $L$ = Mounting distance
• $\theta$ = Effective beam angle (typically 90-120°)

Multi-zone control: Divide large areas into independently controlled zones:

• Preheat zone (moderate power)
• Process zone (maximum power)
• Cooldown zone (low power or off)

Electrical Requirements

Power Supply Sizing

Single-phase limitation: Maximum ~12 kW (50A @ 240V)

Three-phase for larger loads:

• Balanced loading across phases
• 208V, 240V, 480V common
• Delta or wye configuration

Example: 36 kW infrared system

• Three-phase 240V: 87A per phase
• Requires 100A circuit breaker, #2 AWG wire

Voltage Drop Considerations

Acceptable voltage drop: < 3% for stable operation

$$\Delta V = \frac{2 \times I \times L \times \rho}{A_{wire}}$$

Impact on power:

$$P_{actual} = P_{rated} \times \left(\frac{V_{actual}}{V_{rated}}\right)^2$$

5% voltage drop → 10% power reduction

Mitigation: Proper wire sizing, minimize run length, use higher voltage where possible.

Ground Fault Protection

GFCI required:

• Wet or damp locations
• Outdoor installations
• Within 6 ft of water source

Industrial GFCI: 30 mA trip threshold, meets NEC requirements

Safety Considerations

Burn Hazards

Surface temperatures:

• Quartz lamp: 2000°F (1090°C) - severe burn on contact
• Quartz tube: 1500°F (815°C) - severe burn on contact
• Metal sheath: 1000°F (540°C) - burn hazard
• Ceramic panel: 700°F (370°C) - burn hazard

Guards and shields:

• Wire mesh guards (prevent direct contact)
• Transparent quartz shields (allow radiation transmission)
• Minimum 3-6 in clearance around elements
• Warning labels on equipment

Electrical Safety

Grounding:

• All metal components bonded to ground
• Ground fault protection in wet locations
• Proper conduit and junction box installation

Overcurrent protection:

• Circuit breakers sized for 125% of continuous load
• Branch circuit protection per NEC Article 424

Insulation:

• Ceramic standoffs for high-temperature sections
• Heat-resistant wire insulation (200°C minimum)
• Periodic insulation resistance testing (> 1 MΩ)

Fire Hazards

Clearances to combustibles:

• Minimum 18-36 in (depends on element temperature)
• Never direct radiation at stored combustible materials
• Automatic shutoff if over-temperature condition

Ignition temperature concerns:

• Paper: 450°F
• Wood: 500-700°F
• Textiles: 400-600°F
• Keep element temperature and radiant flux below material ignition

Maintenance

Routine Inspection

Quarterly:

• Visual inspection of elements (cracking, deformation)
• Reflector cleaning and condition
• Electrical connection tightness
• Control system function test

Annually:

• Insulation resistance testing
• Reflector reflectivity measurement
• Power output verification
• Temperature calibration

Element Replacement

Failure indicators:

• Open circuit (complete failure)
• Reduced output (element degradation)
• Hot spots (uneven heating)
• Visible cracks or breaks

Replacement cost:

• Quartz lamp: $15-50 per lamp
• Quartz tube: $25-80 per tube
• Metal sheath: $30-100 per element
• Ceramic panel: $50-200 per panel

Expected life:

• Quartz lamp: 5,000-10,000 hours
• Quartz tube: 8,000-15,000 hours
• Metal sheath: 15,000-25,000 hours
• Ceramic panel: 20,000-40,000 hours

Reflector Maintenance

Cleaning procedure:

1. De-energize and cool completely
2. Remove loose dust with compressed air
3. Wipe with damp cloth (water or mild detergent)
4. Rinse thoroughly, dry completely
5. Avoid abrasives (damage reflective surface)

Frequency:

• Clean environments: Annually
• Moderate dust: Semi-annually
• Heavy dust/contamination: Quarterly

Performance recovery: Proper cleaning restores 85-95% of original reflectivity.

Economic Analysis

Operating Cost

Energy consumption:

$$C_{annual} = P_{rated} \times t_{operation} \times C_{electric}$$

Example: 10 kW system, 2000 h/year, $0.12/kWh

• Annual energy: 10 kW × 2000 h = 20,000 kWh
• Annual cost: 20,000 kWh × $0.12 = $2,400/year

Comparison: Electric vs. Gas

Electric infrared:

• Efficiency: 85% (quartz lamp to target)
• Operating cost: $0.12/kWh ÷ 0.85 = $0.141/kWh delivered
• Maintenance: Low
• Capital: Moderate

Gas high-intensity:

• Efficiency: 65% (combustion to target)
• Operating cost: $1.00/therm ÷ 0.65 = $1.54/therm delivered = $0.045/kWh delivered
• Maintenance: Moderate
• Capital: Moderate
• Operating cost advantage: Gas = 1/3 of electric

When electric makes sense:

• No gas available
• Clean environment required (semiconductor, food)
• Precise control essential
• Intermittent operation (instant on/off advantage)
• Small capacity (< 10 kW)
• Process heating (specialized wavelength requirements)

When gas makes sense:

• Large capacity (> 25 kW)
• Continuous operation
• Space heating (comfort applications)
• Operating cost critical

Electric infrared heaters provide clean, precise, instantly controllable radiant heating through resistance elements spanning near-infrared to far-infrared spectra, enabling optimized performance for process heating, spot comfort, and applications requiring wavelength-specific absorption characteristics unavailable from combustion-based systems.