Hard Candy Production HVAC Systems

Overview of Hard Candy HVAC Requirements

Hard candy production demands rigorous environmental control throughout the manufacturing process. The hygroscopic nature of sugar-based confections requires maintaining specific temperature and humidity conditions to prevent moisture absorption, surface sticking, crystallization, and quality degradation. HVAC systems must accommodate high sensible heat loads from cooking operations while simultaneously maintaining low relative humidity levels incompatible with standard comfort cooling systems.

The production sequence generates distinct climate control challenges: initial cooking at 143-154°C releases substantial moisture and heat, batch roller cooling requires rapid heat removal without condensation formation, die forming operations need precise temperature stability, and finished product handling demands sustained low humidity to prevent surface tackiness.

Process Stage Environmental Parameters

Hard candy manufacturing transitions through multiple thermal zones, each requiring dedicated climate control strategies.

Sugar Cooking Area Requirements

The cooking zone operates at the highest temperatures and generates the most significant moisture release. Syrup concentration from approximately 75% solids to 98-99% solids liberates water vapor at rates proportional to batch size and production throughput.

Overhead exhaust hoods capture steam plumes before moisture disperses into the production space. Hood face velocities of 0.75-1.0 m/s provide adequate capture without excessive air removal. Make-up air systems must temper outdoor air to prevent thermal shock while dehumidifying to prevent humidity spikes during high outdoor dewpoint conditions.

Batch Roller Cooling Section

Post-cooking candy mass at 130-145°C transfers to water-cooled batch rollers for initial temperature reduction. The cooling process must proceed rapidly to maintain plasticity for forming operations while preventing surface moisture condensation that causes sticking and quality defects.

Critical HVAC Requirements:

• Air Temperature: 18-21°C (maintains cooling rate without condensation)
• Relative Humidity: 25-30% RH maximum (prevents surface moisture film)
• Air Velocity Over Product: 1.5-2.5 m/s (enhances convective cooling)
• Dewpoint Temperature: -2 to +2°C (minimum 8-10°C below product surface)

The batch roller zone requires dedicated air handling with deep cooling coils (apparatus dewpoint 2-4°C) followed by reheat to achieve the low humidity levels. Direct expansion systems with hot gas reheat provide efficient moisture removal. Desiccant dehumidification systems offer an alternative for extreme low humidity requirements (below 25% RH).

Cooling air distribution employs laminar flow diffusers positioned to direct air across the roller surface and emerging candy ribbon. Avoid turbulent mixing that draws higher humidity air from adjacent zones. Physical barriers or air curtains separate the cooling zone from upstream cooking operations.

Cooling Rate Calculation:

The convective heat transfer from candy surface follows:

Q = h × A × (T_surface - T_air)

Where:

• h = convective heat transfer coefficient, 15-25 W/m²·K (turbulent air flow)
• A = exposed candy surface area, m²
• T_surface = candy surface temperature, °C
• T_air = air temperature, °C

For a typical 30 kg batch at 140°C cooling to 80°C forming temperature:

Q_total = m × c_p × ΔT = 30 kg × 1.26 kJ/kg·K × 60 K = 2,268 kJ

At 1.5 minutes cooling time: Required cooling capacity = 25.2 kW

Die Forming and Shaping Zone

Candy ribbon at 70-90°C feeds into forming dies or rope sizers. Temperature stability within ±2°C maintains consistent viscosity for accurate dimensional control. Excessive cooling accelerates hardening and causes incomplete die filling; insufficient cooling results in slumping and dimensional loss.

Temperature control employs variable air volume (VAV) systems with precision DDC control. Supply air temperature modulation (typically 16-19°C) maintains zone temperature against variable process loads. Individual die stations may require local cooling or warming depending on candy composition and forming speed.

Die surfaces themselves require temperature control separate from ambient conditions. Chilled water circulation at 15-20°C removes heat from the die mass without over-cooling that causes candy adhesion. Proportional control valves modulate water flow based on die temperature measurement.

Low Humidity Dehumidification Systems

Maintaining 25-35% RH throughout hard candy production areas presents the primary HVAC design challenge. Standard cooling-based dehumidification cannot achieve these conditions without excessive refrigeration and energy waste.

Cooling Coil with Reheat

The most common approach uses deep chilling of process air to condense moisture, followed by reheating to the desired supply temperature.

System Configuration:

1. Cooling Coil: Apparatus dewpoint 1-4°C, chilled water 1-7°C or DX refrigerant
2. Reheat Coil: Hot water, steam, or electric resistance, 30-50 kW typical
3. Supply Air Temperature: 16-22°C (zone dependent)
4. Moisture Removal: 40-60 grains/lb air (outdoor conditions dependent)

Energy consumption for this approach is substantial due to the cooling-reheating cycle. Heat recovery from refrigeration condensers, if available from process refrigeration systems, improves efficiency. During winter months in cold climates, outdoor air may provide “free” dehumidification when properly conditioned.

Example Psychrometric Calculation:

Outdoor conditions: 32°C, 65% RH (dewpoint 24°C) Required room: 21°C, 30% RH (dewpoint 2°C)

Moisture removal = 0.0187 kg/kg - 0.0047 kg/kg = 0.014 kg moisture/kg dry air

For 10,000 m³/hr air flow (12,000 kg/hr dry air): Dehumidification load = 12,000 × 0.014 = 168 kg/hr water removal Latent cooling = 168 kg/hr × 2,450 kJ/kg = 114 kW latent Sensible cooling = 12,000 kg/hr × 1.006 kJ/kg·K × (32-4°C) = 93 kW sensible Reheat load = 12,000 kg/hr × 1.006 kJ/kg·K × (21-4°C) = 51 kW

Desiccant Dehumidification

Solid or liquid desiccant systems remove moisture through adsorption rather than condensation, enabling lower humidity levels with reduced energy consumption in specific operating conditions.

Solid Desiccant Wheel Systems:

• Process Air Stream: Passes through rotating silica gel or molecular sieve wheel
• Regeneration Air: Heated to 100-140°C to drive off adsorbed moisture
• Achievable Humidity: 15-40% RH depending on wheel speed and regeneration
• Energy Source: Natural gas, steam, or electric resistance for regeneration

Desiccant systems excel when required humidity levels fall below 25% RH or when waste heat sources (process exhaust, boiler exhaust) can provide regeneration energy. Capital costs exceed cooling/reheat systems by 40-60%, but operating costs may be lower depending on energy rates and climate.

Hybrid Systems:

Combining cooling coil pre-dehumidification (to approximately 50% RH) with desiccant polishing achieves very low humidity levels with optimized energy consumption. The cooling coil removes bulk moisture load; the desiccant wheel provides final humidity reduction. This approach minimizes both refrigeration capacity and desiccant regeneration heat.

Cooling Conveyor Design

Post-forming candy pieces at 60-80°C transfer to cooling conveyors for final temperature reduction to ambient (20-25°C) before packaging. Conveyor length, speed, and airflow design determine cooling effectiveness.

Conveyor Configuration Parameters

Airflow Direction Options:

1. Upward Flow: Supply air plenum beneath conveyor, exhaust above

   • Advantages: Even air distribution, simple construction
   • Disadvantages: Belt resistance limits velocity, harder access for cleaning

2. Downward Flow: Supply diffusers above, collection beneath conveyor

   • Advantages: Gravity assists airflow, easier maintenance access
   • Disadvantages: Uneven distribution without careful diffuser design

3. Cross-Flow: Horizontal air movement perpendicular to conveyor travel

   • Advantages: Accessibility, adjustable by zone
   • Disadvantages: Non-uniform cooling across belt width

Cooling Load Calculation

For 150 kg/hr hard candy production (glucose-based formulation):

Initial temperature: 75°C Final temperature: 25°C Specific heat: 1.3 kJ/kg·K Temperature drop: 50 K

Q_product = 150 kg/hr × 1.3 kJ/kg·K × 50 K = 9,750 kJ/hr = 2.7 kW

Total system capacity including safety factor and ambient gains: 4-5 kW refrigeration

Air flow rate for convective cooling (ΔT_air = 8°C): m_air = Q / (c_p × ΔT) = 2.7 kW / (1.006 kJ/kg·K × 8 K) = 0.335 kg/s = 1,000 m³/hr

Distribute this airflow evenly across the 8-15 m conveyor length using zone control dampers to account for decreasing temperature differential as candy cools.

Sticking Prevention Through Environmental Control

Hard candy surface sticking results from moisture absorption creating a thin syrup layer that bonds individual pieces or adheres candy to contact surfaces. Prevention requires controlling both air humidity and product temperature to prevent exceeding the critical relative humidity for the specific candy formulation.

Hygroscopic Equilibrium

Each candy formulation has a characteristic moisture sorption isotherm relating environmental humidity to surface moisture content. Generic hard candies (95-98% sugar solids) exhibit critical relative humidity around 45-50% RH at 20°C—above this level, the candy surface absorbs moisture and becomes tacky.

Design Safety Margins:

• Operating RH should remain 10-15% below critical RH
• For standard hard candy: maintain ≤30-35% RH
• For hygroscopic formulations (high corn syrup): maintain ≤25-28% RH
• Temperature stability: ±1-2°C (RH increases ~3%/°C at constant dewpoint)

Anti-Sticking Air Distribution Strategy

Cooling Tunnel Design:

Supply air delivery must ensure uniform humidity distribution without dead zones where moisture accumulates. Perforated duct diffusers running parallel to conveyor travel provide even distribution. Monitor and control humidity at multiple points (inlet, mid-point, outlet) to verify uniform conditions.

Surface Drying Enhancement:

Final conveyor sections may incorporate slightly warmer, drier air (23-25°C, 20-25% RH) to actively dry candy surfaces before transfer to packaging. This “polishing” zone removes any residual surface moisture from earlier cooling stages.

Static Elimination:

Electrostatic charge accumulation attracts dust and promotes sticking between pieces. Ionizing bars positioned above the final conveyor section neutralize surface charge. Maintain air velocity below 2 m/s in this zone to prevent dust entrainment.

Packaging Area Climate Control

The packaging environment represents the final critical control point before product leaves controlled conditions. Even brief exposure to elevated humidity can initiate surface deterioration visible as hazing, sticking in wrappers, or loss of gloss.

Positive pressure relative to adjacent areas (10-15 Pa) prevents humid air infiltration. Airlocks or vestibules at personnel entry points maintain pressure differential. Rapid-acting doors with air curtains protect against forklift traffic.

Storage and Warehouse Conditions

Finished hard candy in moisture-barrier packaging tolerates moderate storage conditions, but unwrapped product or packaging with incomplete seals requires continued environmental control.

Recommended Storage Conditions:

• Temperature: 15-25°C (avoid thermal cycling)
• Relative Humidity: 40-50% RH maximum (wrapped product), 30-35% RH (unwrapped)
• Air Movement: Minimal, prevent dust accumulation
• Maximum Storage Duration: 6-12 months depending on formulation and packaging

System Integration and Control Strategy

Hard candy production HVAC systems integrate multiple subsystems requiring coordinated control to optimize product quality and energy efficiency.

Primary Control Loops:

1. Space Temperature Control: VAV or zone-level modulating control, ±1°C setpoint
2. Space Humidity Control: Dewpoint-based control preferred over RH sensors (less temperature sensitivity)
3. Cooling Coil Capacity: Chilled water valve or DX expansion valve modulation
4. Reheat Staging: Step or modulating control to achieve supply air temperature
5. Outside Air Economizer: Enable when outdoor dewpoint below return air dewpoint

Monitoring Points:

• Supply air: temperature, RH, dewpoint, flow rate
• Zone air: temperature, RH (multiple locations), static pressure
• Cooling coil: entering/leaving water or refrigerant conditions
• Production equipment: batch roller temp, die temp, conveyor speed

Energy Optimization:

Humidity control dominates energy consumption in hard candy facilities. Optimization strategies include:

• Heat recovery from process cooling to provide reheat
• Variable refrigeration to match actual dehumidification load
• Night setback to relaxed humidity limits (35-40% RH) during non-production
• Demand-controlled ventilation based on occupancy (cooking area only)
• Free cooling/dehumidification during favorable outdoor conditions

Equipment Selection Criteria

Air Handling Units

Purpose-built low-humidity air handlers incorporate features specific to confectionery applications:

• Stainless steel construction or epoxy-coated carbon steel (corrosion from sugar dust)
• Deep cooling coils (6-8 rows) for low apparatus dewpoint
• Dual-stage or modulating reheat (precise humidity control)
• High-efficiency filtration (MERV 13-14) to minimize sugar dust
• Insulated casing with vapor barrier (prevent condensation in walls)
• Access panels sized for coil cleaning (sugar dust accumulation)

Refrigeration Systems

Chilled water systems (1-7°C supply) offer operational flexibility and centralized refrigeration efficiency. Direct expansion systems provide simpler installation and lower first cost for smaller facilities. Select based on facility size and existing infrastructure.

For facilities with multiple refrigeration loads (process cooling, HVAC), centralized ammonia or low-GWP refrigerant systems improve efficiency and reduce refrigerant inventory compared to distributed DX systems.

Dehumidification Equipment

Select between cooling/reheat and desiccant based on:

• Required humidity level (below 25% RH favors desiccant)
• Available regeneration energy (waste heat reduces desiccant operating cost)
• Climate conditions (humid climates increase cooling/reheat load)
• Capital budget (desiccant carries 40-60% cost premium)
• Space availability (desiccant units physically larger)

Maintenance and Operational Considerations

Critical Maintenance Tasks:

• Cooling coil cleaning quarterly (sugar dust accumulation reduces capacity)
• Air filter replacement monthly during production (sugar loading)
• Humidity sensor calibration every 6 months (drift affects product quality)
• Belt conveyor cleaning weekly (prevent sugar buildup, maintain airflow)
• Refrigeration system inspection per manufacturer schedule

Performance Verification:

Monitor actual vs. design humidity levels during production. Humidity excursions indicate system capacity insufficiency, control problems, or building envelope leaks. Product quality issues (sticking, hazing) correlate directly with environmental exposure—use product defects as additional process indicators beyond sensor measurements.

Document seasonal variations in system performance. Summer peak humidity loads may exceed design capacity if outdoor conditions worsen or production increases beyond original design basis. Winter conditions may enable energy savings through increased outdoor air usage when properly conditioned.

Conclusion

Hard candy production HVAC systems must simultaneously address high heat removal loads and stringent low humidity requirements incompatible with standard commercial systems. Success requires integrated design of dehumidification equipment, precision controls, and process-specific air distribution coordinated with production equipment. Proper system design prevents moisture-related quality defects while optimizing energy consumption in an inherently energy-intensive application.