HVAC Systems Encyclopedia

A comprehensive encyclopedia of heating, ventilation, and air conditioning systems

Commercial Refrigeration

Commercial refrigeration encompasses a broad range of equipment and system architectures designed to preserve perishable goods in retail, foodservice, and institutional settings. Unlike residential or industrial refrigeration, commercial systems prioritize product visibility, accessibility, and operational flexibility while maintaining precise temperature control across multiple zones.

System Architecture Classifications

Commercial refrigeration systems are categorized by their configuration approach, which directly impacts refrigerant charge, energy efficiency, installation complexity, and maintenance requirements.

Multiplex Systems

Multiplex (centralized) systems consolidate multiple compressors in a single machinery room, with refrigerant piping distributed to numerous display cases and walk-in units throughout the facility. This architecture dominates large supermarket installations.

Key characteristics:

  • Central compressor rack with 4-16 compressors operating in parallel
  • Refrigerant charge typically 2-3 lb per linear foot of display case
  • Single or dual suction groups serving medium-temperature (28-35°F) and low-temperature (-10 to 0°F) loads
  • Common discharge manifold with air-cooled or evaporative condenser
  • Satellite compressors for isolated loads or capacity modulation

Advantages:

  • Centralized maintenance access
  • Efficient compressor staging and capacity control
  • Heat reclaim opportunity from single point
  • Reduced in-store noise levels

Disadvantages:

  • High refrigerant charge (3,000-6,000 lb typical supermarket)
  • Extensive piping installation and leak potential
  • Total system shutdown risk from machinery room failure
  • Refrigerant leak detection complexity across distributed piping

Distributed Refrigeration Systems

Distributed systems employ self-contained condensing units located near the refrigerated load, either on the sales floor or in back rooms. Each unit serves one or a small group of cases.

Configuration types:

  • Remote condensing units with line-set connections (20-100 ft typical)
  • Rooftop condensing units with vertical piping penetrations
  • Self-contained units with integral condenser (plug-in cases)

Refrigerant charge reduction:

  • 50-75% less refrigerant compared to multiplex systems
  • Typical charge: 5-15 lb per condensing unit
  • Reduced leak impact and regulatory compliance burden

Performance considerations:

  • Compressor staging limited to individual unit capacity steps
  • Higher cumulative energy consumption (no system-level optimization)
  • Increased condenser fan energy in moderate weather
  • Distributed maintenance requirements across multiple units

Secondary Loop Systems

Secondary loop (indirect) systems use a primary refrigeration circuit to cool a secondary heat transfer fluid (glycol, brine, or CO₂), which is then pumped to display cases and walk-ins. This architecture minimizes refrigerant charge while maintaining centralized refrigeration.

Primary refrigeration circuit:

  • Central chiller with 200-600 lb refrigerant charge (typically HFC or ammonia)
  • Plate-and-frame or shell-and-tube heat exchanger interface
  • Single refrigeration system serves entire facility load

Secondary fluid distribution:

  • Propylene glycol (25-45% concentration) most common for medium-temperature applications
  • Calcium chloride or potassium acetate brine for low-temperature service
  • Pumped circulation at 2-6 ft/s velocity in insulated piping
  • Supply temperatures 20-25°F for medium-temp, -15 to -20°F for low-temp applications

Energy penalty:

  • Secondary loop temperature approach reduces evaporating temperature by 5-10°F
  • Compressor power increase of 8-15% compared to direct expansion
  • Pump energy addition (0.5-1.5 hp per 10,000 Btuh cooling)
  • Offset by reduced piping heat gain and optimized central plant operation

Display Case Technologies

Display cases represent the primary refrigerated load in retail environments, designed to balance product preservation with visibility and customer access.

Open Display Cases

Open (vertical or horizontal) cases provide unrestricted customer access without doors or covers, using air curtains to minimize infiltration.

Air curtain design:

  • High-velocity discharge air (300-500 fpm) creates thermal barrier
  • Return air grille captures curtain and entrained ambient air
  • Discharge-to-return air temperature rise: 8-12°F typical
  • Humidity infiltration drives 40-60% of total case load

Performance factors:

  • Ambient temperature and humidity directly impact load (psychrometric mixing)
  • Air curtain effectiveness degrades with cross-drafts >50 fpm
  • Night covers reduce off-hours infiltration by 60-80%
  • Product loading above load line increases temperature and energy consumption

DOE 2017 regulations:

  • Maximum daily energy consumption (MDEC) limits for all case types
  • Open vertical cases: eliminated from regulation for new installations
  • Existing open vertical cases must meet 2017 baseline efficiency
  • Anti-sweat heater (ASH) controls required (dewpoint or proportional)

Closed Display Cases

Closed cases incorporate transparent doors or lids to eliminate air curtain infiltration, reducing energy consumption by 40-65% compared to open cases.

Door configurations:

  • Single-pane low-E glass (U-factor 0.35-0.45)
  • Double-pane insulated glass (U-factor 0.20-0.30)
  • Triple-pane for low-temperature applications (U-factor 0.12-0.18)

Anti-sweat heater management:

  • Frame and glass heating prevents condensation formation
  • Energy consumption: 10-30% of total case load in humid climates
  • Dewpoint control modulates heater based on ambient conditions
  • Zero-energy doors use heated discharge air for condensation control

Customer acceptance factors:

  • Sales impact varies by product category (beverages minimal, grab-and-go significant)
  • Door type selection (hinged, sliding, gravity) affects accessibility perception
  • Lighting integration and glass clarity influence product visibility

Walk-In Coolers and Freezers

Walk-in units provide bulk storage and product staging in controlled environments, sized from 6×6 ft reach-in style to 30×50 ft warehouse-scale installations.

Construction standards:

  • Insulated panels: 2-4 in polyurethane or polystyrene (R-20 to R-35)
  • Cam-lock or tongue-and-groove panel joining systems
  • Thermal breaks at all structural penetrations
  • Floor insulation for freezers below 32°F space temperature

Refrigeration configurations:

  • Top-mount unit coolers (most common, gravity condensate drainage)
  • Ceiling-mount evaporators (larger spaces, better air distribution)
  • Penthouse remote evaporators (equipment isolation from food zone)

Capacity determination:

  • Transmission load: Q = U × A × ΔT (walls, ceiling, floor, door)
  • Infiltration load: air changes method or open-door calculation
  • Product load: sensible cooling + latent freezing + respiration
  • Internal loads: lights, motors, occupants
  • Safety factor: 10-20% for door openings and load variation

Defrost methods:

  • Coolers (>32°F): off-cycle air defrost
  • Medium-temperature freezers (0-32°F): electric or hot gas defrost every 4-6 hours
  • Low-temperature freezers (<0°F): hot gas or electric defrost every 6-8 hours
  • Defrost termination: time, temperature (45-60°F typical), or demand-based

Reach-In Refrigerators and Freezers

Reach-in units serve as the primary food storage equipment in foodservice kitchens, providing accessible refrigeration in standardized footprints.

Configurations:

  • Single, double, or triple section (27, 54, or 81 in widths)
  • Solid or glass doors (half-door and pass-through options)
  • Top-mount or bottom-mount refrigeration systems

Temperature classifications:

  • Refrigerators: 33-38°F (average 36°F)
  • Freezers: -10 to 0°F (average -5°F)
  • Dual-temperature units: separate compartments

Energy efficiency considerations:

  • DOE 2017 standards mandate improved insulation and compressor efficiency
  • ECM evaporator and condenser fans required
  • Maximum daily energy consumption based on unit volume and configuration
  • Energy Star certification provides additional 10-15% efficiency improvement

Refrigerant Management and Regulations

Commercial refrigeration faces increasing regulatory pressure to reduce refrigerant emissions and transition to low-GWP alternatives.

Leak detection requirements (EPA Section 608):

  • Systems with 50+ lb charge: annual leak inspection
  • Leak rates >30% (comfort cooling) or >35% (commercial refrigeration): mandatory repair
  • Trigger calculation: (refrigerant added annually / full charge) × 100%

Refrigerant phase-down (AIM Act):

  • R-404A production allocated at 15% of baseline (effective 2024)
  • R-22 production eliminated (2020), service stocks declining
  • Transition refrigerants: R-448A, R-449A, R-452A (GWP 1300-1400)
  • Future targets: R-455A, R-454C (GWP <150), CO₂, ammonia, propane

Charge minimization strategies:

  • Distributed systems reduce total facility charge
  • Secondary loops limit primary refrigerant to machinery room
  • Microchannel condensers reduce charge by 30-50% versus tube-and-fin
  • Suction line accumulators and liquid receivers sized to actual needs

Heat Reclaim Applications

Commercial refrigeration systems reject substantial thermal energy to condensers (compressor work + refrigeration load), creating opportunity for heat recovery.

Available heat quantity:

  • Heat rejection = refrigeration load × (1 + 1/COP)
  • Typical COP: 1.5-2.5 for commercial refrigeration
  • Heat rejection: 130-165% of refrigeration capacity

Heat reclaim applications:

  • Space heating: hydronic coils or forced-air heat exchangers (winter months)
  • Domestic hot water: desuperheaters provide 120-140°F water continuously
  • Booster water heating: refrigeration heat raises temperature to 100-110°F, auxiliary heater tops off
  • Snowmelt systems: glycol loops embedded in entrance areas
  • HVAC reheat: dehumidification applications in humid climates

Desuperheater configuration:

  • Installed in compressor discharge line before condenser
  • Refrigerant-to-water heat exchanger (double-wall for safety)
  • Recovers 15-30% of total heat rejection at high temperature (140-180°F gas)
  • Three-way valve bypasses desuperheater when heat not required

System integration considerations:

  • Heat reclaim reduces condenser load, lowering condensing temperature
  • Compressor efficiency improves with reduced head pressure
  • Heat storage tank required for temporal load mismatch
  • Economic analysis: simple payback typically 2-5 years in high-usage applications

Energy Code Compliance

Department of Energy (DOE) regulations establish minimum efficiency standards for commercial refrigeration equipment.

DOE 2017 Walk-In Standards:

  • Insulation: R-25 minimum coolers, R-32 minimum freezers
  • Doors: R-12 minimum coolers, R-16 minimum freezers
  • Evaporator fan power: 0.4 W/°F per 1,000 Btuh (coolers), 0.7 W/°F per 1,000 Btuh (freezers)
  • Lighting: LED with automatic shut-off

DOE 2017 Display Case Standards:

  • Maximum daily energy consumption (kWh/day) by case type and volume
  • Testing per AHRI 1200 standard (ASHRAE 72)
  • Anti-sweat heater controls mandatory
  • LED lighting required

California Title 24 (more stringent):

  • Closed cases required for new installations (limited open case exemptions)
  • Walk-in insulation: R-28 coolers, R-36 freezers
  • Demand defrost required (temperature termination)
  • Occupancy-based lighting controls

Commercial refrigeration systems must balance competing demands of energy efficiency, food safety, product visibility, and operational reliability. Understanding the fundamental physics of heat transfer, psychrometrics, and refrigeration cycles enables proper system selection, design, and optimization for each unique application.