Refrigerated Display Case

Overview

A refrigerated display case is a point-of-sale merchandising cabinet that holds perishable goods (deli meats, dairy, prepared foods, ice cream) at controlled temperature while displaying them behind transparent glass doors to customers. Unlike walk-in coolers, display cases are compact (0.9–2 m wide) and designed to attract impulse purchases by showcasing products with bright LED lighting.

The thermal challenge is maintaining constant product temperature while repeatedly opening doors (heat ingress) and supporting high customer traffic. An air-curtain evaporator coil mounted horizontally across the top of the cabinet directs a laminar flow of cold air downward across shelf faces, creating an invisible thermal barrier that minimizes heat loss during door openings. A remote condenser (roof or back-wall mounted) is connected via long refrigerant lines, eliminating compressor heat from the sales floor.

How it works

Air-curtain principle: The evaporator coil (4–6 aluminum fin-tube rows) is mounted horizontally at the top of the cabinet interior. The EC blower motor (0.2–0.5 kW) forces cold air downward at 1–2 m/s through the coil outlet slot, creating a uniform sheet of chilled air across the entire shelf face height. This air descends the product faces, picks up sensible heat, and recirculates back to the coil inlet. The laminar flow prevents warm room air from entering and warm product air from escaping, maintaining shelf temperature within ±2 °C of setpoint even during frequent door openings.

Evaporator heat exchange: Refrigerant enters the coil at -15 °C via the thermostatic expansion valve, evaporates across the fin surfaces while air flows through, and exits as low-pressure vapor (~20 psi) back to the remote condenser. The evaporator drain pan collects condensate; melting ice (during automatic defrost) drains via the PVC drain line to the floor or a sump.

Glass door sealing: The tempered-glass swing or sliding doors are critical. A magnetic or adhesive gasket compresses when the door closes, creating an air-tight seal. The electric heater wire (200–300 W, embedded in the gasket) warms the frame to prevent condensation and frost around the edges. A thermostat inside the heater circuit monitors frame temperature; when it drops below dew point, the heater turns on automatically.

Remote condenser operation: Long refrigerant suction and discharge lines (10–30 m typical) connect the display case evaporator to a roof-mounted or external condenser unit. This decouples compressor heat from the retail sales floor, improving customer comfort and reducing whole-store cooling load. The condenser operates autonomously with its own EC fan, rejecting heat to outdoor air (or rooftop ambient).

Refrigerant circuit: Superheated vapor returns from the display case evaporator (~25 °C, 20 psi) via the suction line to the remote condenser inlet. The EC fan cools the gas to saturation; liquid condenses and collects in a receiver tank. Liquid at 30 °C and 200 psi flows back to the display case via the liquid line, where the thermostatic expansion valve meters flow into the evaporator, completing the cycle.

Temperature control: A Pt100 RTD sensor mounted on one shelf monitors product temperature. The microprocessor thermostat cycles the compressor via a contactor when temperature exceeds setpoint (e.g., +4 °C). A programmable defrost timer cycles the system every 6–12 hours, energizing the solenoid valve to reverse hot discharge gas through the evaporator, melting ice on the coil.

Lighting: LED shelf lights (5 W/m, 5500 K daylight white) illuminate products on each shelf; a front-facing fascia lamp (20–40 W) provides additional visibility. These draw minimal power (30–60 W total) compared to older incandescent or halogen designs, which generated excess heat and reduced cooling efficiency.

Defrost Strategies

Ice accumulation on the evaporator coil is inevitable in food-contact applications due to high humidity. Two methods address it:

1. Hot-gas defrost (automatic):

• Timer triggers every 6–12 hours.
• Solenoid valve diverts discharge gas (200 psi, 80 °C) directly into the evaporator, bypassing the thermostatic expansion valve.
• Ice melts; water drains via condensate line.
• Duration: 15–30 minutes. Compressor is de-energized during defrost to save energy.
• After defrost, airflow cools the now-empty evaporator back to -15 °C within 10 minutes.

2. Off-cycle defrost (simpler):

• Timer simply de-energizes the compressor for 30–60 minutes every 8 hours.
• Ambient room air (25 °C) gradually warms the evaporator coil from -15 °C to 0 °C; ice melts.
• Slower than hot-gas defrost but lower energy consumption.
• Risk: If room air is cold (e.g., case located near door in winter), defrost may be incomplete.

Refrigerant Options

Modern display cases increasingly use low-Global Warming Potential (GWP) refrigerants:

• R-404A: Traditional, high efficiency, GWP = 3921. Phase-out scheduled 2030 in EU.
• R-290 (propane): Natural refrigerant, GWP = 3, but flammable; requires special training and containment.
• R-32 (difluoromethane): GWP = 677, moderate flammability; gaining adoption in self-contained units.
• R-290a (isobutane): GWP = 4, emerging option for hybrid high-efficiency systems.

Manufacturers are transitioning to R-290 and R-32 in new designs to comply with EU F-Gas Regulation (2014/517/EU) and similar policies.

Installation & Field Setup

Structural considerations:

• Display cases are typically modular; multiple units bolt together to create longer runs (4–8 m).
• Cases must rest on level flooring; if uneven, condensate may pool and freeze at one end.
• Vibration isolation is essential; some cases include elastomer pads under compressor mounts (if self-contained) or under the cabinet base.

Refrigerant lines:

• Suction line is insulated (1.5 m foam wrap minimum) to prevent sweating.
• Discharge line is bare; should be routed away from high-traffic zones.
• Liquid line is parallel-run with suction for subcooling benefit.
• Lines are pressure-tested at 350 psi nitrogen before refrigerant charge.

Electrical:

• Power entry is typically a standard 120/208/230 V plug.
• Some locations allow soft-start (reduces inrush current); single-phase contactors usually suffice for <3 kW loads.

Drainage:

• Drain line must have minimum 0.25 in/ft slope to prevent ponding.
• In humid climates, drain lines may freeze if routed outdoors in winter; insulation or heat tracing is recommended.

Energy Efficiency & Load Reduction

Display case energy consumption is dominated by:

1. Conduction through walls: 300–500 W (minimized by insulation thickness).
2. Door infiltration: 500–800 W during peak shopping (many door openings).
3. Internal heat generation: Lights (~50 W) + evaporator coil defrost (~1 kW for 20 min/cycle).

Total 24-hour load: 2–5 kW depending on usage pattern.

Efficiency strategies:

• Night blinds/covers: Insulated covers reduce infiltration by 30–50% during closed hours.
• EC fan modulation: Proportional-integral controllers vary blower speed based on room temperature.
• LED lighting: 70% less heat than halogen, minimal cooling load impact.
• Door gasket maintenance: Cracked gaskets increase infiltration 50%; regular replacement is cost-effective.

Maintenance & Cleaning

Daily: Visual inspection for temperature alarm (buzzer), frost on exterior, or condensate backup.

Weekly: Clean glass doors (inside and outside); wipe shelves to prevent mold.

Monthly: Vacuum remote condenser coil (especially in retail environments with airborne dust or grease). Clogged condenser raises discharge pressure and increases compressor amperage.

Quarterly: Inspect door gasket for cracks or deformation. Replace if sealing is compromised.

Annually:

• Check defrost timer operation and duration.
• Verify door heater element function.
• Inspect drain line for algae or mineral buildup; flush with dilute bleach solution.

Standards & Compliance

• NSF/ANSI 7: Commercial refrigerated storage cabinet performance and safety.
• AHRI 1200: Refrigerated display case rating conditions and efficiency.
• EU F-Gas Regulation (517/2014): Phase-down of high-GWP refrigerants; no new R-404A systems after 2030.
• Local codes: Many jurisdictions mandate secondary containment for self-contained units (compressor inside case); remote condenser units avoid this requirement.

Display cases are the customer-facing interface of a cold chain; appearance, temperature reliability, and food safety depend on regular maintenance and proper refrigerant handling.