Cold store

A cold store is an enclosed, thermally insulated building or compartment in which the internal air temperature is maintained within a controlled band below ambient, for the storage of perishable food, pharmaceuticals or other temperature-sensitive goods. The term is generally applied to the complete facility — envelope, structural frame, refrigeration system and supporting services — whereas a cold room usually describes a smaller compartment within a larger building, and a refrigerated warehouse describes the logistics function of the building as a whole.

Unlike a conventional warehouse, the envelope of a cold store is a primary performance element rather than a weather skin: it carries the thermal duty, controls moisture movement through the construction, and is typically the largest single determinant of operating energy.

[edit] Temperature classifications

Cold stores are classified by the temperature band they maintain:

• Chilled — approximately 0 to +8 °C, for fresh produce, dairy, meat, cut flowers and pharmaceutical products.

• Frozen — approximately −18 to −25 °C, for long-term storage of frozen goods.

• Blast freezing and rapid chilling — air temperatures of −30 to −40 °C, used to freeze or chill product quickly before transfer to a storage compartment.

• Ante-room or airlock — an intermediate space between the store and the outside, buffering door movements and reducing heat and moisture ingress.

• Multi-temperature — partitioned compartments with independent refrigeration, served by a common envelope and loading dock.

The design temperature of each compartment governs panel thickness, floor insulation, door specification and refrigeration duty, and should be fixed before the envelope is designed.

[edit] Forms of construction

Three broad arrangements are used:

1. Site-erected cold store. Insulated panels are installed as the envelope, either within a separate structural frame — often described as a building within a building — or spanning directly between the structural members.

1. Pre-assembled or modular cold store. Factory-made panels and cold room components are delivered as an engineered kit and assembled on a prepared floor. Modular systems are demountable and extendable, and are commonly used for smaller footprints, phased capacity, leased premises and temporary facilities.

1. Conversion. An existing warehouse or industrial building is adapted by installing an independent insulated envelope inside the existing shell.

A key structural decision is whether the frame sits outside the insulated envelope (a warm frame) or inside it (a cold frame). A warm frame keeps the steel at near-ambient temperature, which simplifies material selection and reduces thermal bridging, but requires the envelope to be continuous around the structure and increases the enclosed volume. A cold frame reduces volume and envelope area but exposes the structure to low temperature and to a continuous vapour drive from the warm side, and demands careful detailing of every support and penetration.

[edit] The insulated envelope

Cold store envelopes are normally formed from factory-made, double-skin metal-faced insulated panels with a rigid core — polyisocyanurate (PIR), polyurethane (PUR), expanded polystyrene (EPS) or mineral wool — manufactured and tested to EN 14509. Thickness is selected against the required U-value and the temperature difference across the panel; chilled stores commonly use 100–120 mm, frozen stores 150–200 mm.

Panels perform a structural role, spanning between rails, purlins or structural members and resisting wind and internal pressure as well as their own weight. Achievable span is often governed by deflection limits rather than by strength.

Performance depends more on the joints than on the panel face. Joints are typically tongued and grooved, with a cam-lock or concealed fixing and two gasket lines; the joint geometry, the compression of the gaskets and the continuity of insulation across the junction determine real thermal performance. Wall-to-roof junctions, corners, door openings and the floor connection are the main sources of thermal bridging and are normally detailed as standard tested components rather than fabricated on site.

Where an internal ceiling is required — to reduce the cooled volume or to create a separate temperature zone — it is suspended from the structure. Its fixings penetrate the vapour and air control layers, so each penetration requires a sealed detail.

[edit] Vapour control and airtightness

In a frozen store, water vapour moves from the warmer, more humid side towards the cold side. If it reaches the cold face of the insulation it condenses and then freezes, progressively reducing thermal performance and, in severe cases, damaging the panel. The control strategy is a continuous vapour control layer on the warm side, with lapped and sealed joints and as few penetrations as possible.

Airtightness matters more than vapour resistance. A small air leak carries many times more moisture than diffusion through the same area, and it deposits that moisture inside the construction, where it forms ice at joints and around penetrations.

Envelope airtightness is therefore verified before refrigeration is commissioned, by pressure testing the completed envelope. Defects found at that stage are far cheaper to correct than after pull-down.

[edit] Floors and frost heave

The floor is usually the most heavily engineered element of a cold store. Insulation — commonly extruded polystyrene or PUR board — is placed beneath the structural slab, with the slab designed for racking leg loads, forklift wheel loads and point loads imposed by the storage system, and finished to the flatness tolerance required by the racking supplier.

Beneath a frozen store the ground will freeze unless heat is supplied, and the resulting frost heave can lift and crack the slab. Protection is provided by a low-temperature heating system under the insulation — electric heater mats or a glycol circulation loop — or by a ventilated air space. It must operate continuously for the life of the building, not only during cold weather.

Drainage needs particular care: gullies and traps must not create a path for vapour or a thermal bridge, and floor falls must clear wash-down water without ponding.

[edit] Structural frame

Single-storey steel portal frames are the usual structural form, with spans and eaves heights set by the storage and handling system. Structural steel exposed to low temperature loses ductility and becomes more susceptible to brittle fracture; where the frame operates cold, steel grades with specified impact toughness at the service temperature are selected, and connections are detailed to avoid severe stress concentrations. Thermal breaks are required wherever a cold structure meets a warm one, both to limit heat gain and to prevent condensation on the warm-side steel.

[edit] Doors and access

Door openings are the largest single source of heat and moisture ingress in operation. Common arrangements include rapid roll doors, sliding doors, air curtains, strip curtains and an ante-room or airlock between the store and the outside. Heated door frames and thresholds prevent condensation and ice at the opening, while dock levellers and insulated dock shelters limit losses at the vehicle interface. Door discipline in operation typically has more effect on energy use than any single envelope improvement.

[edit] Refrigeration interface

Envelope design is inseparable from refrigeration design. Unit cooler positions and weights, defrost method, pipe and drain routes, and plant room location all impose loads on, and penetrations through, the envelope, and should be coordinated before panels are ordered. Defrost drainage in particular must be traced and insulated, or it will freeze.

[edit] Commissioning

Commissioning includes verification of envelope airtightness, a controlled pull-down of temperature to avoid thermal shock and moisture migration, and temperature mapping of each compartment. Alarms, door interlocks and monitoring should be demonstrated at handover, and a maintenance regime established for gaskets, door seals and heater circuits — the three components most often responsible for gradual loss of performance.

[edit] Operation and energy

The dominant energy drivers are door openings, defrost cycles, the temperature lift across the refrigeration system, lighting heat gain and fan energy. Low-heat LED lighting, EC fans, floating head pressure control, staged defrost and sub-metering by compartment are commonly specified. Roof-mounted photovoltaic arrays are increasingly included where the roof structure and the planning context allow.

[edit] Related articles

Sandwich panel. Insulated metal panel. Metal profile cladding. Air tightness in buildings. Thermal insulation for buildings. Thermal bridging. Portal frame. Braced frame structures. Steel frame building systems. Cold-formed steel.

[edit] External references

Design data and typical envelope and structure details for site-erected refrigerated warehouses, including insulated floor and frost heave protection arrangements: [refrigerated warehouse design data]