Cold Storage Refrigeration Design: Load, Temperature, Refrigerant

A cold storage plant is the most honest refrigeration customer: it runs 8,760 hours a year, its load walks through a door every shift, and every design compromise shows up on the electricity bill or in a product temperature within weeks. That is why cold storage refrigeration design rewards getting three things in order — the load, the temperature, the refrigerant — in that order. Most schemes fail by starting with the third.

The reasoning below is the one an engineer should be able to defend line by line before a machine is quoted; the cold storage solutions line at the end shows how a published package answers it.

The load: four numbers, one of them dishonest

Cold storage heat comes from four places, and the design estimate must include all four honestly:

• Transmission through walls, roof and floor. Set by insulation conductance, the temperature difference across the envelope and — often dominant — the ground beneath a floor slab that was never insulated. A store full of frozen product on an uninsulated slab pulls heat from the earth all year.
• Product, and here the dishonest number lives. Product entering above storage temperature carries sensible heat down to the set point and, for anything frozen, latent heat through the freezing range. New intake is a pulse load, not a steady one; design for the worst realistic intake day, not the average week.
• Infiltration and ventilation — warm humid air through doors, airlocks and forklift traffic. Small per event, continuous in a busy distribution store, and the defrost load on the evaporators is its invoice.
• Internal gains: lighting, people, forklifts, and the fan motors themselves, which dump their own heat inside the cold box.

Two capacities come out of the estimate and should be kept separate on the drawing: the pull-down duty that takes a room from warm to set in a defined time, and the maintenance duty that holds it there. Sizing evaporators for maintenance alone is the classic failure — the room never arrives on time and the compressor answers by running warm and long.

The temperature: a zone, not a set point

Cold chains run at least three distinct refrigeration duties: chilled (near 0 to +5 °C), frozen (−18 to −25 °C) and the deep or blast duties that freeze product quickly. Each asks a different evaporating temperature from the machines, and evaporating temperature is the number that sets compressor power — every kelvin lower costs power, year-round. Good design keeps deep-cold rooms and chilled rooms on separate stages rather than dragging one compressor down to the coldest requirement of the worst room.

The refrigerant: the safety decision that was always the design decision

Only then does the refrigerant question arise, and in cold storage it is mostly a charge-and-occupancy question. Ammonia does the low temperatures efficiently at low pressure, but its toxicity caps how large a charge may sit inside an occupied building — the limit is in the standards, and it is the reason large ammonia stores moved their machinery outside or moved to cascades. CO2 is non-flammable and non-toxic, tolerates being piped close to the product, and handles −30 °C evaporation in the low stage of a cascade; its cost is pressure and charge mass. The NH3/CO2 cascade splits the two problems: a small ammonia inventory doing the heat rejection, the CO2 carrying the deep cold where people and product actually are.

Six checks before the design is frozen

1. Pull-down time, agreed in writing. How fast must a warm room reach temperature, and with what intake? The answer sizes evaporators and compressor staging; silence on this point is where commissioning disputes are born.
2. Floor and ground loads. Compute the slab heat flow and check the insulation and any underfloor heating or vapour barrier interaction. A cold floor that heaves or a warm slab that sweats both trace back here.
3. Door traffic, honestly counted. Openings per shift, airlock or not, curtain or not. Infiltration load from traffic is designed at the layout stage or paid forever in defrost cycles.
4. Defrost strategy and its heat bill. Hot-gas, electric or air defrost each dump a known heat load into a room that was just cooled. Confirm the condensate path, and confirm the drain cannot freeze shut — an iced-up drain is a floor failure waiting.
5. Redundancy by staging. Multiple smaller machines covering N+1 duties, or one large frame? The answer is a business-continuity decision — what does a warm room of product cost per hour?
6. Charge limits against occupancy. Before the refrigerant is fixed, confirm the classification of the machinery space, occupied areas above or beside it, and the ventilation and detection the local rules will require.

How a published package answers this

The cold storage line is built on the AIST CO2 module unit: an NH3/CO2 cascade rated 271–526 kW at −38 / +40 °C nominal, with the ammonia charge held to a published 110–220 kg while the CO2 low stage (2,700–5,200 kg, non-flammable) carries the deep-cold duty. It arrives skid-mounted in a container housing with no separate refrigeration control room required, integrates monitoring, alarm and defense systems, and starts on frequency conversion so the store's electrical supply never sees an impulse current. Modular capacity steps match the staging question directly: build the plant as the rooms fill, not as one frame on day one. MOON-TECH has built low-temperature refrigeration since 1956 for food processing and cold chains in 120+ countries and regions.

Questions cold store owners ask

Why does a cold store's energy bill grow over its first year?

Three usual causes, all design-time preventable: ground and floor loads that were estimated optimistically, infiltration from door habits that outgrew the airlock design, and defrost cycles absorbing more heat than assumed as frosting accelerates. Meter the four load components separately from commissioning so drift shows up as a number, not a suspicion.

Is CO2 refrigeration safe inside a cold store?

Its risk profile suits occupied cold boxes well: CO2 is non-flammable and non-toxic at small-leak concentrations, so piping can run near product and people where ammonia may not. The real hazard is volume — a large charge in a sealed room displaces oxygen — which is why occupied spaces served by CO2 need ventilation and oxygen monitoring regardless of the machine's safety record.

Should the whole store be one refrigeration system?

Rarely. Chilled and frozen duties want different evaporating temperatures, and forcing one stage to serve the coldest room pays deep-cold power bills on every warm room. Staged systems — separate circuits or modular units per temperature zone — also answer redundancy: a failed module cools down one zone, not the building.