Sizing the Refrigeration for a Mini Cold Store: What a Heat Load Calculation Includes

Two mini cold rooms of the same size can need very different refrigeration equipment, because “size” is only one input. What actually determines the compressor and evaporator you need is the heat load—the total amount of heat the system has to remove per day to hold the room at temperature. Get the heat load calculation right and the plant runs comfortably, holds spec and lasts. Get it wrong on the low side and the compressor runs non-stop, never pulls down in hot weather, and burns out early. This article explains the components of a heat load so you can sanity-check what a supplier quotes you.

Why you cannot size by volume alone

A supplier who quotes a condensing unit purely from the room’s cubic metres is guessing. The same box holds a different load depending on whether it is a chiller at 4 degrees or a freezer at minus 20, whether it is in shaded or full sun, how often the door opens, what product goes in and at what temperature, how warm the ambient gets in a Bangladesh summer, and how many hours a day you want the plant to run. All of those feed the calculation. Our note on how much space you need covers the volume question; this is the refrigeration-capacity question, and they are different.

Component 1: transmission load through the envelope

Heat conducts through the walls, roof and floor from the warm outside to the cold inside, continuously. The size of this load depends on the panel insulation—its thickness and material—the surface area of the envelope, and the temperature difference between inside and ambient. A freezer in a hot climate has a large temperature difference, so its transmission load is high and its panels need to be thicker. This is exactly why panel thickness and PUF versus PIR are not cosmetic choices—they set a permanent baseline load the compressor pays for every hour of every day.

Component 2: product load (pull-down and holding)

When you put product into the room, you have to remove the heat it brings with it. There are two parts. Sensible heat is cooling the product from its entry temperature down to storage temperature. Latent heat, for anything you freeze, is the much larger amount of heat removed as the water in the product changes from liquid to ice at its freezing point. A freezer that receives warm product and has to freeze it—rather than just hold already-frozen stock—carries a big product load and needs serious capacity. The daily intake rate matters as much as the total: freezing five tonnes over 24 hours is a far bigger instantaneous load than holding fifty tonnes that is already cold.

Component 3: respiration heat for fresh produce

Fresh fruit and vegetables are alive in storage and generate heat as they respire. Leafy greens, broccoli, peas and sweetcorn respire fast and add a meaningful continuous load; potatoes, onions and apples respire slowly and add little. If your cold room is for fresh produce, the respiration load has to be in the calculation, and it is one reason a produce chiller and a frozen store of the same size need different plant. This connects to the whole post-harvest cold-chain question.

Component 4: infiltration through the door

Every time the door opens, cold dense air falls out and warm humid air rolls in, and that warm air then has to be cooled and, worse, dehumidified—its moisture condenses and freezes on the evaporator coil. A room with heavy traffic, or one without a strip curtain or air curtain, has a large infiltration load and frequent defrost demand. A low-traffic store with good door discipline has a small one. This is a design input, not an afterthought: tell your supplier honestly how often the door will cycle.

Component 5: internal loads

Everything inside the room that produces heat counts: the evaporator fan motors (which run for hours), the lights, the defrost heaters, forklifts or pallet trucks that come inside, and the people working in there. Individually small, together they are a real slice of a small room’s load, and fan motor heat in particular runs almost continuously.

Putting it together: safety factor and run hours

The calculation adds these components into a total daily heat load, then divides by the number of hours per day you want the plant to run—typically designed around 16 to 18 hours, leaving headroom for defrost cycles, hot days and pull-down after a big intake, rather than assuming 24-hour running. A modest safety margin is added on top. Size the compressor and evaporator to that hourly figure. A plant sized to run 24 hours flat out on a normal day has nothing left for a heatwave or a large delivery, and it will lose temperature exactly when you need it most.

Two related decisions follow from the load: the compressor type that suits the duty, and the refrigerant. And once the room is built, a temperature mapping study confirms the plant actually holds spec in every corner.

How the components combine over a day

The five loads do not all peak together, which is why the calculation works on a daily total and a design run time rather than a single worst instant. Transmission through the envelope and the evaporator fan motors run essentially all day, a steady baseline. Infiltration spikes whenever the door opens and then tails off. Product load arrives in bursts—large when a delivery comes in and you are pulling it down or freezing it, near zero when you are only holding cold stock. Respiration heat, for fresh produce, is a slow continuous addition that falls as the product cools. Lights and people come and go with the working shift.

You add the day’s totals for each component to get a total daily heat load, then divide by the hours per day you want the plant to run—designed around 16 to 18, not 24—so there is spare capacity for defrost cycles, a heatwave, and pull-down after a big intake. A modest safety margin goes on top. Size the compressor and evaporator to that hourly figure. A plant that has to run flat out for 24 hours on an ordinary day has nothing in reserve for the day you actually need it.

Signs your plant was undersized

• The compressor runs almost continuously and rarely cycles off, even in cooler weather
• The room holds temperature in winter but drifts above setpoint on hot afternoons or after a delivery
• Pull-down after loading warm product takes far longer than the supplier promised
• Heavy ice build-up on the evaporator and frequent or extended defrosts, pointing to high infiltration the coil cannot keep up with
• Rising electricity bills as the struggling compressor runs longer hours
• Short compressor life — a unit that never rests wears out early

If you see these, the fix is rarely a setting—it is capacity, insulation, or door management, and it is much cheaper to get the heat load calculation right before the room is built than to retrofit a bigger condensing unit afterwards. A temperature mapping study after commissioning will confirm whether the installed plant genuinely holds spec in every corner under a real load.

What to ask a supplier

• Show me the heat load calculation, with each component listed, not just a compressor model
• What ambient temperature and what internal temperature did you design for?
• What daily product intake and entry temperature did you assume?
• How many run hours per day is the plant sized for, and what safety factor did you use?
• What door traffic and what defrost schedule does the design assume?

A supplier who can answer those has done the engineering. One who cannot has sold you a box with a guess bolted on. We are not going to publish a capacity figure or a price for your room, because the honest answer genuinely depends on your temperature, your climate exposure, your product and your intake rate—a number from a blog post would be the wrong number. Give us those inputs and we will produce a proper heat load calculation. For mini cold storage designed and sourced from China with the calculation done properly, contact DE International — see our cold-storage services, our China buying-agent service, and the catalogue.