Two kinds of pull-down
- Cabinet pull-down: the time an empty unit takes to reach its set point after switching on, starting at room temperature. It is a useful check of a new or repaired unit.
- Product pull-down: the time warm product takes to reach storage temperature after it is loaded. This is what matters day to day, and it is usually much longer, because product holds far more heat than air.
Storage refrigerators hold; they do not blast-chill
Most reach-ins, visi coolers and deep freezers are designed to hold product at temperature and to cope with moderate warm loading. Their spare capacity, beyond what is needed to fight heat leaking in, is limited. Hot or large batches of food need a blast chiller or a cooling method in your food safety plan; putting them in a storage cabinet warms everything else around them.
A simple estimate
Heat to remove (kJ) = mass (kg) × specific heat (kJ/kg·K) × temperature drop (K). Minimum time (s) = heat (J) ÷ spare cooling capacity (W). Spare capacity is the unit's cooling output at your conditions minus the heat it is already fighting from walls and door openings.
This gives a lower limit. Actual time is longer, because heat must also travel from the centre of each bottle or pan to its surface and then into the air, which depends on airflow, packaging and product thickness.
Worked example: restocking a visi cooler
Assumptions: 100 bottles of 600 ml (60 kg of drink, specific heat close to water at 4.18 kJ/kg·K) loaded at 35 °C and cooled to 5 °C; spare cooling capacity of 300 W at the shop's temperature.
| Step | Working | Result |
|---|---|---|
| Heat to remove | 60 kg × 4.18 × 30 K | 7,524 kJ |
| In joules | 7,524 × 1,000 | 7,524,000 J |
| Minimum time | 7,524,000 ÷ 300 W | 25,080 s, about 7 hours |
| If only 50 bottles are loaded | half the heat | about 3.5 hours |
What the example means in practice
Even with generous assumptions, 100 warm bottles need most of a working day to get cold. Restocking little and often, putting new stock behind cold stock, and pre-chilling in a back-room unit all work with the physics instead of against it. For cold rooms, staggering deliveries and pre-cooling product before it enters storage achieves the same thing.
Large cold stores specify pull-down explicitly. India's National Horticulture Board standards for produce cold storage, for example, set crop-specific pull-down rates and daily loading rates, because loading too much warm produce at once overwhelms the plant.
What to ask a supplier
- Pull-down time from what starting temperature, at what room temperature, to what end temperature?
- Was it measured empty or with a stated load?
- What is the unit's cooling capacity at a 43 °C ambient, compared with its holding load?
- For freezers, how long does recovery take after the lid has been open for loading?
Frequently asked questions
- Why is my new visi cooler slow to cool drinks?
- Usually because a large amount of warm stock went in at once. Check the cabinet pull-down empty; if that is reasonable, the unit is working and the load is the issue. Restock in smaller batches and keep the condenser clear.
- Does a lower set point speed up pull-down?
- Very little. The compressor is already running flat out while it is pulling down. A lower set point mainly makes the unit run colder once it gets there, which wastes energy and can freeze product near the evaporator.
- Is pull-down slower for frozen products?
- Much slower per kilogram. Freezing removes latent heat, roughly the product's water fraction × 334 kJ/kg, on top of cooling it, and freezers have less capacity because they evaporate colder. Loading unfrozen product into a storage freezer is a heavy load; freeze in a blast freezer where possible.
- How long should an empty cabinet take to reach temperature?
- It depends on the design, the ambient and the set point, so there is no single figure. Ask the maker for the expected empty pull-down at a stated room temperature and use it as a benchmark when commissioning or after a repair.
