A badly sized chiller shows up quickly. If it is too small, the water warms up in summer and cycle times get longer. If it is oversized, it starts and stops constantly, uses more energy than needed and ages sooner. Sizing an industrial chiller correctly is not difficult: it takes a simple calculation, reliable process data and a couple of corrections that are almost always forgotten. This guide goes through them with a real plastic injection moulding example.
Chiller, water chiller or process cooler: what we mean
It is the same machine with different names. In Spain it is called an enfriadora, a water cooler or, increasingly, a chiller. In Mexico and much of Latin America people say chiller, water cooler or chilled-water unit. In every case it is a refrigeration machine that cools a fluid (water or water with glycol) and sends it to the process in a closed circuit. Do not confuse it with a temperature control unit (TCU), which keeps the mould stable at a set temperature, normally above that of the mains water.
The formula: Q = m · cp · ΔT
The cooling capacity the process needs is the heat that has to be removed per unit of time:
- Q: cooling capacity (kW).
- m: mass flow rate (kg/s) of the product being cooled or of the circulating water.
- cp: specific heat (kJ/kg·K). Water: 4.18. Water with 30% glycol: approx. 3.8.
- ΔT: temperature difference (K or °C).
In plastic injection moulding it is more practical to work with the enthalpy of the polymer: the heat each kilo releases as it goes from melt temperature to ejection temperature. This avoids calculating sensible heat and, in semi-crystalline materials, the heat of crystallisation separately.
Indicative enthalpy by polymer
Approximate values between melt temperature and demoulding temperature. They are fine for a first calculation; for a specific project, check them against the material data sheet.
| Polymer | Heat to remove (kJ/kg) | Equivalent (kWh/kg) |
|---|---|---|
| PS | 250–300 | 0.07–0.08 |
| ABS | 300–380 | 0.08–0.11 |
| PC | 300–380 | 0.08–0.11 |
| POM | 420–500 | 0.12–0.14 |
| PA6 / PA66 | 550–650 | 0.15–0.18 |
| PP | 550–650 | 0.15–0.18 |
| PE-HD | 600–700 | 0.17–0.19 |
Semi-crystalline materials (PP, PE, PA, POM) need almost twice as much cooling per kilo as amorphous ones (PS, ABS, PC). That is why two identical injection moulding machines can need very different chillers.
Example: injection moulding machine processing 50 kg/h of PP
- Heat from the material. 50 kg/h × 600 kJ/kg = 30,000 kJ/h. Divided by 3,600 s: 8.3 kW.
- Additional loads. Hot runner, pipe heat gains and pump: in a mould with a hot runner it is reasonable to add 10–20%. We take 15%: 8.3 × 1.15 = 9.6 kW.
- Safety margin. Between 15% and 25%, depending on how stable production is and how likely an expansion is. With 20%: 9.6 × 1.2 = 11.5 kW.
- Correction for real conditions (water and ambient temperature, see below). If the unit works outside its rated conditions, its useful capacity drops and a larger model must be chosen.
If the same chiller also has to cool the hydraulic oil of the injection moulding machine or the hopper throat, add those loads. The reliable figure comes from the machine manufacturer; guessing it usually leads to an undersized unit.
The water flow rate
With the capacity calculated, the flow rate comes from the same formula. With 11.5 kW and a 5 °C difference between supply and return: m = 11.5 / (4.18 × 5) = 0.55 kg/s, about 2 m³/h. The chiller pump must be able to deliver that flow against the real pressure drop of the circuit: mould, hoses, fittings and manifolds. A larger temperature difference means less flow, but the mould then works with a bigger difference between inlet and outlet. For the mould itself, see our guide on how to calculate the cooling flow rate of a mould.
Water temperature: every degree counts
Catalogues give the capacity at rated conditions, often 12/7 °C water and 35 °C ambient. Outside those conditions the capacity changes:
- Colder water: every degree below the rated point reduces capacity, by roughly 3% per degree. Asking for 5 °C when the process runs well at 12 °C is expensive.
- Below about 5 °C glycol is needed, which reduces heat transfer and forces you to oversize.
- Condensation: if the water enters the mould below the dew point of the air in the hall, the mould sweats and defects and corrosion appear. In summer, with high humidity, this can already happen at 12–14 °C.
Set the highest water temperature the process allows, not the lowest the unit can deliver. For reference, see the mould temperature table by polymer.
Air-cooled or water-cooled condensing
| Criterion | Air-cooled condensing | Water-cooled condensing |
|---|---|---|
| Installation | Simple, electricity only | Needs a cooling tower or well water |
| Heat rejected | To the air: into the hall or outdoors | To the cooling tower water circuit |
| Effect of summer | Loses capacity at high ambient temperature | More stable if the tower is correctly sized |
| Efficiency | Lower in hot weather | Normally higher |
| Maintenance | Clean the condenser coils | Treat the tower water against scale and legionella |
An air-cooled chiller installed inside the hall returns all the heat it extracts to it, plus the heat from the compressor. In a small plant that may be acceptable; in a closed hall in August, it is not. If it is air-cooled, calculate with the real maximum ambient temperature at the installation site, not the rated one.
Typical sizing mistakes
- Calculating with average output instead of peak output.
- Forgetting the hot runner, the hydraulic oil or the loads that will be added within a year.
- Choosing by rated capacity without correcting for water and ambient temperature.
- Not checking that the pump can overcome the pressure drop of the whole circuit.
- Installing an air-cooled chiller in a room without ventilation.
- Oversizing by double "just in case": the unit runs at low load and controls worse.
Frequently asked questions
How many kW of chiller does an injection moulding machine need?
It depends on the material and the kilos per hour, not on the clamping force. Multiply kg/h by the enthalpy of the polymer, divide by 3,600 and add the additional loads and the margin.
What safety margin is reasonable?
Between 15% and 25% above the calculated load. Beyond that, the unit runs at part load almost all the time.
Can I use a single chiller for several machines?
Yes, it is common with a centralised circuit. Add up the loads of all of them, apply a realistic simultaneity factor and make sure each machine receives its flow.
Do I also need a temperature control unit?
If the mould works above the chilled-water temperature, normally yes. We explain it in TCU or chiller: differences and when to use each.
We help you size it
Send us the material, the kg/h, the water temperature you need and where the unit will be installed, and we will review the calculation with you. You can see our range of industrial chillers, try the equipment selector or talk to a technician.