Chiller selection: capacity calculation, water temperature and key factors

Choosing a chiller starts with one question: how much heat must the unit remove, and at what water temperature? Capacity is calculated from the water flow and the difference between supply and return temperatures, using the formula Q = m · c · ΔT. After that come the leaving water temperature and the deviation you can accept, the need for glycol, redundancy, part-load behaviour, the installation site and the noise limit. A selection made before these points are settled ends either with a unit that is larger and more expensive than necessary, or with one that falls short in the middle of July.

This article is about selection and sizing. If you want to know the parts of the machine and how its cycle works, see our sister company's article on how a chiller works.

Capacity calculation: Q = m · c · ΔT

A chiller's job is to take heat out of water. The heat removed is the product of three quantities:

• Q: cooling capacity (kW)
• m: mass flow rate of the water (kg/s)
• c: specific heat of water, about 4.18 kJ/kg·K
• ΔT: water temperature difference between the unit's inlet and outlet (K)

On site, flow is usually known in cubic metres per hour. Since one cubic metre of water weighs about 1,000 kg, the formula becomes in practice: Q (kW) ≈ flow (m³/h) × ΔT (K) × 1.163.

Example: on a production line, the water going to the machines is at 12 °C and the water returning from them is at 17 °C (ΔT = 5 K), and the line circulates 6 m³ of water per hour. Q = 6 × 5 × 1.163 ≈ 34.9 kW. A margin of 10–20 % is usually added for the heat the pump puts into the water, heat gains in the piping and a machine to be added soon; for this line you would look for a unit of about 40 kW. That puts it in the mini chiller class (5–60 kW), which serves a single line or a few machines.

If the flow and temperature difference are unknown

• The cooling requirement given in the machine manufacturer's catalogue (in kW or kcal/h) is the quickest source.
• In a running installation, a flow meter and two temperature readings give the real load within a few days; the measurement must be taken during the busiest shift.
• The load can also be found from the product side: the amount of product cooled per hour, its specific heat and its temperature drop go into the same formula.
• In buildings, the heat gains from the sun, people, lighting and equipment are calculated; rough per-square-metre figures only give a first idea.

Older specifications may state capacity in kcal/h or in tons: 1 kW is about 860 kcal/h, and 1 ton of refrigeration (TR) is 3.517 kW.

Leaving water temperature and precision

A capacity figure is incomplete unless it states the water temperature it applies to. The same unit gives less capacity when it produces 7 °C water than when it produces 12 °C water, and it uses more electricity per unit of cooling. Asking for water colder than needed is therefore a hidden loss of capacity. In comfort installations, 7 °C leaving and 12 °C return is a common design pair; in production, the machine and the product set the temperature.

How stable the temperature must be is a separate question. In applications such as lasers, measuring instruments, moulds or fermentation tanks, a fluctuation of a few degrees shows up directly in the product. For work that needs a ±0.5 °C band, the choice is a mini chiller with an inverter compressor or the process version of the industrial chiller. The water volume in the circuit also adds to stability: the larger the tank, the more sudden load changes are smoothed out and the less often the compressor starts and stops.

A building or a machine?

A building's cooling load changes with the sun, the season and occupancy; for most of the year the unit runs below full load. That is why part-load efficiency and stepped capacity matter most in hotel, hospital, shopping centre and office tower projects. Commercial chiller units (50–500 kW) run in steps with several scroll compressors, and with two independent circuits the building is not left entirely without cooling when one circuit stops.

In production the picture is reversed. The load is often constant, the unit runs every day of the year and, in most plants, every hour of the day, and the leaving water is held within a narrow band. Here full-load efficiency, endurance in continuous operation and precision are what count. The industrial chiller is built for this work in the 300–2,000 kW range; for a single machine, a mini chiller is enough. We cover application-specific details in our articles on injection moulding machine cooling and concrete cooling.

Is glycol needed?

If the leaving water is to go below +5 °C, or if the pipes run along a route where they could freeze in winter, glycol is added to the circuit. Food plants use non-toxic propylene glycol; other applications mostly use ethylene glycol. The concentration is set by the lowest temperature the circuit will see.

Glycol comes at a price: the mixture has a lower specific heat than water and a higher resistance to flow. Carrying the same heat takes more flow and a stronger pump, and the unit's capacity drops somewhat. The glycol concentration should therefore be stated at the quotation stage, and capacity calculated for that mixture. The glycol version of the mini chiller delivers water down to −5 °C; in the industrial chiller, −10 °C with glycol is standard, and the low-temperature version goes down to −30 °C.

Redundancy and part load

If one hour of lost production costs more than the unit itself, redundancy goes into the project from the start. The common arrangement is N+1: one unit more than those carrying the load. Example: a 600 kW load can be met by one 600 kW unit or by two 300 kW units. When a third 300 kW unit is added to the second arrangement, full capacity is maintained even if one of the units stops. In this arrangement, industrial units are managed together as master/slave.

Several units also help at part load: when the load drops, one goes offline and the others run in their efficient range. In catalogues, look at seasonal efficiency as well as full-load EER; under the European ecodesign rules this figure is given as SEER for comfort chillers and SEPR for process chillers. In plants that run continuously and have cold winters, a free-cooling coil stops the compressor when the outside air is cold enough and cools the water directly with outside air.

If the heat the cooling rejects can be used for another job, state that in the selection too. The heat recovery or heat pump version of the commercial chiller produces 55 °C hot water; we compare the two approaches in our article on the difference between a heat pump and a chiller.

Condenser type, location and noise

Whether the condenser is cooled by air or by water from a cooling tower is a decision that affects efficiency and the installation; our sister company explains the difference in its guides on air-cooled units and water-cooled units. Our mini and commercial units are air-cooled; the industrial class offers both options, and the choice depends on the water and space available at the plant.

With an air-cooled unit, the installation site is part of the selection too:

• Summer design temperature of the region: catalogue capacities are mostly given for 35 °C outside air; where summers exceed 40 °C, the unit is selected for that temperature.
• Air flow: there must be free space around the unit; a unit squeezed against a wall or into a pit draws back the hot air it has just rejected.
• Structural load: the weight and vibration isolation of a unit placed on a roof are assessed together with the structural design.
• Noise: close to a neighbouring façade or bedroom windows, a low-noise version or an acoustic enclosure is needed.
• Winter operation: with a winter kit, the commercial units run down to −12 °C outside air; for jobs that need chilled water in winter as well, this limit is checked.

Two examples from the field

At Viskon Rubber Factory, a mini chiller with two refrigeration circuits, a tank and a pump was used for the rubber production process. The unit was assembled and tested in the workshop, then installed on a galvanised frame on site and commissioned. In a design like this, two circuits keep the process from being left entirely without cooling when one stops, and having the tank and pump inside the casing shortens the connection work on site.

At Bursa Gelatin Factory, we prepared the plant's chiller project and handed over the system. In food industry plants such as gelatin production, chiller selection is usually driven by the temperature the production line needs and by the need for uninterrupted operation.

What information is needed for a quotation?

1. Cooling load (kW), or flow with supply and return temperatures
2. Required leaving water temperature and the acceptable deviation
3. Whether glycol is needed, and the lowest temperature the circuit will see
4. Operating pattern (shifts or 24/7) and expected redundancy
5. Installation site, summer temperature of the region and noise limit
6. Power supply; connection to building automation or SCADA
7. Any requirement for heat recovery or free cooling

Send this list through our quotation form and we will select the unit that suits your load from our chiller range and propose it together with its technical documents.