Mechanical cooling: how it works, what it's for, and when to choose it

What is mechanical cooling and how does it work?

Mechanical cooling operates via the vapor compression cycle. This consists of four continuously repeated steps.

1. EvaporationThe refrigerant flows at low pressure through the evaporator and evaporates. In doing so, it absorbs heat from the space or medium to be cooled. This is where the cooling effect is generated.

2. CompressionThe compressor increases the pressure of the gaseous refrigerant. The temperature rises accordingly. The greater the pressure difference between the evaporator and condenser, the more energy the compressor consumes.

3. CondensationThe hot refrigerant flows through the condenser and releases heat to the environment, via air or cooling water. The refrigerant condenses back into liquid.

4. ExpansionThe expansion valve reduces the pressure of the liquid refrigerant. The temperature drops sharply, and the cycle begins again. The efficiency of this process is expressed as the COP (Coefficient of Performance): the ratio between the delivered cooling capacity and the electrical power consumed. In industrial applications, the COP typically ranges between 2 and 5. With a COP of 3 and a desired cooling capacity of 100 kW, the system consumes 33 kW of electricity. The COP decreases at lower process temperatures and also decreases with higher ambient air temperatures when used as a cooling medium.

The four main components of a mechanical refrigeration system

1. CompressorThe compressor is the driving force of the system. It increases the refrigerant pressure and ensures circulation. Four types are common in the industry: the reciprocating compressor (smaller capacities), the scroll compressor (quiet, suitable for medium-sized systems), the screw compressor (dominant from 10 kW to 10 MW, robust and easily controllable), and the centrifugal compressor (very large capacities, high efficiency at full load). In industrial refrigeration, the screw compressor is by far the most commonly used type.

2. CondenserThe condenser dissipates heat to the environment. Three versions are relevant: air-cooled (simple, no water connection required, lower efficiency at high ambient temperatures), water-cooled (higher efficiency, requires a cooling tower or chilled water circuit), and evaporative-cooled (most efficient at high ambient temperatures, as it consumes both air and a small amount of evaporative water).

3. Expansion ValveThe expansion valve regulates the flow of refrigerant from the high-pressure side to the low-pressure side. The electronic expansion valve (EEV) is the current standard for industrial installations. It responds quickly to changing loads and increases the COP at partial load.

4. EvaporatorThe evaporator absorbs heat from the space or medium to be cooled. The type depends on the application: a plate heat exchanger for liquid-to-liquid cooling (chillers), an air evaporator for cold rooms and climate chambers, or a falling-film evaporator for large industrial installations with high cooling capacity.

Applications of mechanical refrigeration

Cold stores and storageCold stores for food, flowers, and pharmaceutical products operate at temperatures between +1 °C and +15 °C, with specific requirements for humidity and air circulation per product. Nijssen has many years of expertise in the storage of fresh produce, where the interplay between cooling, ventilation, and control is crucial for product quality.

Freezer storage and IQFDeep-freeze storage requires temperatures from -18 °C to -25 °C. For food processing, IQF (Individual Quick Freezing) freezers are common: they operate at -35 °C to -40 °C to freeze products quickly and individually. Both applications require systems with a high pressure ratio and a refrigerant that still provides sufficient efficiency at low temperatures.

Industrial process coolingReactors, extruders, molds, and chemical processes require precisely controlled chilled water temperatures, typically between +5 °C and +25 °C. A chiller manages the chilled water circuit. The same principle applies to data centers: heat dissipation from server racks via a chilled water circuit. Availability and temperature accuracy are crucial here, not the lowest temperature.

Climate simulation and test chambersMechanical cooling is at the core of climate chambers and test chambers for R&D, product development, and qualification testing. Temperature ranges down to -70 °C are possible, depending on the refrigerant and system design.

Potatoes and other stored productsFor the storage of potatoes, onions, and other agricultural products, mechanical cooling is active at ambient temperatures above approximately 12 °C. Below that threshold, outdoor air ventilation suffices. Mechanical cooling, as a supplement to ventilation, ensures that the storage condition is maintained throughout the entire season, even during warm periods in spring. The desired storage temperature for potatoes is between 4 °C and 8 °C, with narrow tolerances.

“At a potato storage facility in the Flevopolder, we upgraded an existing ventilation system with mechanical cooling. The client didn't need a system for the first two years. After the third warm summer, the business case was made within three months.”

When should you choose mechanical cooling, and when should you not?

Mechanical cooling is the preferred technology in four situations. First: if the desired cooling temperature is below +10 °C, as adiabatic cooling cannot reach that range. Second, for cooling capacities above 10 kW, as smaller systems are served by refrigerators or thermoelectric cooling. Third, when continuous cooling is required, regardless of outdoor temperature and humidity. And finally, for cold storage and process cooling, where space heating and comfort air conditioning are not a consideration.

In specific situations, other technologies are a better fit.

Adiabatic cooling works efficiently if the desired cooling medium can remain above +15 °C. In dry climates, the COP of an adiabatic system is significantly higher than that of a mechanical installation. No refrigerant is needed, CAPEX is lower, and maintenance costs are smaller. The limitation lies in its dependence on outdoor temperature and humidity: at high relative humidity, the system loses effectiveness.

Absorption cooling is interesting if a substantial supply of waste heat is available, such as steam or flue gas. The COP of absorption cooling is lower than mechanical cooling, but the electricity costs for the chiller itself are negligible. For industries with high process temperatures and inexpensive waste heat, this can present a good business case.

Free cooling is not an alternative to mechanical cooling, but a supplement. At low outdoor temperatures, outside air or water is directly used as a cooling medium. In temperate climates like the Netherlands, this results in 30 to 40% energy savings for the total cooling installation. Nijssen integrates free cooling as standard in new designs where the temperature range and application allow it.

Mechanical cooling is the most versatile and proven cooling principle for industrial use. It works at any outdoor temperature, across a wide capacity range, and at temperatures that no alternative cooling technology can achieve. The choice lies in the implementation: refrigerant, compressor type, cooling capacity, and integration with heat recovery or free cooling. Nijssen designs and builds custom cooling systems for food, industry, and climate simulation, with expertise in EN 378, the F-Gas Regulation, and the most efficient refrigerants for each application area.

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