Complete guide to defrosting in industrial evaporators: analysis, energy efficiency and defrost systems
In industrial refrigeration systems, frost formation on evaporators is a direct consequence of the cooling process, especially in low-temperature (LT) applications typical of food logistics, the storage of frozen products and sub-zero industrial processes.
The very mechanism that removes heat from the environment can, over time, become the main driver of efficiency loss. As ice progressively builds up on the finned coil, the evaporator’s heat-exchange capacity falls, the airflow drops and the refrigeration system is forced to work in increasingly unfavourable thermodynamic conditions.
It is not just about restoring the heat exchanger’s performance, but about properly managing one of the most significant items of industrial refrigeration OPEX.
The physics of frost: how it forms and why it is critical
Frost formation is a phenomenon that simultaneously involves heat transfer and mass transfer. When humid air passes through the evaporator coil, water vapour condenses on the metal surfaces and, if the temperature is below 0 °C, solidifies into ice.
The rate of formation depends on three factors: the relative humidity of the air (influenced, for example, by the stored product, frequent door openings and cleaning processes), the difference between air temperature and evaporating temperature (ΔT) and the fin surface temperature.
The reason frost is critical in industrial evaporators lies in its low thermal conductivity compared with the metals used in heat exchangers. While for copper it is about 380 W/mK and for aluminium 200 W/mK, for frost/ice it is only 2 W/mK.
This means that even a relatively thin layer of ice can create significant additional thermal resistance, triggering a decline in the evaporator’s performance.
- Reduced overall heat-transfer coefficient (U): the ice layer acts as a barrier that slows heat transfer between air and refrigerant.
- Air-side pressure drop: as the gaps between the fins narrow, aerodynamic resistance rises and the fans’ airflow falls, causing higher outlet temperature and a higher operating ΔT of the evaporator.
- The system responds by lowering the evaporating temperature to maintain the required thermal load. This leads to a higher compression ratio and a lower COP (Coefficient of Performance) of the plant.
- In extreme conditions, uneven refrigerant distribution can lead to the risk of liquid return to the compressor, with consequences for system reliability.
Frost prevention: operational strategies
The strategic role of fin spacing
One of the most important design parameters for managing frost is fin spacing, i.e. the distance between the fins of the exchange coil.
A narrow spacing, typically between 4 and 6 mm, allows more compact machines for the same rated evaporator capacity. However, this configuration is extremely sensitive to ice build-up, since small frost thicknesses can quickly reduce the available airflow. For this reason, narrow spacings are better suited to air-conditioning or medium- and high-temperature refrigeration, where the frost risk is limited or nil.
In low-temperature applications, wider spacings are preferred instead, generally between 10 and 12 mm. Here, the greater space between the fins allows frost to accumulate for longer without immediately compromising the airflow.
This configuration works like a real “buffer reservoir”, allowing the evaporator to maintain acceptable performance for more operating hours.
More advanced solutions also feature differentiated fin spacing, designed to even out ice build-up between the air inlet and outlet zones.
Digitalisation and intelligent defrost control
Traditionally, defrost cycles were triggered by time-based logic: for example every 6 or 8 hours of operation. This approach does not account for the evaporator’s real conditions and can generate inefficient defrost cycles, wasting energy and destabilising the cold-room microclimate.
Modern systems instead adopt on-demand logic, based on monitoring operating conditions. Among the most common methods:
- air-side differential-pressure (ΔP) sensors
- monitoring of the ΔT between air and refrigerant
- analysis of fan running times
Integration with remote monitoring systems also makes it possible to use predictive algorithms to anticipate frost formation, correlating defrost cycles with operational events such as door openings or thermal-load variations.
Defrost systems compared
When ice build-up reaches critical levels, a defrost cycle must be activated to restore the evaporator’s exchange capacity. Several technologies exist, each with advantages and limits.
Electric defrost
This is one of the most widespread systems. Armoured heating elements are installed inside the coil and activated periodically to melt the ice.
The main advantage is construction simplicity. However, this method entails significant direct energy consumption and introduces heat into the cold room, temporarily increasing the plant’s thermal load.
Hot-gas defrost
This system uses the compressor’s discharge gas, diverted into the evaporator to heat the coil.
The main advantage is the use of energy already present in the refrigeration cycle, reducing the additional energy requirement. However, the design requires particular care to avoid flow-control problems and possible water hammer in the circuit.
Glycol or hot-water defrost
In some configurations a secondary circuit is used to circulate hot fluid inside the heat exchanger.
This solution offers precise control of the defrost temperature, but requires a more complex plant infrastructure.
Air defrost
It is possible only when the air temperature in the cold room is above about 2 °C. In these conditions it is enough to temporarily stop the refrigeration cycle, run the fans and use the ambient air to melt the frost.
Managing frost, then, is not just a technical matter: it has a direct impact on the plant’s economics, on the thermal and mechanical stresses on its components and on the quality of the stored product.
For this reason, in industrial refrigeration design, frost must be considered not as an occasional event but as a structural variable of the plant’s Life Cycle Cost.
At Refteco we analyse the plant’s real operating conditions to design industrial unit coolers optimised for frost management and Life Cycle Cost reduction.
If you are designing a new refrigeration plant or considering the revamping of an existing system, share your operating data and running conditions with us: correct evaporator design can significantly affect energy efficiency, operational continuity and plant lifespan.
REFrigeration TEchnological COmponents
For more information