A novel method for frost detection

Frost growth on the evaporator coils of a refrigeration plant can dramatically reduce its efficiency. A novel indirect detection method that would avoid frost built-up has been developed.

In a refrigerator, the evaporator's function is to produce cold by transferring heat from the food being stored in the cold compartment into the circulating refrigerant. Frost stacking on the evaporator’s coils can dramatically reduce the efficiency of refrigeration systems. This results in higher energy consumption since the heat transfer between the air flow and the evaporator is reduced by the frost stacked. Hence, unless defrost processes are performed periodically, the evaporator cannot be efficient. It could even stop working.

As reminded by the authors of a recent article (1) in the International Journal of Refrigeration (IJR), it has been demonstrated that defrost process can consume up to 25% of the total energy demand of a refrigerator with an electric heater as defrost system. Unlike other defrost processes which are launched at pre-set times – which can lead to unnecessary defrosting operations – launching defrost process only when frost is accumulated on the evaporator can significantly lower energy consumption.

Thus, many studies focus on frost detection and estimation of frost level. However, measuring frost layer thickness through direct methods is generally expensive and not practically feasible.

Nevertheless, a novel indirect frost detection method developed and experimentally tested by the authors of this IJR article (1) could answer this problem. This new technology can be applied to both walk-in freezers run by remote condensing units and multiple cold rooms fed by a rack of compressors. It allows to accurately quantify the frost level regarding the initial dry state. Two set-ups are used to carry the study: a walk-in freezer unit whose refrigerant is R404a and a refrigerated multi-chamber set-up, run by a multi-compressor rack and using a R134a-CO2 cascade system.

A new parameter called Thermal Variation Easiness (TVE) is introduced to accurately detect and quantify the frost accumulation on the evaporator. When the frost layer grows, the evaporator and frost total mass (Mevap) increases and the overall heat transfer coefficient (U) decreases, hence TVE value decreases.

The results of the study provide evidence that the presented method works properly in a quantitative way to monitor the level of frost stuck on the evaporator when working on multiple cold rooms run by a rack of compressors.

This accurate frost detection method provides energy savings by reducing the amount of defrosting cycles and avoid using extra energy to cool down the cold room after each defrost. It also prevents any possible evaporator blockage due to an excessive level of frost.

(1) J. M. Maldonado et al; Frost detection method on evaporator in vapour compression systems. Available in FRIDOC database (free for IIR members)