Experimental study on the feasibility of falling film absorbers for dehumidification in cold stores

Frost formation on air coolers in cold stores significantly reduces the energy efficiency of the refrigeration process. It blocks the airflow, insulates the air cooler surface, and ultimately transfers heat to the cold store due to the required defrosting cycles. According to literature, frost prevention can save more than 15 % of the required energy for refrigeration.
This work presents an experimental investigation of a novel approach to prevent frost formation on the air coolers in cold stores. The proposed solution uses a liquid desiccant falling film absorber to dehumidify the air. By this, the dew point temperature of the air upstream of the air cooler is lowered below the surface temperature of the cooler. As a result, no water is condensing on its surface and thereby no frost is formed.
Current liquid desiccants, such as LiCl and LiBr, have the problem of crystallization at low temperatures and high concentrations. They also cause corrosion when used with metals. To avoid these problems, an ionic liquid is used as the desiccant. By using the waste heat from the refrigeration system to regenerate the desiccant, no additional heat source is required.
To demonstrate the feasibility of this approach, two finned heat exchangers were transformed into falling film absorbers using selfmade 3D printed parts. A test rig was constructed, capable of conditioning the air, testing the advantages and limitations of this method and evaluating the effectiveness of the liquid desiccant absorption and regeneration. The results show the viability of the falling film absorber, where it was possible to reduce the dew point of incoming air at 7 °C and 80 % relative humidity to -2.5 °C. The system showed that the desiccant can still form a falling film at around 0 °C and that carry over of desiccant into the air is not an issue at air speeds up to 2 m/s. The regeneration could be achieved at air temperatures of 30 °C with relative humidity of 40 % while the desiccant was heated to 42 °C. The results of this experimental investigation show that it is possible for the liquid desiccant falling film absorber to prevent frost formation on the air coolers, reducing the need for defrosting.