Defrost cycles in cold stores

The basic problem is not a new one. Applications that have evaporation temperatures of 0°C or below regularly experience ice build-up on the surface of the air cooler’s fins. The heat transfer is thus reduced, resulting in a decrease of the air cooler’s efficiency. The build-up of ice is caused by the dehumidification of the refrigerator room, when the air cooler draws the moisture from the air and the goods stored in the room, which in turn settles as ice on the outer surface of the air cooler. How quickly the ice builds up depends on several different factors: The stored goods and the level of humidity, the frequency of entries as well as the general humidity in the surrounding air at the respective location.

Different methods of defrosting exist in order to counteract this negative effect of ice during the heat transfer. However, regardless of whether you use air circulation, electric heating or gas – the defrosting of the air cooler requires extra energy and therefore negatively impacts the efficiency of the entire system.

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This is why the following rule generally applies to defrosting: As often and as long as needed – but as seldom and short in duration as possible.

However, how can you ensure the optimum duration and frequency of defrosting in systems that are subject to constantly changing circumstances and parameters due to the variables of stored goods and frequencies of entries?

The solution: The HBDF sensor from HB Products will detect the ice thickness and supply an analogue output signal 4…12 (20) mA. The 4 mA signal is emitted when no ice is on the evaporator. At a 12 mA output signal, the ice thickness will be 1…1.5 mm and defrost is required. The defrost time is controlled by the sensor’s defrost timer, which can be set in the PC tool.  

HBDF de frost sensor (Large)

The product advantages:

  • Provides energy savings of up to 10 %, compared with conventional defrostning.
  • Reduces downtime and minimises system defrost time.
  • Split design, which makes the sensor easy to install and service. The electronic part can be separated from the mechanical part.
  • Simple installation. Can be carried out in an hour without system shutdown.
  • Can be installed on all evaporator types from all manufactures.

 

Calculation example:

Below we present an example of the benefits of our sensor for increasing energy efficiency when the evaporators operate without ice and up to 1.5mm ice thickness.

Our example is based on the use of a 220kW cold storage chamber, cooled by 4 NH3 Güntner evaporators, fed by a SES-Colfax compressor unit with a Howden compressor. Estimated operating range: -32º C-+35º C.

Flooded evaporator model: AGHN 080.2F/310-HND/8P.E; capacity: 55 kW/unit

Ice thickness (mm): Evaporation temperature (º C): Compressor evaporation capacity (kW): Power input (kW): COP COP decrease (%)
0 -32 262 143.5 1,826 -
0.5 -32.4 254 142.5 1,782 2.41
1 -32.8 249 141.9 1,754 3.94
1.5 -33.0 246 141.5 1,738 4.82

This table illustrates power requirements and COP decrease resulting from intake temperature variation induced by ice.

Read more about the defrost sensor at http://www.hbproducts.dk/en/products/de-frost-of-evaporator/de-frost-sensor