A blast chiller or bust freezer is a chamber for foodstuffs in which the cooking or storage temperature is lowered to freezing temperatures in a very short time, allowing the foodstuffs to retain their organoleptic properties in an optimal way and ensuring food quality.
Unlike other refrigeration units, the blast chiller achieves the cooling objective by means of a very high rotation of cold air, which prevents bacterial growth and the proliferation of microorganisms, as well as avoids the formation of crystals on the surface of the food.
Evaporator for a blast chiller
The evaporator or heat exchanger is the key part of a blast chiller. Its function, as in any other evaporator coil, is to absorb the hot air by means of a coil containing a refrigerant gas that evaporates below the chamber temperature.
But unlike other applications, in the case of a blast freezer it is to take the function of an evaporator to the extreme, it is to lower the air temperature at a very fast rate with high air renewals in the chamber and with a high-temperature differential between the inlet and outlet.
To achieve this, it is necessary above all to increase the ventilation and the ratio between the volume of the chamber and the size of the coil and the exchange surface. This is a proportionally much higher ratio than other ventilated evaporators such as those used for storage, e.g., under counter or supermarket refrigerated display cabinets.
Specifications for a blast chiller evaporator
To explain the specifications that an evaporator for a blast chiller must have, let’s use as an example a 27 m3 chamber that we need to work at -18°. We use R744 (CO2) as refrigerant gas, with an evaporation temperature of -25° and a cooling capacity of 34 kW. At Sereva, we would use our TD-110X model.
It is a battery of 288 tubes, placed with a geometry that 32 tubes would form the height of the battery and 9 tubes the width; a finned surface of 213 m2, and an internal volume for gas of 95.5 litres. The dimensions are 1,880 mm long, 2,100 mm high, and 710 mm deep.
The ventilation train is one of the most important evaporator components, as the operation of a blast chiller consists of a high rotation of cold air. In the case of the TD-110X model, we installed 3 fans of 450 mm diameter, AC type with a power of 1.5 kW, generating an airflow of 21,000 m3/h and circulating at a speed of 2.46 m/s.
With these air data and the volume of the chamber (27 m3), we achieve 777 very cold air renewals in one hour.
How to use a blast chiller?
A blast chiller user must take into account two essential aspects when placing the product to be quick-frozen or refrigerated:
- For maximum efficiency, it is important to fill the chamber or arrange the products in such a way that air is forced to pass through them. If we cannot fill it in such a way as to achieve a “screen” for the passage of air, we must cover the free areas to prevent the air from circulating freely and going through the easiest way. In other words, we must prevent air from recirculating without passing through the product.
The space between products must be sufficient to allow air to circulate between them and cool them as evenly as possible. However, the space between products should not be too large because this would cause the bypass effect and a large part of the air would pass through the chamber without practically coming into contact with the product.
- If the installer or manufacturer has installed a product temperature probe, its use is recommended to achieve cooling without freezing