Computational Fluid Dynamics (CFD) of

Refrigeration Cooling Room.

Posted by george vakfaris on January 21, 2018

This paper presents the computational fluid dynamics (CFD) analysis of airflow, inside a Cold Storage Room. It contains frozen fruits in cartons, deployed into pallets with an approximately height of 5meters. The cold air is delivered by roof mounted cooling units, who distribute cold air to maintain fruits temperature below -18oC.

This study focuses into the air flow between the pallets gaps. Its purpose is to examine and investigate various techniques to achieve uniform air circulation between them. Therefore, the main characteristic to be measured and observed is the air flow velocity U(m/s). There are other parameters also important to the cold storage design such as air temperature, pressure and humidity, but these are not subject to the specific study.

Initially, we examine the airflow circulation into a conventional storage. The cold air is recirculated through the cooling unit and distributed as shown below. Air circulation inside the cold storage room. Velocity diagram, U(m/s).

We can observe that the air flows tangentially to the roof, while the distribution among the pallets is negligible. This can cause adulteration because many cartons receive hotter air than others, even higher than -18oC. Further, in some lower spots, the air is either not or minimum recirculated.

In this installation the air travels to the top, up to the rear wall and then slips down, where it spreads inside the storage.

An alternative solution, examined in this article is to install flaps on the top, with their length being increased in each row. They are placed over the gaps between the pallets. Their purpose is to change the direction of the air and drive it among them. They change partially air-volume direction, while the rest of it continues laminar flow to the next flap. The design and air flow results are shown below.

Air re-circulation with roof mounted flaps. Velocity Diagram, U(m/s).
Air flow streamline at the flaps (laminar flow).

The following image shows a top-view of the air trace. It is obvious that the flaps, don't only direct the air among the pallets vertically, but also spread it to the right and to the left as well. This is important since the cooling units are placed either in the middle or in case of multiple, symmetrical to the midplane. So, they mainly distribute the air into the front of their fans.

Top view. Air distribution to the side walls.

Conclusions

We simulated a technique to improve the air recirculation inside a cold storage room. The scope was to increase the distribution between the pallets, so the cold air would freeze equally all sides. With this type of distribution we avoid to leave some of the cartons less cool than others and even some of them to reach temperatures higher than -18oC.

What is traditionally met in similar installations, is that the users set the Thermostat at 2 or 3 degrees less than required, in order to ensure that all cartons will receive cold air, below required set point. So, they spend more energy to achieve this result, while the flaps can deliver colder air into those spots.

What is important to mention is the length of the flaps. They are smaller at the front flaps, so they can allow most of the air to continue laminar flow to the next one. Therefore, their length increases in each row. We must pay attention when designing them, not to decrease enough the gap between them and the top of the pallets because it will cause turbulances which are not desirable. For the same reason, the installation angle should not excheed the 45deg - although could vary among rows.

The material for these flaps is Stainless Steel, AISI 304 and the thickness is considered to be 2.5mm. They are placed in fixed positions, since the cooling units operate in single speed in almost all cold storages.


Text and images by george vakfaris.