Forced Air Pre-cooling is being used more and more, especially with concerns about phyto-sanitary conditions of water-based pre-cooling methods (hydro-cooling and vacuum-cooling.)
Where do we start with design of forced-air pre-cooling?
How much air do we need? At what static pressure? How wide should the center tunnel be? Do we need a center tunnel? Do we automatically turn off the fans and cooling when we achieve desired temperature? Should we reverse airflow at 3/4 cooling? Are tarp systems the best? Or the MACS automatic system? ($100,000 versus $900,000!)
And how do we measure those variables, and where, and how often, and how does that impact the control system? Just for starters…
Let’s take this one step at a time….
Airflow – otherwise known as CFM (cubic feet per minute, or volume). This can be extremely variable from design to design, but as a rule of thumb, we use 2,000 CFM per pallet. This results in airflow per pound, of 1 CFM for heavy commodities such as bananas and grapes, and 2.5 CFM per pound for more delicate items. such as berries. We can provide fans delivering 3,000 CFM per pallet, which is 1.5 CFM per pound for heavy items, and almost 4 CFM per pound for berries and leafy vegetables.
Static Pressure – this is the “Holy Grail” of pre-cooling, what will be the static pressure (SP), in inches of water column (or pascals or millimeters) ? Our pre-cooling systems all have a minimum SP of 1.1″ SP. and options are available up to 3″ SP.
SP influences fan motor HP, the higher the SP, the greater the HP fan motors required. Larger diameter fans normally require less BHP input, saving on electrical costs to operate.
Benefits of reversing airflow pre-coolers. Joint research conducted with a leading AG.Edu, concluded that pre-cooling time could be reduced by 17% by reversing airflow at 3/4 cooling time, thus cooling the warmest fruit on each pallet faster, than one-way airflow could.
Vertical airfow pre-coolers are the best way to do this. They can also be installed outdoors, feeding into a cold storage room, with loading from one end, and unloading from the other, so they are a “Zero Footprint” design.
Interestingly, pre-cooling in 17% less time, not only improves the quality and shelf-life of the produce, it also reduces the capital cost per unit throughput of pre-cooling itself
