It is my firm belief that airflow and air distribution is every bit as important in refrigeration design as load calculations. What good is a properly sized system that can’t deliver the air to areas that need it? Poor air distribution and airflow lead to stratification in temperature and humidity as well as poor heat transfer.
For those of you who may not know, air distribution in refrigeration is quite a bit different from air distribution in HVAC. The old rule of thumb in HVAC is 400 cfm/ton. In refrigeration we typically deliver well over 2,000 cfm/ton. Where in HVAC we are distributing air through a system of ducts, in refrigeration we are blowing air freely, in most instances, as hard as we can down aisles in cold storage facilities. In refrigeration, selection and placement of the correct equipment is critical. (It’s critical in HVAC too, but I will save that for an upcoming article.)
Evaporators need to be sized and placed for both air turns and air distribution. I typically try to shoot for 20+ air turns in an hour in cold storage coolers/freezers. To calculate air turns we need room volume and total cfm. Most manufacturers publish their evaporator cfm. If the space square footage and ceiling height are known, we can calculate room volume. From there, given evaporator cfm and room volume, we can calculate air turns in an hour. In a process facility where people are present, I still like to see several air turns but have no problem in selecting low velocity evaporators which will produce in the area of 15 air turns/hour.
As far as air distribution is concerned, we must ensure that we mitigate dead air for maximum air mixing and heat transfer and minimum stratification. Typical cold storage facilities have long aisles. It is critical to check the air-throw distance of an evaporator to ensure that air can be thrown the entire length of the aisle. If need be, most manufacturers offer long throw adapters to increase the length of air-throw for a given evaporator.
In process facilities it is a little different. Typically, low velocity evaporators are used in these spaces in an effort to reduce the wind chill factor to improve comfort level for the employees working in the space. Most manufacturers will publish a radius of air throw or a side-to-side air throw distance for their low velocity evaporator offerings. At this point it becomes critical to select the right amount and place the evaporators in locations which will minimize any dead air locations. In pull-down with blast coolers or freezers all bets are off. The name of the game here is maximizing convective heat transfer. I will take as much airflow as possible. In many instances our cfm is only limited by the size of the blast cell. Blast cells are typically small in cubic volume. Evaporators in these applications tend to take up a lot of room so proper sizing of the blast cell itself is critical. I have designed systems in which we are turning air several times a minute with these applications.
As a side note for blast freezer and blast cooler air distribution, the airflow won’t mean a thing if the product is already packaged and in bulk. Best practices for blast cells where we are trying to get as much heat out of the product as quickly as possible requires that the product be spaced in a manner in which air can fully envelop the individual items to be cooled or frozen, unhindered by the insulating effects of packaging. Packaging is available with holes pre-cut, but even this hinders the convective heat transfer which is typically required to cool the product in the time desired.
One last thought that any designer should pay attention to is that sometimes in blast applications, the air velocity is high enough to blow the product off the trays or conveyor. I always try to hit the product on the return side of the airflow, keeping the high velocity, more turbulent air high, turning the air down, and then pulling the return air through the product.
Publication date: 3/21/2016