Install Baffles and Load Freezer to Achieve Uniform Airflow. Builders and operators sometimes forget -- or ignore -- the design lessons learned over the years. Many operations need at least some tuning to prevent the extended operating time needed to ensure that the last package is frozen. A shorter cycle saves energy.
Control Fan Speed. Installing a variable-frequency drive (VFD) can significantly reduce electric power driving the fan with minor effects on freezing time. This saves refrigeration power as well because fan heat can represent 25 percent to 30 percent of the total heat load.
Food products in stationary batch freezers typically have freezing times of several hours. Partway into the cycle, when surface layers are frozen, a diminishing heat flow rate is increasingly controlled by the internal conduction resistance; that is, the Biot number becomes large. As this occurs, you can reduce fan speed and, therefore, air velocity with only a minor effect on freezing time. Air velocity will vary directly with fan speed, but power driving the fan varies with the cube of the fan speed. Cutting speed to one half will reduce power to one eighth. So a minor slowing of the fans will lead to a major decrease in fan power and resulting heat.
Using that performance as a baseline, table 1 shows the effects of several fan reduction trials. The best fan slow-down schedule reduced fan energy by 44 percent and total energy by 11 percent while freezing time increased by 8 percent. With an energy cost of $0.10/kW-hr, the $9,000 VFD installation would be paid back after about 850 freezing cycles.
This and other design and demonstration projects have helped to define steps that processors can follow to evaluate whether energy savings can be achieved by changing their existing blast freezer configuration.
Step 1: Evaluate the Current Freezer Airflow. Test and adjust uniformity of airflow through the pack. Velocities can be used to evaluate flow uniformity or bypass, and they can be used in freezing time calculators. Although extreme turbulence makes very accurate measurements difficult, use of an inexpensive hand-held wind velocity meter gave adequate readings (figure 2). By fastening to a wand and attaching a piece of cassette tape to indicate flow direction, an operator can manually record velocities inside the room. Because velocity will not vary significantly with temperature, measuring prior to refrigerating will give good results in a more comfortable environment.
For the blast cell described in this article, initial flow measurements showed about 15 percent of the air bypassing over the top and 50 percent around the ends; velocities through the pack ranged from 300 to 1,000 ft/min. Baffling eliminated bypass air. Velocities through the pack became more uniform (600 to 800 ft/min) and averaged around 700 ft/min.
One result of baffling is that it will decrease the total flow rate as the systems curve shifts up, with a possible decrease in fan horsepower (figure 3). A product change or a partial loading can affect flow uniformity and throw off performance. In that case, the production manager must optimize performance with a change in product distribution and baffling. And, even when flow is uniform, product freeze-times at the exit end of the pack will be longer than those at the front because the air temperature increases as it flows through the pack. Experienced designers have recommended a flow length of no more than three pallets or racks.
Knowing air velocities and freezing times enables the use of freezing time calculators to approximate the effects of other freezing scenarios -- for example, lower air velocities, thinner boxes and higher air temperatures. Links to two web sites describing such calculators are located at the bottom of this page.
Step 3: Install the Variable Frequency Drives. Install the VFD in the circuit driving the fans; in our project, there were three parallel axial flow fans. Sizing is best done by first measuring full-load amps, then selecting a drive based on that rating plus some safety factor. (One expert recommended 20 percent, but work with your drive and fan suppliers to determine the best safety factor for your freezer application.) Remember that for motors cooled in the blast airstream, power draw will likely be far higher than the name-plate rating with service factor. The three 7.5 hp fans in our project actually drew 30 hp in the cold, dense air of operation.
Another approach to saving energy without using a VFD was shown in an experiment where the center fan was shut down after an initial period of freezing. We had first determined that a lower but uniform airflow would result if only the two outer fans were operated. (Note that this caused some reverse rotation of the idle center fan.) When shutdown occurred after 3.75 hr, the maximum freeze time increased by 13 percent while total energy use decreased by 8 percent.
Our trials looked at three schedules, with two runs for each; fans were slowed in steps (table 1). Fan energy is an integration of measured electrical power (kW) multiplied by time steps. You can approximate the refrigeration energy required to remove fan heat by using an effective system coefficient of performance (COP). So, the total electrical energy associated with fans is approximated by
(Energy Consumed) = (Fan Energy)(1 + [1/COP])
Design models developed during the demonstration project suggest further savings that could result from other scenarios. In one simulation, the fan was slowed continuously as the heat load to the evaporator decreased. When the speed was decreased to maintain a constant 4oF air temperature change across the evaporator coils, the theoretical increase in freezing time was just 2 percent, while total energy saved remained at 11 percent. A second simulation showed that when fan direction is periodically reversed, freezing uniformity improves, resulting in shorter cycles and saved energy. Such a scenario would require use of modified fan blades.
Using a VFD to control fan speed can save energy while increasing the freezing time marginally. The ability to slow fans can bring other benefits as well, including a programmed low-speed startup to minimize initial air pressure loads on the room or to limit initial refrigeration demand. Other potential benefits include the ability to reduce air speed and noise when workers need to be in the room. And in the event that unloading must be delayed, reduced fan speed saves energy and decreases moisture loss from unpackaged products.
AcknowledgementsThis article was partially funded by the NOAA Office of Sea Grant and Extramural Programs, U.S. Department of Commerce, under grant number NA16RG103 (project number R/SF-30-NSI-TEC), and by appropriations made by the Oregon State legislature. The views expressed do not necessarily reflect the views of any of those organizations.
The authors also want to thank the following for additional support: Columbia Colstor Inc., Woodland, Wash.; Cowlitz PUD, Longview, Wash.; and Pacific Seafood, Woodland, Wash.