Blast Freezers: Frozen Solid
July 01, 2009
by Shane Whitaker
Whether you sell your baked foods frozen or fresh, blast freezers can be an excellent way to extend the shelf life of your products. Freezing products also can make them easier to handle, and it preserves their flavor, texture and nutritional attributes well beyond that of fresh baked foods.
Bakeries generally use spiral conveyors in large insulated rooms to automate the process of lowering temperatures of products below the freezing point; however, companies also may choose to use blast cells or inline conveyor belt freezers. Freezers feature either mechanical or cryogenic freezing systems.
Mechanical freezers consist of compressors, condensers with internally recirulated refrigerants, heat exchangers (coils) and fans. Refrigerants such as Freon, Freon replacements and ammonia are commonly used in the condensers (See “Freon or Not to Freon” on Page 68). Cryogenic freezing, on the other hand, employs liquefied nitrogen or carbon dioxide that is sprayed directly onto the product and into the product areas to affect heat transfer. Cryogenic systems are generally used for products that must shed their sensible and latent heat as quickly as possible to maintain product and structural integrity.
Although mechanical freezers typically require a greater upfront cost and need greater space than cryogenic freezers, they are more widely used because the cost of gases used in cryogenic freezing can be prohibitive to large bakery operations. Cryogenic freezers, however, can work as much as 10 to 30 times as fast as mechanical blast freezers. Nevertheless, manufacturers of mechanical blast freezers have increased production cycles of their freezers to assist bakeries.
One of the main issues bakeries encounter with mechanical blast freezers is the need to defrost the refrigeration coils. Because coils are the coldest material within the blast freezer, moisture is attracted to the coil fins and tubes, according to Peter White, president of I.J. White Systems, Farmingdale, NY.
This moisture forms into starfrost and ice on the coil surfaces and adversely affects heat transfer. This ice buildup causes the blast freezer to run less efficiently to the point where the blast freezer cannot hold air temperature, and product discharges not cold enough.
The production line must stop the blast freezer to defrost, and this defrost cycle takes approximaltey an hour and can occur as frequently as every eight hours. This translates into losing three hours of production every day and 18 hours during a 6-day workweek, according to Mr. White. I.J. White recently introduced a Continuous Production System (CPS) that incorporates Ultra Series Thermal-Pak coils, which are unique independent evaporator coils. These coils offer optimal heat transfer at low operating costs. With CPS, the coils are engineered to defrost sequentially around the clock, so there is no need to stop production for defrost, according to Mr. White. A bakery running a blast freezer with CPS is able to run 24/7 for weeks at a time without losing production time.
The company also offers an Extended Production System (EPS), which is designed to control moist air that infiltrates blast freezing systems. Blast freezers with EPS are capable of running for three to four days and sometimes longer between defrost cycles, according to Mr. White.
A blast freezer with EPS uses I.J. White’s Automated Pressurization Systems (APS) to balance the pressure differential at the infeed and discharge openings of blast freezers . By balancing the pressure at these areas, APS allows less moist air to enter the blast freezer, and it is this moisture in the air that contributes to the frost buildup on blast freezers’ refrigeration coils.
In one plant that I.J. White installed, APS increased production efficiency by 32%. The low infeed opening continually had cold air spilling onto the plant floor making it difficult for the employees to work in the area, and the opening and spiral framework were also continually building up with ice and snow. When the snow melts, the water can drip down onto the product, he pointed out. APS solved all of these problems for the bakery, according to Mr. White.
He pointed out that existing blast freezers can be retrofitted with APS, a computer-controlled and -modulated system that lengthens the time needed between defrosts. Plants will save energy and increase production time, Mr. White said.
Mechanical blast freezers generally operate with temperatures between 0° to -40°F, depending on what the baker wants to do with their product, according to Guy Irwin, vice-president, engineering, G&F Systems, Roosevelt, NY. Some bakers only want to crust freeze the product so they can handle it during bulk packaging, so they don’t need -40°F air.“The colder the air, the more expensive the process because you have to use more horsepower to compress the gas to get that cool,” he explained.
The control technologies used on freezers are also continuing to evolve, according to Mr. Irwin. Bakeries nowadays are installing computer-based control technologies, as opposed to relay-based control systems. The newer technologies allow companies to have greater process control. “Basically, they make the equipment more efficient to operate,” he said. “They have diagnostics attached to them so you can track what is going on and the freezing curve of product as well as maintenance intervals.”
The liquefied gases used in cryogenic systems are known as cryogens, and their boiling points are so low — liquid nitrogen evaporates at -321°F and carbon dioxide’s boiling point is -109°F — that it can result in greater quality product retention by minimizing starch retrogradation and minimizing ice crystal sizes within the dough or crumb structure. Because of this, cryogenic freezing is often used for high-end baked foods.
Linde, Inc., Murray Hill, NJ, sells the equipment and gases for cryogenic systems. Linde is a relatively new name in the North American market, having merged with The BOC Group, several years back.
In October 2007, Linde introduced a new line in the US, known as Cryoline, and these systems offer improved sanitation and controls the cryogenic freezing, according to the company. The tunnel freezers feature airflow patterns that make them up to 40% more efficient than comparable cryogenic systems.
Linde has concentrated on easy-to-clean freezers, without any flat surfaces internally or externally for water to pool during sanitation. Cryoline freezers also open completely for internal access during sanitation.
I.J. White’s Ultra Series blast freezers are also available with Total Cleaning Systems to rinse, foam and wash the blast freezers from top to bottom. This automates the task of sanitation and greatly reduces the labor and man hours needed to clean a blast freezer, according to Mr. White.
New freezers designs should assist bakeries in increasing production runs, as defrosting coils could become a thing of the past.