We All Scream for Ice Cream
We are all familiar with ice cream; it is a welcome treat on a hot day. But the creamy, satisfying, mouthfeel of ice creams, sorbets, and gelatos are all made possible by careful combination of ice, flavorings, butterfat, and air. That’s right, tiny pockets of air in the mix allow all the frozen water, flavors, and fats to separate and coat your mouth with deliciousness.
In my region of North Carolina, there are quite a few people who pride themselves on their homemade ice cream. The classic image of a small container, rotating in a bucket of ice/salt brine, everyone taking turns with the hand-crank is part of many childhood memories (image courtesy of Pinterest).
The principal of these hand-cranked ice cream makers is simple. Use ice melted by salt around the rotating container to make a brine that is colder than the freezing point of water, use the stationary paddle inside the container to scrape the walls so that you don’t freeze the concoction into a block, and cool the mix until it thickens and emulsifies into your favorite treat. As time moved on, so too did the home ice cream maker. Newer designs look more like this (image courtesy of Whynter).
In this design, the inside container rotates in a refrigerated chamber and the internal container is driven by an electric motor; otherwise, the process is the same. Automating the process makes it far less labor intensive and therefore, any belief that the effort to produce your treat offsets the calories is laughable.
The packaged ice cream you find in your grocery store or even at your local ice cream shops is made in a barrel freezing machine that looks similar to this (image courtesy of Tetrapak).
A modern barrel chiller (or scraped surface heat exchanger) is arranged in a horizontal manner. A blend of sugars, flavors, dairy products, and possibly (gasp) fruit called the mix is pumped to the freezing barrel. The outside of the barrel is cooled in a bath of freezing-cold refrigerant. Inside the barrel sets of rotating blades scape the ice crystals formed on the barrel interior and feed the slush back to the center of the barrel chamber. Compressors feed air into the mix with precise timing and controlled flow to give just the right texture.
The rotating element that carries the scraping blades is known as the dasher. It has holes within it that allow the mix to flow into its interior and outward to the barrel wall. This provides agitation to the mix and aids in the emulsification of the products. A video of this product flow is seen here (courtesy of Tetrapak).
Commercial food processors use centralized refrigeration systems that support many different users. See the diagram below. Before each of the compressors is a liquid separator. These have been excluded for clarity.
In the cooling cycle, low pressure refrigerant vapor is compressed and directed to the condenser as high-pressure vapor. In the condenser, heat is removed from the refrigerant and a mixture of high-pressure liquid and gas is sent to the liquid separator. The liquid refrigerant is then supplied to the various users. By controlling the backpressures at the evaporator coils, these systems can provide a wide variety of temperatures for food processors.
In most cases, ammonia is used as a refrigerant because of its excellent thermodynamic properties. Ammonia has high heat transfer coefficients in equipment and allows for efficient compressor operation. Ammonia is inexpensive, does not deplete ozone, is sustainable and provides self-alarming in the event of a leak. (i.e. its odor is obvious before harmful quantities are released).
However, ammonia absorbs into water with great speed and creates caustic solutions. It corrodes copper and copper alloys and is very corrosive to human tissue on exposure. For these reasons it is not found in most consumer refrigeration products.
Issues with Ice Cream Production
Like most mechanical equipment, barrel chillers are susceptible to fracture in driveline components. In the event of a fracture, metal debris may contaminate foodstuffs and locating the debris in containers may be troublesome. Another common malfunction is the loss of control of refrigerant by improper setting or mis-operation of backpressure valves, leading to excessive cooling and freeze-up of chillers or refrigerated storage areas.
Compressor malfunctions, ammonia releases and loss of cooling control can lead to large losses where product temperature is crucial. Ammonia refrigeration systems are robust, well-established technologies, but accidents involving these systems occur and the damage and injuries from large releases of ammonia can be severe.
EDT can help you with food processing losses, isolation of contaminated product, root cause determinations, subrogation, and product liability defense along with many other consulting services.
About the Author
Rod Turk, P.E. is a Consulting Engineer in our Charlotte, NC Office. Mr. Turk provides consultation in the areas of assessment of industrial and utility incidents. These assignments include damage assessment, root cause investigation, valuation of loss, and project management for repairs. You may contact him for your forensic engineering needs at email@example.com or (704) 523-2520.