"Frost-free" Freezing
Experiments from Team Labs

Experiment Profile

Connections:		Physical Science, Chemistry, Mathematics, Technology
Skills:		Graphing, Analyzing, Measuring, Inferring
Duration:		1 Class Period with a 24-hour overnight data collection trial
Team Size:		2-3 students per group, or whole class demonstration
Content Standards:		Science Standard A (grades 9-12) Science Standard B (grades 9-12) Science Standard E (grades 9-12) Math Standard 1, 4, 5, 13 (grades 9-12)

Summary

In this experiment, students will take data collected from their home or school freezers to determine how they work. You will use a temperature probe to analyze what is taking place inside the freezer when left closed and undisturbed. The process we seek to understand is how a "frost-free" freezer manages to avoid the collection of frozen water on the sides of the freezer's walls. Generally speaking, older models of freezers had to be periodically defrosted as the ice build up on the walls can become several inches thick or in extreme cases, one big block of ice that can take over the space available for food in the freezer compartment. If your freezer is deemed a "frost-free" model, this investigation will provide the clues to its function.

Materials

Computer

ThinkStation SP16 Interface Kit

ThinkStation Interface

Power Supply

Communications Cable

Excelerator 2000 Software

Standard or Extended Temperature Probe with API (Automatic Probe Identification)

Background

Using a freezer in the past--especially those handy little dorm fridge/freezer combinations--was always a challenge to make room for the food around the accumulating ice sculpture that was being created on the walls of your freezer. Where does this ice come from?

The accumulation of ice was due to the warm humid air outside of the freezer entering the freezer compartment when you opened the door, condensing the water vapor from its gas state into a liquid because cold air cannot hold as much water vapor as warm air. This phase change was due to the water molecules condensing up against the cold coils that run along the inside of the freezer's walls (much like the "sweat" on a glass of ice cold tea on a warm and humid Summer day). After the moisture from the air was condensed on the inside walls of the freezer, it quickly again changed state to its solid form, ice, due to the below-freezing temperatures of water maintained inside the freezer. This layer of "frost", which is really the solid form of water--usually referred to as ice-then acted as a layer of insulation between the cooling coils and the contents of the freezer. This is not desired, as it makes the freezer work harder to maintain the temperature of the contents, and over time the build-up of ice will continue on the walls, only exacerbating this effect and lowering the available volume in which to freeze your goods.

This problem of ice build-up in the freezer was solved with the advent of the "frost-free" freezer that became popular in the early 1980s. "Frost-free" freezers are now the most common type of freezer purchased. Why? Because they offer the consumer time savings by avoiding the "defrosting" procedure of the past. They also help lower the food cost involved due to periodic thawing or having to use up your freezer's contents before defrosting it. Using Team Lab's probeware and computer-based, highly accurate data collection, we can discover how this "frost-free" function works.

Procedure

Collecting Data with the Temperature Probe

1. Prepare your experiment by finding an appropriate "frost-free" freezer that you can monitor without disturbance for approximately 24 hours.
2. Attach a Temperature Probe (either Extended or Standard) to the ThinkStation Interface. Position the temperature probe inside your freezer compartment with the sensor tip of the probe exposed in the center of a shelf area, and not touching the sides or any items within. (Try using a rubber band or tape and a plastic cup to prop up the probe.)
Show me	3. Next, launch Excelerator 2000 and click on the Connect&GOTM icon. Excelerator will automatically identify the temperature probe and create a graph of Temperature vs. Time. The software also sets a default sample rate and duration for the experiment, which you will change in the next step.
	4. To change the sample rate and duration of the experiment, click on the Edit Clock icon located on the Excelerator toolbar.
Show me	Set the sample rate to 12 samples per minute (this equates to one sample being taken every 5 seconds) and the duration to 24 hours (86,400 seconds).
	5. Plug your API temperature probe into the Thinkstation Interface. At this point, you are ready to record some data. Click the green GO button on the left side of the Excelerator toolbar.
View screen	6. Excelerator will record the Temperature emitted from the probe for exactly 24 hours at a sample rate of once every 5 seconds (12 samples per minute) and will display a graph similar to the one at left. Different models and configurations of freezers will vary in the time and temperature between cycles, however, the data should be similar in trend. It may be necessary to click on Rescale in the Tools menu to see all of the data. Save the trial after completing.

Analysis of the Data

1. Use your Fast Graph "Analysis" toolbar and choose to "Select" each of the peaks from the warming and cooling cycles to determine the time it takes between cooling cycles (those trends shown well below 0 degrees Celsius) as well as between the heating cycles (those that go above 0 degrees Celsius).
View screen	Maintaining Cooling Cycle Temperature: This graph shows that the freezer cycles on to bring the temperature down to approximately -21 degrees Celsius from a maximum allowed temperature of approximately -9 degrees Celsius every 82 minutes or so during the maintenance of below freezing temperatures or the cooling cycles. (37,340s - 32,405s = 4,935s / 60s/minute = 82.25 minutes / 60min./hour = 1 hour 12.25 minutes)
View screen	The Warming Cycle: This graph shows how the freezer periodically warms up to above the freezing point, or O degrees Celsius, approximately every 7 hours. (43,670s - 18,100 s = 25,570s / 60s/minute = 426.16 minutes / 60minutes/hour = 7.1 hours
2. The graphs above show that a "frost-free" freezer uses cooling cycles that simply maintain the freezer at a temperature sufficiently below the freezing point of water by initiating the freezer's cooling coils to turn on periodically, and with the help of an internal fan in the freezer compartment, spreading around the very cold air inside to maintain the food at below freezing temperatures. This is occurring in our example above at approximately every 82 minutes. 3. The graphs above also show that a "frost-free freezer actually uses a heating cycle that brings the temperature of the freezer above the freezing point of water from time to time in order to maintain a "frost-free" compartment. Temporarily heating the coils above the freezing point temperature warms any frost that may have accumulated on the sides of the walls near the coils. With the frost now in a liquid state, the liquid droplets fall due to gravity to a drain hole and then out to an evaporation tray under the freezer, usually to simply evaporate back into the air surrounding the freezer. This is how the freezer can maintain its "frost-free" environment. The internal workings that play a part in this process are shown below (from american-appliance.com):

Conclusions

A. "Frost-free" freezers maintain their "frost-free" environment by using cycles of cooling and heating in order to control the water vapor from the air that enters the compartment each time the door is opened.

B. The cooling cycles simply maintain the freezer at a sufficient temperature below the freezing point of water to keep the compartment goods truly frozen. However, the freezer occasionally-in our example, every 7 hours or so-warms up to combat any ice formation that may be occurring due to the condensation of warmer and more humid air from outside the freezer, as well as the solid form of this humid air that we call ice, on the walls near the cold cooling coils.

C. These warming and cooling cycles maintain a "frost-free" compartment, eliminating the need to periodically "defrost" the freezer, thus saving time and possible food cost to the user. However, these warming and cooling cycles subject the food inside to many warming and cooling cycles as well, which may lower the shelf-life and stability of the items inside. This is especially noteworthy to scientists in the medical field, which generally use what we may consider to be more "old-fashioned" models of freezers, that maintain their temperatures at a more constant level, one that is always well below the freezing point of whatever they seek to stabilize and truly freeze for a length of time. This is why vitamin and drug companies sometimes label their products with warnings that "frost-free" freezing is not recommended. Why? Because it may contribute to destabilizing and lowering the shelf-life of their product due to the periodic and frequent changes of state possible with the warming and cooling cycles of a "frost-free" freezer.

Note: Frozen food should not thaw under the controlled conditions of a properly functioning "frost-free" freezer, but such things as drainage blockage will cause water accumulation and small items with a lot of surface area (like vitamins) are more prone to thawing.

Extensions

Compare the temperature cycles and trends in freezers of different sizes and styles.

• Are they all the same?
• Which ones are more cost effective?
• Which ones are more likely to lower the shelf-life of your freezer's goods due to repetitive thawing and freezing cycles?

It is also noteworthy to look at the constantly below-freezing point cycles of an older or large chest freezer that allows for the formation of "frost"-- in actuality, the buildup of condensing water vapor from the air that changes state from gas to liquid to solid, forming ice on the sides of the freezer near the cooling coils. What are the associated costs to the consumer and the environment in using "frost-free" freezers, as they do generally use more energy to maintain the freezing compartment when compared to a "traditional" or older freezer? (The older models simply maintained the temperature of the compartment to always be below the freezing point without expending the energy necessary to periodically heat the coils.) What is your time worth if you had to "defrost" your freezers periodically? What would be the energy savings due to power consumption be when comparing a "traditional" or chest-style freezer with a "frost-free" product?

Download a PDF of this experiment (249 KB)

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