How convective heat transfer works and why it matters
In the complex world of cold chain shipping, where temperature-sensitive goods are transported across vast distances, maintaining the integrity of products is paramount and convective heat transfer plays a critical role. In this article, we delve into the workings of convective heat transfer, exploring how it influences the success of cold chain shipping and its vital role in preserving the quality and safety of perishable products throughout their journey.
TSS Thermalist
September 19, 2023 | R value
Controlling Convection in Cold Chain Shipping: Strategies for Success
In the complex world of cold chain shipping, where temperature-sensitive goods are transported across vast distances, maintaining the integrity of products is paramount, and convective heat transfer plays a critical role.
In this article, we delve into the workings of convective heat transfer, exploring how it influences the success of cold chain shipping and its vital role in preserving the quality and safety of perishable products throughout their journey. Understanding this fundamental concept is crucial for stakeholders in the cold chain industry, as it paves the way for implementing effective temperature management strategies, minimizing spoilage, and ensuring customer satisfaction.
Thermodynamics 101
Thermodynamics is the scientific study that explores the connections between heat, work, temperature, and energy. In essence, it examines the movement of energy as it is transferred between various locations and transformed from one state to another.
There are three laws of thermodynamics that form the fundamental principles behind the behavior and interactions of energy within a system.
1
First Law of Thermodynamics:
The Law of Energy Conservation
Energy cannot be created or destroyed within an isolated system. This law highlights the principle of energy conservation, where the total energy within a closed system remains constant. Energy may be converted between different forms, such as heat, work, or internal energy, but the overall amount remains unchanged.
2
Second Law of Thermodynamics:
The Law of Increasing Entropy
This law focuses on the concept of entropy, which quantifies the level of disorder or randomness in a system. It states that the entropy of an isolated system tends to increase over time. In simpler terms, natural processes generally lead to higher entropy, signifying an increase in disorder. The second law also introduces the concept of spontaneous heat transfer from hotter objects to colder ones in the absence of external influences.
3
Third Law of Thermodynamics:
The Law of Absolute Zero
This law pertains to the behavior of systems as they approach absolute zero temperature (0 Kelvin or -273.15 degrees Celsius). It states that it is impossible to reach absolute zero in a finite number of steps. Additionally, it asserts that as a system approaches absolute zero, its entropy approaches a minimum value that is unique to each pure substance.
In addition to these three classic laws of thermodynamics, there is also a “zeroth law of thermodynamics,” which states: When two systems are each in thermal equilibrium with a third system, the first two systems are in thermal equilibrium with each other. This property makes it meaningful to use thermometers as the “third system” and to define a temperature scale.
What is convective heat transfer?
Convection is a mode of heat transfer that plays a significant role in thermodynamics. Convection involves the transfer of heat energy from one place to another through the actual movement of fluid or gas. Convection occurs when warmer areas of a liquid or gas move to cooler areas in the liquid or gas. Cooler liquid or gas then takes the place of the warmer areas. This results in a continuous circulation pattern. Water boiling in a pan is a good example of these convection currents.
In the context of thermodynamics, convection is relevant because it contributes to the overall energy transfer within a system. It is one of the mechanisms by which energy is transported from one location to another, along with conduction and radiation. Convection often occurs in fluids due to the uneven distribution of temperature, causing regions of higher temperature to have lower density and rise, while regions of lower temperature have higher density and sink.
Why is convection an important factor to consider?
Convection can have a significant impact on the effectiveness of insulation. Insulation works by slowing the transfer of heat which can move in three ways: conduction, convection, and radiation.
In situations where there are air gaps or voids within or around the insulation, convection can become a significant factor. Air movement within these gaps can promote heat transfer through convection. Warm air tends to rise due to its lower density, creating convective loops that carry heat from the warmer areas to cooler regions. This can result in increased heat loss or gain, reducing the effectiveness of the insulation.
To mitigate the impact of convection on insulation, it is important to minimize or eliminate air gaps and ensure proper sealing of insulation materials. This can be achieved by using insulation materials that are designed to fit tightly and evenly into the spaces they are intended to insulate.
Let’s take a deeper dive into how convection affects insulation, but before we do, we need to explain something called “R-value.”
R-Value Explained
Insulation is measured using a unit called R-value, which stands for resistance to heat flow. A higher R-value indicates greater resistance to heat flow and, consequently, better insulation. R-value can be expressed per inch or as a total R-value.
Measuring R-Value of cold-chain insulation systems is like measuring gas mileage while you are rolling downhill. R-value is a measure of a material’s resistance to heat transfer. The test method requires a technician to measure the thermal resistance of a 12”x12”x1” specimen placed between a cold plate and a hot plate while measuring the energy transfer required to maintain a certain temperature gradient. This does not take into account air movement, moisture levels, or size of the shipping container. If you shrink the exterior surface of a container, you are reducing the amount of area exposed to conduction, convection, and radiant energy. By shrinking the interior surface, you achieve even better performance.
Convection can affect insulation through voids, compression, air movement, and moisture.
Voids
Are empty spaces where insulation is missing. These voids enable heat transfer through convection and radiation.
Compression
Negatively affects the effectiveness of insulation by eliminating the small, trapped air pockets that contribute to its insulating properties.
Air movement
Reduces its effectiveness by increasing convective heat loss. To effectively slow down heat transfer, it is crucial for the air pockets within insulation to remain still.
Moisture
Reduces the R-value of insulation by making the insulation more conductive and by causing settling, compression, and voids.
How does convective heat affect cold chain?
Controlling convection is the most overlooked element in cold chain shipping. The objective of thermal packaging solutions is to minimize and prevent heat transfer to maintain the desired temperature for sensitive products and packages throughout packaging, shipping, and delivery processes.
Proper cold chain management practices are crucial in avoiding temperature fluctuations that can lead to undesirable outcomes. Excessive warmth can result in spoilage of products like meat, flowers, or biologics. On the other hand, extreme cold can cause separation, freezing, or chemical alterations in products such as milk, paint, or medications. Therefore, maintaining optimal temperature control is vital to preserve the quality and integrity of these sensitive goods.
To prevent the temperature fluctuations that can quick ruin cold chain products, convective heat has to be effectively managed.
What products best reduce convection?
Unlike traditional boxes, the Kangaroo Mailer self-inflates, squeezing out air and enabling standard ice packs to cool the smaller area more efficiently. This mitigates convection heat-loss and locks the gel pack into place. The outside surface of the Kangaroo Mailer is also much smaller than the boxes it replaces, so convective heat is greatly reduced. And by eliminating the void by the expanding foam, the Kangaroo Mailer eliminates internal convection that shortens gel pack duration.
