PHARMABIO TRANSPORT: Thermally Protective Packaging: Five Essentials of Gel Refrigerant Design and Specification
Water-based gel refrigerants exhibit properties that are unique to the thermodynamic properties of water.
Karen K. Greene, CPP, Technical Director, DDL Inc.
Table I. Gel pack geometry and weight influence phase-change time.
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Temperature-controlled packaging is used throughout many industries to protect temperature-sensitive products. Designing such packaging requires a level of analysis as detailed as any other type of shipping package. Items to consider include product design requirements, route and destination, and the anticipated hazards that compromise product safety and efficacy.
The temperature-controlled packaging industry has developed some very effective thermally insulating designs utilizing several forms of containers and refrigerant packs. This article will focus on five essentials of gel refrigerant design and specification that are vital to optimizing thermal protection for temperature-sensitive product in a timely and cost-effective manner.
THE GEL PACK
The refrigerant pack, or gel pack, is used to maintain a thermal environment in an insulated shipping container sufficient to meet the product’s temperature requirements. A few thermodynamic concepts are involved here: heat transfer, heat absorption, and phase change. These principles are some of the components of the “zeroth law” of thermodynamics, commonly known as thermal equilibrium. That is, all systems attempt to reach a state in which heat energy is equally distributed. If an object with a higher temperature comes in contact with a lower-temperature object, it will transfer heat to the lower-temperature object.
The objects will approach the same temperature and then maintain a single constant temperature. Therefore, a product is maintained at its stable temperature range by attaining thermal equilibrium with a gel pack for some predetermined period of time.
Water is commonly used in refrigerant packs because it is relatively cheap and ubiquitous. If a water-based refrigerant pack is used, “cold temperatures” can be achieved by freezing the water-based refrigerant pack. Water freezes, or goes from a liquid to a solid, at 0°C (32°F). This change from a liquid to a solid (change in the state of matter) is called a phase change. Water has a relatively high level of heat capacity as it goes through its freezing and thawing process and can be relied upon to maintain that 0°C temperature for long periods of time.
Introducing water into a protective shipping package means, of course, that the package design must also protect the product from water. To decrease the potential for water damage, the common approach is to mix additives with the water to cause gelling, or an increase in viscosity. The higher-viscosity contents of the refrigerant pack will allow the pack to keep its shape longer and not leak from a damaged container as easily.
The fact that water gel will maintain 0°C can also be a problem in that many temperature-sensitive goods cannot be frozen. Further protection usually involves designing the shipping package to buffer the contents from the gel pack so that there is a resultant temperature offset and the product material stays above 0°C. Another solution is to completely replace the water gel with another material that has a freeze/thaw phase-change temperature that matches the product’s stable temperature range. (Design of custom phase-change materials will not be explored in this article.)
Commonly used water-based gel packs can meet the thermal-protection requirements of many products. Refrigerant-pack engineering can be optimized for the greatest consistency in thermal protection for the maximum time (phase-change) period of distribution and shipping. In addition, development time and total package costs can be minimized. Several laboratory studies were designed, developed, and executed at TCP Reliable (Edison, NJ) to illustrate the importance of specific design inputs when designing and specifying a gel refrigerant.
ESSENTIAL #1: GEOMETRY, WEIGHT, AND THERMAL PROTECTION
Table II. Testing empty, lined, and insulated containers illustrates the principle of heat transfer through conduction, also shown as an equation in Figure 1.
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An experiment was conducted to collect temperature-mapping data from different gel packs, varying in size and geometry, during a freeze/thaw cycle for a water-filled gel pack. The objective of the study was to illustrate that a frozen gel pack is not a homogeneous temperature block as it moves through a typical freeze/thaw cycle that it might encounter during real-world distribution and shipping.
The gel packs were placed lying down in an environmental chamber and five thermocouple temperature probes were inserted into them to form a diagonal pattern to measure the thermal penetration across the gel pack. The probes were located at the opposite corners of the gel pack and at the center, with additional probes located at the intermediate distance between the corners and the center, forming a straight line across the entire gel pack. The study illustrates that the geometry and mass of the gel pack did not influence this freeze/thaw temperature progression pattern. Each gel showed a progression of the thermal penetration moving from the opposite corner through the intermediate probe to the center probe, which consistently showed the longest time to complete its phase change. Looking only at the center probes and comparing the 2-lb gel packs that were square versus rectangular shows that the square gel takes longer to freeze/thaw completely and consequently remains longer at the phase change temperature than the rectangular gel, owing to the difference in the distance (gel pack thickness) that the thermal penetration has to cover in order to complete the phase change. The phase-change elapsed-time difference, illustrated by the mapping across the gel, is significant. This effect of gel package geometry on the phase-change time should be considered when designing shipper container packaging around these sorts of refrigerant packs. Table I summarizes the highlights of the study.
ESSENTIAL #2: PACKAGE DESIGN AND FREEZE/THAW TIME
Table III. A time-lapse photographic study of melting refrigerant designs.
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The selection of an insulative shipper design is a key component of a temperature-controlled package. Selecting the appropriate phase-change material when designing or selecting an insulating shipper will determine how long thermal protection will last.
Our experiment included the use of an 8 × 8 in. gel pack (phasing at 0°C), weighing 2.0 lb. One gel pack was placed into the center of the bottom of the shipper, with a thermocouple probe inserted into the geometric center of the gel pack. The shipper designs were at room temperature and then placed into a –20°C chamber, freeze cycle. This was repeated for a total of three different shipping configurations and a naked gel as the control:
- Empty RSC shipper.
- Expanded polystyrene (EPS) panels lining the RSC.
- Vacuum-insulated panels lining the RSC.
Readings were taken from the thermocouples inserted into gel packs, recording the time that the instrumented gel packs took to freeze (phase change) and then come to equilibrium with the environmental chamber (control, ambient thermocouple), at –20°C. Table II summarizes test results.
In practical terms, the study illustrates the effect of an insulating shipper design on the thermal performance characteristics of your package design. The conduction effect is the strongest when insulation is involved; the better the insulation, the slower the heat transfer rate. The convection effect, i.e., heat transfer, that occurs between a surface and a moving fluid when they are at different temperatures is an additional source of heat transfer, but even an unlined RSC mitigates the convection effect.
ESSENTIAL #3: SIZE AND NUMBER OF GEL PACKS AND THEIR PERFORMANCE
Figure 1. This equation calculates heat transfer rate.
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To determine how to specify gel packs relative to the total mass of the refrigerant and its orientation, refrigerant size and orientation were considered in the next study. Outcomes are shown in a table posted to www.pmpnews.com. The data suggest that a single gel pack is slightly more effective than two gel packs of equivalent mass remaining at the phase-change temperature longer. There appears to be greater heat transfer with the two side-by-side units. The possible winner here is a slightly lower cost of purchasing one refrigerant as opposed to two, for the same mass.
ESSENTIAL #4: GEL STRUCTURE AND GEOMETRIC STABILITY
Figure 2 . Melting Photographic Study.
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As mentioned previously, gelling agents are added to water to increase viscosity and add structure to the gel pack. The attribute of structure can also be achieved in several different ways. First, let’s focus on the importance of structure, or the opposite of free-flowing water, in a bag.
We performed a time-lapse photographic study of several different refrigerant designs by placing previously frozen refrigerants on end at room temperature (20°C) and photographing the thawing process every 30 minutes. The test units are described in Table III and shown in Figure 2.
The significance of these time-lapse photographs is the loss of shape that occurs as the refrigerant melts if it is not assisted by a gelling agent to increase viscosity, or the use of a rigid bottle. The larger the gel refrigerant mass, the greater the potential to lose symmetry. The loss of shape creates nonuniform thermal coverage and may negatively affect the thermal protection of the payload. When designing a package and specifying a gel pack as the refrigerant, considerations must include whether a pack that maintains its shape is needed. The important thing to note from this study is that while the structured and unstructured gels compared here are both properly referred to as gel packs, they each have very different performances from the perspective of geometric stability. It must be understood that water will shift in size and configuration when undergoing a phase change.
ESSENTIAL #5: PROTOTYPING THE PACKAGE
Using the four essentials outlined in this paper, develop the design input requirements for your temperature-controlled package design.
Plan for thermal and distribution simulation feasibility testing, as this is vital to determining if product requirements and thermal protection requirements are being met. Then proceed to prototyping as follows.
- Create physical samples for testing to allow to investigation of the first four essentials outlined in this paper.
- Follow “rule of thumb” guidelines for temperature-controlled package design:
- Explore how to specify gel packs and their use in the package in the context of these studies.
- Describe other options available.
- Cover as much surface area as possible with refrigerant.
- Use insulation to slow the heat transfer into the package.
- Buffer your refrigerated product from the frozen gels.
- Make sure the design is robust enough to survive shock and vibration during shipment.
- Include enough refrigerant in the package so that it is always undergoing phase change during the shipment time.
- Specify a gel for your design.
- To ensure best performance, the gel should be shaped in a uniform symmetric way, preferably square in shape.
- Understand that the amount of insulation used in your package directly affects how long it will work. Consider the cost balance between adding more gel and more insulation when trying to improve performance.
- When possible, use single larger gels instead of many smaller gels. Depending on the size and geometry of your product, this will be more or less practical.
- Remember the importance of the structural properties of the gel material itself. If your package requires gels to stand on end, make sure they will maintain their shape.
- Consider other options available to solve these problems:
- Rigid bottles: The bottle will not change shape but the gel can still flow inside of it and cause voids in your package.
- Phenol foam brick packs: The performance per pound may not be as high as gel material since the foam does change phase.
- Gel blanket: Spreads the refrigerant evenly across the payload, trading surface area coverage for pure weight performance.
- Custom phase-change materials: The phase change will better match your product’s stable range but the material itself is generally much more expensive and, depending on the chosen material, toxicity issues must be understood and guarded against.