Five_Essentials_of_Gel_Refrigerant_Design_and_Specification.pdf

7/28/2019 Five_Essentials_of_Gel_Refrigerant_Design_and_Specification.pdf

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Thermally Protective PackagingFive Essentials of Gel Refrigerant Design and Specification

Anthony Alleva, Manager Technical Services

TCP Reliable, Inc.and

Karen K. Greene, CPPVice President Sales and Technology

Life Packaging Technology LLCwww.lifepackagingtechnology.com

Introduction:

Temperature controlled packaging is used throughout many industries to protect

temperature sensitive products. These are products especially sensitive to fluctuationsin temperature. The industries utilizing temperature sensitive packaging range fromhighly engineered biotechnology products to gourmet cakes and fish. Designingtemperature sensitive packaging requires a level of analysis as detailed as any othertype of shipping package. What are the design requirements of the product/device/itembeing shipped, where is it going and what are the anticipated hazards that can renderyour product at the minimum, unfit to eat and at the extreme, a compromisedbiotechnology product, combination drug/device or pharmaceutical which can no longerguarantee the safety and efficacy of the medical product for the patient. Thetemperature controlled packaging industry has developed some very effective thermallyinsulative designs utilizing several forms of insulated containers and refrigerant packs.

This report will focus on five essentials of gel refrigerant design and specification vital tooptimizing thermal protection for your temperature sensitive product while achieving thisin a timely and cost effective manner.

The refrigerant pack or gel pack is used to power your insulated shipping container tomaintain a thermal environment sufficient to meet the products temperaturerequirements. A few thermodynamic concepts are involved here: heat transfer, heatabsorption and phase change. These principles are some of the components of thezeroth law of thermodynamics, commonly known as thermal equilibrium. That is, allsystems attempt to reach a state in which heat energy is equally distributed. If an objectwith a higher temperature comes in contact with a lower temperature object, it will

transfer heat to the lower temperature object.


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Page 2 of 10DDL, Inc.TCP Reliable, Inc.

The objects will approach the same temperature and then maintain a single constanttemperature. Therefore, your product is maintained at its stable temperature range byattaining thermal equilibrium with your gel pack for some pre-determined period of time.

Water is a common material used in refrigerant packs as it is relatively cheap andubiquitous. We know that if we use a water-based refrigerant pack that we can begin to

achieve cold temperatures by freezing the water based refrigerant pack. Waterfreezes or goes from a liquid to solid at 0 C / 32 F. The change from a liquid to a solid(change in the state of matter) is called a phase change. Water has a relatively highlevel of heat capacity as it goes through its freezing and thawing process and can berelied upon to maintain that 0 C temperature for long periods of time. Unfortunatelyintroducing water into a protective shipping package means that the design must alsoprotect the product from the new hazards represented by the water. To decrease thepotential of product damage caused by water escaping from a damaged container thecommon approach is to mix additives with the water to cause gelling or an increase inviscosity. The higher viscosity contents of the refrigerant pack will allow the pack tokeep 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 for many temperaturesensitive goods they also have a requirement of Do Not Freeze. Protecting them witha refrigerant that by its nature promotes freezing can result in damage to the productmaterial. The two ways in which this is usually accomplished is to design the shippingpackage to buffer the contents from the gel pack so that there is a resultant temperatureoffset and the product material stays above 0 C.

Another solution is to completely replace the water gel with another material that has afreeze/thaw phase change temperature that matches the products stable temperaturerange. Design of custom phase change materials is a topic in and of itself and will notbe explored in the context of this paper. The focus of this paper is the properties of the

more commonly used water based gel pack and the derived benefits to your productsthermal protection requirements. These benefits include optimized engineering of yourrefrigerant packs for the greatest consistency in thermal protection for the maximumtime (phase change) period of distribution and shipping, while minimizing developmenttime and total package cost. Several laboratory studies were designed, developed andexecuted at TCP Reliable, Edison, NJ to illustrate the importance of specific designinputs when designing and specifying a gel refrigerant

Essential #1: Geometry, Weight and Thermal Protection

An experiment was conducted to collect temperature mapping data from different size

and geometry gel packs during a freeze/thaw cycle for a water filled gel pack. Theobjective of the study was to illustrate that a frozen gel pack is not a homogeneoustemperature block as it moves through a typical freeze/thaw cycle that it mightencounter during real world distribution and shipping.


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The gel packs were placed lying down in an environmental chamber and fivethermocouple temperature probes were inserted into them to form a diagonal pattern tomeasure the thermal penetration across the gel pack. The probes were located at theopposite corners of the gel pack and the center with additional probes located at theintermediate distance between the corner and the center forming a straight line acrossthe entire gel pack. The study summarized below did illustrate 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 oppositecorner through the intermediate probe to the center probe which consistently showedthe longest time to complete its phase change. Looking only at the center probes andcomparing the 2lb gel packs that were square vs. rectangular shows that the square geltakes longer to freeze/thaw completely and consequently remains longer at the phasechange temperature than the rectangular gel owing to the difference in the distance(gelpack thickness) that the thermal penetration has to cover in order to complete the phasechange. The phase change elapsed time difference, illustrated by the mapping acrossthe gel, is significant. This effect of gel package geometry on the phase change timeshould be considered when designing shipper container packaging around these sorts

of refrigerant packs.

The following table summarizes the highlights of the study inputs:

Gel Pack Size Thermocouple Detail Freeze/ThawCycle

TimeDuration atPhaseChangeTemp(0 C)

Group A =Geometric

EquivalenceSquare, Massdifference6 X 61 lb. Ea. TC probe into the gel

pack. 5 locations2 corners,geometric middle, midpointbetween middle and corners.

(+) 20 C (-)20 C.

4.2 hours

8 X 8---2 lb. Ea. TC probe into the gelpack. 5 locations2 corners,geometric middle, midpointbetween middle and corners.

(+) 25 C (-)25 C.

5.3 hours

Group B = MassEquivalence 2.0 lbs.,geometric difference

6 X 122 lbs. Ea. TC probe into the gelpack. 5 locations2 corners

(+) 20 C (-)20 C.

3.8 hours

8 X 82 lbs. Ea. TC probe into the gelpack. 5 locations2 corners

(+) 25 C (-)25 C.

5.3 hours


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Essential #2: Package Design and Freeze/Thaw Time

The selection of an insulative shipper design is a key component of your finaltemperature controlled package. The answer to your question, how long will mythermal protection last can be answered by selecting the appropriate phase change

material (when the gel pack is frozen at what temperature does it remain in a frozenstate) and designing/selecting an insulative shipper.Our experiment included the use of an 8 X 8 gel pack (phasing at 0 C), 2.0 lbs. One(1) gel pack was placed into the center of the bottom of the shipper, with thermocoupleprobe inserted into the geometric center of the gel pack. The shipper designs were atroom temperature and then placed into a (-) 20 C chamber, freeze cycle. This wasrepeated for a total of 3 different shipping configurations and a naked gel as the control:

Empty RSC shipper

EPS(expanded polystyrene) panels, lining the RSC

Vacuum insulated panels lining the RSC

Readings were taken from the thermocouples inserted into gel packs, recording the timethat the instrumented gel packs took to freeze (phase change) and then come toequilibrium with the environmental chamber (control, ambient thermocouple), (-) 20 C.

The following is a summary of the test results:

Insulating Shipper Study and Thermal Performance

Shipper Design GelPack

FreezeCycle

Thermocouple Time tocool to 0C

Time at PhaseChange(Freeze)

Control Gel,

No Container

8 X

8, 2.0lb.

(-) 20 C Probed into

geometric center

1.5

hours

4 hours

Unlined RSC, 12X 12 X 12

8 X 8 (-) 20 C Probed intogeometric center

3.6hours

11 hours

EPS panel linedRSC, 12 X 12 X12


6 hours 19 hours

Vacuuminsulated panellined RSC, 12 X12 X 12


13 hours 48 hours

The data very simply illustrates the principle of heat transfer through conduction.

qx = conductive heat transfer ratek is the thermal conductivity constant of the insulation

A is the total surface area of the insulation

x

TkAq x

=


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T is the temperature difference form one side of the insulation to the otherx is the thickness of the insulation

In practical terms, it provides an illustration of the effect of an insulative shipper designon the thermal performance characteristics of your package design. The conductioneffect is the strongest when insulation is involved and the better the insulation, the

slower the heat transfer rate. The convection effect, heat transfer that occurs betweena surface and a moving fluid when they are at different temperatures, is an additionalsource of heat transfer and even an unlined RSC mitigates the convection effect.

Essential #3: Size and Number of Gel Packs and Their Performance Effect

The question here is how to specify your gel packs relative to total mass of therefrigerant and orientation. The following study was conducted:

Refrigerant size and orientation

Refrigerant

Size

Freeze

Cycle

Outcomes

8 X 9, (1)-2 lb

(+) 20 Cto (-) 20C

Expected thermal mapping, center of refrigerant longest timeat phase change temp. Illustrates corners reaching tempequilibrium with chamber fastest.

4 X 9, (2)-1lb.

(+) 20 Cto (-) 20C

Thermal mapping demonstrated the perimeter is moreinfluenced by heat transfer from the neighboring gel packand thus heat transfer happens more easily and the overallsurface area of the refrigerant has more variability.

(2) 4 X 9 1lb.

(+) 20 Cto (-) 20C

Thermal mapping demonstrates a performance more like the8 X 9, (1) - 2 lb. The stacking of the refrigerants minimizesheat transfer during thaw/freeze cycle, thus longer time is

spent at the phase change temperature.

The data suggests that a single gel pack is slightly more effective than 2 gel packs ofequivalent mass at remaining at the phase change temperature longer. There appearsto be greater heat transfer with the 2 side by side units. The possible winner here is aslightly lower cost of purchasing one refrigerant as opposed to two, for the same mass.

Essential #4: Gel Structure and Geometric Stabili ty

As mentioned previously, gelling agents are added to water to increase the viscosityand add structure to the gel pack. The feature of structure can also be achieved in

several different ways. First, lets focus on the importance of structure or the oppositeof 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) andphotographing the thawing process every 30 minutes. The test units were as follows:

Melting Photographic Study


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DDL, Inc.TCP Relia

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Plan for thermal and distribution simulation feasibility testing as this is vital todetermining if product requirements and thermal protection requirements arebeing met.

o Creating physical samples for testing allow for investigation of the first 4essentials outlined in this paper.

o Rule of Thumb Guidelines for Temperature Controlled Package Design

Explore how to specify gel packs and their use in the package incontext 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 yourrefrigerated 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.o Specifying a Gel for your Design

To ensure best performance the gel should be shaped in a uniformsymmetric way, preferably square in shape.

Understand that the amount of insulation used in your packagedirectly affects how long it will work. Consider the cost balancebetween adding more gel and more insulation when trying toimprove performance.

Essential #5:, continued

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 makesure they will maintain their shape.

o Other Options Available to Solve these Problems Refrigerant Geometric Stability: 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.o 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 temperature 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.


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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 products stabile range

but the material itself is generally much more expensive and

depending on the chosen material toxicity issues must beunderstood and guarded from.


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Summary:

It is the intent of this paper to alert packaging engineers of temperature sensitiveproducts that water based gel refrigerants exhibit properties that are unique to thethermodynamic properties of water. For the successful development of a water basedgel refrigerant, we have outlined five(5) essentials that will help you achieve success in

your temperature controlled package development. It is our goal to highlightperformance outcomes of your phasing material so that your thermal protection is notcompromised.The five (5) essentials of water based refrigerants:

1. Geometry, Weight, and Thermal PenetrationTemperature distributionacross your gel pack

2. Package Design and Freeze/Thaw Time--- The effect of shipper insulationon thermal protection.

3. Size/Number of Gels and Performance---2(1) lb. gel packs vs. 1 (2) lb.gel packs and thermal protection.

4. Gel Structure and Geometric Stability---Physical structural integrity of yourgel pack and why it might be important.

5. Prototyping the Package--- Plan for thermal and distribution simulationfeasibility testing as this is vital to determining if product requirements andthermal protection requirements are being met.

Five_Essentials_of_Gel_Refrigerant_Design_and_Specification.pdf

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