Kevin O'Donnell04.02.08
It's been unseasonably cold, dreary and snowy in Chicago this winter. We have had 19 days below -18C (0F) and a recent 22-day run with temperatures never getting above the freezing mark, making it the third coldest winter in the past 20 years. It's also been the third cloudiest winter in 137 years. We haven't seen the sun around here but 31% of the time. Average annual snowfall for winter in Chicago is 37 inches. We're at 56 inches so far, expecting another six inches overnight, and we're not yet out of February, historically the second snowiest month of the year. On the optimistic side -- at least the Cubs are in spring training.
My whining about the weather actually has a point. My wife noticed the other day when the temperature outside reached a balmy 5C (40F) that the ice on the driveway didn't appear to be melting.
"Too cold," I said.
"But it's above freezing," she replied.
"Too MUCH cold," I said, correcting myself. "The ice hasn't yet had a chance to reach its phase change temperature because the ground beneath it is still way below freezing."
She looked at me like a dog that just heard a new sound.
"Forget it," I said. But it got me to thinking.
* * *
Frozen gel pack refrigerants, a critical component of temperature controlled packaging, can also affect performance of the package and the product temperatures they are meant to protect by altering their freezing temperatures.
A well-designed thermal package works on the principle of regulating the absorption of heat penetrating the package. Frozen gel packs in a well-designed system are able to help keep the product within a specified temperature range and duration regardless of its exposure to fluctuating external temperatures.
The answers to these questions provide very valuable information when designing temperature controlled packaging for pharmaceuticals or biologics for which very tight temperature ranges must be maintained.
How frozen do the gel pack refrigerants have to be?
When gel pack refrigerants are removed from the freezer they immediately begin to transition to their phase change temperature, roughly 0C in the case of water-based gel packs, those most commonly used. If the gel pack refrigerants are removed from the freezer and immediately confined to a hermetically sealed insulated package, there is often a "thermal shock" that occurs -- a brief but significant dip in internal air temperature within the package. This effect can be verified by placing a battery-operated data logger monitoring device within the package or by probing the actual product within the package using thermocouple wire integrated with a temperature data logger. The lower the temperature to which the gel pack refrigerants are conditioned, the more dramatic the initial dip in temperature. The effect is only compounded with the addition of the insulation. The better the insulation barrier, the more pronounced the effect and the longer the duration.
Recordings of this phenomenon have been the cause of countless deviations, non-conformances and subsequent explanations among quality organizations within the pharmaceutical industry.
Is there a risk of freezing the product payload?
Yes. But there are a few ways to address this issue. One elegant method to avoid this danger is to shed the shock by allowing the gel pack refrigerants to relax for 30 minutes or so before packing or sealing the packages closed. Some wait until the frost on the outer surface of the gel pack liquefies or dissipates. The World Health Organization (WHO), for example, communicates to its field personnel that the frozen mass inside the bag or plastic bottle will slide when shaken. This assures that the gel pack refrigerants have neared or reached their phase change temperature. The risk of thermal shock, if not eliminated entirely, is significantly reduced by the time the product is placed into the package with the gels so as to stay within the acceptable range: 2-8C for example. This is a critical step in distribution for the WHO, since its vaccines cannot be allowed to freeze.
Not everyone has the ability to do this within their daily operation. Others don't employ this solution due to an inability to control and document the process. Still, others allow for the dip to occur and design around it, and invest the time and expense to qualify their packages accordingly.
What impact will conditioning gel pack refrigerants to various frozen temperatures have on overall performance longevity of a package?
Figure 1 illustrates that the temperatures at which the frozen gels are conditioned have a direct and variable influence on the package's performance and longevity.
The graph represents an average culled from multiple tests at each freeze temperature and illustrates:
1) time equilibrium at various frozen temperatures,
2) time to attain phase temperature of 0C +/-1C, and
3) the heat of fusion and consequent longevity of the gel pack.
In each test, identical 16 oz., 0C phase change gel packs were probed with thermocouples and conditioned simultaneously in freezers set at -10C, -20C, -30C and -80C. They were removed simultaneously after 48 hours and placed into a controlled temperature chamber at 23C (+/- 1C). Data were then logged at 15-minute intervals over a 48-hour period.
There is evidence of an obvious performance difference. The lower the starting temperature, the more exaggerated the effects of thermal shock. It is more pronounced and of longer duration. This decreases the air temperature within the package and increases the threat of lowering the product temperature, possibly below 0C, until the gel pack refrigerants come up to their phase change temperature. To attempt to artificially increase the longevity of the package by freezing the gel pack refrigerants at lower temperatures is a risky proposition if you have freeze-sensitive product.
All gel packs contain a finite amount of energy. All 0C phase change gel packs filled with 16 oz. of water contain roughly 179 kilojoules of energy. Regardless of what temperature the gel packs are frozen, once they reach their phase change temperature, the graph shows that they all performed the same and melted essentially at the same rate -- their heat of fusion is equal -- if not the same time along the continuum.
Practitioners of insulated package design attempt maximum utilization of the phase change portion of the graph while minimizing the effects of what happens before and after (the curves on either end of the graph).
What impact might this have on the product payload temperatures?
Suppose your company has multiple distribution sites across the country. Each site is responsible for preconditioning its gel pack refrigerants for controlled temperature packaging shipments. Do all sites have freezers set to the same set point with the same tolerances?
The example in Figure 2 reveals the relative changes in product temperatures when the staging temperature of the frozen gel pack refrigerant is manipulated. The data shown are theoretical and based on numerous assumptions made in order to create the following computer simulation model. The trendlines indicate there is variance between both product temperature extremes and average product load temperatures if frozen gel pack refrigerant is preconditioned at temperatures other than the commonly applied -20C. Although design tools can theoretically predict package performance quite accurately, this is only one specific example. Results can vary depending on the input data and any theoretical modeling should be confirmed with physical test data.
Is there an optimum temperature to condition frozen gel packs?
But before you go changing all of your freezers to lower temperatures to possibly optimize package performance, consider the mechanical impact. One of the more remarkable properties of water is that it has the highest specific heat of any common substance: 1 calorie/gm C = 4.186 J/gm C. It's cheap, abundant and easy to work with -- which is what makes it such an attractive source of coolant in insulated packaging systems. It does however; require a significant amount of energy to freeze. Developed and published by Amgen, Figure 3 calculates the trade-offs in operating costs for freezing 1 kg of gel. All things considered, the Amgen study concluded that the most efficient operating temperature is generally accepted to be around -18C.
Source: Amgen Process Development Dept.
What precondition temperature should be used for gel pack refrigerants when designing and qualifying temperature controlled packages?
Many variables play a critical role. Therefore, there is no single, correct answer. It is important to specify and document the temperature at which your gel pack refrigerants are conditioned for design and qualification tests. Make certain that the conditioning temperature and tolerances of the freezers are commensurate with the freezers used for conditioning gel pack refrigerants within your commercial operation. If multiple sites contain freezers with various set-points, design and qualification tests should be done over the entire range. Otherwise, you may see a difference in performance between the package configurations you qualify and those you send from your distribution sites and periodically monitor. This is especially critical for products which must maintain a narrow acceptable temperature range.
Kevin O'Donnell is director and chief technical advisor to industry at Tegrant Corp., ThermoSafe Brands. He blogs about cold chain issues at Where Cooler Heads Prevail.
My whining about the weather actually has a point. My wife noticed the other day when the temperature outside reached a balmy 5C (40F) that the ice on the driveway didn't appear to be melting.
"Too cold," I said.
"But it's above freezing," she replied.
"Too MUCH cold," I said, correcting myself. "The ice hasn't yet had a chance to reach its phase change temperature because the ground beneath it is still way below freezing."
She looked at me like a dog that just heard a new sound.
"Forget it," I said. But it got me to thinking.
* * *
Frozen gel pack refrigerants, a critical component of temperature controlled packaging, can also affect performance of the package and the product temperatures they are meant to protect by altering their freezing temperatures.
A well-designed thermal package works on the principle of regulating the absorption of heat penetrating the package. Frozen gel packs in a well-designed system are able to help keep the product within a specified temperature range and duration regardless of its exposure to fluctuating external temperatures.
- But how frozen do the gel pack refrigerants have to be?
- Is there a risk of freezing the product payload?
- What impact will conditioning gel pack refrigerants to various frozen temperatures have on overall performance longevity of a package?
- What impact might this have on the product payload temperatures?
- Is there an optimal temperature for conditioning gel pack refrigerants?
- What precondition temperature should be used for gel pack refrigerants when designing and qualifying temperature controlled packages?
The answers to these questions provide very valuable information when designing temperature controlled packaging for pharmaceuticals or biologics for which very tight temperature ranges must be maintained.
How frozen do the gel pack refrigerants have to be?
When gel pack refrigerants are removed from the freezer they immediately begin to transition to their phase change temperature, roughly 0C in the case of water-based gel packs, those most commonly used. If the gel pack refrigerants are removed from the freezer and immediately confined to a hermetically sealed insulated package, there is often a "thermal shock" that occurs -- a brief but significant dip in internal air temperature within the package. This effect can be verified by placing a battery-operated data logger monitoring device within the package or by probing the actual product within the package using thermocouple wire integrated with a temperature data logger. The lower the temperature to which the gel pack refrigerants are conditioned, the more dramatic the initial dip in temperature. The effect is only compounded with the addition of the insulation. The better the insulation barrier, the more pronounced the effect and the longer the duration.
Recordings of this phenomenon have been the cause of countless deviations, non-conformances and subsequent explanations among quality organizations within the pharmaceutical industry.
Is there a risk of freezing the product payload?
Yes. But there are a few ways to address this issue. One elegant method to avoid this danger is to shed the shock by allowing the gel pack refrigerants to relax for 30 minutes or so before packing or sealing the packages closed. Some wait until the frost on the outer surface of the gel pack liquefies or dissipates. The World Health Organization (WHO), for example, communicates to its field personnel that the frozen mass inside the bag or plastic bottle will slide when shaken. This assures that the gel pack refrigerants have neared or reached their phase change temperature. The risk of thermal shock, if not eliminated entirely, is significantly reduced by the time the product is placed into the package with the gels so as to stay within the acceptable range: 2-8C for example. This is a critical step in distribution for the WHO, since its vaccines cannot be allowed to freeze.
Not everyone has the ability to do this within their daily operation. Others don't employ this solution due to an inability to control and document the process. Still, others allow for the dip to occur and design around it, and invest the time and expense to qualify their packages accordingly.
What impact will conditioning gel pack refrigerants to various frozen temperatures have on overall performance longevity of a package?
Figure 1 illustrates that the temperatures at which the frozen gels are conditioned have a direct and variable influence on the package's performance and longevity.
FIGURE 1 |
The graph represents an average culled from multiple tests at each freeze temperature and illustrates:
1) time equilibrium at various frozen temperatures,
2) time to attain phase temperature of 0C +/-1C, and
3) the heat of fusion and consequent longevity of the gel pack.
In each test, identical 16 oz., 0C phase change gel packs were probed with thermocouples and conditioned simultaneously in freezers set at -10C, -20C, -30C and -80C. They were removed simultaneously after 48 hours and placed into a controlled temperature chamber at 23C (+/- 1C). Data were then logged at 15-minute intervals over a 48-hour period.
There is evidence of an obvious performance difference. The lower the starting temperature, the more exaggerated the effects of thermal shock. It is more pronounced and of longer duration. This decreases the air temperature within the package and increases the threat of lowering the product temperature, possibly below 0C, until the gel pack refrigerants come up to their phase change temperature. To attempt to artificially increase the longevity of the package by freezing the gel pack refrigerants at lower temperatures is a risky proposition if you have freeze-sensitive product.
All gel packs contain a finite amount of energy. All 0C phase change gel packs filled with 16 oz. of water contain roughly 179 kilojoules of energy. Regardless of what temperature the gel packs are frozen, once they reach their phase change temperature, the graph shows that they all performed the same and melted essentially at the same rate -- their heat of fusion is equal -- if not the same time along the continuum.
Practitioners of insulated package design attempt maximum utilization of the phase change portion of the graph while minimizing the effects of what happens before and after (the curves on either end of the graph).
What impact might this have on the product payload temperatures?
Suppose your company has multiple distribution sites across the country. Each site is responsible for preconditioning its gel pack refrigerants for controlled temperature packaging shipments. Do all sites have freezers set to the same set point with the same tolerances?
The example in Figure 2 reveals the relative changes in product temperatures when the staging temperature of the frozen gel pack refrigerant is manipulated. The data shown are theoretical and based on numerous assumptions made in order to create the following computer simulation model. The trendlines indicate there is variance between both product temperature extremes and average product load temperatures if frozen gel pack refrigerant is preconditioned at temperatures other than the commonly applied -20C. Although design tools can theoretically predict package performance quite accurately, this is only one specific example. Results can vary depending on the input data and any theoretical modeling should be confirmed with physical test data.
FIGURE 2 |
Is there an optimum temperature to condition frozen gel packs?
But before you go changing all of your freezers to lower temperatures to possibly optimize package performance, consider the mechanical impact. One of the more remarkable properties of water is that it has the highest specific heat of any common substance: 1 calorie/gm C = 4.186 J/gm C. It's cheap, abundant and easy to work with -- which is what makes it such an attractive source of coolant in insulated packaging systems. It does however; require a significant amount of energy to freeze. Developed and published by Amgen, Figure 3 calculates the trade-offs in operating costs for freezing 1 kg of gel. All things considered, the Amgen study concluded that the most efficient operating temperature is generally accepted to be around -18C.
|
Freezing Time
Hours
|
Freeze Temp C |
% Cost Freezers |
| 4.75 | -30 | +35% |
| 5.5 | -25 | Base Line |
| 6.75 | -20 | Base Line |
| 7.25 | -15 | Base Line |
| 11.25 | -10 | -20% |
Source: Amgen Process Development Dept.
What precondition temperature should be used for gel pack refrigerants when designing and qualifying temperature controlled packages?
Many variables play a critical role. Therefore, there is no single, correct answer. It is important to specify and document the temperature at which your gel pack refrigerants are conditioned for design and qualification tests. Make certain that the conditioning temperature and tolerances of the freezers are commensurate with the freezers used for conditioning gel pack refrigerants within your commercial operation. If multiple sites contain freezers with various set-points, design and qualification tests should be done over the entire range. Otherwise, you may see a difference in performance between the package configurations you qualify and those you send from your distribution sites and periodically monitor. This is especially critical for products which must maintain a narrow acceptable temperature range.
