Technical Glossary
- Thermal Conductivity (k)
- Thermal Resistance (R)
- Thermal Resistivity (1/k)
- Thermal Conductance (C)
- Thermal Transmittance(U)
- Phase Change
- Heat Capacity
- Specific Heat Capacity
- Latent Heat
- Energy Density
- Enthalpy
- Delta T (δT)
- Wall Thickness
- EPS (Expanded polystyrene)
- PU (Polyurethane)
- Vacuum Insulation Panel (VIP)
- Closed Cell
- Thermal Mass
- Condition or Stabilise Product
- IQ OQ PQ (Installation Qualification, Operational Qualification & Performance Validation)
- Ambient
- Temperate Product
- Chilled Product
- Frozen Product
- Volumetric Weight
- Phase Change Materials (PCM)
- Isothermal
- Dry Ice
1. Thermal Conductivity (k)
The Thermal Conductivity of a material is a measure of its ability to transmit heat. It is a specific property of that material and is defined as the quantity of heat in Watts, which will flow through one square metre of surface area of the material one metre thick, when there is a temperature difference of one degree Celsius between its faces, i.e;
Wm/m2°C - By cancelling out m above and below the expression becomes;
W/m °C - Thus the units of k are expressed in Watts per metre per degree Celsius (W/m °C).
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2. Thermal Resistance (R)
Thermal Resistance is a measure of the resistance to heat flow, either of a material of any given thickness, or of a combination of materials. It may be regarded as the number of hours required for the transmission of one Watt through one square metre, when the difference between the outer surfaces is one degree Celsius. The units of R are expressed in m2 °C/W. If the thickness of the material is increased, there is a corresponding increase in thermal resistance and if several materials are placed together in parallel layers, the total thermal resistance of that combination may be obtained by adding together the resistances provided by the particular thickness of each component. The unit of thickness to be used is the metre and if thickness is expressed in millimetres, then this must be converted to metres.
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3. Thermal Resistivity (1/k)
Thermal Resistivity is the reciprocal of Thermal Conductivity and is expressed as m °C/W.
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4. Thermal Conductance (C)
Thermal Conductance is the rate of heat flow through a material of any given thickness, or through a combination of materials. It is the reciprocal of total thermal resistance i.e. C=1/R and is the quantity of heat in Watts which will flow throughout a square metre of a material or a combination of materials, in one hour, when the difference between the two surface temperatures is one degree Celsius. The units of C are expressed in W/m2 °C. This term is normally used to express the insulating efficiency of a polished metallic surface such as aluminium foil, used in conjunction with an air space. The former may be used alone or stapled or glued onto building boards or some other type of insulation. However, to function effectively, the foil must have a cavity with a minimum depth of 19mm. If the C value of a material or combination of materials is given, this can readily be converted to thermal resistance R units as follows: 1/C = R
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5. Thermal Transmittance (U)
The k, R and C values defined above, have been related to the temperatures at the surfaces of materials. The surface temperatures of a building are not usually known and for purposes of heat loss calculations, it is the inside and outside air temperatures that are used. The heat is first transferred from the inside air to the structure, then through the structure and finally, from the structure to the outside air. Both the inside and outside surfaces provide some resistance to heat flow and the thermal transmittance, or U value, takes into account these surface resistances.
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6. Phase Change
The process by which a substance transforms from solid to liquid or liquid to gas, as well as transformations in the reverse direction. Most common is Temperature Controlled Packaging (TCP). A CoolPac would be placed into the shipper in a frozen state and allowed to change phase to a liquid, thereby absorbing energy in an isothermal manner and stabilising the internal temperature.
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7. Heat Capacity
Heat Capacity is the amount of heat required to raise the temperature of a specific quantity of a substance by one degree Celsius. Symbol C (C p at constant pressure, C v at constant volume). This value is useful to calculate the temperature change in TCP when no phase change is taking place. The higher the value, the more stable the internal temperature.
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8. Specific Heat Capacity
Specific Heat Capacity is the quantity of heat needed to raise the temperature of unit mass of a particular substance by one degree Kelvin (Units W/Kg K).
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9. Latent Heat
Latent Heat is the energy required to change a substance from one state to another. Heat of fusion (Hf) = solid to liquid or liquid to solid. Heat of vaporization (Hv)= liquid to gas or gas to liquid. This is an important value for the design of Temperature Controlled Packaging (TCP), since it defines the amount of heat that can be absorbed isothermally.
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10. Energy Density
Energy Density is the amount of energy per unit volume. The term is used commonly to refer to food or batteries. In TCP design, it is used commonly to refer to the sum of the Heat Capacity and Latent Heat.
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11. Enthalpy
Enthalpy is the quantity of heat necessary to raise the temperature of a substance from one point to a higher temperature. The quantity of heat includes both latent and sensible.
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12. Delta T (δT)
This is short-hand for difference in temperature, usually referring to the difference between the ambient, or external temperature and the product temperature or system interior within the TCP. A high Delta T requirement, means that a system with a high performance is required.
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13. Wall Thickness
Thickness of the shipper walls being used in the TCP.
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14. EPS (Expanded polystyrene)
EPS is unlike other thermoplastic processes. The production of EPS products requires that the raw materials be pre-conditioned prior to their final "tooled" moulding process.The raw material (also known as "expandable polystyrene" or "bead"), has a spherical shape and is similar to sugar in appearance. This is the most commonly used material for TCP within Europe and has a thermal conductivity in the region of 33 mW/mK.
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15. PU (Polyurethane)
PU is used in a foamed state within TCP design. PU foam is made by adding water to polyurethane plastics. The material foams up after mixing and spraying and a tool is used to define the finished shape. The thermal conductivity is lower than EPS, in the region of 23 mW/mK. The finished material is also more robust and therefore more suitable for packaging which is to be reused.
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16. Vacuum Insulation Panel (VIP)
Vacuum Insulation Panels are made by placing a core into a bag of barrier material (film or foil) and evacuating and sealing the bag. The core provides a framework to give the panel thickness and the vacuum minimises the heat transfer by conduction and convection. The thermal performance and lifetime of the panel varies significantly with the materials used.
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17. Closed Cell
This describes the nature of foamed material used in the manufacture of insulated containers. The closed cell structure reduces the convection within the wall thickness. Normally the smaller the cell size, the better the insulation.
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18. Thermal Mass
Thermal Mass is the amount of potential heat storage capacity available. This term is commonly used in architecture and has been adopted by the TCP industry. Usually, it refers to the idea that a high thermal mass will have high temperature stability, thereby requiring more energy transfer to change temperature.
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19. Condition or Stabilise Product
To stabilise or condition products or CoolPacs they are stored at a temperature range. So, to condition CoolPacs, they are left at ‘factory temperature’ (typically in the range +15°C to +25°C) for a fixed period of time.
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20. IQ OQ PQ (Installation Qualification, Operational Qualification & Performance Validation.)
The process through which equipment and instruments must go, for them to be acceptable for use by the pharmaceutical industry.
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21. Ambient
The ambient profile is the temperature profile that any TCP is tested against, to show its performance. The ambient temperature is the temperature outside the packaging at any time. ‘Ambient product’ has been used in the past as a term for product which could be stored and shipped with no temperature control.
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22. Temperate Product
A term for product to be stored and shipped between temperatures such as +15°C and +25°C or +2°C and +25°C.
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23. Chilled Product
A term for product to be stored and shipped between +2°C and +8°C.
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24. Frozen Product
A term for product to be stored and shipped in a deep frozen state. Temperature ranges vary but generally below -18°C.
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25. Volumetric Weight
Calculation used in the logistics industry to charge for large parcels that weigh little. Various formulae are used but generally, Litres/6 = kg (volumetric).
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26. Phase Change Materials (PCM)
A Phase Change Material is matter which undergoes a phase change at a specific temperature and gives off, or takes in, large amounts of energy in the process. They are usually engineered in order to achieve a certain amount of temperature consistency within a system. Common usage of this expression is normally for materials other than water, where the phase change does not happen at 0°C. These materials usually have a lower latent heat.
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27. Isothermal
Isothermal means ‘at one temperature’. Ice will melt isothermically, thereby absorbing energy, but not increasing temperature.
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28. Dry Ice
Solid Carbon Dioxide, used to maintain product in a frozen state. Dry Ice changes directly from a solid to a gas (or sublimes) in normal atmospheric conditions at -78.5°C, without going through a wet liquid stage. Therefore, it gets the name "dry ice". It does need to be shipped as a miscellaneous dangerous good UN1845.
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