Cold. What is it all about?
From a physical point of view, heat is a form of energy which causes the disorderly agitation of molecules within a substance. The more the matter is cold, the less the molecules are agitated and the matter is more solid. The more the matter is hot the more the molecules are agitated and the lighter the matter is. We will see further on that this impact on density makes warm air lighter than cold air.
This energy which we call heat, transmits itself from a hot matter to a cold matter. This transfer does not stop until the two matters share the same temperature.
Why use cold supply with an isothermal container?
An isothermal material will slow down these heat intakes until the inside and outside environments are at the same temperature. Therefore, an isothermal container alone cannot maintain the stability of the desired temperature. A cold supply is necessary: active cooling, passive cooling or cryogenic cooling…
What is an isothermal container? How to compare using lambda and the coefficient K?
To evaluate the performance of an insulating material we use the value of the lambda thermal conductivity coefficient (λ) in W/m.K. The lower the λ, the more insulating the material.
The most commonly used materials in isothermal systems and their associated lambda values are as follows:
|Polyurethane PUR :||0,020- 0,025|
|Expanded Polystyrene PSE:||0,029-0,034|
|Extruded Polystyrene XPS:||0,031-0,038|
|Vacuum insulated panel VIP:||0,007|
To evaluate the performance of an isothermal container, we use the value of the thermal transmission coefficient K in W/m.K which presents the loss of the container as a whole. The lower the value of the k coefficient, the more efficient the isothermal container is.
The reference values in the cold chain are:
• K < 0,70W/m².K for the transport of fresh products;
• K < 0,40W/m².K for the transport of frozen products.
How to produce cold? Why are we talking about active cooling and passive cooling?
Several physical phenomena can be used to produce cold. We present two of them to you:
We only observe this phenomenon when we use an aerosol. The compressed gas in the tank quickly expanded. This expansion is accompanied by a cooling that we feel when we touch the aerosol nozzle. On the other hand, when we inflate the tyres on a bicycle, we compress the gas. The bicycle pump becomes hot.
This phenomenon is used in the case of active cooling. A motor with an electrical or mechanical power source, compresses and expands a gas in a closed circuit. The part of the circuit where the expansion takes place cools the isothermal container. A conventional refrigerator works on this principal.
The material can be found in three states: solid, liquid and gaseous. At constant pressure, the material changes state by absorbing or releasing heat.
Therefore, when iced water melts, it absorbs heat to move from solid (ice) to liquid (water). This phenomenon is called fusion. The environment is then cooled.
It is this phenomenon that is used in passive cooling when the supply of previously frozen eutectic plates releases cold in an isothermal container.
Active cooling uses a cooling unit that requires a continuous source of energy, either electric or using fuel.
Passive cooling uses an energy source such as a battery previously charged with cold: PCMs or eutectic plates.
Cryogenic cooling or the sublimation of dry ice
The fusion of PCMs and the eutectic plates or why is it necessary to completely freeze the gels or plates used in passive cooling?
The graph below shows the rise in temperature of a block of iced water over time.
- In the solid state (segment 1), iced water rises very quickly from -18°C to 0°C.
- In the two-phase state of an ice + liquid mixture (segment 2), the water remains constant and for a long time at its melting temperature: 0°C. This is the phase change stage. This stage stops as soon as the last gram of solid material has melted. It is this level that is used in passive cooling.
- In the liquid state (segment 3) the water temperature rises rapidly from 0°C to 15°C.
Point A on the graph
Therefore, whilst a completely frozen plate melts and its contents turn into a liquid state, it will cool its environment to a constant temperature, here 0°C for 40 minutes.
Point B on the graph
Whilst the plate is in a two-phase state it will cool its environment to a constant temperature but over a shorter time than its total mass can allow, here 0°C for 25 minutes.
Point C on the graph
When the eutectic plate is in a liquid state, it will bring a certain inertia. By acting as a buffer it will slow down the rise in temperature of the thermosensitive product (segment 3 on the graph) but it will not maintain its environment at a stable temperature of 0°C.
Two parameters influence the melting time and the rise in temperature of the eutectic:
- The temperature of the environment: the higher the outside temperature, the less time it will take to become 100% liquid eutectic.
- The overall mass of the ice plate: the larger the mass, the longer it will take to become 100% liquid eutectic.
Therefore, like a fully charged battery, it is important to freeze the eutectic and PCM plates completely to the core in order to take advantage of the cold recovery phase to its highest potential.
Heat is transmitted in three ways…
- By conduction: the heat is transferred by molecule to molecule contact. We can feel this transmission when we heat a metal ruler at one end and feel the heat arrive at the other end. The agitation of the metal molecules has been conducted from one molecule to the next and therefore from one end of the ruler to the other. Air is often used as thermal insulation because it is a very poor conductor of heat and therefore a good insulator.
Therefore, insulation materials – such as polystyrene foams, polyurethane, glass wool or cotton wool – trap air in the form of a bubble. A vacuum being the best insulator. Since no molecules are present no thermal conduction is possible.
- By radiation: heat is transmitted as an electromagnetic wave. We feel this phenomenon when we move from a shaded area into direct sunlight or when we are in front of an open fire. The parts of our body exposed to the sun or fire feel the heat whereas the part of the body which remained unexposed remains cool.
Materials with a silver/gold coating and a very smooth exposure surface reflect radiation and absorb only a small part of the heat. These materials guarantee good quality thermal insulation.
- By convection: this phenomenon only concerns fluids (gas and liquid). The difference in fluid density associated with gravity results in a less dense and ‘lighter’ upward movement of the hot fluid when the denser, lower fluid falls. We observe this phenomenon in a pan of heated water. The water is moving.Insulating foams prevent heat transmission by convection since no air movement is created between the trapped air bubbles of the structure. The convection phenomenon is created from a certain distance between the hot and cold fluid.
A short distance such as in double glazing, where the air gap between the two panes does not exceed 2cm, no thermal convection is possible.
What is a PCM? What is a eutectic?
A PCM is a material that, under the effect of temperature, changes from a solid to a liquid state (and vice versa) while maintaining the same temperature. PCMs therefore store energy in the form of latent heat. Heat is absorbed or released when the solid state changes to the liquid state.
It is this battery phenomenon that we use in passive cooling to cool an isothermal container. This results in keeping the isothermal container cold or cooling.
The PCMs can be classified into three main categories:
- Organic PCMs
These are paraffin and fats. They have the advantage of offering a wide range of holding temperatures but their latent heat relative to volume is rather limited.
- Inorganic PCMs
These are mainly hydrated salts. Their latent heat relative to volume is high but the temperature for the holding phase (solid and liquid) of the cooling may be more unstable.
- Eutectic PCMs
This is a mixture of at least 2 pure matters (organic or not). The resulting melting point temperature is lower than that of either the two substances that have been mixed. The most well-known eutectic is the water + salt eutectic (H20 + NaCl) often used on icy roads to lower the melting point temperature of the ice.
What is temperature?
- The centigrade or Celsius scale: at atmospheric pressure at the coast, 0°C corresponds to melting and 100°C corresponds to boiling.
- The Kelvin scale, whose lower limit 0 K corresponds to -273°C, is the absolute rest state of molecules. There is no agitation in the molecules of matter.
- The Fahrenheit scale, the reference frame of which can be recalculated on the Celsius scale using the relationship T(°F) = 9/5 T(°C) +32
Which PCM to choose? At what temperature should the eutectic / PCM plates be prepared?
- When we are looking to transport frozen foods we use negative PCMs such as PCM-26. Its phase change stage will hold the temperature at -26°C. To melt, the PCM-26 must first be solid. It must therefore be conditioned at a temperature below -26°C. Our recommendation is to place it at least 8°C below its phase change level – therefore -34°C.
- For the transport of perishable foodstuffs we recommend using PCM-3 or PCM 0 (gelled water). Their respective phase change stages of -3°C et 0°C will hold the temperature between 0°C and 4°C/6°C. Similarly the PCM must be solid beforehand. We recommend to condition it at -18°C.
- In the case of thermosensitive products that must be held at positive temperature levels, such as 15-25°C, we use a PCM +20. The conditioning temperature of this PCM will depend on the outside temperature. If the external temperature to which the isothermal container will be subjected is higher than the melting level – i.e. a temperature higher than 20°C – the PCM must be made solid. If the outside temperature to which the isothermal container will be subjected is below the melting point – i.e. a temperature below 20°C – the PCM must be made liquid. Our recommendation is therefore to place the PCM 20°C in the refrigerator during the summer and 30°C during winter.
Type of liquid
Recommended freezing temperature
-29°C to -23°C
-12°C to 0°C
0°C to +4°C
PCM 0 (eau gélifiée)
0°C to +12°C
2°C to 8°C
0°C maximum in summer 12°C minimum in winter
15°C to 25°C
4°C maximum in summer 30°C minimum in winter
How and for how long should the PCM and eutectic plates be reconditioned?
To accelerate conditioning, the eutectic plates must be separated from each other inside the freezing chamber. Stacked storage or packing them in cardboard boxes or on pallets prevents the circulation of cold air and considerably increases their freezing time. It takes up to 1 week to freeze the eutectic plates to the core.
To facilitate the efficient conditioning of the PCM and eutectic plates we recommend the use of plate distribution trolleys or shelves to organise and spread out the eutectic plates.
To ensure the plates are always available, a double set of eutectic plates makes it possible to set up a system of rotation: when one set of plates is being used the other set is being recharged.
Soft gels, absorbent gels, solid gels, plates… how do you find your way around?
- Soft gels, also known as gel packs are flexible bags made up of an opaque or transparent film containing eutectic or PCM in gel form. These packs are lighter and more economical than a rigid pack due to the saving made from not having a rigid shell. However, they must be frozen flat to give them a homogenous frozen shape.
- Absorbent gels are flexible gels whose film making up the pouch is made of absorbent material. Therefore, the condensation and water drops that appear when melting conventional gels, are absorbed by this woven coating. We recommend this coating to protect heat sensitive products from moisture. As with traditional soft gels they should be frozen flat to give them a homogenous frozen shape.
- Rigid gels have a rigid outer shell often made of high-density polyethylene (HDPE) to resist impact, compression and puncture. These shells are then filled with the desired eutectic liquid or PCM and sealed with a crimped cap. They are available in a variety of different sizes and weights from 250g to 1kg.
- Eutectic plates have the same characteristics as rigid gels. With weights ranging from 2kg to 4kg, eutectic plates have more standardised sizes called Gastronorm. Currently we find:
> GN 1/1 plates with dimensions 530 x 325 x 30 mm
> GN 1/2 plates with dimensions 325 x 265 x 30 mm
What is eco-responsible urban distribution?
- Reduction of gases, pollutants and particles related to fuel consumption to improve air quality.
- Reduction of noise pollution.
- Relieving traffic congestion by pooling transport, developing night deliveries, deliveries by electric mopeds or bicycles and multimodal deliveries.
Our CarryTemp solutions are noiseless and autonomous from an energy point of view. Moreover, they can easily adapt to clean and new last miles vehicle: cargo bike, electrical scooter, electrical or CNG (Compressed Natural Gas) vehicle.
The ATP, what should I know?
This United Nations treaty, which entered into force in 1976, was signed by 48 countries.
In European countries having contracted the ATP, the transport of perishable foodstuffs is subject to two types of regulation:
- An obligation to achieve results: the food must be transported at the temperature indicated in the legislation.
- A duty of care requiring the use of a transport system with a certificate of ATP technical conformity. The text is available here: Text of the ATP Agreement on UNECE website.
The ATP sets out the thermal insulation and refrigeration performance requirements necessary to transport perishable foodstuffs. This obligation may be waived for transport of less than 80km without reloading i.e. without opening the doors of the refrigerated unit.
In other words, the distribution of perishable foodstuffs for a distance of less than 80km and with several delivery points must:
1. Justify correct temperatures for maintaining perishable foodstuffs;
2. Use ATP approved solutions, insulating sufficiently and with good refrigeration capacity.
How to benefit from an ATP certificate?
A measure of the K coefficient, insulation coefficient, determines the isothermal quality of the equipment:
The container does not have insulating quality.
The container is ‘Normally Insulated’ (known as ‘IN’), sufficient for the transport of products in positive cooling.
The container is ‘Heavily Insulated’ (Known as ‘IR’), necessary for the transport of products in negative cooling.
For an outside temperature of 30°C, a refrigerating unit will be classified into 4 classes if it lowers and maintains the inside temperature for more than 12 hours:
Class B (K Coefficient shall be less than 0.4W/m²K)
Class C (K Coefficient shall be less than 0.4W/m²K)
For example, ColdandCo’s CarryTemp containers have obtained the ATP agreement as a reinforced class C and D refrigerated unit.
For example this ATP label on a ColdandCo CarryTemp box indicates that it is a reinforced isothermal unit or a reinforced refrigeration unit – if combined with eutectic plates – of class C.
This CarryTemp container was assembled in June 2019 and its first 6-year approval is valid until June 2025.
Temperature conditions to be observed for the carriage of chilled foodstuffs:
|Either at +6°C or at temperature indicated on the label and/or on the transport documents|
|Either at +2°C or at temperature indicated on teh label and/or on the transport documents|
|On melting ice or at temperature of melting ice|
Temperature conditions to be observed for the carriage of quick (deep)-frozen and frozen foostuffs: