The quest for cold, a matter of temperature and pressure -
© Glassghost / Flickr CC BY-NC (via The Conversation)
Without refrigerators, no anti-Covid vaccine (in any case, that of Pfizer / BioNTech which must be kept at a temperature of -72 ° C), according to a study published by our partner The Conversation.
These devices achieve the permanent feat of maintaining, or even increasing, a temperature difference between their contents and the room in which they sit.
The analysis of this phenomenon was carried out by Jacques Treiner, theoretical physicist, researcher associated with the LIED-PIERI laboratory of the universities of Paris.
A refrigerator, however banal the object is today, achieves a permanent feat: to maintain, even increase a temperature difference between two spaces "in thermal contact" - the interior of the appliance and the room where it is located. installed.
The issue has taken on a new dimension since the development of the anti-Covid vaccine developed by
Pfizer / BioNTech
, because it must be maintained at a temperature of -72 ° C.
A freezer in a Milan hospital that keeps Covid vaccines at a temperature of -75 ° C © Nicola Marfisi / AGF / SIPA
Obtaining and maintaining such a temperature difference with the ambient temperature seems a priori contrary to natural processes: indeed, when two bodies at different temperatures are in contact, a heat transfer takes place spontaneously from the hottest to the coldest, so that the temperature difference ends up canceling out, even though there is insulation between the two.
But when food is placed at room temperature in a refrigerator, it cools, in other words the device ensures a transfer of heat from the inside (the food) to the outside (the room). that is, from the coldest to the hottest.
How is it possible ?
To understand it, we have to go back to some fundamentals of thermodynamics.
Questions of principle
Let us imagine to begin with a fluid contained in a pipe in contact with a medium outside the temperature.
With the help of a motor, the pressure in the pipe can be varied.
It is first assumed that the fluid is and remains in a gaseous state.
If the pressure of the gas is increased, its temperature also increases (think of a bicycle pump), and when this temperature becomes higher than
T
ext
, heat transfer occurs spontaneously from the fluid to the external environment, through the wall. of the pipeline.
If the pressure of the gas is reduced, its temperature decreases, and when it becomes less than
T
ext
, heat transfer occurs spontaneously from the external environment to the fluid, always through the pipe.
Now suppose that we place a valve at a point in the pipeline, which opens when the pressure rises above a chosen value.
When the valve opens, the gas undergoes an expansion, as when opening a camping gas canister.
This expansion is accompanied by a sudden decrease in temperature, as can be seen by touching the metal near the cartridge valve.
Two very distinct modes are thus obtained along the pipe: a high pressure and high temperature part downstream of the engine and produced by the latter, and a low pressure and low temperature part downstream of the valve.
That's all we need to make a refrigerator.
First, put the high temperature part of the pipe in thermal contact with the kitchen - this is the function of the grill found at the back of a refrigerator.
By touching it, it is observed that its temperature is higher than ambient temperature, and its large surface area promotes heat transfer.
A refrigerator therefore warms the kitchen.
Next, a valve, placed just before the pipe enters the rear wall of the appliance, so that the low temperature part of the pipe is in thermal contact with the food.
The heat transfer therefore takes place spontaneously here too.
Thus, by two spontaneous heat transfers, an overall heat transfer is achieved which is not.
Refrigeration cycle © Modified by Elsa Couderc from Homer Landskirty, Wikipedia, CC BY (via The Conversation)
From a practical point of view, to avoid pressures that are too high or too low in the pipe, arrangements are made so that, in the high pressure part, the gas liquefies.
Thus, the pressure remains constant - as during any change of phase - and the heat necessary to pass from the gaseous state to the liquid state, called "latent heat of condensation" is added to the heat transfer to heat the gas. outside.
Similarly, downstream of the valve, the fluid passes from the liquid state to the gaseous state: the “latent heat of vaporization” is taken from the food in the refrigerator.
Make sure to stay above the temperature of the "triple point" of the fluid
It should always be ensured that the lowest temperature and pressure reached by the refrigerant in the circuit is such that it does not freeze: if it becomes solid, this would block its circulation and paralyze the entire system.
Take the example of water.
The liquid-vapor equilibrium depends on the pressure.
It is known that water boils, at atmospheric pressure, at 100 ° C.
But if you decrease the pressure, the boiling temperature drops.
Thus, at the top of Mont-Blanc, the boiling point is only 85 ° C, and it is 72 ° C at the top of Everest.
Water phase diagram. The triple point of water is at T = 0.01 ° C and P = 611 Pascal (i.e. 0.006 atmosphere) © Maghémite, Wikipedia, CC BY-SA
Rather than planning a mountain expedition, we can explore this temperature / pressure relationship in the laboratory.
It suffices to place the water in a closed vessel connected to a suction pump.
It is then observed that the boiling temperature and pressure decrease together, and that when the pressure reaches the value of 0.006 times atmospheric pressure, the boiling temperature is no more than 0.01 ° C.
Better still, a new phenomenon then appears: if the gas continues to be sucked in, the temperature no longer decreases, but a small ice cube forms.
As long as the three phases coexist, temperature and pressure remain constant.
The liquid turns into both a gas and a solid, the latent heat of vaporization being provided by the latent heat of solidification.
This point of coexistence of the three phases is called the "triple point".
When all the liquid phase has disappeared, only a solid phase (ice) remains in the vessel, in equilibrium with water vapor.
If we then continue to suck in the steam, the temperature and pressure decrease again.
This experiment teaches us that one cannot observe a liquid phase at a temperature lower than that of the triple point.
All bodies behave this way, with, of course, different triple points.
It also follows that the lowest temperature that a refrigerator can reach cannot be lower than the triple point of the refrigerant.
For example, the temperature of the triple point of carbon dioxide is - 56.6 ° C, that of freon is - 103.3 ° C, that of ethane is - 183.3 ° C.
Keep vaccines cold, but how cold?
The temperature at which a vaccine should be stored before use is related to the chemical stability of the active molecule.
If the molecule is too fragile, simple “thermal agitation”, that is to say the movement of atoms relative to each other under the effect of heat, can degrade it.
Usual vaccines can be stored in a simple refrigerator at a temperature close to 0 ° C.
They are relatively stable proteins.
But messenger RNA molecules, which form the chemical basis of the vaccine developed by Pfizer / BioNTech, are more fragile molecules.
This is the reason why they should be stored at a lower temperature - 72 ° C in the case of Pfizer / BioNTech.
Our "Temperatures" file
This does not pose a problem in principle: there are sufficient refrigerant fluids which can be used to reach these temperatures using refrigerators operating according to the principles described above.
On a practical level, however, several difficulties must be overcome.
For example, as we have seen, the lower the temperature, the lower the pressure.
Now, the fluid must then be compressed to reach the temperature allowing good heat transfer to the outside of the device.
In an apparatus suitable for the Pfizer / BioNTech vaccine, it is necessary to go from -72 ° C to a temperature above ambient, say 30 ° C, in order for the heat transfer to the outside of the refrigerator to take place.
The lower the starting pressure, the more powerful and larger the engine will be.
Sometimes this compression has to be done in two steps, which increases complexity and cost.
In addition, the engine contains moving mechanical parts which must be oiled.
On leaving the compression, the oil must be separated from the refrigerant fluid, otherwise it will solidify in the low temperature part and disturb the system.
In the case of Moderna's vaccine, also authorized in Europe, it can be stored at - 20 ° C.
Other candidate SARS-CoV-2 vaccines should be able to be stored in refrigerators rather than freezers.
Electricity consumed by domestic refrigeration
One of the first electric refrigerators with its heat exchanger on top © Birmingham Museums Trust, CC BY
In a refrigerator-freezer, to obtain a temperature in the freezer compartment different from that of the refrigeration part, we used to aim for the freezing temperature, and we placed a heating resistance in the door of the refrigeration part!
This was obviously a waste of energy.
Today, devices are equipped with two motors, one for each function - the energy savings achieved outweigh the cost of the additional motor.
Significant progress has also been made in limiting heat exchange through walls.
Today's appliances are therefore much more energy efficient than 50 years ago.
However, the overall consumption of refrigerators has not decreased, it has even increased.
Indeed, energy savings have been more than absorbed by the increase in the size of devices and the increase in the fleet: this is what economists call the “rebound effect”.
To give an order of magnitude, the consumption of all domestic food refrigeration (37.5 TWh) represents today in France five times more than the electricity consumption of the entire SNCF train network ( 7.5 TWh)!
It takes more than 4 nuclear power plants (around 1 GW, but they are not turned on 100% of the time) to keep our yogurts and beers cool ...
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This analysis was written by Jacques Treiner, theoretical physicist, researcher associated with the LIED-PIERI laboratory of the universities of Paris.
The original article was published on The Conversation website.
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Covid 19
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