Salt Hydrate Based Positive Temperature PCMs
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These PCMs are water-based like the sub-zero product range, but they freeze and melt at temperatures higher than 0 °C (32 °F). They also use different salts to alter the phase change temperature, in fact in some cases the same salt can be used in different concentrations in water to produce both positive temperature and negative temperature PCMs. The positive temperature PCMs utilise the ability of some salts to form complexes with water and to bind it up as water of crystallisation, hence "salt hydrates". This means that although the salt crystal is solid, it may contain over 50% water. When the crystal is heated it melts, releasing the water of crystallisation and allowing the salt to dissolve in this water. This melting process is similar to that of water, in that it absorbs large amounts of energy in the form of latent heat, which is reversed when the solution freezes.
The prefix "S" in the product range stands for " Hydrated Salt Based PCM Solutions"
PlusICE Salt Hydrate PCM Solutions
Using this phenomenon it is possible to obtain water based PCMs that freeze and melt at temperatures up to 117 °C (242.6 °F), thereby making water based PCMs that are still frozen at temperatures higher than the boiling point of water!
Some salt hydrate compositions freeze and melt cleanly, and the liquid phase is exactly the same chemical composition as the solid phase. These are said to be congruent and exhibit a eutectic point. Others are semicongruent, in that on melting a lower salt hydrate and an aqueous solution are formed, which need to be recombined before freezing to avoid loss in performance. However, semicongruent melting PCMs can be modified with additives to render them congruent. The final type of salt hydrates exhibit incongruent melting, in which the anhydrous salt and an aqueous solution are formed on melting. Unless modified, the anhydrous salt settles out due to gravity and is difficult to recombine on freezing, thereby resulting in a gradual loss in performance of the PCM. The most suitable method of preventing this is to thicken the salt solution, so that when anhydrous salt crystals are formed they remain dispersed throughout the PCM, making it easier for them to redissolve in the aqueous solution.
The PCMs can be supplied in prefilled icepacks or balls as described on the Products page, or can be used on site to fill the customer's own containers. Some the phase change materials available from PCM Products have been thickened to the extent that they are a thick paste. Bearing this in mind, it is sometimes difficult for the customer to handle these materials when filling their own containers. Others can be poured like water when melted. We therefore recommend checking with us how the PCM is supplied, and discussing usage options before obtaining large quantities. Salt hydrate PCMs must be stored and used in air-tight containers to prevent exposure to the atmosphere. Failure to do this would result in loss of water via evaporation which would severely affect the performance of the PCM.