We are often challenged with terminology that seems too technical to bother about and we dismiss the words and fail to understand the concept behind them... but often these key terms drive our Freeze Dry system performance so it’s worth thinking about some more.
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Triple Point |
A low pressure point where water, steam, and Ice can all exist. Below this point Water cannot exist and when sublimation energy is added, Ice will transform from a solid to a vapour without moving through the liquid phase |
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Saturated |
Where the item described as saturated is in close contact to its liquid or solid state - Saturated vapours are capable of condensing back into their parent body (e.g.: a gas bottle containing some liquid also contains saturated vapor/ ice vapours leaving ice are saturated vapours). It is not possible to superheat a saturated vapour |
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Latent heat |
Latent heat does not add to the temperature of product. It is the energy necessary to change of form (water to ice/ water to steam/ steam to water/ ice to water) without any increase in temperature of the water (or product) .Latent heat energy is also consumed in saturated refrigerants. |
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Sublimation |
Vaporising ice to vapour form at low pressure to prevent liquid phase collapsing product |
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Super heat |
When vapour leaves the presence of its liquid or solid form and is further heated it is said to be superheated – this can occur with refrigerants and sometimes in the flow of sublimated ice vapours in ‘Secondary drying’ |
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Sub cooling |
More common in the refrigeration systems connected to the freeze dryer. Sub cooling is important because it can create an increase in refrigerant performance , and used wisely can help deliver better dryer performance |
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Heat recovery |
Heat recovery is a system that captures waste heat that can be used again in process i.e. to heat defrost water or fresh air via a heat exchanger. |
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Sensible Heat |
Sensible heat is energy that contributes to raising the temperature of a product. |
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Vapor Pressure |
Science has publish tables for common items that can be affected by vapour pressure – Water is a good example: water will boil 100°C(212°F) at atmospheric pressure (1000millibar) If we lower the ‘vapour’ pressure then the saturated water will boil at lower pressure. We can use saturated vapour tables to calculate what the new boiling point would be at a different pressure. ‘Water’ will boil at about 1°C at 7millibar. |
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Primary Drying |
This is the term used to describe the first Freeze drying phase where ice vapour is sublimating from the saturated (frozen) product and latent heat change is occurring (Ice to vapour) |
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Glass transition |
Symbol Tg – A point of condition where frozen material becomes slightly malleable. This is before the thaw point and can be different for different products. Components such as salt, oil, fat, sugar,& solvent and change the Tg which is responsible for collapse of shape when the vapor pressure is increased to this point |
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HTF |
Heat Transfer Fluid – a solution used to convey energy. Unlike a refrigerant, HTF can only transfer sensible heat. It may contain anti-freeze properties to work at low temperatures. |