Evaporation and Condensation, or How to Drive Heat 'Uphill'

Vaporization,compression,condensation,expansion Choose a fluid, or refrigerant, which vaporizes at a lower temperature than the space to be cooled. Heat then flows from the cool space (downhill) and vaporizes the refrigerant. This is represented by the section 1 - 2 of the orange line. Then, instead of just letting it boil away and disappear as vapour, it is captured, and pressurized (2 - 3). At the high pressure its boiling point, or evaporating temperature is much higher. So it can condense at this higher temperature, giving up its latent heat which flows (downhill again) to the warm air outside (3 - 4). When it has become a liquid, the pressure is reduced (4 - 1) and the process can start again.

This is the most widely used process for providing cooling. It is called the Vapour Compression Cycle, and it finds application on equipment ranging from domestic refrigerators and freezers to large cold stores and building air conditioning systems.



Basic Pressure Enthalpy diagram showin the saturation curve

Pressure - Enthalpy Diagram (P-h Diagram)
In order to study this process more closely, refrigeration engineers use this pressure - enthalpy diagram. "P" is the symbol for Pressure, and "h" is the symbol for Enthalpy. This diagram is a way of describing the liquid and gas phase of a substance. On the vertical axis is pressure, and on the horizontal, enthalpy. Enthalpy can be thought of as the quantity of heat in a given quantity, or mass of substance. The curved line is called the saturation curve and it defines the boundary of pure liquid and pure gas, or vapour. In the region marked vapour, its pure vapour. In the region marked liquid, its pure liquid. If the pressure rises so that we are considering a region above the top of the curve, there is no distinction between liquid and vapour. Above this pressure the gas cannot be liquified. This is called the Critical Pressure. In the region underneath the curve, there is a mixture of liquid and vapour. In order to understand how this diagram is used, lets consider again water turning into steam in the kettle.


Constant Pressure Process, evaporation

Evaporation
The boiling of water, or vaporization of refrigerant is a constant pressure process. So it is represented by a horizontal line as shown. The water (or liquid refrigerant) starts off on the left side and as it warms up it approaches the left hand side of the saturation curve. At point 1 it starts to boil (like the kettle). Heat is being added so the condition point is still moving along towards the right. When point 2 is reached, all the liquid has turned into vapour. Point 2 is saturated vapour, because it only just contains sufficient enthalpy to be vapour. Between points 1 and 2 the liquid part, and the vapour part of the mixture are termed Saturated. If more heat is added, the vapour becomes hotter, this is called Superheated Vapour.





Constant Pressure Process, condensation

Condensation
Condensation occurs when the vapour turns back to liquid. The vapour starts off on the right hand side and as it cools down it approaches the right hand side of the saturation curve. At point 3 it starts to condense and this continues until point 4 when all the vapour has turned into liquid. Point 4 is saturated liquid. If more heat is removed, the liquid cools. It is then called subcooled liquid.

Condensation is shown at a higher pressure than evaporation, but either process may take place at any pressure below the critical pressure. For refrigeration, when the liquid has been used up by evaporation at low pressure which cools the cold space, the vapour is compressed to the higher pressure where condensation can occur. At the higher pressure, heat can be lost to the atmosphere and the refrigerant becomes high pressure liquid. To complete the process all that's needed is raise the pressure of the vapour by drawing a vertical line upwards, and then allow the liquid pressure to diminish so that the liquid can get to point 1 and start again. Simple as that? Take a look at the Vapour Compression Cycle.

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