Vapour cycle cooling system in aircraft

Explain the vapor cycle cooling system used in aircraft with neat illustration. Also explain the physics behind the scene of working.

Physical principle

Liquids can be vaporized at any temperature by changing the pressure acting on it.  To clearly understand this concept, we will take an example of water contained in a vessel. When the vessel is at atmospheric pressure, the water will be boiling at 100°C when heating. If I am pressurizing the vessel to more than the atmospheric pressure, then water will not boil at 100°C. If I am creating a drop in pressure in the vessel by a vacuum pump, then water will boil at a temperature less than 100°C.

Basic law of thermodynamics states that heat will always flow from hot body to cold body. If I need a reverse of this, I have to add some work.

System operation:

1. We are having the FREON as refrigerant in the vapor cycle cooling system. It has a boiling point of 4°C.

2. At the receiver I am having high pressure, so that FREON will have high boiling point.

3. When the vapor cycle system is switched on, the compressor starts delivering the pressure and thus making flow.

4. The highly pressurized FREON at the receiver is in liquid phase. When the Freon flows through the circuit, first it expands at the Expansion valve. So pressure has been dropped (i.e. Boiling point decreased).

5. The less pressure Freon then goes to the evaporator stage. Evaporator will be exposed to Cabin. We blow the warm air of cabin over the evaporator coils by fan, and thus doing a forced convection.

6. The heat transferred to the Freon makes it to change the phase which is from liquid to vapor.

7. The less pressure Freon vapor is then compressed by the Compressor and thus it delivers high temperature high pressure Freon vapor.

8. Now this high pressure and high temperature Freon vapor enters the Condenser coils where the cool air from atmosphere will be blown over the coils (here too making a forced convection). Condenser will be exposed to the Atmosphere. Because of heat transfer the Freon losses heat and returns to liquid phase.

9. Then it goes to the receiver (high pressure low temperature Freon liquid)

10. The cycle continues as stated. Before doing any type of maintenance activities to the vapor cycle system, we have to purge the system with inert gas in a open atmosphere.

11. Freon is colorless, odorless, and non toxic; however, being heavier than air, it will displace oxygen and cause suffocation. When heated over an open flame, it converts to phosgene which is deadly!

12. To know the Freon level in the circuit a sight glass arrangement will be employed between Receiver to Expansion valve. If the unit requires additional refrigerant, bubbles will be present in the sight glass otherwise steady.

Note:

When using this method air conditioning, pressurization have to be done separately, either by using Cabin superchargers or Engine driven compressor or by using your future design that delivers pressure with least energy taken as input.