HVAC; How it works.

HVAC refers to a complete system of Heating, Ventilation, and Air conditioning. It could refer to the Air conditioner in your room or the large system used in High Rise buildings.

HVAC equipment could perform Heating and/or Cooling. Using the same refrigeration cycle it could act as a heat pump for heating or an air conditioner for cooling.

How it works.

Every refrigeration system has 4 key components that make it work. These are the evaporator, the condenser, the compressor, and the expansion valve. The job of these four main components is explained in the refrigeration cycle.

Key things to understand before we continue.

§  Water boils at 100 °C only at atmospheric pressure, if the pressure is reduced, water would boil at a lower temperature. Boiling point and condensation point temperatures are not fixed because they vary with pressure.

§  A refrigerant absorbs heat from the environment when it evaporates.

§  A refrigerant boils (evaporates ) between -20 °C to 10 °C depending on the type.

§  When water is heated to 100 °C the temperature stops rising, instead, the heat applied from the burner is used to change the state of water from liquid to gas. This is called Latent heat of Vaporisation.

§  When a refrigerant begins to evaporate it gets latent heat from the environment making the surrounding environment cooler in the process. Heat is absorbed when the liquid refrigerant changes to vapor.

§  When a gas expands without doing any external work, the temperature changes. In fact, it becomes cooler.

§  When a gas is compressed rapidly, the temperature increases, it becomes hotter.

§  Hot travels to cold until the temperature becomes equal.

§  Heat is removed through a process of boiling and condensing the refrigerant in the refrigeration cycle.

The Refrigeration Cycle.

I would start the refrigeration cycle from the Compressor. The compressor is a device used to increase the pressure of a gas by reducing the volume. So when we first turn on the Air conditioner, the compressor immediately picks up low-pressure refrigerant gas. 

As it compresses the gas, the temperature of the refrigerant gas (vapor) rises along with the pressure. The refrigerant leaves the compressor as a hot, high-pressure gas. The temperature at the compressor outlet is far higher than the atmospheric temperature.

The hot refrigerant vapor now flows into the condenser coil (Outdoor Unit). The condenser coil is basically a heat exchanger. A fan blows across the condenser coil to cool the refrigerant. Remember that hot travels to cold which means that the condenser would dump the refrigerant heat into the environment because it is hotter than the environment. As the refrigerant vapor cools down within the condenser, it condenses into a warm high-pressure liquid.

At the end of the condenser, we have an expansion valve. The expansion valve allows sudden expansion of the liquid refrigerant and regulates the rate at which liquid refrigerant enters the evaporator (Indoor Unit). The sudden expansion of the refrigerant causes pressure to drop suddenly and the gas to cool as per Joule-Thomsons effect. As the pressure drops some of the refrigerant liquid evaporates (vaporizes or boils). The heat required for vaporization of the liquid refrigerant is absorbed from the warm room air blown across the evaporator coil making the room air cooler.

The refrigerant vapor inside the evaporator coil is now colder than the room temperature as it passes through the evaporator coils.  The evaporator coil like the condenser coil is also a heat exchanger. Remember again that hot travels to cold so as the fan blows the room temperature air over the evaporator coils any refrigerant liquid that was not vaporized by the expansion process absorbs the heat from the room and begins to boil (vaporize). Remember that a refrigerant boils between -20 °C to 10 °C

By the time  the refrigerant gets to the end of the evaporator it is all gas(vapor) again ready to be compressed so that the heat absorbed in the room can be rejected outside and the cycle continues over and over until the room becomes very cold.

EndNote:
Compression increases the temperature and expansion decreases the temperature.  Compressing the gas first doesn't do anything to make the subsequent expansion more effective at cooling the gas.  But if, after you compress the gas, you remove the heat (which is then easier because it's hot), then the expansion will cool it to a lower temperature than it started.  This is basically how refrigeration works.

Joule-Thomson effect is defined by https://www.britannica.com as the change in temperature that accompanies the expansion of a gas without production of work or transfer of heat.