A direct expansion (DX) cycle is the process used in cooling mode which uses refrigerant as the medium of heat transfer. This is also known as the refrigeration cycle. The refrigerant is used to reject or absorb heat from the air to cool the building
The DX cycle is composed of 4 main components that transfer heat or increase/decrease the pressure and temperature of the refrigerant.
- Expansion Valve - reduces the refrigerant pressure and temperature. This device is also important when verifying what the superheat is for the system.
- Evaporator Coil – a heat exchanger allowing the refrigerant to absorb heat from the air that passes over it, cooling the air stream that is delivered to the space. The refrigerant goes from a liquid state back into a vapor. The evaporator not only cools the air but also pulls moisture from the air as well, reducing how humid the building space will feel.
- Compressor – compresses the vapor refrigerant to increase its pressure. It is the heart of the system as this is what moves the flow of refrigerant. The compressor is a vapor pump, meaning it can only pump refrigerant in the vapor state. Liquid refrigerant can damage the compressor.
- Condenser Coil – a heat exchanger that rejects the heat from the refrigerant to the outside air. The outside air is cooler than the refrigerant so the refrigerant will exit as a cool liquid. The refrigerant goes from a vapor state to a sub-cooled liquid state.
All these components are connected with copper refrigerant piping and create a loop for the refrigerant to circulate. When the system is in cooling mode, the refrigerant will circulate through this closed-loop system extracting the heat from the air of the building space and rejecting it to the outside.
A good way to visualize this is to think of having a bowl of hot soup. To cool the soup off, you blow over it. The air is cooler than the hot soup so it absorbs the heat from the soup and transfers it to the air passing over it, leaving the soup to be cooler. This is in essence the same thing that happens with the DX coils. The condenser has hot refrigerant running through it while the cooler outside air is passing over it, absorbing the heat and leaving the refrigerant in a condensed, liquid state. The evaporator has cool refrigerant running through it while the warmer space air is passing over it. The refrigerant absorbs the heat from passing through the coil and leaves cooler air to enter back into the space.
A residential home’s refrigerator also employs a DX cycle. The difference between a refrigerator and an HVAC system is the condenser on the refrigerator is not located outdoors. Ever wondered why the refrigerator is unable to cool a home when the door is opened? It is because the condenser is in the back of the refrigerator and is rejecting that heat into your home instead of to the outside.
Note: Leaving your refrigerator door open will actually make your home hotter.
The DX cycle is also used in heat pumps. An advantage of a heat pump is that it uses the DX cycle to provide cooling or heating depending on the needs of the space. The heat pump accomplishes this through a reversing valve in addition to the 4 other components of the DX cycle. The reversing valve allows the evaporator and condenser to change roles (since they are both heat exchangers) based on the space thermostat’s signal for heating or cooling.
When it is in cooling mode, a heat pump functions like the DX system functions as previously discussed. With a heat pump, the evaporator (indoor coil) is now the condenser and the condenser (outdoor coil) becomes the evaporator. When heating mode is enabled, the heat pump uses a component called a reversing valve to alter the refrigerant flow. This allows the heat exchanger located in the space to be hot and reject heat, while the heat exchanger outside the building now absorbs heat from the outside and adds it to the refrigerant. In other words, instead of rejecting the heat from the refrigerant to the outside, the heat is now rejected into the space.
It is important to note that the outdoor air is not actually doing the heating or cooling, it is simply providing the opportunity for the refrigerant to reject or absorb heat, just like how we described the example of the bowl of soup. The other cycle components do the work in providing the heating or cooling. Outside air temperatures do impact the efficiency and capacity of the DX cycle since it becomes more difficult to absorb really cold air or reject heat into already very hot air. This is why it is a common concern of a heat pump when the outside air temperatures could get low enough that the unit heating will not be enough and will need to be supplemented to maintain desired leaving air temperatures.