Get ready for the laws of thermodynamics. Don’t worry. We’ll go easy on you as we explain how heat exchangers work in the HVAC industry.
What are heat exchangers?
Just as the name suggests, heat exchangers are devices that enable us to transfer or exchange thermal energy (also known as heat energy).
Most people assume that heat exchangers only produce heat. No, that’s not the case. Heat exchangers can also cool fluids.
Nevertheless, there is a condition—There must be a temperature difference for heat exchange to occur. Also, heat always flows from a hot to a cold region.
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Methods of heat transfer
There are only three methods of heat transfer:
- Convection.
- Conduction.
- Radiation.
Most HVAC heat exchangers rely a lot on conduction and convection. Nonetheless, radiation still occurs, but only a little bit.
We will now go ahead and explain these three methods of heat transfer.
Convection
Conduction is heat transfer via fluids. By fluids, we are referring to both liquids and gases. The process occurs both naturally and mechanically.
With the help of devices like fans, we can initiate convection by mechanical force. The best example is when you try to cool a hot spoonful of soup by blowing air onto it. The air carries away the heat, and the soup cools.
Conduction
Conduction involves the transfer of heat in solids.
For example, take a cup of hot water and place it on a table. After a few minutes, lift the cup of water and feel the table. You’ll notice that the table now is warm.
Radiation
Radiation is the transfer of heat energy via electromagnetic waves. Unlike the others, radiation doesn’t require a medium. It’s also safe to say that radiation is the transfer of heat through space.
Did you know that you also emit some thermal radiation?
The working fluids in heat exchangers
Heat exchangers have several fluids that do the “heat exchange” work—That is why we refer to them as the “working fluids.” These fluids include water, steam, a water-glycol solution, and a refrigerant.
For heat purposes, we use hot water and steam. On the other hand, refrigerants and chilled water are for cooling purposes.
Types of heat exchangers
There are two main types of heat exchanges:
- The coil heat exchangers.
- The plate heat exchangers.
The coil heat exchangers
The coil heat exchangers are the most commonly used. They involve several tubes coiled in insulated containers.
Most of the time, the insulated container contains cool fluid. Hot air is then sucked from your living space and passed through the coiled tubes. Since the coiled tubes are in contact with the cool fluid, the cool fluid will absorb the heat and cool the air. The now cooled air is then pumped back into your living space.
The plate heat exchangers
Instead of coiled tubes, we now use plates to separate the two fluids.
Remember, for heat exchange to occur, the fluids must be of different temperatures (One cold and the other hot). They must also flow in opposite directions to further improve the heat transfer between them.
During a hot day, the fan sucks the hot indoor air and conveys it through the plates. Remember, the other fluid is still cold. The cool fluid will then absorb the heat from the indoor air, thus lowering its temperature. The now cooled indoor air is then pumped back into your living space.
HVAC application of the types of heat exchangers
Finned tube coil
You must have seen them somewhere. We also refer to finned tube coils as heating or cooling coils.
Have you ever looked at the back of your refrigerator? Those are the finned tube coils. You’ll also find them in ductwork and air conditioning systems (As evaporators and condensers).
Their operation is simple: Hot air flows outside as the refrigerant (cooling fluid) moves on the inside, and vice versa.
When heating, the cool indoor air flows in the inside of the tubes. We then convey hot air on the outside. The cool indoor air then absorbs heat from the hot air. We then pump the now hot air back to your living space.
Duct plate heat exchangers
This is an application of the plate heat exchangers. They are common in air handling units. They involve passing two streams of fluids through a system of plates. The streams have to be of different temperatures and moving in opposite directions. We use them for both cooling and heating purposes.
How air conditioners use heat exchangers
We use air conditioners to cool homes during the hot summers.
The refrigerant is the crucial fluid in air conditioners. Therefore, it is always on the move as it transfers heat from one point to another.
At the start, the refrigerant is at low pressure in the evaporator coils. When you turn on your air conditioner, the fan sucks and blows air across the evaporator coils. The refrigerant then absorbs the heat and cools the indoor air. The fan then sucks the now cold indoor back to your living space.
After absorbing heat, the refrigerant turns to vapor. The refrigerant is then transferred to the compressor and converted to a high-pressure gas. From there, the refrigerant flows over the condenser. The condenser takes away the heat, and the refrigerant becomes a high-pressure cold fluid. Once again, it’s ready to absorb heat from indoor air.
The process goes on and on in a cycle.
NB: The evaporators and condensers are the heat exchangers.
How gas furnaces use heat exchangers
We use gas furnaces to heat homes during the freezing winters.
First, the burners in gas furnaces produce combustion gases. The gases then pass through the first heat exchanger (usually at the top). As that happens, the blower forces the cool indoor air over the outside of the heat exchanger.
The heat exchanger then facilitates the heat transfer from the hot combustion gases to the cool indoor air. The now warm air is then pumped back into your living space.
Do you now understand the importance of heat exchangers in the HVAC industry?
