How Heat Exchangers Work

A heat exchanger enables your HVAC unit to provide heating or cooling and your water heater to provide hot water. The basic principle behind how a heat exchanger works is that it moves heat from one place to another. Using a hot fluid, it can provide heat; a cold fluid lets it cool a space.

In a heat exchanger, a fluid can be in liquid or gaseous form. To further understand the inner workings of a heat exchanger, it helps to know heat always flows from hot to cold, and a temperature difference is needed for this flow to occur. A heat exchanger enables this to happen with no physical contact between the air and the medium (water, steam, refrigerant, etc.). The two elements always remain separate from one another.

How Heat Exchangers Transfer Energy

The methods by which a heat exchanger can transfer thermal energy include:

• Conduction: Involves physical contact between two materials of different temperatures. In the case of a heat exchanger, two fluids remain separate, but conductivity between separate elements (i.e., tubes for coolant and fluid/air passing outside them) drives the process.

• Convection: Thermal energy is physically transported away by fluids. You can trigger convection by blowing onto a spoon full of soup. The heat is carried away by air currents, thereby cooling the soup enough for you to eat. Most HVAC fans work in this fashion.

• Radiation: Radiation is a process in which heat is emitted via electromagnetic waves. The hotter something is the more thermal radiation it will release. Radiation is just a small factor when it comes to HVAC systems, although it can play somewhat of a role.

Heat Exchanger Function By Type

The types of heat exchangers most commonly used in residential settings include:

• Coil Heat Exchanger: The general concept involves tubes running back and forth several times through the unit. While one fluid passes through the system inside the tube, a separate fluid flows through the heat exchanger outside the tube. Convection allows heat to transfer from a heated inner fluid, via the tube wall, and into the outer fluid, which then transports the heat away.

A common example is the fin tube coil integrated into air handling units, ductwork systems, and AC evaporators and condensers. You’ll also find a fin tube coil behind a refrigerator. Some electric heaters in ducts and furnaces use an open coil design; it’s placed directly in the path of airflow.

• Plate Heat Exchanger: In this design, the two fluids are separated by thin plates of metal. These fluids usually flow in opposite directions. The hotter fluid convects heat onto the plate wall. The heat is then conducted to the other side and absorbed by the lower temperature fluid, which then uses convection to move the heat away.

This concept is used in air handling units with duct plate heat exchangers, usually made of aluminum; thermal energy is exchanged between the intake and exhaust streams without transferring moisture.

• Shell and Tube Heat Exchangers: Typically used in the evaporator or condenser, this design features tubes that carry fluid through a shell, where a second fluid is contained. The fluids (usually water and refrigerant) never mix but thermal energy passes through the tube walls. Where each fluid is found in the heat exchanger depends on its design.

More Examples of How Heat Exchangers Work

Radiators found in older homes to heat rooms are heat exchangers. Usually connected to a boiler, they circulate hot water to provide heating. Water heaters have a heating element with a metal coil to generate heat, which conducts into the surrounding water. Rotary wheel heat exchangers can be used in air handling units to reclaim heat from exhaust steam; a thermal wheel captures heat or cold air from the intake stream, depending on the season.

A more advanced design, a microchannel heat exchanger builds on the finned tube coil concept. It uses convection as its primary method of heat transfer and is used for refrigeration and air conditioning, including air-cooled chillers, air dryers, and rooftop units. Furnaces also use heat exchangers. While traditional units use only one, high-efficiency furnaces use a primary heat exchanger that handles the hottest flue gas and a secondary heat exchanger to take in combustion exhaust and extract latent heat from water vapor that forms.

Contact Black Hills Home Services

How heat exchangers work is an important consideration for just about all home heating and cooling systems. You can depend on Black Hills Home Services to manage your AC, furnace, and heat pump installation, repair, and maintenance requirements. To schedule AC and heating, plumbing, or electrical services in Olympia or the surrounding area, contact us online or call 888-530-7677 today.