Heat Exchangers




Heat exchangers are mechanical devices designed to exchange or transfer heat from a hot fluid to a cold fluid. Heat exchangers are used heavily throughout the HVAC and Refrigeration field, for example a condenser or evaporator in a chiller is simply a heat exchanger. A cooling or heating coil is a heat exchanger that transfers heat from one fluid to another fluid. A chilled water air handling unit transfers heat from the hot air to the chilled water.

There are many different types of heat exchangers that will be briefly discussed, but first it is important to understand the two classifications of heat exchangers, counter-flow and parallel flow heat exchangers. These two classifications describe the relation of the direction of flow between the cold and hot fluid. First the parallel flow heat exchanger, this heat exchanger has both the cold and hot fluids entering at the same end of the heat exchanger. At the beginning of the heat exchanger there is a large difference between the cold and hot fluids and at the end of the heat exchange the difference between cold and hot is reduced, refer to the figure below.

Heat Transfer Parallel Flow Heat Exchanger for the Mechanical PE Exam

The counter-flow heat exchanger is opposite of the parallel flow heat exchanger. The cold and hot fluids enter at opposite ends. The figure below shows the counter-flow heat exchanger, notice the change in directional arrows.

Heat Transfer Counter Flow Heat Exchanger for the Mechanical PE Exam

LOG MEAN TEMPERATURE DIFFERENCE (LMTD)

In heat exchangers that do not have a phase change, heat is transferred from the hot fluid to the cold fluid through the temperature difference between the cold and hot. However, in a heat exchanger as shown in the previous figures, the temperature difference between the cold and hot fluids is not always constant and depends on the location in the heat exchanger. Thus the log mean temperature difference is used. The LMTD describes the logarithmic average temperature difference between the cold and hot fluid through a generic heat exchanger (counter or parallel). LMTD cannot be used for heat exchangers with a phase change like a boiler or condenser. The equation for LMTD is shown below.

Heat Transfer R-Value for the Mechanical PE Exam

The LMTD is then used to calculate the total heat exchanged by the heat exchanger through the following equation. The U-value is the heat transfer coefficient of the heat exchanger which is given by the heat exchanger manufacturer. The Area value is the total area where heat exchange occurs, which is given by the heat exchanger manufacturer.

Heat Transfer R-Value for the Mechanical PE Exam

HEAT BALANCE

Often times in the HVAC and Refrigeration field, a heat balance is conducted on a heat exchanger to show that a balance of heat loss from the hot fluid is shown as a heat gain to the cold fluid. For example, cooling coils are heat exchangers that transfer heat from air to water. The heat balance governing this heat transfer would be as shown below.

Heat Transfer R-Value for the Mechanical PE Exam

If there is a phase change, then the following equation can be used. Heat balances are discussed further in the Refrigeration Section, Mechanical Systems and the Psychrometrics section. Basically, heat balances are integral to the HVAC and Refrigeration field, but luckily the equations governing a heat balance are fairly simple.

Heat Transfer R-Value for the Mechanical PE Exam