How to Calculate Heat Exchanger Efficiency

Air conditioners use spiral heat exchangers to transfer heat and create cool air.

Heat exchangers heat or cool stream flows from one piece of equipment to another; transferring heat to and from liquid, gas, or multi-phase mixtures in vapor or condensate form without mixing them. Heat exchangers are found in boilers, furnaces, refrigerators, air conditioning units and other pieces of mechanical equipment. You use temperature, moisture and enthalpy formulas to calculate heat exchange efficiency. Alternatively, plug necessary data into an online calculator.

Things You'll Need

• Engineering formulas
• Data on temperature, moisture, and enthalpy

Instructions

1. Calculate temperature transfer efficiency. Temperature transfer efficiency is equal to the temperature (degrees Celcius) of outside air after the heat exchanger minus the temperature of the outside air before the heat exchanger, divided by the temperature of the inside air before the heat exchanger minus the temperature of the air outside before the heat exchanger. The resulting percentage reflects the efficiency of the heat exchanger in maintaining the inside and outside air temperatures during the heat exchange process.

2. Calculate moisture transfer efficiency. Moisture transfer efficiency is equal to the moisture of the outside air (expressed as kilograms per kilograms) after the heat exchanger minus the moisture of the outside air before the heat exchanger divided by the moisture of the inside air before the heat exchanger minus the moisture of the outside air before the heat exchanger. The resulting percentage reflects how efficiently humidity is transferred from inside a building to the outside environs.

3. Calculate the enthalpy transfer efficiency. Enthalpy accounts for the total energy contained within the system in addition to the energy transferred to the environmental resulting from the expansion of the system. Enthalpy transfer efficiency (expressed as kilograms per kilograms) is the enthalpy of the outside air after the heat exchanger minus the enthalpy of the outside air before the heat exchanger divided by the enthalpy of the inside air before the heat exchanger minus the enthalpy of outside air before the heat exchanger. The percentage reflects the amount of energy lost during the transfer of heat from one state to another.

4. Calculate air flow through the heat exchanger, which is an optional calculation. Air flow is expressed as cubic meter per hour of air flowing through the heat exchanger. Air flow will affect the efficiency rate of transfers in that particulate deposits create resistance, decreasing efficiency rates.

5. Develop percentage values. The percentage values from these calculations represent how efficiently a heat exchanger transfers temperature, moisture and enthalpy between different states of matter. The efficiency value tells you how well the heat exchanger operates, for example, how efficiently it removes heat from water to create cold water.

Tips & Warnings

• Heat exchangers can be important components for building energy efficiency. A heat recovery system on top of a building can recover heat and then pump it back inside, where it can be recirculated to heat the building.

References

• Engineering Toolbox: Heat Exchanger Efficiency
• University of New South Wales: Heat and Mass Transfer in Air to Air Enthalpy Heat Exchangers
• University of California: Modeling Particle Deposition on HVAC Heat Exchangers

Resources

• NJ Tech News : Energy Management Systems