Air-to-air Heat ExchangerComponent

General Overview

Air-to-air heat exchangers transfer energy between two airstreams that must be at different temperatures for sensible heat transfer and different moisture contents for latent heat transfer.

In a building, an air-to-air heat exchanger can be used to recover latent or sensible heat either during the cooling or heating season. Recovering energy from the exhaust stream lessens the energy load on the air handling plant. Typical air-to-air heat exchangers include fixed plate heat exchangers and rotary wheel heat exchangers, with the former typically only recovering sensible heat.

Table 1 shows the plant and system configurations that may contain an air-to-air heat exchanger.

Evaluation of Heat Transfer

Table 2 provides a summary of measurements needed to quantify the annual energy transfer and operating characteristics of an air-to-air heat exchanger.

Measurement Strategy

The measurement strategy for an air-to-air heat exchanger is to measure the heat transfer (Btu/h) from airstream to airstream within the heat exchanger, as well as any energy consuming components associated with the heat exchanger, such as the energy wheel motor.

Figure 1 shows the measurement locations in a fixed plate heat exchanger.

Figure 2 shows the measurement locations in a rotary wheel heat exchanger.

Measurement Equipment

Table 3 provides the equipment required to carry out the measurements of this component.

Table 3. Measurement equipment.

Equipment	Description	Measurement (Units)
DENT ELITEproXC Portable Power Data Logger (EXCUNC) DENT 16” RoCoil Flexible Rope Current Transformers (CT-R16-A4-U)	Provides a measurement of true RMS power from voltage and current inputs and records long-term power (kW) and energy (kWh) measurements. Requires ELOG19 software and a USB connection cable for programming and downloading data files.	True RMS Power (kW)
Onset HOBO MX1101 logger	Measures and records temperature and relative humidity. Using bluetooth technology you can transmit your data wirelessly to your mobile device using the HOBOconnect app. Stores 84,000 measurements.	System Air Temperature (F) Relative Humidity (%)
Onset HOBO Temperature/Relative Humidity Weatherproof Data Logger (MX2301)	Records outdoor air temperature and relative humidity using internal sensors. Requires HOBOware software and a USB connection cable for programming and downloading data files.	Outdoor Air Temperature (F) Relative Humidity (%)
Hot Wire Anemometer (Blue) (TSI 9545)	Simultaneously measures and logs data for several ventilation parameters using a single probe with multiple sensors. It measures velocity, temperature, and relative humidity, and calculates flow, wet bulb, and dew point temperature.	System Air Temperature (F) Relative Humidity (%)

Calculation Methodology

The general methodology for quantifying the energy impact of an air-to-air heat exchanger is to determine the temperature differential on the supply side of the heat exchanger and the supply airflow. If latent energy is also transferred, this is determined by the humidity differential across the heat exchanger. The supply flow rate can be measured or assumed, depending on available resources. These energy transfer values can be regressed against a controlling variable (such as outdoor air temperature) to develop a regression model. Depending on the variability of operations, daily or weekly models may be developed to better characterize the component.

This methodology only applies to systems with rotary wheel heat exchangers.

Click the button below to go to the calculators for this component.

Air-to-air Heat Exchanger Heat TransferCalculation

For a rotary wheel heat exchanger, the equations and calculator in this page estimate the annual sensible and latent heat transfer across the rotary wheel in an energy recovery ventilation (ERV) unit.

Further Reading

• ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 1. HVAC SYSTEM ANALYSIS AND SELECTION. I-P Edition.

• ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 26. AIR-TO-AIR ENERGY RECOVERY EQUIPMENT. I-P Edition.

• CenterPoint Energy (2022). “Runaround Loops.” Energy Recovery Systems. https://www.centerpointenergy.com/en-us/Services/Pages/Runaround-loops-MN.aspx?sa=MN&au=bus

• Heatex (2022). “Heat Exchanger Basics.” https://www.heatex.com/knowledge/heat-exchanger-basics.