USING AIR-T0-AIR HEAT EXCHANGERS TO ACHIEVE YOUR CLEAN AIR QUALITY NEEDS

Poor air quality is a risk to human health. The average human inhales over 220 million particulates of matter under the size of 2.5 micrometers per day. That’s over 2,500 per second. 

Luckily, through heat exchange and ventilation, there are steps that your facility can take to limit the amount of pollutants and airborne contaminants inhaled when indoors.  Discover how using Air-to-Air Heat Exchangers (AAHX) in your building or home can improve indoor air quality and reduce your PM2.5 intake.

How Energy Recovery Systems Work

Most Air Handlers, especially in large buildings, implement some type of “energy recovery” heat exchanger to help condition incoming air being used to control the climate of the building. During this process, a filter removes PM contaminants so that the outside air being pushed inside is safe to breathe. 

The heat exchange process, however, requires a considerable amount of energy. Choosing the right heat exchanger technology is a significant operational and economic cost driver.

HVACs, for instance, are considered long-term energy recovery systems because they can operate efficiently while reducing long-term energy costs. Therefore, the three main design considerations for recovering energy with heat exchangers are

1. Thermal Performance
   • The heat exchanger’s ability to efficiently condition air and heat or cool it to the desired building temperature.
2. Energy Recovery
   • The energy consumption level of the component that’s required for efficient heat exchange.
3. Operating Cost
   • The costs required of the energy recovery system based upon its thermal and energy recovery processes.

The more effective your heat exchanger, the closer you can get the incoming air to the desired building temperature, which leads to reduced operating costs.

These heat exchangers typically utilize passive (no energy) or low-energy solutions. Therefore, the operation of the heat exchanger is much cheaper than allowing heaters or cooling coils to fulfill the entire heating/cooling process.

Implementing an air-to-air heat exchanger at this point relieves the load on the higher operating cost heaters and cooling coils.

Types of Energy Recovery Systems

The best energy recovery heat exchangers typically use passive heat exchange, which requires low or no energy consumption. This makes operating costs much cheaper than allowing heaters or cooling coils to fulfill the entire heating/cooling process.

Two of the more effective options in terms of energy recovery systems for side-by-side airflow systems are enthalpy wheels and Air-to-Air Heat Pipe Heat Exchangers (AAHX).

Enthalpy Wheels

An enthalpy wheel is a type of air-to-air heat exchanger that essentially mixes fresh outdoor air with the conditioned exhaust air of the building through rotary ventilation. 

What makes them unique is their ability to transfer sensible and latent heat, allowing for the recovery of thermal energy and moisture. The latent heat exchange helps reduce operating costs that would otherwise be required to humidify/dehumidify incoming air.

As shown in Figure 1., the direct contact mixing is effective from a heat exchange perspective, however, it’s less efficient in removing contaminants in fresh air that are collected through air circulation within the building. Additional filters and maintenance are often required to meet clean airflow requirements, which can lead to downtime or added costs.

Figure 1. Energy (Enthalpy) Wheel Operation

Enthalpy Wheels At a Glance

Advantages:

• High heat recovery efficiency.
• Transfer temperature (sensible) and humidity (latent).
• Work year-round — can use exhaust energy to heat or cool incoming air. 

Disadvantages:

• Imperfect sealing results in leaks or contamination between ducts. Therefore, they do not always meet zero-transfer requirements.
• Geometric limitations — require side-by-side and equal-sized ducts.
• Require maintenance personnel to keep all moving parts operating. 
• Parts need to be replaced when they wear out.

Air-to-Air Heat Pipe Exchangers (AAHX)

An alternative option to enthalpy wheels is AAHX — a technology that completely isolates the two airstreams, resulting in zero cross-contamination between airstreams. 

This makes them an ideal heat recovery heat exchanger for HVAC systems in hospitals, cleanrooms, and other high occupancy facilities where air cleanliness is of critical importance.

The heat pipes provide extremely efficient heat exchange to fin stacks that span the entire volume of the air duct. While the heat pipes span both airstreams, the AAHX is designed such that no air enters the opposite duct. See Figure 2 for the operating principles of ACT’s AAHX.

Figure 2. Air to Air Heat Exchanger Operation

As outside air enters the heat exchanger, the liquid in the heat pipe boils, reducing the temperature of the heat entering the air conditioning system. This means less energy consumption for your air conditioner to reduce the heat to a comfortable indoor temperature. 

Additionally, any debris, dust, bacteria, etc. that is collected within the building, is safely exhausted to the outside without any risk of recirculating or contaminating the fresh incoming air. 

Many buildings with clean requirements often state “no cross-air contamination” in their HVAC specification. The AAHX can meet this requirement while providing efficient heat exchange that requires no energy consumption. 
Plus, they can be implemented into almost any geometric configuration. They can be installed flat, vertically, horizontally, or in split-loop thermosyphon formations.

AAHX At a Glance

Advantages:

• High heat recovery efficiency using heat pipe technology. 
• No cross-contamination between incoming and outgoing air streams. 
• No size or space requirements.
• Little to no maintenance compared to other heat recovery systems.

Disadvantages:

• Less effective for latent heat exchange.
• Temperature limitations — only work when ambient air is below the system’s minimum operating temperature.

If you’re looking for cost-efficient, reliable clean-air building technology, the AAHX should be the top consideration.

This blog was originally published on July 7th, 2020. It was updated on November 27th, 2023.

Explore all energy recovery solutions at ACT.