Optimize interior ventilation and humidity

Introduction

As residential, commercial and industrial sectors understand the essential nature of fresh air to the health and safety of the occupants in each of the buildings, technologies to recuperate energy from outgoing exhaust air for transfer to incoming cold fresh air has become more and more common. Thermal wheels, runaround loops and plate exchangers are but a few examples of technologies being used to create comfortable and safe work environments for occupants. However, a nontrivial question becomes evident when dealing with these technologies: how do we manage the humidity in the air given that all systems recover heat in various settings? To answer this question, we shall use counterflow plate (air to air) exchangers as a practical example. A counterflow plate exchanger is like a crossflow design in terms of orientation of the plates, however the difference lies in the direction of the fluid flows. Figure 1 demonstrates the flows visually, where in a crossflow exchanger, fluids cross at 90 degrees; counterflow exchanger fluids flow in opposite directions.

In counterflow plate (air to air) exchangers, two types of technologies are readily available to ventilate spaces efficiently: heat recovery and energy recovery. The type of technology selection is entirely based on the application and should be looked at through the lens of humidity requirements and control.

Heat Recovery Ventilation (HRV)

Some industries do not require the control humidity while introducing fresh air, making HRV an ideal choice. Sectors such as commercial laundries, textile washing plants, and greenhouse agricultural facilities often do not need controlled humidity levels to maintain safe and efficient operations, thus applicable to HRV technology.

HRVs utilize impermeable plates to transfer sensible heat while preventing airstream mixing, ensuring effective thermal exchange without transferring moisture. As a result, any condensation in the exhaust air stream must be carefully managed and this to avoid any frost buildup in winter months.

Frost-Prevention and Defrosting Strategies in HRV

1. Water Drainage: Eliminates the accumulation of condensation through a series of channels and pipes. Though there is a risk of freezing, a properly designed system can avoid such a problem.
2. Preheat Coil: Warms incoming air above the frost threshold using electric, steam, or hot water heating, reducing heat recovery efficiency but ensuring continuous ventilation.
3. Performance Modulation: Adjusts heat exchanger effectiveness—increases the flow of outgoing stream and reduces the flow of incoming stream—to minimize frost formation at the cost of reduced heat recovery and efficiency.
4. Exhaust-Only Defrosting: Temporarily shuts the supply fan, using exhaust air to melt ice. Simple and cost-effective but may interrupt ventilation and cause negative pressure.

Energy Recovery Ventilation (ERV)

On the other hand, some industries and especially commercial and residential sectors need to maintain humidity for comfort and to limit disease spreading. This can be explained because mucous membranes dry out when relative humidity levels hit 20%, and this allows for disease to spread at exponential speeds throughout population centers. Tighter humidity control is also important for clean rooms such as in the pharmaceutical or microelectronic sectors (to help limit contamination). In this case, when changing the air, one can maintain humidity control (saving on humidification costs) all while also eliminating frost buildup in the exchanger core.

For this, Energy Recovery Ventilation is very important. Instead of impermeable metal plates that only transfer sensible heat, ERV technology allows for the transfer of humidity through small porous openings in the plates that allow for the vapour particles to pass from a humid air stream to the adjacent drier one by vapour pressure gradient, allowing for moisture and latent heat without phase change. Because both heat and moisture are transferred, it reduces stress on humidifiers in winter and dehumidifiers in summer (depending on climate).

Considerations in ERV technology efficiency

To best select the right technology, we must better understand the technical subtleties of each technology. Because HRV technologies use highly conductive, optimized and large surface areas (aluminum or polymer), they are extremely efficient at transferring sensible heat. However, ERV technologies need to incorporate perforations to allow for the humidity to be transferred between streams. This requires small frames or cell walls that inevitably reduce active surface area and optimum sensible transfer design. However, this less efficient sensible transfer allows the ERV exchanger to gain latent transfer, thus allowing for better overall transfer efficiency, and lowering the burden and humidification and heating equipment.

So back to the original question, which technology is most optimal? For cold and dry climates or where humidity is not an issue, the HRV technology is more useful and applicable. Additionally, if one wants to save considerable costs, HRVs are also much less expensive. Where humidity and heating systems are solicited and where humidity is an important factor, ERV is essential, will be significantly more efficient overall and allow for much quicker payback periods.

Conclusion

Choosing between Heat Recovery Ventilators (HRV) and Energy Recovery Ventilators (ERV) depends on both the application and the efficiency requirements. ERVs typically come at a higher cost but offer enhanced moisture control, which can impact overall energy efficiency and greenhouse gas reduction strategies. Selecting the right system requires evaluating ventilation needs to ensure optimal performance and sustainability. If you have any questions regarding these technologies, you may contact your Energir energy efficiency advisor for more information.