In ASHRAE’s guidance: “Well-designed and well-maintained air-to-air energy recovery systems should remain operating in residences, commercial buildings and medical facilities during the COVID-19 pandemic. This is because maintaining at least normal outside air ventilation rates, with proper temperature and humidity conditioning of the inside space, is important for maintaining health and combatting infectious (bio)aerosols.”1
This article will touch on the correct design and application of energy recovery devices. The two main focal points will be 1) how to reduce the amount of cross contamination of exhaust/return air into the outside/supply airstream, and 2) what are the implications if you increase the amount of outside air. It is the last of a three-part series that included filtration and dilution in response to the COVID-19 pandemic.
ASHRAE Standard 1060 “Performance Rating of Air-To-Air Heat Exchangers for Energy Recovery Ventilation Heat Equipment’ established certified performance ratings for different type of energy recovery devices. The standard defines ‘exhaust air transfer’ (EAT) as the amount of cross contamination from the exhaust/return airstream into the outside/supply airstream. The rate of EAT is referred to as ‘exhaust air transfer rate’ (EATR). Below is a depiction of EATR in an energy recovery unit.
In general, the EATR is dependent on the amount of airflow in the two airstreams as well as the differential pressure between the two airstreams. During these times of the pandemic, one of the recommended methods to improving IAQ is by increasing the amount of OA. From the diagram above exhaust/return air from the space can be considered to not be as ‘clean’ as the outside air being brought into the space. Therefore, EATR entering the outside airstream would reduce the amount of outside air and therefore diminish the indoor air quality. Please note that depending on the level of contamination of the exhaust/return air, ASHRAE Standard 62.1 (Ventilation for Acceptable Indoor Air Quality) allows EATR to be up to 5% and 10% of the outside airflow.
The Building Readiness document has three recommendations to be incorporated into a ‘well designed’ ER system. These three items are:
- The supply and exhaust fans are located correctly for pressure control at the exchanger.
- Size the ERV for an appropriate velocity and air pressure drop.
- Specify appropriate seals and purges for the application.
Each of these recommendations are means to meet the minimum ASHRAE 62.1 requirements for a) outside air ventilation rates and b) the 5 to 10% acceptable EATR levels.
The TC5.5 document has recommendations on how you can EXCEED 62.1 guidelines while using an ER device. The two methods on how to further improve your indoor air quality are 1) reducing the amount of EATR and 2) increasing the amount of outside air.
One method to reduce the amount of EATR to negligible levels (as opposed to the 5% to 10% allowable by ASHRAE 62.1) is to specify an appropriate energy recovery device. One such product with ultra-low EATR’s is Semco’s model #TC3 ‘True 3A’ Energy recovery wheel. It has ASHRAE 1060 certified performance for an EATR of 0.045 % when properly applied. Semco’s true 3A ER wheel is available in multiple size up to 70,000 cfm; and can be applied singly, in self-contained units, or as retrofits of existing wheels. In addition to its low EATR, it also incorporates an anti-corrosion and anti-stick coating to reduce cross contamination. However, in these times of heightened IAQ concerns, its best feature is probably the fact that the wheel faces and transfer media contains an anti-microbial coating which actively limits microbial growth.
To further ensure achieving ultra-low EATR’s during all operating conditions and not just at design airflows is to incorporate Semco’s Health and Safety Monitor Control System to their ERW. This system actively monitors the varying airstream temperatures and pressure while adjusting the speed of the ERW to ensure that cross contamination does not occur.Please contact your HCN salesperson for assistance in designing and applying Semco’s products.
A method to eliminate all cross contamination (or an EATR of 0.00) is to design an ER recovery system that has the two airstreams separated from one another. Two examples of this type of ER systems are 1) refrigerant based heat pipes and 2) a ‘Run-Around’ Loop that utilizes water or glycol. The refrigerant heat pipe system uses the heat of vaporization and condensation to transfer energy between the two airstreams. A ‘Run-Around’ loop places a hydronic coil in the two different airstreams and then has associated piping, pumping, and controls to transfer energy between the two airstreams. HC Nye represents both types of systems though Heat Pipe Technologies (HPT). Please contact your HCN salesperson for assistance in designing and applying HPT’s products.
Increasing the amount of Outside Air through an ER device is the other method to improve IAQ. Increasing the amount of OA beyond 62.1 ventilation rates could result in your equipment being even more oversized during minimum operating conditions. At all operating conditions it is important that you still maintain comfort conditions with respect to temperature and humidity levels. Please recall that the epidemic task force is asking you to consider designing and operating equipment at three very different conditions:
- with OA quantities greater than ventilation code minimums
- at minimum ventilation code rates
- at minimum operating conditions.
At a minimum, you should specify equipment that can operate in a stable and accurate fashion at these three different points. In a packaged piece of DX equipment, it is desirable to provide these components:
- Compressors – two-stage, digital scroll, or inverter driven.
- Fans – direct drive with EC motors or VFDs.
- Space humidity – modulating hot gas reheat.
- Heat – Electric heaters with SCR control or gas heat (modulating or staged) .
These components also allow for the greatest flexibility for potential operating conditions. Given that all these components are variable and operate both simultaneously and independent of each other (within limits), the control system must be robust enough to accommodate the diverse operating points with diverse operating sequences. We recommend that these controls should be factory installed.
In the ‘Overall System Considerations’ in the TC5.5 document, the first recommendation is to evaluate if your existing ER system can accommodate this increase. Packaged DX equipment located on a roof is the most common type of ER system installed today. Most of these ER systems are installed where there is recirculation of the return air from the space. Please see the diagram below that depicts an ER unit with recirculated air as well as an ER device for the outside air.
Providing a solution to increasing airflow across the heat wheel are the Aaon model RN and RZ rooftop units - available from 6 to 240 tons. For each of the different cabinet sizes there are two (2) different sized ER wheels available, a ‘small’ or ‘large’ face area. Each of the wheels has different maximum airflow available, however, in each instance the height and width of the two airstreams remains the same. If one wants to increase the amount of outside air, there may be a situation where a ‘small’ wheel was previously installed but a ‘large’ wheel is now required. In these instances, it could be relatively easy to replace the wheel. The wheel/cassette assembly would just be swapped out because the overall face area is the same. Please note that in some instances the depth of the wheel would change; and this would also have to be evaluated to make sure there is enough space.
Another method to reduce the EATR and increase the OA ventilation rate would be to evaluate the replacement of the existing ER device. In most instances these devices are an energy recovery wheel that recover sensible and latent energy. ER wheels come in many different types of construction, face areas, depths, air pressure drops, EATR’s, and thermal effectiveness.
HC Nye represents four (4) different energy wheel manufacturers that are available for replacement work. Each of these manufacturers have different strengths and weaknesses that could be investigated to determine the best possible solution. If you increase the amount of outside air you would have to also consider the following items:
- Can the existing fan and air distribution system handle the higher internal APD?
- Can you still maintain comfort conditions for space temperature and humidity?
- Are you still maintaining the necessary air pressure relationships? Do you also have to increase the amount of exhaust air (EA)?
Prior to COVID-19, two of the industry’s driving forces were energy conservation and Indoor Air Quality (IAQ). Sometimes these two forces were at odds. Ventilation rates had been decreasing over time and energy recovery had been mandated by codes adopting ASHRAE standards. Now with ventilation rates precipitously increasing as a reaction to the pandemic, ASHRAE has provided guidance on energy recovery systems.
For almost 40 years HC Nye has been a strong advocate for indoor air quality and has many products available to assist the HVAC community. Your H C Nye sales engineer will be glad to assist as your address your IAQ concerns.