Energy recovery ventilation: A technology for the 21st century

June 28, 1999
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As we draw near the end of a millennium, the North American hvac market is constantly undergoing challenges. This includes the need to improve overall system efficiency while providing indoor building environments with acceptable air quality levels.

Rapid growth in communications has changed how we do business, and has created a consumer awareness that far exceeds the knowledge of the 1980s.

In short, IAQ is no longer a buzzword. It has become part of the building industry vocabulary.

Indoor building environments have undergone major changes over the last two decades. The energy crisis of the 1970s stimulated change on how our structures were erected and systems were designed, operated, and maintained.

Unfortunately, many of these changes have led us to the situation that we have had to deal with in the mid to late 90s — inadequate indoor air quality.

With more than 60,000 registered chemicals used in industry, many of which can be found in building products today, the need to displace those contaminants with clean, fresh air is imperative.

ASHRAE (American Society of Heating, Refrigeration and Air-Conditioning Engineers) Standard 62-89, “Ventilation for Acceptable Air Quality,” has been the hvac guideline to minimize any adverse health effects due to improper levels of IAQ. More than 95% of the market currently applies this standard as a baseline for minimum levels of ventilation.

As most people are aware, these levels are much higher than the levels applied in the previous decade. Therefore, equipment that was designed for ventilation loads in the past, is not necessarily designed to meet the new loads created from ASHRAE 62-89.

However, traditional equipment has been applied to try and meet these requirements. Nonetheless, buildings are faced with a whole new problem: an increase in indoor relative humidity.

Handling Humidity

In areas where wetbulb summer designs (based on new ASHRAE 97 Fundamentals) rise above 70°F, providing up to three times the amount of outside air into conventional equipment can create a situation where indoor relative humidity levels become a problem.

Increasing humidity levels above the ASHRAE comfort zone of 40% to 60% may cause new indoor air quality problems, namely microbial growth.

The demands for increased energy efficiency ratios of air conditioning systems and more outside air do not balance out in the overall building equation. Higher energy efficiency ratios are typically achieved with higher evaporative temperatures, thus reducing the capability of removing moisture out of the airstream.

Introducing more outside air further disables the system to remove moisture down to levels that are considered safe and comfortable. Therefore, the equation below does not work.

Energy reduction + Improved indoor air quality = Traditional hvac systems

Unfortunately, there are motivations to reduce ventilation rates in ASHRAE 62-89 in an attempt to lessen the overall energy impact traditional systems are imposing in North America. What we are failing to do is find the root cause of the problem.

It is not the current ventilation rates that are creating our energy load problems and humidity problems; it is the systems that are applied that are causing the problems. If anything has to change, it is the system, not the ventilation rates.

Technologies

Energy recovery ventilation technology has grown tremendously in the last 10 years. Many manufacturers provide high-quality products that are cost effective. This technology solves the issues of energy loads and indoor air quality and humidity.

Three prominent energy recovery technologies are commonly applied. The enthalpy (rotary) wheel and plate heat exchanger is typically applied in commercial applications, whereas the heat pipe is more common in the industrial marketplace.

All technologies can reduce the overall heating and cooling loads due to ventilation by 65% to 80%. This reduction, coupled with the fact that operational conditions can be reduced by 50% to 70%, is the solution to our dilemma of energy consumption vs. indoor air quality.

Flat-plate heat exchangers: Plate air-to-air heat exchangers transfer mainly sensible energy from the exhaust airstream to the supply airstream.

The exchanger consists of a series of alternating plates creating two separate air passageways. These plates can be constructed of various materials, including aluminum, polymers, and paper. Plate exchangers can have various airflow patterns, including crossflow, counterflow, and in some cases, parallel flow.

The most common exchanger uses crossflow technology. The only time latent energy can be transferred is when the exhaust air goes below dewpoint and the latent heat of condensation is transferred over to the supply side in the form of sensible heat transfer.

Plates are known for their low cross-contamination between the supply and exhaust airstreams, since the exchanger has no moving parts.

Rotary (enthalpy) wheels: Rotary (enthalpy) wheels can transfer both sensible and latent energy from the exhaust airstream to the supply airstream. The rotating wheel (15 to 35 rpm) is split to allow outdoor air to pass through one half and exhaust air through the other. Latent energy is transferred in the form of moisture transfer, since the vapor never changes state.

Moisture is adsorbed by a desiccant applied on the surface of the wheel matrix, enabling the transfer of moisture based on vapor pressure differential between the two airstreams.

With the rotating wheel, exhaust air can be carried over to the supply airstream when the air that is entrained within the volume of the wheel passes a seal and is carried across by the rotating wheel. To eliminate this carryover, a purge section, which purges the contaminated air back into the exhaust airstream, can be applied.

Heat pipes: Heat pipes are similar to the plate in that they transfer mainly sensible energy from the exhaust airstream to supply airstream. The exchanger consists of individual tubes charged with a refrigerant.

By separating the exchanger so that the outdoor air travels across one half and the exhaust air over the other, a passive evaporator and condenser are formed based on temperature differentials.

The heat pipe may transfer latent energy if the exhaust air goes below its dewpoint and condensation occurs. This latent heat of condensation is transferred over as sensible energy through the heat pipe.

ERV Applied

Any time outside air must be brought into a building envelope and the design conditions for summer and winter require heating and/or cooling, energy recovery should be seriously considered.

Depending on the location of the building, the application, and the first cost budget constraints, various technologies may be applied.

For hot, humid climates, an enthalpy wheel is a natural choice; the latent energy transfer dramatically reduces the summer cooling loads for ventilation. However, if the application is for source control, you may choose to apply a plate exchanger or heat pipe to ensure that no cross leakage occurs between the two airstreams. Source-control applications could include air that has toxic fumes, or gases such as laboratory exhaust.

For cold climates, all technologies can be applied. The flat plate and heat pipe operate extremely well for cost-effective dehumidification applications. The enthalpy wheel provides humidification regain during winter operation, minimizing the humidification load on the building.

Keep in mind that in any cold climate, it is important to pay attention to the defrost strategy employed by the manufacturer.

First of all, ensuring that there is a defrost mode is essential for outside winter conditions of 20°F and colder (depending on the technology). Ensuring that the defrost strategy requires no additional energy will provide you with the most energy efficient design. Examples include exhaust only and recirculation defrost.

Note that the ventilation is interrupted by a percentage of time during defrost. Therefore, to offset this interruption, you can increase the ventilation rate by the same percentage to maintain the same diluted ventilation rate.

For industrial source-control applications, consider applying a heat pipe with virtually 0% cross leakage and the ability to apply 500°F process air over the heat exchanger. With this type of energy typically exhausted to the atmosphere, the energy savings induced from capturing some of that energy back for heating purposes is tremendous.

Dilution ventilation (“general ventilation”) requires no specific minimum cross leakage between the two airstreams. Therefore, any technology can be applied, providing the net outdoor air volume is accounted for.

For source-control applications where the exhaust air is contaminated, plates, heat pipes, or enthalpy wheels with purge sections can be applied. The severity of the application will determine which one of these options should be selected.

If first cost is the only important factor, then the plate heat exchanger will likely be the selection. However, do not count out the other technologies such as the enthalpy wheel, as the first cost is always the cost of the energy recovery equipment less the heating, cooling, and humidification load savings (as a function of capital cost savings).

One technology may seem more expensive at first glance, but if you do the math, you may be pleasantly surprised. These calculations are relatively easy to do. Equations can be found in ASHRAE handbooks or by consulting a manufacturer.

Maintenance

All technologies require periodic maintenance for filter replacement and general inspection of components. Typically, no additional maintenance is required when compared to conventional heating-cooling equipment.

Since the product is dealing with outside air, the heat exchangers should be inspected to determine if cleaning is required. However, the same could be said for heating-cooling coils on conventional equipment with outside air mixed with return air.

In addition, all energy recovery manufacturers have cleanable media that does not degrade the overall performance of the heat exchanger.

ARI Certification

In 1997, the Air Conditioning & Refrigeration Institute published a standard for energy recovery components. Included are the rotary wheel, plate exchanger, and heat pipe.

This standard, ARI Standard 1060, provides the testing requirements that should place all manufacturers on a level playing field for performance at a summer and winter conditions.

The standard follows ASHRAE 84-91, “Method of Testing Air to Air Heat Exchangers,” for testing requirements. In addition to the standard, a certification program will be launched in 1999, which should enable users to select product that has been certified by ARI for performance.

This program will enable specifiers to select equipment with comfort, so that loads can be sized accurately and, as a result, heating and cooling systems will operate much more effectively.

Oversizing of cooling equipment due to safety factors built into the equation will cause humidity problems, which in turn directly effect the level of IAQ. With this program, effectiveness values can be taken for par, which will size the cooling systems at accurate levels needed for the building envelope.

Better IAQ With Less Energy Consumption

As discussed earlier, there is motivation to reduce ventilation rates by as much as 50% to respond to the indoor air quality problems stemming from high humidity levels. Let’s compare two different approaches:

System A — 15 cfm per person with approximately 75% total effectiveness from an energy recovery ventilation system; and

System B — 7.5 cfm per person with conventional equipment assuming outside air on supply system through minimum openings of economizer section.

Assume a 56-Btu/cfm cooling load, and a 75-Btu/cfm heating load.

According to Table 1, here are the System A advantages:

  • Conforms to ASHRAE 62-89 @ 15 cfm/person;

  • 50% less cooling and heating design loads; and

  • 39% savings in annual operating savings.

Comparing energy recovery ventilation at ASHRAE 62-89 ventilation rates with conventional systems at half of those rates, energy loads and operating costs are still reduced significantly.

This reduction, coupled with the ventilation rates that are known to be effective in the industry, are the solutions in meeting our IAQ needs for the 21st century.

Maury Wawryk is the marketing manager for Venmar CES, Saskatoon, Sask., Canada, and can be reached at mwawryk@ venmarvent.com (e-mail).

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