Ventilation and its role in IAQ:
The HVAC community plays a vital role in providing healthful indoor environments in which to live, learn, work, and play. With half of all illnesses attributable to indoor airborne contaminants, the EPA has declared indoor air quality a public health priority. Ventilation with outdoor air is the only strategy that can simultaneously reduce the levels of all indoor pollutants. This strategy, in general accordance with the Dilution Principle, is shown in the illustration below.
Each doubling of the ventilation rate results in a 50% reduction in the concentration of all constant source air pollutants evenly mixed within the space. At 1.0 ACH, pollutant concentrations are reduced by a factor of 5. National, state and local codes mandate minimum outdoor air ventilation rates based on ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality. The challenge is to intro-duce the outdoor air at the levels required by the codes while maintaining indoor comfort and conserving energy.
Why Energy Recovery Ventilation:
Building Code requirements for increased outdoor air ventilation rates have placed new demands on HVAC equipment and on building operating budgets. Energy recovery ventilation reduces the load on the system due to outdoor air by taking advantage of the work that has already been done to heat, cool, humidify or dehumidify the space. Instead of exhausting building energy to the outside, it is temporarily captured on the surfaces of the enthalpy wheel heat exchanger and then released to pre-heat, pre-cool, humidify or dehumidify the incoming air. Enthalpy wheels do this with exceptional efficiency and are the leading technology for achieving energy conservation while ventilating for health and comfort.
The next Two figures illustrate the power of enthalpy exchange. The first chart shows the result of just adding the required load from increased ventilation to the normal air conditioning process line.
The chart below shows the new process line when using enthalpy recovery. As one can see, the cooling load (work) saved is a direct result of the difference in the two enthalpies
The Airxchange Matrix Media
All Airxchange Energy Recovery Ventilation (ERV) components utilize a unique parallel plate energy transfer matrix design that optimizes the energy recovery surface area for a given diameter and depth of a rotary heat exchanger. In addition, a polymer film matrix offers ideal properties that limit counterproductive axial conduction of heat. This combination achieves the required performance in a thin, light weight configuration.
All Airxchange desiccant-coated enthalpy wheels are corrosion resistant. They are washable due to patented and proprietary processes that secure the desiccant to the matrix substrate with a permanent mechanical bond without the use of adhesives. Recognizing the different needs of the unitary packaged and air handling segments of commercial space conditioning equipment, Airxchange components are available in Standard Matrix and Channel Matrix configurations.
The standard construction employs a series of small conical internal dimples (standoffs) to separate the film layers and define the geometry of the matrix.
The standard construction is always suitable for ventilation in comfort applications and is generally specified for:
Accessories, or integration into packaged unitary HVAC Equipment
Airxchange 3-inch depth components are available in the Channel Matrix configuration with an optional adjustable mechanical purge sector. Channel matrix wheels employ the same ideal parallel plate geometry as the standard configuration, however, the internal standoffs are axial ridges that separate intake and exhaust air streams while determining the matrix geometry.
With the channel matrix configuration, a mechanical purge sector may be required to control the amount of exhaust air that transfers to the supply air stream by the carryover mechanism. Using an adjustable purge sector, carryover can be reduced to less than 1% while limiting excess fan energy to less than 10%. This configuration is ideal for air handlers and other high pressure applications where the Standard Matrix might allow higher carryover or excessive fan energy losses. It is also responsive to applications and engineering specifications in which it is necessary to limit the recirculation of exhaust air.
Frost control is required in extremely cold climates to preserve performance and assure the continuous supply of outdoor air. Enthalpy wheel frost control strategies take advantage of inherently low frosting thresholds, which result in minimized energy use and maximized design load reductions.
Frost formation causes reduction of airflow through the heat exchanger. Without frost control, energy recovery and airflow may be significantly reduced. The frost threshold temperature is the point at which frost begins to accumulate on heat exchanger surfaces. It is a function of both outside temperature and indoor relative humidity. The following figure compares the frost threshold of a plate-type sensible heat exchanger with that of an enthalpy wheel.
Note that while frost forms at between 22° F and 30° F in a plate-type exchanger, frost thresholds for enthalpy wheels are generally 20 to 30 degrees lower. This is because the enthalpy wheel removes water from the exhaust air stream, effectively lowering the dewpoint of the exhaust. The water removed is subsequently picked up through desorption, re-evaporation or sublimation by the entering outdoor air.
For Frost Control strategies please refer to our technical note: Click here
Silica Gel Desiccant
What is it?
Silica gel is a highly porous solid adsorbent material that structurally resembles a rigid sponge. It has a very large internal surface composed of myriad microscopic cavities and a vast system of capillary channels that provide pathways connecting the internal microscopic cavities to the outside surface of the sponge. Silica gel enthalpy wheels transfer water by rotating between two air streams of different vapor pressures. The vapor pressure differential drives water molecules into/from these cavities to transfer moisture from the more humid air stream to the drier air stream.
Adsorption: Silica Gel vs. Molecular Sieve
The following figure shows the characteristic curve for adsorption of water on silica gel. It shows the percent weight adsorbed versus relative humidity of the air stream in contact with the silica gel. The amount of water adsorbed rises linearly with increasing relative humidity until R.H. reaches near 60%. It then plateaus at above 40% adsorbed as relative humidity approaches 100%. For contrast, the curve for molecular sieves rises rapidly to plateau at about 20% adsorbed at 20% R.H.
The graph explains the following application considerations:
- Molecular sieves are preferred for regenerated applications such as desiccant cooling and dehumidification systems that must reduce processed air streams to very low relative humidity.
- Silica gel has superior characteristics for recovering space conditioning energy from exhaust air and handling high relative humidity outside conditions. Another key point is that the transfer of water by sorption/desorption is not dependent on temperature. Thus, the silica gel enthalpy wheel works to reduce latent load at difficult part-load conditions.
Purge removes exhaust air that would be otherwise carried to the supply air stream by the rotating wheel matrix. Outdoor air is used to clean or purge the wheel matrix before it rotates from
the exhaust air stream to the supply air stream. The driving force for the purge stream is the pressure differential between the outdoor air and return air compartments adjacent to the wheel. Purge is accomplished by utilizing the wheel matrix or by mechanical means.
Two Choices of Purge Technology
Airxchange offers two distinct technologies to support the purge process.
- Through-Matrix Purge using Standard Matrix components
- Channel Matrix with optional Mechanical Purge
- If purge is desired, the design considerations include:
- Use of outdoor air to flush carryover through the open matrix
- Adjusting fans and pressures so that any seal leakage is from supply to exhaust
- Ensuring that pressures are not excessive resulting in wasted fan energy
- The following diagram defines the terminology of airflow for consideration of purge.
- EATR (%) is composed of carryover leakage resulting from the rotation of the wheel from Return air to Supply air and any seal leakage in that direction, minus the impact of purge. Purge airflow removes return air from the wheel volume before it enters the supply side of the component.
- Outdoor Air Correction Factor (OACF) is the difference in airflow measured between OA and SA, presented as a ratio. The OACF includes air lost through purge and leakage from the outdoor air stream to the exhaust stream. Accordingly, OACF is used to size the fans
This technology employs the Airxchange standard energy transfer matrix. It is well suited for comfort applications where EATR values of 4% to 6% are acceptable. The performance of through-matrix purge is competitive with mechanical purge sectors supplied with fluted wheels, however, through-matrix purge is less expensive and simpler for most field applications. Effective purge yielding an EATR of 1% or less can be achieved with through-matrix technology whenever static pressure differences are positive from supply to exhaust on both sides of the wheel. By making best use of system characteristics and fan placement, EATR (cross-leakage) can be held to 1% or less. For example, in the draw-thru, draw-thru configuration shown below, where all static pressures are negative, nominal wheel delta P is 1″ on both sides.
Typical data for standard wheels with through-matrix purge is given in the following table which indicates that in order to obtain less than 1% EATR, as much as 14% of the flow entering
the outdoor air hood will be used to purge the matrix and seals into the exhaust compartment. To achieve this, the exhaust fan must be sized to result in 114% of normal flow.
Channel Matrix with Mechanical Purge
Airxchange channel matrix technology is the option of choice to limit excessive leakage of air in critical higher pressure applications. Channel technology results in greater energy savings
throughout the system. These designs require a mechanical purge if it is necessary to minimize the impact of carryover of exhaust air. This is a wedge-shaped sector that captures and redirects the supply air to the exhaust side.
In summary, channel matrix designs:
Limit excessive leakage of air and limit resulting additional fan energy requirements in high differential pressure applications
With adjustable purge, limit EATR (exhaust air transfer, or cross-leakage) to less than 1% in sensitive applications over a wide range of pressure differentials
The following diagram defines airflows and delta P:
The driving forces for OACF and purge are provided by the plenum pressures in the wheel compartment and adjusted by system and component pressure loss and fan placement.
EATR of 1% or lower can be provided whenever pressures are positive from supply to exhaust on both sides of the wheel.
Application specific leakage ratings can be obtained by utilizing the Airxchange AIRX 3.0 selection software. Click here to learn more
ARI Certified leakage ratings can be obtained via the ARI Directory
Performance Impact of Unequal Air Flows:
When outdoor air and exhaust airflows are equal at the ARI rated condition, the system is balanced and energy recovery effectiveness is equal for both flows. This results in maximum recovery effectiveness of the entire ventilation system. As shown in the adjacent graph, when the airflows are unequal, the effectiveness of the higher airflow decreases while that of the lower airflow increases. This building system imbalance is made up by infiltration or ex-filtration having an energy recovery effectiveness of 0%. Thus, unbalanced flow reduces the benefit of energy recovery for the building system. The effect of unequal airflow on efficiency is potentially significant and can be calculated, as follows:
- Determine the Flow Ratio.
- Flow Ratio = CFMmin / CFM max
Use the adjacent graph to find the effect on effectiveness
by locating the flow ratio value on the X-axis and drawing
a vertical line from that point. Emax will be the lower line
and Emin will be the upper line.
Outdoor air = 5000 CFM; Exhaust Air = 4000 CFM
For a flow ratio of 4000 CFM / 5000 CFM = 0.8,
the adjacent figure results in:
Emax = 12% lower and Emin = 6% higher.
Thus, if base effectiveness for the system is 80%, these unequal flows result in Emax = 68% and Emin = 86%.
The Airxchange AIRX 3.0 performance modeling software automatically calculates performance for unbalanced flow conditions. For more information please click here
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