Using louvers to prevent snow intake

Northern climates, with the extreme temperature and precipitation conditions experienced, present many unique design challenges for the mechanical engineer involved in the design of heating, ventilation and air conditioning (HVAC) systems. One of these is ensuring perimeter surfaces are provided with heat to increase mean radiant temperature and improve comfort to satisfy ASHRAE 55: Thermal Environmental Conditions for Human Occupancy requirements.

Other HVAC design challenges include assuring good mixing of outdoor air with return air to prevent the tripping of freeze stats or the freezing of coils, and making sure that ceiling spaces above entrance vestibules do not drop below freezing to prevent fire suppression piping from bursting. There are many other cold weather challenges, some of which may have not yet been discovered.

Snow penetration of louvers and snow intake into air handling systems is a challenge that can cause many problems, including damage to filters; promotion of microbial growth as a result of repeated wetting of plenum, duct and air handling unit surfaces; leaks caused by melting snow in ducts and air handlers; and corrosion of AHUs and ductwork.

Air handling louvers are designed to protect air-intake openings from the infiltration of unwanted water while allowing air to pass into the system, with different louver models offering different levels of performance.  ANSI/AMCA Standard 500-L: Laboratory Methods of Testing Louvers for Rating defines test procedures for certifying louvers. Most recently published in 2015, ANSI/AMCA Standard 500-L is the standard to which louvers are tested and its test protocols are what the AMCA certified rating program uses to certify a louver’s performance.

ANSI/AMCA Standard 500-L covers five testing protocols: pressure drop, airflow leakage, water penetration, wind-driven rain penetration and wind-driven sand penetration. These tests are conducted at an AMCA-accredited laboratory. Once a louver has been tested and the accuracy of its ratings proven, the manufacturer can display the test results, along with an image of the AMCA seal that corresponds to the testing in its literature.

Selection of louvers in the Midwest, for example, is usually based on preventing water penetration through the louver due to rain falling on its face. Louvers are typically selected at a maximum of 500 feet per minute airflow velocity through the louver free area. Looking at a typical water penetration rated stationary louver with drainable blades, it is found that the required air velocity for the beginning point of water penetration can typically be above 900 fpm through the free area, so a commonly used selection point of 500 fpm is relatively conservative with respect to water penetration.

The outdoor air plenum should be provided with a bottom sloping to the outdoors or provided with a drain to relieve any splashing water that makes its way through the louver. As an extra measure of precaution and good practice, the first 10 feet of outdoor air duct beyond the plenum should be sloped back toward the plenum and sealed watertight. With these considerations implemented in the design, rarely are there rain intrusion or water leakage problems in outdoor air intake systems.

Other parts of the country, especially throughout the South where hurricanes are common, may require hurricane louvers or wind-driven rain louvers. Hurricane louvers are designed to be used in locations subjected to extreme weather conditions from hurricanes or tropical storms where the louvers may be subjected to high-velocity debris. Wind-driven rain louvers are typically rated with a 29 mph wind directly at the louver face and with a simulated 3 inch per hour rainfall and are also rated at a 50 mph wind with a simulated 8-inch per hour rainfall, much higher than an AMCA water penetration louver.

Preventing snow intake

Prevention of snow entry into an air handling system requires a different approach in louver selection and more importantly in plenum design. Snowflakes vary infinitely in density, shape and size — all of which influences snowflake fall velocity and wind/air entrainment. Numerous studies have been conducted using everything from optical instruments to radar modeling to determine the fall speed of hydrometeors and snowflakes. Yet this information is not widely applied by many engineers to one of the most troubling aspects of air intake in a northern climate — snow entrainment.

Should you visit mechanical equipment rooms in the spring, at the end of the snowy season before filter changes have occurred, you will most likely find filters with obvious signs of water damage and signs of previous water ponding in intake plenums, intake ductwork and inside of AHUs. All of this causes damage to equipment and provides opportunities for microbial growth which could affect indoor air quality.

ASHRAE Fundamentals Handbook includes louver selection criteria based on cubic feet per minute and a face velocity of roughly 400 to 500 fpm through the louver free area is recommended. ASHRAE Systems and Equipment Handbook indicates “plenums in cold regions may require a snow baffle to direct fine snow to a low velocity area below the dampers.” ASHRAE Standard 62: Ventilation for Acceptable Indoor Air Quality states:

“Where climate dictates, outdoor air intakes that are part of the mechanical ventilation system shall be designed to manage water from snow, which is blown or drawn into the system.”

It is not possible to prevent every snowflake from passing through the louvers. Even with the air handling equipment off, the wind can blow accumulating snow through the louver and deposit it in the plenum. The goal of outdoor air intake design in northern climates should be to:

• Limit the amount of snow brought in through the louvers
• Not allow the snow to penetrate beyond the intake plenum
• Allow for proper draining of water within the plenum once the snow melts
• Allow access to the plenum for regular cleaning.

For most mixed air system AHUs operating in a northern climate, the intake louver selection is based upon 100% outdoor air economizer operation for when the outdoor air temperature is approximately 55°F to 60°F and cooling of spaces is required. The louver would be selected based on a 500 fpm velocity through the louver free area. Note that when it is 55°F to 60°F outside, snow intake is not an issue and the system typically would not be operating at full fan capacity with a variable air volume system. So, the louver selection at 500 fpm through the free area would be considered conservative.

With more emphasis placed on building operating costs, modern HVAC systems are being optimized by looking at the heating and cooling function and at the ventilation function of HVAC systems separately. This allows each to be handled in the most efficient manner possible. Terminal equipment such as active chilled beams or induction units provide the heating and cooling, while dedicated outdoor air systems with energy recovery devices provide the ventilation.

Other systems are designed to provide the outdoor air directly to the spaces by way of a DOAS and use other forms of heating-only equipment or passive chilled beams. Any such use of a DOAS will typically significantly reduce the cost to heat and cool the incoming ventilation air and the use of these system types is increasing. These dedicated outdoor air systems supply 100% outdoor air.

Louver selection at 500 fpm will prevent water penetration during a typical rainstorm, but the potential for snow intake will be increased significantly as the supply air volume in a DOAS may not be reduced as it may be in a mixed air economizer system in economizer mode.

Louver sizing to prevent snow entrainment

Snowfall typically occurs at 32°F and below — much above that and it would be rain or a heavier rain/snow mixture. Considering a mixed air system of 72°F return air and 32°F outdoor air, the designer can calculate that roughly 30% outdoor air is needed to make a 60°F mixed air condition. Using a louver sized for 500 fpm velocity with 100% outdoor air flow in the economizer mode, the system would have roughly 150 fpm velocity through the louver free area at 30% outdoor air flow through the louver. At this velocity, there is no concern for rain intake, but snow will still enter the plenum and, therefore, the plenum design must consider that.

Snowflake size, shape and density vary considerably based on temperature and myriad other factors that occur during the formation and life of a snowflake. This size, shape and density will affect a snowflake’s terminal or fall velocity. Various studies have shown vertical fall velocities of between 1 and 8 feet per second. For this example, we will assume that it is 20°F and the snowflake’s vertical fall velocity is 1 meter per second or roughly 200 fpm.

Assume the system has a DOAS and the louver has been sized for 500 fpm through the free area to prevent rain penetration (see Figure 1). Snow captured by the suction pressure at the louver face and entrained in the airstream would travel from the louver, where it is sent on an upward trajectory at 500 fpm to the outdoor air ductwork in 0.18 seconds.

Looking at a mixed air system, the louver would be sized at 500 fpm through the free area at full supply air volume to prevent rain entrainment while in the economizer mode. In the winter, the system may be operating at a minimum outdoor air position of approximately 30%, with a resulting velocity of 150 fpm through the free area (see Figure 2). Snow captured by the suction pressure at the louver face and entrained in the airstream would travel from the louver, where it is sent on an upward trajectory at 150 fpm to the outdoor air ductwork in 0.60 seconds.

From these illustrations, it is easy to see how snow accumulations inside the plenum, outdoor air ductwork and most likely the AHU and filter bank are likely to occur. Figure 3 shows snow accumulation in the outdoor air ductwork with the louver and plenum visible in the background. Figure 4 shows snow making it all the way into the AHU. This accumulation of snow when melted will wet the filters, cause corrosion of the ductwork and AHU, allow bacterial growth within the air handling system and possibly leak onto the floor and into building areas below.

Figure 5: Designing the outdoor air plenum with a vertical chamber with an upward airflow velocity of less than 200 fpm will prevent snow from rising up and into the outdoor air ductwork. Courtesy: Peter Basso Associates[/caption]

Problems with snow entrainment can be extremely difficult to resolve after construction is complete and the building is occupied. A survey of air handling system filters in the springtime in northern climates will often show water damage to the filters as a result of previous snow event accumulations. If the snow accumulation is large enough, it can result in filters being pulled through the filter racks, allowing unfiltered air to penetrate to the coils and be recirculated to the spaces. Slowly over time, corrosion of ductwork and air handling equipment will occur.

In the worst case scenario, melting of the snow will result in dripping and damage to building contents and finishes, leading to dissatisfied occupants. Correction of these problems can often require significant rework or replacement of the intake plenums or portions of the air handling systems.

Thoughtful analysis of air intake system design will reduce the likelihood of this occurring, preserve filters, extend the life of ductwork and equipment, improve the quality of the air being supplied to the occupied spaces and will allow the owner and engineer to go outdoors to enjoy the snow, rather than spending time indoors figuring out how to resolve a snow intake problem.

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