HVAC Design - Displacement Ventilation

TURNER BUILDING SCIENCE LLC.

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HVAC Design - Displacement Ventilation

Displacement Ventilation Design Concept:

The vertical displacement ventilation concept is different from conventional ventilation, mixing ventilation systems in several important ways. These differences offer many potential benefits over conventional mixing type HVAC systems typically found in facilities.

Displacement Ventilation Design Differences

No Drafts. Air is typically supplied near the floor in the space at extremely low velocity, which results in no “throw” of air and subsequently little risk of “drafts”.

Stratified Room Air. Supply air is purposely not uniformly mixed throughout the space. It is intentionally stratified vertically to provide a better quality of air in the occupied part of the facility. Supply air is delivered during occupancy at temperatures slightly lower than desired area temperature. The supply air moves horizontally across the floor until it naturally rises, driven by convective currents as it warms due to internal heat from the process, people, lights, computers, etc.

The stratification as observed in representative Carbon Dioxide monitoring data from a school classroom application is illustrated in Figure #1.

In addition to the stratification of Carbon Dioxide, from the concentration decay rate, it can be seen that there is rapid flushing of the room as the students leave, confirming the achievement of displacement air movement. This room achieved superior flushing of room generated contaminants with an overall room ventilation rate of only 1.5 air changes per hour. This is less than half the ventilation rate that would be needed with a conventional mixing design, drastically lowering electrical energy use through reduced fan horsepower consumption due to less air movement.

Improved Effective Ventilation. Because of both people and process convective currents, in a high density or industrial application, there is a general upward flow of effluents above the occupied zone, as long as it is not greatly disturbed by fan forced air streams (as happens in conventional mixing distribution systems). Air rises from the lower level of the room around stationary processes due to the development of convective currents over power consuming machinery. This means that occupants located in the lower levels of a room will breathe air closer to supply air conditions , rather than the air being exhausted from the space at the ceiling level, improving ventilation effectiveness.

No Recirculated Stale Air. In our designs for school classrooms and industrial applications, all supplied air is “preconditioned” 100% outdoor air. All air removed from the facility is exhausted outdoors–none is recirculated. Only the sensible and latent energy is captured from the exhaust air stream and recycled when needed.

Individual Room VAV Not Needed. When there are few or no internal loads, such as an unoccupied area with the lights off and little solar gain, the area air will be slowly displaced upward by the air beneath it. In this case, the area will eventually be approximately the same temperature as the supplied air, which is only slightly cooler than the desired temperature, thus the need for individual area VAV to prevent overcooling is virtually eliminated. “Demand control” of the total air supply to particular areas via temperature and Carbon Dioxide sensors is utilized to minimize energy use and fan horsepower during cold weather or low occupant density, and to supply higher rates of ventilation only when needed.

Reduced Cooling Capacity Needed. Thermal stratification also allows for some reduction of internal cooling requirements, because about 50% of the heat from the lights and other sources located above the occupants does not reach the occupied zone and, in this design, is exhausted outdoors when not needed.

Less Fan Horsepower Needed. In this design approach, supply airflows needed to achieve adequate temperature control and provide adequate ventilation are often lower than conventional system. Thus, typically lower fan horsepower than conventional mixing type systems is needed. Less Room Noise. Low velocity supply of air cannot be accomplished using conventional ceiling mounted mixing type diffusers, conventional heating ventilators (air make-up units), or non-ducted fan coils. With reduced total air flow quantities and low exit velocities, there is reduced noise when compared to mixing type systems because there is no need to forcefully mix air in the room and less total airflow is needed.

Less Inter-zone Pollutant Transport. The supply air “quality” to individual areas is also improved because, with a 100% outdoor air, the supply air is not already premixed with contaminated air which has been transported from other areas or zones of the building.

Commercial Office Space Application:

In July of 1998, we occupied our new corporate headquarters in Concord, New Hampshire. Designed and constructed by our firm, this office building is one of the first in the United States to exclusively utilize the concept of Displacement Ventilation and Demand Controlled Ventilation as the primary means of providing high performance design aimed at good indoor air quality, improved thermal comfort, and reduced energy costs. As with our school designs, the integrated “sustainable” design concepts of the facility address other important factors including: siting, programming, social dynamics, lighting, acoustics, energy efficiency, and access for planned HVAC preventive maintenance.

Unlike our Advantage Classroom ™ school design, which is dominated by high-density classroom occupancy, only the large conference room training area utilizes demand controlled with 100% outdoor air. The remainder of the facility is served by a displacement design concept, which will provide a minimum of 25% Outdoor Air during design heating and cooling conditions, and up to a maximum of 100% Outdoor Air during most daytime operation. However, unlike conventional office design, all of the air that leaves spaces where known irritants are likely to be generated is always exhausted out of the facility after passing through an energy recovery exchanger i.e., whenever the facility is occupied, emissions generated from reprographics, major printing operations, and food prep are captured at their source and removed from the space.

As with our other designs, an innovative ceiling/roof structure provides a high vaulted ceiling in many areas. Additionally, reflected daylighting is delivered to most core areas. With this office building design we have combined a radiant floor heating system along with the use of the vertical displacement ventilation concept. This approach will also enhance all of the expected benefits of the displacement design such as: no drafts, stratified room air for improved ventilation effectiveness, enhanced comfort with minimal VAV zones, reduction of cooling capacity needs by 1/3 over conventional design and less fan horsepower needed during normal or design condition operation.

Where the office design does not use 100% outdoor air, we have incorporated the use of 95% (ASHRAE Dust Spot) efficiency particle air filters in the central IAQ Package air handlers. We also chose to incorporate factory installed and tested direct digital controls on the HVAC units when assembled at the factory. Only the radiant floor controls required major field controls installation at each of the tubing manifolds.

Decreasing the likelihood of Indoor Air Quality problems and enhancing the working environment in an office facility must begin with sound, conceptual building system designs, and include a cost- effective, easy to maintain HVAC system. In an office facility, the use of 100% outdoor air is generally not justifiable due to the cost of energy. With an office where all known office process emissions are removed with point-of-use exhaust systems, and the building is designed for 20 cfm (minimum of outdoor air per person), heat recovery; enthalpy control; and dehumidification of outdoor air in the cooling season, the goal has been achieved.

Industrial Assembly Area Application:

In January of 1997, we were asked to work with a major manufacturer of construction-related products to assist them with improvements in air quality in an assembly area. After careful analysis of the situation including the temperatures of the materials that were being assembled, the need for full climate control in the assembly area, and the necessary locations of the operators, we chose a modified displacement ventilation design approach. For this situation we were able to use the same exhaust rates which currently were in place. The major change that we implemented was to use a displacement ventilation flow to move the emissions generated in the assembly process away from the operators .and to blanket the assemblers with clean, conditioned 100% outdoor air. This concept performed so well that all assembly and press operations in the plant have been changed to the same design. This approach reportedly lowered operator exposures to product emissions, and is expected to improve product quality. By utilizing a modified displacement ventilation design, we were able develop a high performance HVAC system compared to all other conventional approaches that had been attempted by the manufacturer.

School Classroom Applications:

In 1996, a new 48,000 sf, (4,460 square meter) Elementary School was occupied with 400 students and staff located in Boscawen, New Hampshire. Designed by our firm, this school is the first in the United States to utilize the concept of displacement ventilation and demand controlled ventilation as the primary means of providing high performance design aimed at both good indoor air quality and thermal comfort and reduced energy costs. The integrated “sustainable” design concepts of the facility address other important factors including: siting, programming (specific design elements that consider the educational plan), social dynamics, lighting, acoustics, energy efficiency, classroom computer usage, and good access that allows for planned HVAC preventive maintenance. Ventilation and thermal comfort objectives are achieved through a combination of 100% outdoor air which is delivered near the floor in the classroom and exhausted at the ceiling (the directed air principle), and demand control (providing the correct amount of ventilation air when it is needed).

An innovative ceiling/roof structure, that provides a vaulted ceiling, was incorporated into the classroom design to enhance both the classroom space use and to provide room volume to enhance the performance of the displacement distribution system. This approach also facilitates

appropriate lighting and acoustical design.

Humidity Control: (Sidebar)

With traditional mixed air systems, the supply air may be cooled to 55ºF (95%+ saturated) or below in order to provide enough dehumidification to keep space conditions between 50-60% RH at 72-76ºF. The use of 50ºF supply air is acceptable in a mixed air system.

With a displacement ventilation system, air delivery is close to the occupants, so the supply air temperatures should be accurately controlled at warmer temperatures closer to the desired room temperature. If warmer saturated air is supplied to a room with high occupant loading, such as a classroom or a restaurant, the resulting relative humidity at 74ºF will exceed 70%. This is too high for comfort and may result in conditions favorable to fungi growth in the indoor environment

In order to deliver air to a room which is dry enough to maintain room relative humidity of 50%-60% at 74ºF, some or all of the supply air stream must be cooled below the dewpoint. The supply air must be cooled to a low enough temperature so that the target room relative humidity is obtained once the supply air is warmed up by the room loads. For all outside air applications, the use of enthalpy recovery devices greatly reduces the dehumidification load of the cooling process.

Using enthalpy recovery (total energy recovery) devices with 70% efficiency or better, the enthalpy of outside air conditioned by the energy recovery (ER) device is very close to the exhaust air enthalpy from displacement ventilation.

The results of the application of enthalpy recovery on the performance of the system is dramatic. The exhaust air stream passing through the enthalpy recovery device acts to pre-cool and dehumidify the supply air prior to it reaching the cooling coil. In the heat of the day this can amount a 10 degree or greater differential in supply air temperature, a substantial savings in cooling capacity for the mechanical cooling system.

We have now completed the application of the advanced ventilation design principles in multiple school projects, two office facilities, and an industrial complex located across the Northeast in buildings as large as 165,000 sq. ft.. Just recently we completed the design of a 600 seat performing arts auditorium complex which will also be conditioned with a demand controlled, displacement ventilation system. Based on our experience in designing healthy buildings, we see the application of advanced ventilation technology to be an intregal part for promoting enhanced learning and working environments.

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