Credit Creatas

Air-Distribution Systems for Schools

Nov. 30, 2015
A look at fully mixed, fully stratified, and hybrid air-distribution systems and which is right for your new or existing school.
Credit: Creatas

Whether for new construction or a major renovation, the choice of air-distribution system requires consideration of a number of factors, including acoustics and thermal and ventilation comfort. This article will discuss various air-distribution systems and how to choose the right one for both new and existing schools.

System Types

Air-distribution systems commonly used today can be categorized according to how they stratify a space, falling into one of three groups: fully mixed, fully stratified, and hybrid.

Fully mixed. The system most commonly used in North America over the last 60 years is fully mixed (Figure 1).

FIGURE 1. Fully mixed air-distribution system.

Fully mixed systems are the most popular for schools because they offer lower costs and employ technology with which most facility managers are comfortable. They use high sidewall-mounted grilles or ceiling-mounted diffusers, which means that during cooling operation, there is minimal stratification, if temperature is measured from the floor to the return.

Conventional overhead systems provide comfort through room air mixing. In a typical cooling application, 55°F air is supplied from ceiling diffusers or high sidewall grilles. The mixing created by the outlet discharge jet results in room air motion. This so-called “secondary air motion” is caused by the entrainment of room air into the supply jet where mixing occurs. With a fully mixed system, the supply air is cooled to below dew-point temperature. This is one reason we typically need a 20°F differential temperature on supply air.

With fully mixed systems, it is common to use ceiling diffusers in the core of the building and plenum slot diffusers on the perimeter. While fixed-geometry diffusers meet minimum standards and code requirements, comfort and energy costs can be improved by using an auto-changeover perimeter slot diffuser. This provides all heated air down windows for heating and all cool air across the ceiling for cooling.

Active chilled beam is a type of fully mixed system allowing designers to decouple sensible and latent loads so sensible loads can be managed in a more energy-efficient manner. Primary air supplied to a beam is fed through a bank of nozzles, which induces warm room air through a dry sensible cooling coil. Reconditioned room air then is mixed with the primary-air jet and supplied to the room, similarly to a mixed jet from a linear supply diffuser. The primary-air supply must be sufficient to meet ventilation and latent space-load requirements. The lower volume of air required by the primary-air system means supply fans and air ducts can be downsized to take less ceiling-cavity space. Sensible cooling is piped to the coil from a chilled-water supply, which is more energy-efficient than an all-air system.

While the upfront cost of chilled beams typically is higher, designers like them because they usually result in lower operating cost. Designers can overcome fears of condensation by sealing the skin of the building to minimize infiltration and conditioning outdoor air used for ventilation to control the dew-point temperature inside of the space. The result is a quieter and more comfortable learning environment.

While the fully mixed system may be a solution for nearly all air-distribution applications, other methods may be more effective at addressing the more stringent ventilation and acoustic requirements of schools.

Fully stratified. Another type of system is fully stratified, also known as thermal-displacement ventilation (TDV) (Figure 2). TDV systems have desirable indoor-air-quality benefits that contribute to lower absenteeism rates among both students and teachers. For example, lightweight contaminants (e.g., from sneezes) are carried to return outlets located in the ceiling, where they are removed by the filtration system before air is resupplied to the space.

FIGURE 2. Thermal-displacement ventilation.

With displacement diffusers, cool air commonly is introduced into a space at or near floor level and travels along the floor until it reaches a heat source (e.g., a person or piece of equipment), where it stratifies. In a fully stratified system, temperature slowly increases from floor level to return level. Such a system delivers air to a space at a warmer discharge temperature of about 63°F. By definition, TDV systems are cooling-only systems that require supply air to be at least 2°F cooler than a room to function properly. For most climate zones in North America, supply air must be cooled to below the dew-point temperature and then reheated to the supply temperature.

TDV outlets operate at very low pressure losses. Typically, static-pressure loss is less than 0.04 in. of static pressure at maximum airflow. This is lower than comparable mixed-air outlets. ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, recognizes the fact ventilation air reaches the occupant with ease and has a factor of 1.2. This allows the use of 17-percent less ventilation air, providing a head start toward the 30 percent required for an extra LEED point (see sidebar).

At maximum airflow, most TDV outlets are much quieter than NC 25.

Hybrid. A recent development, hybrid systems use a dedicated outdoor-air system for ventilation and latent-load requirements and hydronics for sensible-load requirements (Figure 3).

FIGURE 3. Hybrid perimeter system for a school.

A top solution for perimeter classrooms is a hybrid heating and cooling unit employing active-chilled-beam technology for cooling and water-source radiant-heat technology for heating. Such a unit is well-suited for classroom applications because cooling and heating demands are met in a space-saving, energy-efficient manner. Demands for energy efficiency are met with water-source supply to cooling and heating coils. Ventilation is exemplary because of the displacement-ventilation cooling distribution system. Moreover, the nozzle induction system guarantees low noise levels in classrooms.

A comparison of the three system types discussed here can be found in Table 1.

TABLE 1. Comparison of system types.

A Wealth of Options—and Challenges

Acoustics and thermal and ventilation comfort are factors that do not get the headlines, but nevertheless are important to a productive learning environment. With updated codes and standards, along with new technology, designers have a wide variety of challenges and choices for today’s school projects.

Jim Aswegan is chief engineer for Titus. He has 15 years of research-and-development experience with Titus and more than 20 years of experience serving on ASHRAE committees.

Did you find this article useful? Send comments and suggestions to Executive Editor Scott Arnold at [email protected].

SIDEBAR: Codes and Standards to Abide By

Before a designer can take into account comfort and acoustical concerns, he or she must comply with any applicable national, state, or local requirements.

For schools, the prevalent rating program used in the United States is Collaborative for High Performance Schools (CHPS). CHPS recommends a design meeting or exceeding:

  • ANSI/ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy.
  • ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality.
  • A maximum classroom background-noise level of 45 dBA, with extra credit for a maximum of 40 dBA. The ultimate goal is to reach the ANSI/ASA S12.60, Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools-recommended 35 dBA.

For schools, the LEED (Leadership in Energy & Environmental Design) green-building certification program requires:

  • Minimum ventilation rates meeting or exceeding those in Section 6 of ANSI/ASHRAE Standard 62.1, with extra credit for going 30 percent or more over.
  • A maximum HVAC-system-related background-noise level in classrooms of 45 dBA, with extra credit for a maximum level of 40 dBA.
  • Thermal comfort meeting ANSI/ASHRAE Standard 55 requirements.

In addition to CHPS and LEED, ANSI/ASHRAE Standard 55 and ANSI/ASHRAE Standard 62.1 serve as benchmarks for many local codes. Comfort in ANSI/ASHRAE Standard 55 is indexed based on space temperature, humidity level, local air motion, occupant clothing insulation, and occupant activity level.

As for acoustics, although there is no direct correlation between A-weighted decibels (dBA) and noise criteria (NC), most sources compare devices with mid-range-frequency sound.

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