Incorporating under floor air distribution (UFAD) and displacement ventilation systems is an increasingly popular way to provide comfort cooling to a variety of spaces. Since their introduction in Europe in the 1950s, and their domestic rise in specification during the 1990s, UFAD and displacement ventilation systems promise significant energy savings as a direct result of a higher supply air temperature that yields an increase in the use of economizer or free cooling hours.
From schools and office buildings to large public spaces and courthouses, these systems can be very successful when designed with IAQ standards for thermal comfort and humidity control in mind. It is crucial when designing UFAD and displacement ventilation systems to consider the relative humidity and enthalpy, or measurement of total energy in the air, as there can be a need to sub-cool and reheat the supply air to make sure the system is meeting optimal occupant conditions and relative humidity requirements.
For this reason, UFAD and displacement ventilation systems aren’t right for every project. A life-cycle cost analysis is useful to de-termine system overall cost effectiveness, and it must take into consideration local climate conditions as well as other building IAQ requirements prior to specification.
While there are a number of benefits to employing UFAD and displacement ventilation systems, the most common (and arguably the greatest) benefit is the improved thermal comfort and IAQ felt by occupants.
Because air is being supplied at or near the floor and passes directly to the occupied or breathing zone, occupant comfort, IAQ and energy efficiency are generally improved or enhanced with UFAD and displacement ventilation systems. As the supply air moves from the floor to the top of the room return where it is exhausted, the air absorbs heat generated by equipment and people and the temperature stratifies. In addition to absorbing heat energy, human CO2 respiration and chemical off gassing of building materials and furniture are also carried up and out of the breathing zone. This is quantified by AHRAE 62.1 as a 20% increase in the ventilation effectiveness of UFAD and displacement ventilation systems as compared to overhead supply mixing systems.
With UFAD systems, where individual diffusers in each workspace provide occupant-controlled airflow, improved productivity and health benefits are directly tied to thermal comfort and IAQ, as research suggests that both satisfaction and productivity increase with individualized temperature and airflow control.
Additionally, UFAD systems have the potential to reduce life cycle building costs, including the reduction of floor-to-floor heights in new construction applications. Depending on occupancy, a UFAD system using the raised floor cavity as a supply air plenum often provides increased modularity and flexibility, and can save on renovation costs during future space modifications as there is no need to relocate ductwork or ceiling diffusers.
The UFAD or displacement ventilation system has the potential to reduce energy use by delivering the supply air to the space as much as 10°F higher than a conventional overhead system (60°F to 65°F, rather than 55°F), which can result in less refrigeration plant energy to cool the air. The higher supply air temperature can also expand the free cooling or economizer hours in many climates when compared with a conventional overhead air distribution system.
However, to achieve the various benefits, one must consider and evaluate the energy associated with maintaining appropriate control of the humidity level in the occupied space. This is not just a hot and humid climate issue.
APPLICATIONS AND OPERATING PARAMETERS
With so many benefits, UFAD and displacement ventilation systems can be specified for a variety of different venues and markets, including: office buildings; call centers and trading floors; lobbies and public circulation spaces; airports; dining areas; casinos and courtrooms, where displacement diffusers are often hidden or disguised in architectural features (i.e. walls and woodwork) or operating elements (i.e. multimedia and signage stations).
With a conventional overhead air distribution system, higher-velocity supply air is typically delivered to the space at 55°F through ceiling diffusers. This supply air has to fight its way down through the mixed air and indoor air pollutants to condition the lower six to seven feet of occupied space. Return air is also removed from the space at ceiling level, with the potential for some of the supply air to short circuit directly to the return air without providing any cooling to the occupied zones, especially of particular concern when at non-peak conditions with a VAV system.
In contrast, UFAD and displacement ventilation systems rely on temperature stratification to achieve success. Supply air is distributed at a low velocity, low turbulence and higher temperature (60°F to 65°F) directly to the occupied zone. The cooler air rises by natural convection from the floor, and displaces warmer air around all the heat generating sources in the space, including people, computers, furniture and other interior fixtures. The warmer return air is removed from the space at the ceiling level and there is no short-circuiting of the supply air to the return — it always passes through the occupied zone.
The occupied zone control conditions can vary depending on the occupancy type, but generally speaking most spaces are main-tained somewhere between 68°F to 78°F, with 50% relative humidity (RH) ± 15% depending on the season.
MOISTURE ISSUES TO CONSIDER AND PSYCHROMETRICS
As mentioned before, a close second to controlling the dry bulb temperature in any space is anticipating the RH or moisture control of the supply air and designing the UFAD or displacement ventilation system according to the needs of the facility and in response to the local climate. This is especially true of facilities in climates with high moisture content and interiors that experience higher latent gains (or moisture from people) in the occupied environment.
In some spaces, the interior finish materials, like fine woodwork, for example, may need closer humidity control than is even required for human occupancy (see the Architectural Woodwork Institute’s Architectural Woodwork Standards, www.awinet.org/aws/). These situations require consistent RH control, usually within a 10% range to make sure the materials meet performance requirements over time. When this is the case, controls for a UFAD or displacement ventilation system must factor in air-handling equipment configurations that can adequately and accurately control supply air and consequently RH in the occupied space.
Densely-occupied spaces like performing arts halls, theaters and gymnasiums experience higher internal latent gains that must be addressed during mechanical system design, as the moisture content in the air is directly related to the number of occupants in the space. In these cases, moisture removal needs to be carefully executed to avoid having to recondition the air from the UFAD or displacement ventilation system, therefore potentially compromising its energy benefits.
When necessary, though, sub-cooling and reheating, or dehumidification by another method, may be required to reach appropriate supply air temperatures and RH control. Examining the psychrometrics to keep the relative humidity in a space within the ASHRAE 55 comfort zone at a given 75°F DB room set point, the supply air at 65°F DB must be below 61.5°F WB.
One of the biggest benefits of a UFAD system is that the higher supply air temperature allows for extended economizer hours. But, how much cooling energy is potentially saved in those magical economizer hours? Let’s look at a 5-story, 100,000 sq-ft office building employing UFAD in both Los Angeles and Chicago. Let’s also assume the following parameters for both offices:
• 1 cfm per square foot (typical high performance UFAD office)
• 400 cfm per ton (typical for office sensible heat ratio)
• 0.9 kW per ton (aggregate water cooled chiller plant)
• The cooling energy for each economizer hour is: 225 kWH=0.9kW/ton*ton/(400 cfm)*(1 cfm)/sq-ft *100,000 sq-ft.
For a seven-day, 7 a.m. to 7 p.m. operating schedule, the number of economizer hours (and kWH of cooling plant savings) in a year from 55°F to 65°F, based on dry-bulb temperature alone are:
• Chicago - 590 hrs (132,750 kWH)
• Los Angeles - 1,970 hrs (443,250 kWH)
Then, let’s look at the psychrometrics. In order to keep the humidity within the same ASHRAE 55 comfort zone during the economizer cycle, the available hours are reduced by:
• Chicago - 10 hrs (2,450 kWH)
• Los Angeles - 62 hrs (13,950 kWH)
At 225kWH per hour, the 62 hours in Los Angeles represents a lost savings of $2,372 per year at $0.17 per kWH. Without taking into account the dehumidification hours, the economizer savings is overestimated.
For those cases where supply air needs to be dehumidified to maintain acceptable RH levels in the occupied space, whether due to outside air conditions or internal humidity gains, there are alternatives to typical reheat configurations that will help to limit carbon emissions. The following are a few alternative configurations for controlling humidity with elevated supply air temperatures associated with UFAD or displacement ventilation systems.
Cooling coil with downstream hydronic or electric reheat — The cooling coil is used to sub-cool the air to 55°F. Downstream of the cooling coil, a conventional reheat coil (hot water reheat or electric coil) is used to then reheat the air back up to 60°F to 65°F, but with an acceptable moisture content. This is the most conventional, technically simple, and controllable configuration. Under certain conditions for this scenario, the UFAD or displacement system could very well use more energy than a conventional overhead air distribution system.
Face and bypass dampers — In this configuration, the AHU is equipped with control dampers at the face of the cooling coil and at a parallel path around the cooling coil. This allows control to move some air through the cooling coil, while the rest takes a parallel path around the coil. For example, the mixed air approaching the cooling coil is at a certain condition (for example, 80°F moist), and two-thirds of it passes through the cooling coil where the supply air temperature drops to 55°F (saturated), while the other one-third of the supply air bypasses the coiling coil and remains at the original mixed air condition, after which the two air streams then mix down-stream of the cooling coil, resulting in 65°F (drier) supply air temperature.
This is a relatively common solution and the fan energy penalty of the pressure drop through the dampers is relatively low, but it does have limitations. If the amount of air that is required to bypass is too high, the desired moisture content of the supply air is not achieved.
Run a round heat recovery for reheat — This configuration recovers heat in the air handling unit return or exhaust air stream and uses it to reheat the sub-cooled air before supplying the space. This requires extra coils that are in each exhaust air stream and downstream of the cooling coil. The 80°F moist return air is first cooled and then reheated to meet required supply air conditions delivered to the space.
This is a more expensive solution, in that two extra coils and a small recirculation pump are needed to move fluid carrying the energy between the coils. The added fan pressure drop due to the recovery coils is an energy penalty, as is the heat recovery pump energy. This solution is often more viable for systems serving densely occupied areas, like theaters, where there are larger volumes of exhaust air to recover energy from, balancing larger outdoor ventilation air volumes for the large number of occupants.
Wrap around heat pipe — Using energy recovery to reheat the supply air, this configuration consists of two coils, one upstream and the other downstream of the main cooling coil. Mixed, moist 80°F air passes through the first heat pipe recovery coil, removing some of the heat energy from the air stream (5°F or so temperature reduction is typical). The supply air then passes through the main cooling coil, which further cools the air down to 55°F. Finally, the energy removed by the upstream recovery coil is transferred to the downstream recovery coil via phase change and used to reheat the sub-cooled 55°F supply air (to approximately 60°F). This much less common configuration has the energy penalty of added fan pressure drop due to the recovery coils and is a bit more complicated to control and operate.
Reheat with solar hot water — Functionally, this is identical to the typical reheat coil but uses hot water generated by a solar hot water system comprised of solar panels, storage tank and pumps. Less common than the others, this configuration is most viable in sunny climates coinciding with periods of high outdoor humidity.
When designing a UFAD or displacement ventilation system, don’t forget to evaluate system operation at seasonal variations, including humidity, in addition to peak heating and cooling conditions, and exercise caution in predicting energy savings, all with an eye at understanding the balance between internal and external loads and gains.
We pose a challenge to the readers of Engineered Systems: How would you rank the alternative configurations of reheat for dehu-midification of comfort cooling for a typical office occupancy using UFAD or displacement ventilation in a Chicago climate? The authors will provide their rankings in the next issue of ES.