Pointing Out the Potential Advantages in UAD

June 9, 2008
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Dennis Stanke, staff applications engineer with Trane, provided the following information regarding underfloor air distribution (UAD) in Engineers Newsletter (2001, volume 30, No. 4):

Some advocates claim that pressurized-plenum UAD systems offer several advantages over traditional overhead VAV systems. Following is a discussion of the benefits most commonly associated with these advantages.

LOWER CHURN-RELATED LIFE-CYCLE COSTS

Most of the savings related to office reconfiguration result from the access floor, which lowers rewiring costs regardless of how the air is distributed. Can underfloor air distribution trim additional expense from “churn”? The answer depends on the type of relocation.

Cubicle rearrangements in UAD applications usually require the relocation of floor-mounted diffusers. By contrast, rearranging cubicles in a space with overhead VAV distribution seldom (if ever) affects the placement of ceiling diffusers. In terms of air distribution alone, then, UAD may actually increase the cost of cubicle-wall “churn.”

Rearranging the walls of private offices is another matter. In this situation, underfloor air distribution avoids the expense of moving and rebalancing overhead ducts and diffusers.

REDUCED FLOOR-TO-FLOOR HEIGHT

Often cited as an initial cost benefit of underfloor air distribution, removing the supply ducts, terminals, and diffusers from the ceiling can reduce overall plenum height, and may reduce slab-to-slab and total building height... perhaps by as much as 10 percent.

IMPROVED COMFORT

A combination of cold plenum air, low-induction floor-mounted diffusers, and reduced airflow can cause excessive (uncomfortable) stratification. However, direct control of supply airflow (a hallmark of most UAD systems) increases the degree of comfort that occupants perceive.

To assure that a UAD application provides the promised improvements in individual thermal comfort, the design of the system must properly account for all relevant parameters, including vertical load distribution, diffuser throw, and floor temperature.

IMPROVED PRODUCTIVITY

As implied above, people express greater satisfaction with thermal comfort when they can control their immediate environment. Adjustable, floor-mounted diffusers contribute to occupant satisfaction because they allow at least some adjustment for individual preferences. Reducing or eliminating the distraction of thermal discomfort in a space increases the productivity of those who occupy it.

IMPROVED INDOOR AIR QUALITY

Indoor air quality (IAQ) relates to contaminant concentrations in the breathing zone. Some studies report lower breathing-zone concentrations for UAD systems than for overhead VAV systems. Here’s why...

In overhead VAV applications, mixing disperses contaminants throughout the space. In UAD applications, contaminants “collect” near the ceiling outside of the breathing zone, so occupants breathe “cleaner” air. Given the higher air-change effectiveness (Eac) of UAD spaces, proper space ventilation requires less outdoor airflow at the diffusers.

REDUCED OUTDOOR AIRFLOW

If better air-change effectiveness in UAD spaces means that each diffuser needs less outdoor air for ventilation, then it follows that the building ventilation system can condition less outdoor air and, therefore, will require less heating and cooling capacity. How much less? That depends. When air-change effectiveness increases from 0.95 (VAV) to 1.10 (UAD), system ventilation efficiency, E, at design conditions also improves - from 0.966 (VAV) to 0.991 (UAD), in this case.

Although the UAD system reduced both outdoor airflow and, therefore, the installed capacity required at the plant, the reductions are significantly less than one might expect. In multiple-space mixed-air applications, improving the air-change effectiveness in the space does not yield an equal improvement in system ventilation efficiency (or airflow reduction) at the outdoor air intake.

Note: System ventilation efficiency improves for UAD at design conditions, which can reduce the installed capacity of the heating/cooling plant. For overhead VAV distribution, system ventilation efficiency improves at part load, which can reduce the required operating capacity if the system is equipped with proper ventilation-reset controls.

LESS FAN HORSEPOWER

If we assume that UAD and overhead VAV systems require the same supply airflow at design conditions, then the absence of supply ducts, terminals, and runouts in a pressurized-plenum UAD system reduces the external static pressure on the supply fan. Less external static pressure results in the selection of a smaller motor (lower initial cost) ... but does it also mean that UAD requires less horsepower (costs less to operate) than overhead VAV distribution?

Many UAD systems supply a relatively constant volume of airflow to both interior and perimeter zones. According to the fan laws, a 50-percent reduction in external static pressure (typical of UAD) yields the same brake-horsepower effect as a 30-percent reduction in airflow. VAV systems that serve both types of zones often operate for many hours at less than 70-percent of design airflow. Which system actually uses less fan energy? Learning the answer requires a careful, case-by-case analysis of part-load operation.

IMPROVED CHILLER EFFICIENCY

In arid climates, 65°F DB supply air may be dry enough during most hours of operation to avoid elevating the relative humidity in the space. If so, then raising the chilled water temperature from 45°F to 55°F, for example, will improve the chiller’s coefficient of performance or COP.

For most climates, however, saturated 65°F DB supply air would unacceptably raise the relative humidity in the space. Therefore, when a cold coil provides dehumidification, the chilled water in most climates must be cold enough to produce a supply-air dew point of 58°F to 60°F, greatly reducing the anticipated COP improvement.

In other words, the warmer supply air temperatures of UAD systems can improve the operating efficiency of chillers applied in dry climates. However, this advantage diminishes significantly in climates that routinely require mixed-air dehumidification (that is, cold water temperatures) at the cooling coil.

Note: Using a separate unit for dehumidification (an active desiccant dehumidifier, for example) allows the chilled water temperature to rise along with the chiller COP ... but perhaps at the expense of overall system efficiency. Again, careful analysis is needed to assess the effects of such a design.

REDUCED ELECTRICAL DEMAND

In UAD applications, the floor slab forms part of the supply duct for one floor and part of the return duct for the floor below. Therefore, the thermal mass of the floor slab can store heat (cooling load) during daytime hours and release it at night.

With proper controls and sufficient slab mass, lower daytime cooling peaks may permit smaller cooling equipment and - when coupled with fan-horsepower savings - may reduce daytime electrical demand peaks and charges. Unfortunately, without dependable models to predict the slab’s thermal performance or a wealth of design experience, it is unlikely that designers will risk reducing the installed capacity of the cooling plant.

MORE HOURS OF ECONOMIZER COOLING

When outdoor air enthalpy is less than return air enthalpy, less energy is required to mechanically cool outdoor air than mixed air. Return air is warmer in UAD systems than in VAV systems - perhaps 80°F versus 77°F at economizer conditions. Therefore, the changeover from “mechanical cooling with minimum outdoor air” to “mechanical cooling with maximum outdoor air” occurs at warmer outdoor conditions, reducing the cooling coil load and increasing economizer hours slightly during warm weather.

UAD systems also supply warmer air than VAV systems - perhaps 65°F versus 55°F. So, the changeover from “mechanical cooling with maximum outdoor air” to “modulated economizer cooling” occurs at a warmer outdoor temperature, reducing the hours of mechanical cooling operation during cool weather.

Finally, because UAD systems usually deliver roughly constant airflow to interior spaces, the change from “modulated economizer cooling” to “heating with minimum outdoor air” may occur at a warmer or cooler outdoor temperature (depending on the building cooling load) than in VAV systems. In other words, heating hours may either increase or decrease during cold weather. Why? Interior zones usually do not require heating during occupied hours. Therefore, while “heating with minimum outdoor air,” the heating coil warms the mixed air to the current cooling setpoint.

Because UAD systems usually require warmer supply air, they may actually use more heating energy for interior spaces than VAV systems ... even if the hours of heating operation decrease.

Stated simply, a UAD system can decrease the cooling coil load during warm weather and decrease the hours of mechanical cooling operation during cool weather (especially in dry climates). During cold weather, however, underfloor air distribution may increase heating energy use and/or hours of heating operation, depending on building loads.

Ultimately, local weather and load conditions, together with system control schemes, will determine how much extra mechanical cooling energy UAD saves and how much extra (if any) heating energy it adds. Once again, careful analysis is needed on a job-by-job basis to quantify the operating cost savings.

Publication date: 06/09/2008

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