With the growth of online shopping and logistics, the demand for warehouses and distribution centers has increased over the past decades. Applying effective heating, ventilation, and air-conditioning (HVAC) strategies in these buildings is important to maintain thermally safe comfortable indoor environments. With large volumes, shifting usage patterns, and sensitive equipment, computational fluid dynamics (CFD) can help engineers optimize HVAC systems in these buildings to meet strict owner requirements. In this blog post, we delve into the specific challenges faced by a Distribution Center and how we were able to apply CFD to analyze and improve the thermal conditions within the space.

Background

The prototype distribution center, spanning 114,000 square feet, is a key project for the logistics company, which constructs such facilities across the country as needed. With a current portfolio including hundreds of facilities, the building serves as the final stop for sorting and loading packages onto delivery trucks for both residential and business deliveries. Inside, there are conveyors, rows of storage shelves, parking areas for carts, and an open office space.

The current prototype design uses rooftop units (RTUs) to provide heating and cooling, dedicated outdoor air systems (DOAS) to provide ventilation, and high volume low speed (HVLS) fans to provide air movement and mixing to improve thermal comfort.

However, the owner’s facilities staff reported frequent complaints about thermal comfort at existing distribution centers built according to the current prototype design, such as:

• During cooling seasons, the stagnant air caused by the obstruction of the shelves leads to under-cooling complaints.
• The morning cool-down process between shelf racks takes a longer time compared with open areas.
• When turning up the included HVLS fans to counter comfort complaints, light packages are often blown off transit carts when passing below the fans.

The owner also sought to achieve a cleaner roof space suitable for installing solar PV panels to help meet corporate sustainability goals. To address comfort complaints and additional desired roof space, the owner identified ground-mounted Air Rotation Units (ARUs) with exterior ground-mounted condensing units as a potential solution to achieving both goals without impacting typical operations. ARUs are large, ground-mounted, vertically oriented HVAC systems using high-volume, low-velocity air circulation to distribute conditioned air to a large open space. ARUs supply air from the top, flowing along the ceiling, then circulating down the walls and finally across the floor back to the return, driven by the Coanda Effect. The systems minimize roof-mounted HVAC equipment, and provide heating, cooling, and ventilation effectively to occupied levels without stratification, requiring additional fans for air mixing. However, due to the owner’s desire to minimize operational impact, together with manufacturer length limits of the outdoor air intake duct and condenser refrigerant lines, the potential ARU installation locations were limited. To ensure the proposed design addressed thermal comfort and operational complaints, the owner sought Baumann Consulting’s CFD services to better inform the design and manage risk.

Baumann Consulting (Baumann) worked with the owner and ARU equipment manufacturer to identify two potential ARU system design scenarios meeting the owner’s roof space and operational impact requirements.

• Option 1: 2 ARUs located outside the project building, supplying air at the same side of the building at 45°, 90°, and 135° angles.
• Option 2: 2 ARU located inside the space, supplying air at 0°, 90°, and 180° angles.

Assessing Thermal Comfort

To evaluate the thermal comfort performance of the HVAC system, Baumann referred to the Predicted Mean Vote (PMV) thermal comfort assessment method defined by ASHRAE standard 55-2023. This methodology predicts likely occupant indoor thermal comfort by considering air and surface temperatures, local airspeed, indoor humidity, occupant clothing, and activity levels.

As previous occupant complaints related primarily to stagnant air during the cooling season, Baumann sought to define acceptable ranges of airspeed and temperature at a given humidity level and defined occupant clothing and activity levels. Using the CBE thermal comfort tool following ASHRAE 55 guidelines, Baumann identified the minimum local airspeed needed to avoid under-cooling complaints at different combinations of indoor temperature and humidity levels, considering typical metabolic rates and clothing levels.

The CFD Modeling Process

To evaluate the thermal comfort performance of the baseline system, the Baumann team performed steady-stage CFD studies for both summer and winter conditions. The team reviewed the prototype design and conducted a series of meetings with facilities staff at existing distribution centers to define outdoor design conditions (Carbondale, IL) and internal heat loads (equipment, people, lighting, and plugged-in equipment), understand building enclosure thermal properties, and confirm RTU supply air flows and temperatures, and HVLS fan speed and operation.

Baumann developed detailed geometry in Autodesk Fusion 360 and completed meshing and analysis using Autodesk CFD 2023. After an independence test on the unstructured mesh with refinement on gaps and edges, it was discovered that the mesh contains a total of 21.3 million elements.

Baseline System Performance

After completing the simulation, Baumann evaluated temperature and airspeed results at various lateral, longitudinal, and horizontal building sections. At the occupancy plane height of 56”, we found that while temperatures are generally around the 75°F expected setpoint, there are warmer pockets reaching 77°F under summer design conditions. Based on the thermal comfort tool, a minimum airspeed of 20 fpm will be needed to meet thermal comfort.

While air speeds are generally elevated, there are several regions with air speeds below 5 ft/min with potential for discomfort at design conditions (75°F/ 50% RH indoors), below 35 ft/min with potential for discomfort on humid days (75°F/ 60% RH indoors), and below 50 ft/min with potential for discomfort in warmer pockets (77°F indoors).

The heating seasons results were similarly assessed, showing the design performs sufficiently with respect to thermal comfort, consistent with the occupant complaint history at existing facilities.

ARU System Performance

The two ARU systems showed distinct airflow patterns. In both cases, the supply air travels along the ceiling, walls, and returns to the bottom of the ARU along the floor as expected.

The ARU case option 1 shows that the proposed design controls space temperature at the occupancy plane height of 56” temperature of 74 °F, but with much slower airspeeds. Despite slower-moving air, the lower achieved temperature means the ARU can still likely maintain occupant thermal comfort.

However, the airflow direction from the ARUs option 1 is not in parallel with the shelf racks. As a result, stagnant air can still be observed in the area between the shelf racks. As a comparison, the ARU case option 2 shows significant improvements in eliminating stagnant air pockets between shelf racks. With the advantage of supplying air in parallel with shelf racks, the airspeed in-betweens is improved to 8-10 ft/min.

Similar to the baseline case, the ARU case simulation results showed both ARU options likely to maintain heating season thermal comfort.

Cool-down Time Comparison

Baumann performed a cooling season transient CFD simulation to compare the morning cool-down time for the baseline and ARU systems. The simulation focused on how long each system takes to cool different locations from an 80°F start to a desired 75°F setpoint. We selected eight observation points representing common comfort complaint locations throughout the floor at a plane  56” above the finished floor. The results showed that both ARU alternative case options 1 and 2 reached the setpoint faster than the baseline at most points. Furthermore, ARU option 2 demonstrated shorter cooldown times than ARU option 1 on most of the points.

Based on the simulation results and analysis, Baumann provided these findings and recommendations to the facility owner, which suggested modifying their prototype design to better meet their indoor environmental and sustainability goals. The recommendations included replacing RTUs and HVLS fans with ARUs to maximize roof space without disrupting operations. Additionally, it was suggested that the ARUs be located indoors, supplying longitudinally to minimize the risk of thermal discomfort during the cooling season. When properly optimized, the indoor, longitudinally-mounted ARUS shows a significant ability to eliminate stagnant air pockets in the studied distribution center. Furthermore, indoor, longitudinally-mounted ARUS reduces the cooldown time in the studied distribution center compared to outdoor-mounted ARUS.