Background: We received a call from a renowned facility management team regarding one of their prestigious buildings in Jumeirah, Dubai. The building had been experiencing high humidity levels during the summer months and early autumn. Despite thorough studies over the years, the issue remained unresolved. This problem was of great concern because high humidity can lead to condensation, compromising occupant health and comfort, damaging interior finishes, and increasing cooling costs. Additionally, the team sought expert advice from OCC on the energy costs associated with HVAC systems.
Analysis: After a careful analysis leveraging OCC’s expertise in humidity control, we identified the main cause of high humidity within the building. Since there were no major infiltration sources, the primary issue stemmed from the Fresh Air Handling Units (FAHUs) and the poor performance of the existing heat recovery wheel, which led to increased operating costs for the FAHUs.
Observations: There are four FAHUs in the building supplying treated fresh air to all the Fan Coil Units (FCUs) above the false ceiling of each apartment and lift lobby. The main components of each FAHU are the heat recovery wheel, the cooling coil, and the heat pipes.
| Parameters | FAHU-01 | FAHU-02 | FAHU-03 | FAHU-04 |
| Outside Air DBT, °C | 38 | 38.3 | 33.9 | 34.2 |
| Outside Air RH | 46.8% | 40.2% | 59.2% | 53.6% |
| Return Air DBT, °C | 25.6 | 25.3 | 26 | 26.5 |
| Return Air RH | 53% | 54.7% | 57.6% | 62% |
| On Precool DBT, °C | 36.1 | 33.3 | 30.3 | 30.8 |
| On Precool RH | 61.8% | 51.7% | 71.2% | 67.8% |
| Off Cooling Coil DBT, °C | 10.2 | 9.8 | 18.5 | 19.2 |
| Off Cooling Coil RH | 91.2% | 98% | 95.9% | 92.7% |
| Supply Duct DBT, °C | 19.9 | 18 | 22.9 | 23.5 |
| Supply Duct RH | 56.6% | 89% | 72.7% | 70.7% |
| Dew Point, °C | 11.1 | 16.2 | 17.7 | 17.9 |
| CHW In, °C | 9 | 8 | 14 | 12 |
| CHW Out, °C | 15 | 14 | 20 | 22 |
| CHW valve position while measurement | 100% | 100% | 100% | 100% |
Findings:
- Air Leakage in FAHU-02:
- There was significant air leakage in FAHU-02 from the return air into the on-pre-cool side through the partition plate between the return side heat recovery wheel and the heat pipe, as well as air bypass across the heat pipes. Due to this bypass, even though the off-cooling coil maintained a temperature of 9.8°C, the supply air became highly humid.
- Inadequate Off-Coil Temperatures in FAHU-03 and FAHU-04:
- The off-coil temperature of FAHU-03 and FAHU-04 could not be maintained as per design conditions due to high chilled water inlet temperatures and lower water flow rates compared to the design values. Any deviation in the chilled water inlet and outlet affected the dehumidification rate, resulting in high humidity air from FAHU-03 and FAHU-04 entering the apartments. This affected occupant health and comfort and damaged interior finishes.
- Poor Performance of Heat Recovery Wheel:
- The average sensible effectiveness of the heat recovery wheels was below 50%.
Solutions and Implementation: To address these issues, we implemented the following solutions:
- Replacement of Heat Recovery Wheels:
- We replaced the existing heat recovery wheels with optimally sized units to enhance their performance and efficiency.
- Water Balancing and Heat Exchanger Modifications:
- We balanced the water flow to ensure adequate rates to the FAHUs, modified the heat exchanger and water circulation pump down the line of water connections to improve chilled water system performance with the help of 3rd party.
- Air Leakage and Bypass Elimination:
- We addressed the air leakage and bypass issues in FAHU-02 by sealing the partition plates and ensuring proper airflow across the heat pipes.
To enhance clarity, the psychrometric processes associated with FAHU-03, both before and after the modifications, are depicted in the chart below. The red line illustrates the processes prior to the modifications, while the green line represents the processes following the modifications.
The points labeled O.A, B2, B3, B4, and B5 correspond to the Outside Air, On Precool, Off Precool, and Off Reheat temperature conditions Before the modifications, respectively. The points labeled A2, A3, A4, and A5 represent the On Precool, Off Precool, and Off Reheat temperature conditions After the modifications.
Results: By implementing these measures, we achieved the desired humidity control and energy efficiency in the building. This ensured a healthier and more comfortable environment for its occupants. Additionally, the energy costs associated with the HVAC systems were significantly reduced, leading to substantial operational savings for the facility management team.
Conclusion: This case study demonstrates OCC’s commitment to delivering expert solutions for complex humidity control issues. Our comprehensive approach, combining technical expertise with practical implementation, ensured that the prestigious building in Jumeirah, Dubai, now operates at optimal conditions, providing enhanced comfort and efficiency for its occupants.
Technical Department, OCC
Ref. Daikin Psychrometric Diagram Viewer 3.2.0
