Published 05/22/2024
The use of Dedicated Outside Air Systems (DOAS) is growing quickly, driven by the adoption of Variable Refrigerant Flow (VRF) systems, active chilled beams, water source heat pumps, and other de-coupled heating and cooling systems. As my colleague Alex Goodwin explained in a previous blog post, enVerid’s Sorbent Ventilation Technology® (SVT®) easily integrates with all types of DOAS designs to offer significant system cost savings and energy efficiency benefits.
One of the DOAS designs that Alex discussed is DOAS coupled with VRF systems providing heating and/or cooling. The goal of this post is to dive deeper into how the IAQ Procedure in ASHRAE Standard 62.1 and sorbent air cleaning such as SVT can be integrated with DOAS/VRF designs to save system cost and reduce energy consumption. Additionally, this optimized design methodology can reduce refrigerant charge and electrical infrastructure required for the ventilation systems.
Traditional DOAS Design
Figure 1 depicts a DOAS providing 100% outside air for ventilation to a multistory building which is heated and cooled by a VRF System.
The design conditions and building parameters are shown in Table 1.
Outside air from the DOAS is ducted directly to the return of the VRF Fan Coil Units (terminal devices) to be distributed to each zone, and exhaust air is exhausted via the grilles in each zone. The general exhaust is brought back to the energy recovery wheel within the DOAS unit to pre-condition the outside air, upstream of the heating/cooling coils, and then relieved to ambient. Exhaust of zones within the building is prescriptively required in state and local mechanical codes and required in Section 6.5 of ASHRAE Standard 62.1. In addition to the prescriptive exhaust requirements, general exhaust air may be provided to balance the outside airflow over the energy recovery device, or to maintain positive building pressurization.
Optimized DOAS Design with SVT
Figure 2 depicts the Optimized Case, which integrates three HVAC Load Reduction® (HLR®) modules that use SVT to clean indoor air with the DOAS unit. We call this type of design a “dedicated clean air system” (DCAS) design because the 100% outside air DOAS system is converted to a mixed air system that delivers clean air (an optimized mixture of outside air and cleaned return air) to each zone.
Incorporating SVT into this design allows the engineer to calculate a new outside air requirement using ASHRAE’s IAQ Procedure that is 68% lower than the traditional design, which uses outside air requirements calculated using the Ventilation Rate Procedure (VRP). This significant reduction in outside air is possible because of the high cleaning efficiency of HLR modules that utilize SVT. As a result, the need for exhaust air above the prescriptive requirements also decreases. The airstream that was used for general exhaust in the traditional DOAS design now functions as return air, of which 100% is brought back to the HLR modules to be cleaned of gaseous contaminants. The cleaned air discharged from the HLR modules is then mixed with the pre-conditioned outside air as it leaves the downsized energy recovery wheel.
Benefits of the Optimized DOAS Design with SVT
Table 2 shows a comparison of the Traditional DOAS design to the Optimized Design with SVT. The table shows that the Optimized Case allows for a reduction in both outside air and total supply air as the volume of clean air added to the system is less than the outside air reduced, a benefit of high efficiency gas-phase filtration.
From a system cost standpoint, the 58% reduction in supply airflow from 12,800 CFM to 7,200 CFM (3,000 CFM of clean air + 4,200 CFM of outside air) can reduce the cabinet size of the DOAS, which reduces overall system cost and system ductwork size. The spatial impact in the mechanical room or on the roof and in overhead and plenum spaces where ducts and equipment are located can be significant.
Table 2 also shows a substantial reduction in the peak cooling and heating load from optimized mixed air enthalpy. The 20.4-ton reduction in peak cooling load on the DX cooling coil allows for smaller compressors and condensers. In a chilled water system, the chilled water coil size can be reduced along with the size of the chillers, pumps, and cooling towers. Similarly, from a heating perspective, a 319 MBH reduction in load allows the electric pre-heat and/or reheat coils to be sized at a lower capacity. In a hydronic heating system, the hot water coil size can be reduced along with upstream downsizing of boilers and primary and/or secondary pumps. Taken together, these peak cooling and heating load reductions translate to 36 metric tons of annual carbon emissions avoided from improved ventilation efficiency. Additionally, total refrigerant charge for optimized VRF DOAS designs can decrease 70-80% coupled with consolidation of power requirements due to downsized outdoor heat pump sections. Often the downsized heat pump sections lead to reduced quantity and amperage of circuit breakers.
We Are Here to Help You
My colleagues and I have worked on countless DOAS/VRF Designs with SVT that deliver similar results to those shown above. We would love to work with you to optimize your next DOAS/VRF design using our “dedicated clean air system” approach.
Contact us for more information.
Ben Goldberg
Application Engineer, enVerid Systems
