Case study: How can DOAS serve a water treatment plant?

Houston, in association with four regional water authorities, is expanding its Northeast Water Purification Plant (NEWPP) from 80-million-gallons-per-day (mgd) to 400 mgd — dramatically increasing its ability to support steady residential and commercial growth while reducing dependency on groundwater. The NEWPP expansion project is the largest water treatment plant project in the world being delivered using a progressive design-build delivery methodology.

This project is being performed by the Houston Waterworks Team, a 50/50 joint venture comprised of CDM Smith and Jacobs operating as one team to bring this exciting project to fruition.

Dedicated outdoor air systems (DOAS) were implemented for spaces that required a high volume of outdoor air. These DOAS units are used to serve chemical buildings, administration buildings and maintenance buildings.

Figure 4: The dedicated outdoor air systems (DOAS) unit shown is serving an administration building at Houston’s Northeast Water Purification Plant. Courtesy: CDM Smith

The first unit in Figure 4 is a ground-mounted, packaged, unit ducted into a chemical building. The unit uses natural gas as a source to heat the air. It is supplying 5,175 cubic feet per minute (cfm) to chemical storage areas. Due to the corrosive nature of the space, the system does not provide any exhaust to the DOAS unit. The unit has a cooling capacity of 204 MBH (thousand BTUs per hour) and a heating capacity of 316 MBH. The unit is supplied with an aluminum mesh and MERV 8 filter.

Some additional accessories include stand-alone controller, plenum curb with horizontal duct connection, stainless steel gas heat exchanger, inverter scroll compressor, electronically commutated motor (ECM) premium efficiency motor and a 1-inch injected foam, R-7, galvanized steel liner.

The unit in Figure 5 is a roof-mounted, packaged, bottom-discharge, unit ducted into an administration building. The unit uses natural gas as a source to heat the air. It is supplying 1,710 cfm to a control room, office and a break room and returning/exhausting 1,575 cfm to the DOAS unit. The unit has energy recovery in the form of an energy wheel, a cooling capacity of 83.80 MBH and a heating capacity of 160 MBH. The unit is supplied with a MERV 8 prefilter and MERV 13 filter.

Figure 5: The dedicated outdoor air systems (DOAS) unit shown is serving an administration building at Houston’s Northeast Water Purification Plant. Courtesy: CDM Smith

Some additional accessories include 1-inch injected foam, R-7, galvanized steel liner, ECM premium efficiency motors, inverter scroll compressor, modulating control on inverter compressor, stainless steel gas heat exchanger, at least 5:1 modulating turndown, provide with horizontal duct connection or plenum curb with horizontal duct connection, stand-alone controller, multiple exhaust airflow conditions based on a control sequence and an internally mounted nonfused disconnect with external service handle.

Comparing the two units, some of the required accessories are similar between them, regardless of the application. Other accessories vary due to the difference in application, serving a chemical storage space versus serving an administrative space. Some of the main differences are due to whether the unit is receiving exhaust/return air, the discrepancy in unit size and installation location.

It is important to note that the chemical building DOAS unit does not require multiple exhaust airflow conditions, but the DOAS unit that serves administrative spaces does and has energy recovery associated with it. The staggered setpoints for exhaust are due to conditions in which kitchen exhaust hoods and exhaust fans are activated.

The unit controller and overall building automation system will be programmed to allow that functionality. This is not necessary for the chemical building DOAS unit as it only has interlocked operation with exhaust fans serving that building and no return air provided to the unit.

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