Certus was asked to help a rural hospital attain low-temperature operating rooms with their existing air handling systems. The operating rooms were having difficulty maintaining the required 68 F room setpoint at 50% relative humidity, and a new doctor requested the spaces to get to 62 F at 50% relative humidity. With these initial requirements, the first task was to determine what kind of dedicated outdoor air system (DOAS) to provide.

Maintenance response time was a key factor in system and equipment selection. The facility wasn’t comfortable with desiccant technology, as there were no other desiccant systems at the hospital or in the region. The hospital had both DX and chilled water systems with qualified service technicians in the area who could respond quickly.

The final decision between DX and glycol chilled water came down to controllability with the system. The facility preferred precise control with glycol-chilled water.

The operating rooms were served by one air handling unit (AHU) on the roof and another in an interior mechanical room, with outside air connection on the same roof. A glycol-chilled water DOAS could be placed on the roof, ducted to the outside air inlet on the rooftop unit and into the outside air duct outside. An air-cooled chiller with integral pumps kept the equipment consolidated to one area. The glycol chilled water temperature was due to the desire to achieve a 40 F discharge temperature off the sub-cooling coil, giving ample room to achieve desired temperatures. Because the chiller was placed near the DOAS unit, the system volume was too little with the piping for the air-cooled chiller. A buffer tank was added to allow for the minimum system volume required for constant temperature back to the chiller.

The DOAS unit was designed to deliver a constant discharge air temperature of 40 F to both existing AHUs. A first filter was provided in the unit to protect the coils from dirt, but there were no final filters, as they were provided in the existing air handling units. The DOAS came with two cooling coils to maximize the energy efficiency. A normal chilled water coil off the existing plant provided the first stage of cooling for the outside air down to 49 F.

An existing water-cooled plant allowed for a more efficient cooling mode, rather than increasing the size of the air-cooled glycol chiller to handle the entire outside air load. The pipe size for the mains did not need to be increased either, as the chilled water load was moved from the existing AHUs to the DOAS unit. A sub-cooling coil then followed the primary cooling coil to achieve the design discharge air temperature. This setup required a heating water preheat coil before the cooling coils to protect the chilled water coil from freezing.

A unique challenge in this project came when ducting to both existing air handling units. One AHU was a constant volume system with no economizer, while the other was a variable volume system with economizer cycle. How do you control different systems when one is calling for a constant quantity of outside air and the other calling for a varying amount? The DOAS was sized for the minimum amount of outside air required for the two units. Motorized dampers in the taps to each AHU and separate outside air intakes in the ducts with motorized dampers were provided to close off the DOAS unit and bring outside air in normally by the existing AHU supply fans.

The economizer cycle for the existing AHU was enabled when the dry bulb was less than 65 F, and the dew point was less than 40 F. Once the economizer was enabled, the DOAS was called to disable, and the motorized dampers switched their respective positions to allow outside air into each unit. This allowed the two separate systems to receive outside air at the necessary dewpoint for the desired condition.

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