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Air Handler Retrofit Turns Regular Hospital Rooms into Negative Pressure ICU Rooms

Coil maker RAE Coils has created a coil box air handler retrofit option that can be used to turn regular hospital rooms into negative pressure ICU rooms for COVID-19 patients. Brad Marshall, Business Development Manager, RAE Coils, volunteered that the concept is straightforward enough that RAE Coils could make the coils, which is the firm’s forte, and somebody else could manufacture the rest of the coil box. A mechanical contractor with a sheet metal shop could do it.

The company recently completed a quick retrofit for a midwestern hospital with the use of its coil box. The hospital did not want to replace the existing air handler, but it needed a quick solution.

The hospital had eight to 10 negative pressure rooms and needed to convert 40 to 60 regular rooms to negative pressure in a week or two. By converting a standard positive pressure space to negative, the hospital can provide additional prevention of COVID-19 migration throughout the hospital while serving the peak load of patients with COVID-19.

The current air handling unit was not designed for 100% outside air, which is necessary for the additional negative pressure rooms. There was also a need to increase the coil cooling capacity without adding additional static pressure in order to keep the existing supply fan. This all had to be done quickly and seamlessly so the hospital had very little downtime.

RAE Coil designed a coil box to work with the existing air handler, explained Marshall. The box contains pre-filter slide racks, a PH Coil and a Cooling Coil with integral drain pans. Ordinarily it would take the company two to three weeks to produce a coil box like this, Marshall said, but time was of the essence and they cranked this one out in less than a week. The assembly shipped in less than 10 business days.

They also minimized downtime by making it easy for the contractor to demo a section of the AHU upstream from the supply fan and to quickly drop the new coil box assembly into place. The manufacturer built the box to the same dimensions as the existing air handler and kept the coil connections in the same location to make it plug-and-play.

Marshall noted that any manufacturer that makes air handlers can do this. The coil design is unique to RAE Coils, however, as they were able to double coil capacity without increasing pressure drop.

The original installation was an economizer air handler designed to take in 80°F dry bulb/67°F wet bulb entering air. When the outside air is cool enough, you can bring in 100% outside, but in the peak of summer you can’t because it would be 95°F and humid. The original air handler mixed return air with outside air for slight positive pressure throughout the hospital building. Most buildings are designed up for 10%-20% outside air, so only need to cool 20% of the air coming in. For example, say the outside air is 95°F db, so you’re mixing 20% 95°F outside air with 80% 72°F return air, so the mixed air going through the air handler is 80°F db/67°F wb, and you want to cool it to 54°F leaving air temperature so by the time it gets to supply air outlets it’s around 60°F.

Negative pressure ICU rooms, however, need to exhaust 100% of the air in the room. Without recirculation air, the air handler needs to bring in 100% outside air, putting strain on a coil that’s not designed to handle that kind of load. If a negative pressure floor is exhausting 20,000 CFM, the air handler should be set up to provide, for example, 19,500 CFM.

The task facing the contractor and RAE was to double capacity on the floor that was being converted to negative pressure from 600,000 Btuh to 1.2 million Btuh. They also had to do that without changing out the existing fan. That meant the static pressure had to stay the same.

The solution was to change from a six-row coil to a 10-row coil, and from 12 fins/inch to 8 fins/inch. Instead of corrugated fins, the new coil has flat fins, and the thickness of the aluminum fins went from 0.006-0.008-in. to 0.01-in. The coil is 5/8-in. copper and the header and all connections are copper. The casing was fabricated from stainless steel. The heat transfer doubled but the static pressure stayed the same.

The fact that the hospital was able to quickly increase the number of infectious disease rooms with zero downtime is a testament to American ingenuity and the role that the HVAC industry can play in combatting COVID-19.

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