Return Air Fan Speed Serving multiple air handling units

[remp]

Im commissioning a central plant air handling system for a commercial building at present. One return air fan serves 3 air handling units. The fan is rated at 45 m3/s and each air handling unit is rated at 15m3/s approx. Its a VAV system so the AHU fans ramp up and down on a VSD to maintain a constant supply air pressure. How can I control the speed of the return air fan?. We have no air flow stations on the return air system. I was thinking of using the return air attenuator for which we can plot a system curve and depending of pressure drop equate the air flow rate. This would be done via the BMS of course. Some people say put a pressure sensor in the return air shaft, and maintain a constant pressure here. I have my reservations about this one. Any ideas?

 

[ChasBean1]

There are many ways to do it and many ways to screw it up. Are there return VAV boxes, or above-ceiling return openings off the shaft? If so, how many, and could it be feasible to put flow stations and return volume dampers at the shaft openings?

 

[remp]

The return air fan delivers the air from a return air shaft which serves 12 floors. Each floor has a shut off damper/sub duct with a 2 position actuator (for smoke control pruposes)connected to the shaft.

 

[ChasBean1]

The two-position actuators could be changed to analog control actuators with in-duct flow stations, with total supply flow sensed at each floor and the return designed to a fixed offset from supply based on a known exhaust volume. Or likewise, the return damper could modulate (without knowing flow) to maintain a differential pressure between the floor and outside air. This would take a pressure sensor at each floor and a tube running down the height of the building, teed off at each floor. Meanwhile, all the return fan has to do is maintain static pressure in the riser.

 

[lilliput1]

Note that ChasBean1 suggestion can only be implemented if the return air riser is ducted. With a return air - plenum shaft however, there will be too much leakage when the return fan is ramped up to provide enough static pressure to operate the airflow measuring stations at the ideal velocity range of 1000 to 1800 FPM. Having the dampers at each floor would have help solve typical stratification problem with high rise buildings.
You can use air measuring stations that install at the inlet of centrifugal fans. You will need two sets for DWDI (double width double inlet fans) and must sum them to get the airflow. You can track track the supply air CFM with an offsdet equal to the design minimum outdoor air but because of the high rise & plenum shaft return riser, expect to have stratification which may be a problem if you don't have proper zoning & supplemental heaters particulary at the buiding entries.

 

[jburrows]

I worked on a building with two return fans and one supply fan. After trying several control strategies, the one that worked the best was controlling the return fans to maintain a set discharge pressure. The set point was just slightly positive (.05" w.g.). This allowed the return fans to supply adequate air through the either the return or relief dampers without overpowering or putting too much of a draw on the supply fan. The relief dampers were controlled off of building static pressure. I don't know why this wouldn't work for one return fan and three supply fans, but there may be a reason. Has anyone tried this?