LABORATORY-VENTILATION CONTROL
Most of the square footage of the building is laboratory space. The nature of the work conducted in that space requires 100-percent outdoor-air ventilation 24 hr a day. Energy use is minimized by utilizing glycol loop heat recovery coils and by using a sophisticated variable-flow control strategy for the entire ventilation system.

Control of the energy-recovery loops (the green piping in Figure 2) in the laboratory- ventilation system was based on three modes:

• Winter heating mode.
If the outside-air temperature is less than the indoorair temperature, and the AHU is supplying heat, then a glycol coil absorbs heat from the exhaust air and uses it to preheat outdoor air entering the AHU. Because the laboratory requires 100-percent outdoor air, even a small temperature change can yield significant energy savings.

FIGURE 2. Composite graphic showing the exhaust fan (top) and air-handling unit (bottom) in summer cooling mode. The twin fans provide a margin of safety through redundancy. Normally, they operate together.

• Summer cooling mode.
If the outside-air temperature is greater than the indoor-air temperature, and the AHU is supplying cooling, then exhaust air is used to cool a glycol coil, which precools outdoorair entering theAHU. Again, even a smalltemperature change canyield significant energysavings (Figure 2).

• Summer dehumidification mode. If the outside-air temperature isgreater than the indoor airtemperature, and thesupply-air humidity ishigh, a “runaround” loopmay be employed. A humidityPID loop sets alow limit for the chilled watercontrol valve, so ifthe need for dehumidificationis greater thanthe need for cooling, thevalve will open far enoughto keep the humiditybelow its upper limit. If this causes thesupply air to fall more than 2 F below itsset point, the system will activate therunaround loop to reheat the supply air.The glycol coil in the AHU outside-air duct is heated by the incoming air. This glycol then is pumped to a reheat coil downstream from the chilled-water coil, where a reheat PID loop modulates the flow to bring the return air back up to the desired temperature.

At the heart of the VAV laboratory-ventilation control are the laboratory fume hoods. These hoods are required to maintain a 100-fps face velocity, regardless of the position of the sash (a vertically sliding glass door). Thus, as the sash is raised or lowered, the fume-hood controls vary the amount of airflow exhausted through the hood to maintain 100-fps flow into the hood. This air is replaced by treated air flowing into the room through ceiling diffusers. This incoming air also is used to control temperature in the room. If the room is too warm, the temperature- control loop increases the amount of incoming air to cool the room. If the room is too cold, the loop will try to decrease the amount of incoming air. This amount cannot be reduced below that required to make up for the flow-hood exhaust; so, when the flow cannot be reduced any further, a reheat coil is used to warm the incoming air.

For containment purposes, the labs have to be maintained at a slight negative pressure at all times.This is accomplished with a separate room-exhaust system, which is controlled so that the total amount of air exhausted by the fume hood and room exhaust is 200-cfm more than the amount of supply air. The room-flow system, therefore, has to keep track of the air being exhausted by the fume hood and the temperature of the room, control the incoming air accordingly, control the exhaust air to maintain negative pressure, and control the reheat coil as necessary. When someone opens a fume hood, all airflows need to change immediately. The incoming air is supplied by a heating and cooling AHU with a variable-frequency-drive- (VFD-) controlled supply fan, while the exhaust air is routed through an independently controlled VFD exhaust fan. A control- system graphic is shown in Figure 3. A typical laboratory room is shown in Photo A.

FIGURE 3. Typical laboratory room with fume hoods and variable-flow control of the supply and exhaust air.

PHOTO A. Fume hood and process chilled-water piping in a typical laboratory room.

As is the case with any modern building, a multitude of other systems affect energy use. The laboratory fume hoods are a major energy consumer. Four process chillers staged to provide chilled water upon request are controlled by the primary BAS. A stand-alone computer-room unit draws chilled water from the process chillers. All or these systems are integrated by the BAS into a single operator interface. The BAS also monitors non-energy-consuming systems, including fire, eye-wash, uninterruptible power, acid neutralization, bottled gas, and back-up generating. This provides a single interface to monitor and control the entire building. An idea of how seamless this interface is can be gained by comparing the BACnet controller in Figure 4 with the Modbus laboratory fume hood in Figure 3.

The laboratory fume-hood controller shown in Photo A uses the Modbus protocol and is integrated into the native-BACnet BAS through a BACnet/Modbus gateway. The office VAV controller shown in Figure 4 is directly linked to the BAS, as it is a native-BACnet controller. Regardless of the language used by the controller, the user has the same access to graphics, set points, thermographic colors, trends, schedules, alarms, and other user-interface features of the front end. The end result is that the building manager has a consistent interface and a consistent set of management tools to use throughout the system.

FIGURE 4. Native BACnet office variable-air-volume zone control.

CONCLUSION
So, what does it take to earn a LEED Gold rating? First, it requires good equipment, as no system can outperform its components. Second, it requires good controls, as no system can outperform its “brain.” Most of all, it requires an intelligent designer, one who can look at the entire building and get all of the pieces to work together, eliminating unnecessary run time here, utilizing waste heat there, and generally exercising common sense. The good news is that a LEED Gold system does not have to be gold-plated. You need good, but not extravagant, controls and equipment.The better news is that a LEED Gold system will save energy and money over the long run. Spend a little silver now, and enjoy your gold for years to come.

REFERENCE
1) Benton, D.J. (2004, May). EPA science & tech center good as gold.
HPAC Engineering, pp. EGB14-EGB17, EGB21

Steve Tom, PhD, PE, is director of technical information for Automated Logic Corp. He began working with pneumatic controls as an engineering trainee with Robertshaw Controls during the late 1960s, then spent 21 years in the U.S. Air Force, working with HVAC systems around the world and teaching courses on HVAC design and HVAC controls. He can be contacted at st@automatedlogic.com.