An Untapped Energy Savings Opportunity
The return on investment from re-staging with an active sensing system can be as low as 12 months.
October 24, 2019 by Steven Graves
Laboratory buildings are the largest energy consuming buildings. To date, most of the attainable lab ventilation energy savings are accomplished through lab air change rate reductions. This is especially true in existing labs that were commissioned years ago, with ventilation levels that are excessive compared to today’s standards.
Recently, it was quite common to operate labs at eight to 12 air changes per hour (ACH), or higher. Today, labs are often commissioned at 6 ACH or less. Efforts to reduce ACH values result mostly in energy reductions associated with the air that is supplied to labs, with very little savings associated with lab exhaust.
Although exhaust CFM in a lab may be reduced, the total exhaust through the lab exhaust fan must be maintained. This is to ensure that safe fan exit velocities are maintained to properly disperse contaminants as they exit the building. Many lab exhaust fans incorporate a bypass damper to enable the exhaust fan to maintain its total flow and exit velocity as lab flows are reduced.
Lab exhaust systems frequently incorporate high plume dilution fans, as well utility sets and other fan systems. Their function is to provide safe dilution of contaminants in the lab exhaust air, and to maintain inlet static pressure to support the exhaust flows from the lab.
The standard for safe lab exhaust fan exit velocity is ANSI Z9.5, which states, “The exhaust stack velocity shall be at least 3,000 ft/min (15.2 m/s) is required unless it can be demonstrated that a specific design meets the dilution criteria necessary to reduce the concentration of hazardous materials in the exhaust to safe levels at all potential receptors.” In many cases, lab exhaust fans run well above 3,000 ft/min.
A critical assumption is that lab exhaust is always contaminated, therefore requiring these high exit velocities constantly. In reality, lab exhaust air is clean conservatively more than 70 per cent of the time. In some facilities, such as teaching labs, the exhaust air may be clean 95 per cent of the time or more. And lab exhaust air is likely to get cleaner as time goes on.
Many research companies emphasize the use of computational analytics before performing experiments in fume hoods. This reduces the cost of chemical disposal and minimizes safety risks. Most companies and universities use “cleaner and greener” chemicals whenever possible. Therefore, the few active fume hoods in labs are exhausting air that is clean the majority of the time.
This substantially clean fume hood exhaust air is typically mixed with the general lab exhaust air either in the exhaust risers or in the plenum of the lab exhaust fan. This mixing with the clean air of the general exhaust provides considerable dilution (conservatively 400:1) before reaching the lab exhaust fans.
As plume height and exit velocities per ANSI Z9.5 are a non-issue when the lab exhaust air is clean, there is a tremendous opportunity for energy savings through active sensing. Active sensing is defined as the process where the cleanliness of the lab exhaust air is continuously monitored for contaminates and the associated exhaust fans are indexed accordingly to drive large energy savings. This is a binary process.
When exhaust air is clean, in which contaminant concentrations are less than a pre-determined threshold, the fan exit velocity is reduced. When the exhaust air is contaminated, that is greater than the threshold setting, the lab exhaust fan is indexed to full dilution.
HOW IT WORKS
Active sensing, as shown in Figure 1, typically incorporates a multi-point air sampling approach, which monitors a location on each riser that is manifolded to the fan set. Monitoring at the plenum is possible. However, monitoring the risers is a more conservative approach. There will be additional dilution in the plenum from other risers, which will likely be clean.
Simultaneous spills in multiple fume hoods are highly unlikely. Air samples are continuously drawn from each riser in a sequential fashion and analyzed by the system using a photoionization detector (PID) sensor technology. The PID is capable of detecting hundreds of compounds commonly found in laboratory facilities. It is also a technology that is recognized by health and safety professionals worldwide.
Once all the monitoring points have been verified to be free of contaminants, the system issues a “setback” command to the specific exhaust fan controls to reduce exit velocity. As soon as contaminants are detected in any of the risers, the system immediately switches out of setback, isolates the PID via a sensor protection mode and signals for the fans to go to full dilution.
Isolating the PID from excessive exposure to contaminants is critical and any active sensing system used for this purpose must have this capability to be safe. Without this capability, continuous exposure to a stream of contaminates will cause the PID to rapidly foul.1 When contaminants are detected, the fan system is commanded to full dilution, usually for 20 to 30 minutes to prevent the “fan hunting.”
There are two opportunities for energy savings with active sensing. One way is through the elimination of bypass air. Bypass air is added to ensure adequate exit velocity (e.g., plume height) is achieved without impacting inlet static pressure. As an example, if a lab is exhausting 20,000 CFM, but 30,000 CFM is required through the fan to maintain adequate plume height, 10,000 CFM will be introduced via the bypass air damper.
When the exhaust air is clean, plume height is a non-issue. The bypass air can be eliminated, saving significant break horsepower.
The second opportunity for savings is via fan re-staging. If there are redundant fans, distributing the load to meet the inlet static pressure over the additional fan can drive very large energy savings. For example, an N+1 fan set, that is one fan operating and one in standby mode, has one fan running at 4,400 ft/min to meet the required plume height.
When the exhaust air is clean, the fan set can be re-staged to run two fans at approximately 2,200 ft/min (exit velocity and flow are essentially linear) to drive demonstrable energy savings. The savings via re-staging are from two sources. One is due to fan laws whereby a reduction in fan speed generates a disproportionately greater reduction in energy use.
The other is related to velocity pressure at the fan discharge. High dilution exhaust fans use a discharge stack and a high velocity discharge nozzle to increase the momentum of the exhaust air to create a high plume for dilution. This high velocity discharge benefit is accompanied by the cost of increased horsepower (hp.)
The pressure loss associated with any high velocity discharge is equal to the velocity pressure at the discharge. The velocity pressure is a square function of exit velocity. Therefore, reducing exit velocity by a factor of two will reduce velocity pressure by a factor of 4. Depending on the fan hp and local power rates, the return on investment (ROI) from re-staging with an active sensing system can be as low as 12 months, often resulting in a 20 to 40 per cent reduction in fan energy use.
A PID detects hundreds of compounds. However, it does not detect every chemical used in labs. An analysis of the chemical inventory is strongly recommended. The analysis reviews the compounds against NIOSH, ACGIH and OSHA standards for toxicity and odour thresholds. These are based on the ease that the compound may evaporate and become airborne during a spill condition. The analysis categorizes the compound into the three segments: 1) compounds detected by the PID, 2) solids or compounds with low vapour pressure that will not evaporate in sufficient quantities to pose issues in the exhaust air. (As an example, sulfuric acid is a hazardous compound, but because of its low vapour pressure, spills of this compound do not generally require high dilution from the exhaust fan), and 3) compounds that may or may not be detected by a PID that require a quantity limitation protocol, due to their toxicity or low odour threshold. This limits the quantity of the compounds allowed in the lab such that if spilled in its entirety, the air changes per hour (ACH) in the lab room provides sufficient dilution.
Active sensing provides a safe method to drive significant savings in laboratory exhaust systems. It monitors the cleanliness of the exhaust air and indexes the associated exhaust fans accordingly. The savings are primarily through the reduction of bypass air or re-staging of fans.
An important element to ensure reliable operations includes a method to protect the PID from fouling. Additionally, a review of the lab’s chemical inventory is highly recommended to confirm all compounds are either detected by the system, benign for odour and toxicity or have a limitation protocol.