Constant Results From VAV Lab Systems
Having said this, it is not uncommon to hear users complain that they do not achieve the savings that the original design promised. This is surely the reason behind the recent interest in low-flow fume hoods. After falling short of the projected savings, the owner's question becomes, "Where did the savings go?" In this article, we will shed some light on the reasons why and suggest some remedies that those either operating or designing vav systems for research laboratories can use to translate the theoretical savings to reality.
Understanding How A VAV Lab WorksBefore we can discuss how best to accomplish energy savings with vav, a quick review is in order of the principles of vav as applied to laboratory spaces. Laboratory ventilation is provided by a once-through hvac system, meaning 100% of the air supplied to the lab is exhausted and therefore must be replenished from outdoors. This outdoor air must be fully conditioned, usually 24 hours a day, year round. Therein is the reason for the inherently high energy use.
The total volume of air required in a particular space is determined by three airflow drivers: air changes, heat load, and makeup air needs. (Figure 1)
Air change requirements dictate how often the air in the space must be replenished and are typically set based on environmental health and safety requirements.
Heat load in a lab space is determined by the experiments being conducted and the heat-producing appliances required to support the research. Electrical "plug loads" typically account for the largest percentage of the heat generated within the space.
Makeup air is the volume of air required above what is needed for air changes and for heat loads. The largest consumer of makeup air will usually be fume hoods, and so if the laboratory does not have hoods, makeup air will not be a driver. The larger and more plentiful the fume hoods in a lab, the greater the makeup volume will need to be and the more it will overshadow the other two drivers.
One other safety-related constraint is space pressurization. NFPA 45 specifies that laboratories with chemicals must be negatively pressured to create a net inflow that will prevent fumes from escaping the lab space. Typically, pressurizing air is a fixed volume, supplied to the corridor and drawn into the room under and around the doors between the lab and the corridor. It is important that the volume of air supplied to and drawn from the corridor by the rooms be equal, or overall building pressurization will be adversely affected.
At any specific point in time, only one airflow driver will establish the airflow settings for the laboratory; however, throughout the day the predominant driver will change as hoods are opened and closed, and the space heat load varies.
Variable-volume controls work to continuously adjust the volumes of supply and exhaust air to maintain airflow balance for space pressurization while ensuring minimum air changes, sufficient volume for fume hood makeup, and comfortable space temperatures.
The extent to which vav controls will save energy is a function of whichever one of the three drivers is predominant. In a large room/small hood scenario, for example, fume hood controls may offer no benefit at all because the maximum volume of the fume hood must be exhausted at all times to maintain air changes.
Yet in the same situation, vav may make sense because of a potentially high heat load. These drivers must be fully understood in order to evaluate the applicability of any hvac solution or potential operational changes.
Getting the Team Moving in the Right DirectionWithin any user organization there are constituency groups, and it will be impossible to meet energy-reduction goals without getting these groups working together. Typically, there is a user group that includes the scientists who are "the clients." The facilities group serves the clients and is responsible for the building and the maintenance of a comfortable work environment. The facilities group is also responsible to the parent organization for operating the facilities efficiently and within a budget. The health and safety group typically consists of industrial hygienists who are responsible to the parent organization for the safety of the workers.
Often these three groups act in isolation, and parochial decisions by one can be viewed as having a negative effect on one or more of the others.
An example of this would be the decision to restrict the size of sash openings: good for facilities and safety, yet all too often considered by the scientists an impediment to creativity.
Facility engineers might favor a "gadget solution" to sense when no one is at the hood and automatically lower the face velocity, but this signals workers that leaving sashes open is acceptable. No one will dispute that leaving sashes open both wastes energy and is unsafe, so the focus must be shifted to what must be done to encourage compliance.
The answer lies in getting a management commitment at the highest levels of each organizational group, and then working as a team to establish realistic goals that all parties are responsible for meeting.
Training of individuals from all three groups will support the goals of the team. It is not uncommon to find scientists who have never been trained in the proper and safe use of a fume hood, or who have no idea of what it costs annually to supply conditioned air to their hoods.
Facilities engineers often blindly apply standard energy-saving techniques without giving a second thought to the impact on lab operations or safety. While the construction of a new building provides the best chance to start this process afresh, there is no reason why collaboration will not work for an existing facility. This team building and goal setting presents a situation where hiring a consultant can expedite the process.
The Nuts and Bolts of Saving EnergyWith the team in place and a solid understanding of the airflow drivers for a particular facility, the process of developing energy-saving strategies and approaches can begin. Fundamentally, saving energy involves impacting each of the airflow drivers, and developing a strategy to prudently minimize the energy requirements of each. We can summarize these individually.
Air Change RatesGuidance on minimum air change rates is often vague and of little help to building owners and designers. OSHA Rule 1910.1450 specifies 4-12 ach is "normally adequate." The ANSI/AIHA Z9.5 laboratory ventilation standard states fresh-air volume, "shall be sufficient, combined with other controls, to control air contaminants to acceptable concentrations." The result is that corporate or site standards for air changes are often set needlessly high on the basis of past practice rather than an engineering justification.
Rather than default to a past practice, a zero-based analysis of the hazard potential in a facility should be done by an industrial hygienist and the proper air change rate determined from that data. The new value can be validated either by doing a field test in a mockup (or existing lab if a renovation) or by computational fluid dynamics. This will usually result in a required air change that is lower than what has been previously used. Table 1 summarizes the effect of incremental reductions of 2 ach on energy use in various areas of the country.
Using variable-volume controls, it is possible to dynamically lower the air change rate during periods when the lab is unoccupied. This can be done by linking air changes to inputs from occupancy sensors, light switches, or the system time clock. Table 2 summarizes the savings possible using air change setback. One argument for high air change rates is that they are needed to purge a chemical spill in the lab. Controls can address this by utilizing a conveniently mounted "purge" button which when actuated by the operator will raise the air change rate, should a spill occur.
Heat LoadsAs stated previously, the majority of heat sources are from plug loads. When designing a new facility, the temptation with plug loads is to use the nameplate data from the electrical devices to determine the watts, and hence the heat load. It has been shown that this method results in grossly overestimating the heat expected and therefore an overdesign of the cooling capacity.
Oftentimes a high heat load prediction will disqualify vav controls from consideration when, in fact, they would produce substantial energy savings. If the user has an existing laboratory space that is similarly equipped to the new space, heat pickup can be approximated by measuring the supply air volume and determining the difference between the supply air entry temperature at the diffusers and the exit temperature at an exhaust diffuser. The result compared to the heat produced by totaling the plug loads will provide a realistic discount factor relative to plug loads (Figure 3).
Another method of reducing the impact of heat load is to analyze whence the heat is coming, and then extract exhaust as close to the source as possible. A "heat map" can be developed by locating heat-generating devices and their potential output on the lab floor plan. The trick is to locate exhaust grilles above large, continuous heat producers such as refrigerators and freezers so that the heat is drawn out of the room immediately rather than be pulled across the center of the lab. Of course, this must be done without negatively impacting supply air distribution patterns.
As with air changes, the space temperature setpoints can be manipulated by the bas based on occupancy to achieve additional savings. How this is done is of critical importance because vav laboratory controls have the ability to both add reheat for heating and increase the supply flow for cooling. Pushing the setpoint up in the summer may inadvertently bring on reheat just as pushing it down may trigger more cooling.
The optimum form of unoccupied control establishes high and low room-temperature boundaries, and allows space temperature to "float" in between without adding reheat or commanding more supply air. For the same reasons, integral control action should never be used for room temperature control because it will result in the use of excessive reheat and/or supply air. Even though it is not perceived as state of the art, proportional-only control provides good temperature regulation without waste.
Direct digital vav controls also allow the dynamic analysis of space load data to reset air-handling system discharge temperature setpoints to reduce both cooling and downstream reheat.
Essentially, by evaluating the outdoor air temperature and humidity, the air volume supplied for cooling to the warmest space, and the extent to which reheat is applied to the coldest space, it may be possible to raise the supply air temperature without negatively impacting the space comfort and humidity in even the warmest zone.
The significance of the savings is largely a function of the number of hours each year when the outdoor air temperature is above 55 degrees F yet the dewpoint is below 55 degrees (Table 3).
Makeup AirAs stated, the need for makeup air is usually driven by fume hoods, and so reducing the volume of air exhausted from a hood has the greatest impact on makeup air.
Several techniques can be employed to reduce the maximum volume a particular fume hood requires. Limiting the sash opening either mechanically by sash configuration or by imposing operational constraints will have the most significant impact. Placing defeatable sash stops on vertical sash hoods at a 14-in. working height will cut the volume required to support a safe face velocity by 40%. Using only horizontal sliding sashes will cut the maximum volume by 60%.
However, restricting the movement of hood sashes is likely to be one area where the desires of researchers and those of the safety and facilities people clash. Researchers typically prefer unrestricted access to the hood face to facilitate experiment setup and teardown.
This is where vav fume hood controls offer benefits to everyone because it is possible to maintain safe face velocities over a wider range of sash openings, and still save energy. Most modern fume hood controls offer multiple levels of warning (normal, caution, alarm) on both face velocity and sash area. Audible alarms can include different tones depending on criticality including persistent but innocuous reminder beeps when alarms have been muted but an unsafe condition still exists. Careful use of these features will allow the maximum cfm to be lowered while enhancing worker safety.
Lowering the face velocity from the generally accepted average of 100 fpm to something lower will have a proportional reduction in exhaust volume, however this must only be done with an understanding of the impact it will have on the fume hood's ability to contain fumes.
Many factors affect the capture performance of a hood such as enclosure aerodynamics, experiment setup, location in a room, and room supply conditions. Room supply influence is the major contributor to turbulence at the hood face and is mostly a factor of diffuser type, placement, and discharge velocity.
In general, the smaller the hood open area relative to the maximum area of the hood face, the better it will capture and the lower the face velocity needed to ensure capture. Unfortunately, most people want to lower the face velocity at the point where it is most detrimental to hood performance: when the sash is wide open. This is why it is so important to teach workers how to position sashes properly.
It also must be noted that no researcher is going to close a sash that doesn't operate freely, and so periodic inspection and maintenance of the sash mechanism is paramount. Routinely cleaning the hood safety glass will also facilitate working with the sash lowered.
Over the years, proactive owners have developed some creative ideas to encourage sash closure and here are a few that have been found to work:
Hood StickersReminders about proper sash use are helpful, emphasizing that sashes should be raised to a point just below the breathing zone when working at the hood and closed when the worker leaves. Stickers showing dollars per year can be placed along the height of a vertical sash to remind the user how much it will cost if the sash is left in a particular location.
Safety AgreementsOne manager developed a "safety agreement" which was signed jointly with the employee stipulating the company agrees to provide safe hoods and the necessary training, and the employee agrees to use the hoods provided safely. A copy of the signed agreement hangs on the lab wall as a continuous reminder.
Monthly ReportsSome users establish energy targets and then monitor both sash and energy use by an individual in a lab. Reports are generated monthly comparing user performance against goals.
Charge Backs for Excessive WasteAnother owner monitors sash position through the bas and charges back excessive energy costs to the particular scientist's budget.
Low-flow HoodsEarlier in this article, it was stated that low-flow hoods have piqued the interest of lab owners. The reason is that they promise to deliver containment at lower face velocities with intrinsically lower exhaust volumes - volumes so low that the need for vav may be eliminated. Whether or not they will deliver on this promise is a function of the specifics of the particular lab to be built and the predominant airflow drivers in that lab.
Usually, low-flow hoods will still require some form of airflow controls, and so any reduction in the control budget may be minimal. The author would caution that these technologies are relatively new and have yet to be deployed in sufficient numbers across a wide enough variety of research settings to understand all of their implications.
Those of us around when auxiliary air hoods were introduced in the 1970s will remember that they were first sold as being safer than conventional hoods. After years of experience, we now know that the opposite is true.
ConclusionIn summary, it should be said that it is possible to achieve substantial reductions in lab energy by using a variable-volume hvac design. The decision to use vav in a laboratory building has implications at all stages of the building life cycle: conception, design, construction, commissioning, and operation. Achieving energy savings is not the responsibility of the designer alone and does not end when the design drawings are done.
Harvesting the benefits requires active involvement of all participating owner groups throughout the building life cycle. Success yields two benefits that every lab owner wants: lower energy costs and a safer work environment.
EDITOR'S NOTE: Some of the images associated with this article do not transfer to the Internet. To review the images or figures, please refer to the print version of this issue.