Humidity Counts—In the Right Amounts
With the increasing focus on keeping processes, equipment and people healthy, indoor-air quality (IAQ) has become a big issue in schools, offices and factories. In this latest installment of our M/E Roundtable, a group of heating, ventilating and air-conditioning (HVAC)-design experts rates the importance of relative humidity (RH) in IAQ issues and discusses the best practices for mitigating i...
With the increasing focus on keeping processes, equipment and people healthy, indoor-air quality (IAQ) has become a big issue in schools, offices and factories. In this latest installment of our M/E Roundtable, a group of heating, ventilating and air-conditioning (HVAC)-design experts rates the importance of relative humidity (RH) in IAQ issues and discusses the best practices for mitigating its negative effects.
Consulting-Specifying Engineer (CSE): How important is RH in relation to IAQ? Have you seen any recent research into humidity's effect on people and the workplace?
RUDKO: RH is one of the critical factors determining IAQ. For example, if humidity content is too low, dust mites and other bacteria are present, creating an environment that promotes allergies and other irritations. On the contrary, if humidity content is too high, mold spores and other microbiological life-forms have the opportunity to breed, again creating potential health hazards. Typically, we categorize "ideal humidity" within the controlled range of 30 to 60 percent.
STONE: As a trend during the last five years, humidity control, as it relates to IAQ, has become more important to owners. By this, we are referring to minimum humidity levels, generally during the heating season. Owners have come to expect dehumidification by mechanical cooling, while humidification has generally been considered more of a luxury. Research is finding that not only people's health and comfort are improving with good IAQ, but that productivity also increases. Most owners will agree that an increase in production and a decrease in missed work due to illness will quickly offset the cost of a humidification system.
GOEBEL: It is a generally accepted fact that a 10-percent increase in RH will make the air feel 1°F warmer.
LEADER: RH is a term I feel has been misapplied to some extent. It's only truly meaningful if it's tied to a dry-bulb temperature. It has a big bearing on what we perceive as a comfortable ambient temperature, affects our respiratory tract and skin, and either helps control or promote growth of mold, fungi and bacteria. It's thus a very important ingredient in the equation but alone cannot guarantee IAQ if adequate filtration, air motion and ventilation are not provided.
Building owners aware
CSE: How many owners are aware of humidity control and IAQ? Are they concerned over health—and perhaps legal—ramifications? Should it be more of a concern than it currently is?
GOEBEL: We have found that only the most sophisticated of clients are concerned with humidity control and IAQ. Typical projects for which we provide humidity control are schools, libraries, art museums, high-tech manufacturing and corporate-office headquarters. Typical developer-type office buildings almost never receive humidity control for first-cost and operating-cost reasons.
RUDKO: To generalize, I would safely say virtually all facility managers are aware of IAQ, but not to the extent we are discussing. The mechanical systems installed today generally control humidity levels in the cooling season; it is during the heating season, particularly in northern climates, that ideal humidity levels are difficult to obtain. It is during this period that we find only 30 percent of facility managers re-adjust humidity levels close to the ideal range. Unfortunately, in older buildings during the coldest days of the year, air-quality issues increase when ideal humidity levels are maintained.
STONE: Not only the owners, but also employees and the general public are more educated on the subject of IAQ due to the increasing publicity it receives. Sadly, most of the publicity is given when poor IAQ exists. Everyone is becoming more sensitive to environmental concerns inside a facility, which requires owners to take action to provide a safe, comfortable workplace.
LEADER: From my perspective, it depends primarily on what industry or market the owner represents. In the health-care arena, they're obviously very aware. Elsewhere, it seems to be a matter of dollars and cents. Unfortunately, the owners' financial wizards too often have taken over the engineering decision process and unless we can demonstrate an adequate internal rate of return on an optional investment, they won't buy it.
Showing a return on a humidifier is easier said than done. It's not a straightforward energy-savings calculation like a more efficient chiller. That's where [more] research would come in handy. We're talking about increases in productivity, decreases in absenteeism, reductions in maintenance costs, etc.
STONE: Many times IAQ problems quickly become associated with lawsuits. What owner wouldn't be concerned? As design professionals, we need to be concerned as well. It is our responsibility to educate our clients on the importance of IAQ.
CSE: What considerations for humidity control do you take into account before designing a facility's HVAC system?
STONE: The greatest considerations center on informing and educating an owner to the options they have and the resultant effects of those options. Discussing the effects that humidity control and IAQ offer to a facility is critical when the building program is being written. It is also important to manage client, owner and user expectation levels, that is, what can be controlled and how accurately a system can be controlled. This is important due to the increasing sensitivity of processes and equipment within the facilities.
LEADER: We utilize a questionnaire/survey form on most projects—that we call a "space-design guide"—that asks the owners' representatives questions about the intended use of the space, ambient temperature and relative humidity requirements, control accuracy, etc. Depending on the type of facility, minimum outdoor-air requirements vary considerably, and tend to dictate the type of airside system to use and whether special considerations must be made to pretreat outdoor air with a dual-path air-handling unit or whether energy recovery is appropriate. For humidification, the amount of outdoor air, 100-percent outdoor-air economizer cycles, water quality, steam availability, air-handling unit type and air-filtration type all have a bearing on the type of humidifier selected and the design of the facility's HVAC system.
RUDKO: The main consideration is whether humidity control is person- or process-related. For instance, printing and research environments often require a higher humidity level due to the work contained within. Another consideration is whether the maintenance staff has the competency to properly maintain a humidification system. Ultimately, the above considerations are matched with an appropriate cost.
CSE: What design concerns go along with the increasing amount of high-tech equipment in facilities? Can these machines actually work to cause air-quality problems?
GOEBEL: High-tech equipment usually increases the sensible heat rejection in the space, but not latent heat. High-tech equipment is also very sensitive to static electricity, which can occur in a low-humidity environment. Therefore, these machines usually need environments with humidity ratios from 50 to 60 percent.
Areas with large quantities of high-tech equipment and the corresponding high sensible heat loads can have indoor RH conditions that are unacceptably low. This can be uncomfortable for the occupants and result in static electricity problems.
STONE: New technology and sensitive electronic equipment require tighter control of both temperature and humidity levels. This requires the engineer to consider spaces containing such equipment on an individual basis to insure that environmental requirements are met. This may require separate HVAC systems, year-round cooling or tight humidity control.
RUDKO: While it causes no direct problems regarding humidity, the remittance of chemicals into the air—through the use of high-tech office equipment such as printers and copiers—can decrease air quality.
STONE: Some new digital technologies eliminate some of the emissions found in older technology equipment. This reduces some concerns.
CSE: How do you design a system for a facility that may have a number of rooms with varying IAQ requirements?
RUDKO: In order to accommodate the different requirements of each area, you would either install special terminal filtration or, if the requirements are severe enough, design a dedicated system.
STONE: Humidity differences can be handled by subzoning the humidification system. A minimum RH level can be introduced at the system level, and additional humidification can be introduced at a zone or room level. The system RH can be controlled by an averaging sensor in a return-air duct, whereas the zone-level humidity level would be controlled by a space humidity sensor. Filtering can be done in a similar method. Generally, in all but extreme cases, the worst case is considered when determining a filtering method.
LEADER: The variation in density of people in a multiple-room facility and the effect on the delivery of outdoor ventilation air is the biggest IAQ challenge. I have the impression that too few engineers are using the [American Society of Heating, Air-Conditioning and Refrigerating Engineers] ASHRAE Standard 62-99, "Multiple Space Equation," to estimate the minimum outdoor-air requirement for systems serving multiple spaces. Putting the controls in place to measure and document the amount of outdoor air being delivered and optimizing controls to reduce the amount of outdoor air delivered at part load are also key ingredients I see lacking.
Local or point-of-use air filtration/treatment and dedicated exhaust equipment are often used for morgues and isolation rooms in hospitals. Duct-mounted booster humidifiers are another solution for special rooms like an operating room which requires 50 to 60 percent RH, whereas the rest of the health-care facility may be maintained at 30 to 40 percent.
CSE: What is the best means of monitoring and controlling humidity in a building once the HVAC system is operating?
RUDKO: A facility-management team must have a significant understanding of the mechanical equipment running in their facility. The system itself must include high-quality humidity sensors, adequate air changes and good modulating devices. Quite often humidity systems are not properly maintained and consequently are shut off.
GOEBEL: Today's sophisticated microprocessor-based energy-management and control systems do an excellent job of monitoring space RH. The majority of owners that commit the up-front resources to install these systems also commit the operating resources to keep these systems running adequately.
STONE: In many cases, smaller buildings do not have the resources necessary to keep a system running. Large companies, such as hospitals and county buildings, are more likely to have a staff dedicated to maintaining their facilities. However, the systems are becoming consistently more sophisticated, and owners are requiring more and more assistance with their buildings and systems.
LEADER: A humidity sensor/transmitter in the controlled space with a high-limit sensor in the supply duct is the most accurate means of RH control, but [it is] not practical. Therefore, usually the RH of the return air is sensed to provide an overall average reading for the entire facility. The type of humidifier used also has a bearing, as does whether its turndown capability is in line with the sensor/controller capability. Humidifier control ranges from simple on/off to highly sophisticated, tight-tolerance systems capable of a 100:1 turndown.
CSE: When should the designer consider a desiccant solution? What are the advantages and drawbacks to desiccant systems?
RUDKO: A desiccant system proposes an ideal solution for extreme environments, either high humidity or unusually dry conditions. The advantages far outweigh the disadvantages if the system is planned for an extended life cycle. In such a case, the solution is extremely energy and cost efficient. However, within a short-term utilization, the up-front installation cost and increased maintenance concerns can be drawbacks.
LEADER: The desiccant solution is relatively straightforward and can easily produce dewpoint temperatures from -20 to -40°F. The initial cost of desiccants and the energy cost of regeneration of the desiccant wheel are considerable, and because the wheel is hot from the regeneration energy, recooling is necessary for most applications. This makes the desiccant solution usually quite costly and complex. A desiccant dehumidifier solution is dictated generally by psychrometrics and the limitations of mechanical refrigeration once you get water or refrigerant coils operating at a temperature that causes frost or ice build-up. This leads to the need to have defrost cycles that put the system out of commission for a while or a stand-by unit to operate while the primary unit defrosts. Neither of these are usually acceptable solutions for a comfort HVAC system. For comfort applications, mechanical refrigeration usually loses out once the room temperature design falls to the mid-60s and RH must be kept below 50 percent. It can be done by mechanical refrigeration, but it takes a careful selection of components; and in low temperature ranges the reliability of refrigeration equipment is compromised.
CSE: What types of facilities have you worked on recently that posed special humidity and IAQ challenges?
RUDKO: Currently, we are designing a class-100,000 clean-room facility requiring a 10-percent humidity level year-round.
GOEBEL: We are currently working on the design of a gaming casino to be located on the shores of Lake Michigan. These facilities have large human occupancies, the majority of which smoke. Therefore, the HVAC system will be required to not only remove large amounts of humidity from the air, but also cigarette smoke. The air-handling systems will be designed to bring in large amounts of outside air for dilution. The systems will also have heating coils in the reheat position for humidity control. In addition, each system will have 30-percent prefilters and 95-percent final filters for removal of smoke particulate.
LEADER: Hospitals are an obvious choice. Another is the type of low-temperature (-40°F) environmental wind-tunnel test facility our firm has designed over the last 10 years. In that case, it was the challenge of keeping ice from building on a 24-row deep cooling coil and keeping the wind tunnel in operation. Our work is mostly in the northern states and the focus is more on humidification than dehumidification, which overall I feel is less challenging than the problems posed by excess humidity.
Commissioned for Greatness
Heating, ventilating and air-conditioning (HVAC) design is a complicated process that requires a great deal of fine-tuning and calculating for the design engineer, especially when facilities require tight indoor-air quality (IAQ) and relative-humidity (RH) controls.
It is understandably frustrating, then, when HVAC systems designed for perfection face haphazard installation and operational practices. Enter commissioning.
"You just can't flip the switch and walk away, although—unfortunately—more and more today, that's what's happening on jobs," explains Philip Leader, P.E., a principal at Albert Kahn Associates in Detroit. "I'd say at least 95 percent of the time I'm called in to do 'damage control' on a project ... the problem is due to workmanship issues or incomplete start-up, testing and debugging of the mechanical systems."
In HVAC applications, commissioning should be the natural step between installation and operation. All of the participants from this month's M/E Roundtable agree on its importance on many levels.
The engineers say that the best way for a system to meet its design potential is through commissioning, where an expert can document the design, construction, installation, system startup and staff training involved with a new building.
There are, however, other important reasons for commissioning a system. One of these, according to Robb Stone, P.E., from Arnold O'Sheridan, Madison, Wis., is that commissioning a system when it is installed provides the building owner and system operator with a benchmark to measure future system performance against. Stone also mentions that it is very helpful for operational staff to be involved in commissioning, as it offers a chance to get trained on proper operation, maintenance and inspection.
Unfortunately, however, commissioning for many projects is done very poorly or not at all. This can be a result of improper planning or simply what happens when budget squeezes happen, as Brian Goebel, P.E., of Goetting and Associates explains.
"Separate commissioning services are outside the scope of many budgets and, therefore, the proper startup of systems is left to the mechanical contractor or his test-and-balance subcontractor," he says.
The solution to these problems, of course, is better education and a greater emphasis on getting systems commissioned before they go operational. As it stands now, a number of engineers contend that problems are quite often a symptom of poor installation and maintenance.
"Typically, the majority of air-quality issues are installation or field issues, rather than design oriented," suggests Dennis Rudko, P.E., a principal with HarleyEllis in Detroit. "Proper commissioning coupled with continued monitoring and maintenance can provide an effective system."
M/E Roundtable Participants
Dennis F. Rudko , P.E., principal, mechanical engineering, HarleyEllis, Detroit, Mich.
Robb Stone , P.E., HVAC engineer, Arnold O'Sheridan, Madison, Wis.
Brian Goebel , P.E., principal, Goetting and Associates, San Antonio, Texas
Philip Leader , P.E., technical director, mechanical engineering, Albert Kahn Associates, Inc., Detroit, Mich.
Jeromie Winsor , moderator