Building humidity control systems are part and parcel of the building automation system (bas) and the fans, ductwork, and heating and cooling coils of the hvac system. As an integral part of the hvac system and its controls, the humidity control system must be designed into the hvac system from the start, installed as intended, and tested in conjunction with other parts of the overall hvac system. Enter commissioning.
Commissioning is the process of quality assurance as applied to the building construction process. It is integrated into the construction project from cradle to grave. Commissioning starts with the planning of the new building and is carried through design, construction, acceptance, and occupancy.
This article will address the commissioning of humidification systems from planning through design checking. This includes documenting the humidification requirements of the building, checking the design to confirm the selection of an appropriate system, and writing the specifications for the testing of the systems during construction.
The Commissioning Design Intent - Why Humidify?As the future structure is studied during planning, so should humidity requirements be studied. During this first stage of the building's life, the future occupants and users of the building are asked what they want in the building. If they aren't being asked, the commissioning authority (CA) should point out to the owner that valuable information is being lost; information that will affect the productivity of the organization.
As the building users discuss their needs, the CA should confirm that the information is recorded in the Design Intent Document (DID). The DID defines the performance that will be used to confirm the correct operation of the building. Some aspects of the DID are easily defined: the site location and fire and life safety codes might dictate the orientation and configuration of the building. For some specialized buildings such as hospitals, interior temperatures are specified by codes.
The problem with humidification is that many occupants may not know they need to regulate humidity. It is up to the CA to ask the right questions during the development of the DID to uncover hidden needs for humidity control.
Humidity Hot SpotsThe most universal need for humidification is to maintain a healthy working environment for people. In a building, a relative humidity (rh) of 40% to 60% provides the greatest resistance to asthmatic and allergic reactions in humans while minimizing conditions favorable to the survival and proliferation of bacteria, fungi, and viruses. Most regions of the United States with any winter heating requirement will have buildings with rh levels below 40% during the heating season unless moisture is supplied to the air. Many buildings will have humidity levels of 10% to 20% rh - far below the optimum.
Can humidification cut down illness-related lost time enough to justify the cost and maintenance of the system? Although the final answer to that question may be just out of reach, if productivity in an office building can be increased by only 1%, it may pay for a humidification system. The owner will have to make the final decision but, as the CA, you must ask the question.
Aside from the people issue, the CA should look for the following hot spots when considering humidification for new commercial and institutional buildings:
- Computer Rooms. Computer rooms can have very low winter rh levels due to the sensible heat generated by the equipment. The lack of moisture sources in the space can result in dangerously low humidity levels of less than 10% rh. This low humidity will cause static electrical charges on people, furniture, and paper-handling equipment. The result will be paper jams in printers and electrical damage to microprocessor computer chips, as well as employee lost time due to respiratory infections.
- Printing Operations. In-house printing operations need humidification to control static electricity buildup and to provide dimensional stability for paper feedstock. Modern offset machines producing 10,000 pages per hour will develop considerable static charges that can result in paper jams and even fires in the presence of flammable inks, solvents, and thinners. Paper can absorb fives times the moisture content at 90% rh that it contains at 10% rh. This causes dimensional changes in the paper that will affect the clarity of continuous roll printing.
- Material handling. The need to control humidity in the greenhouse section of an agricultural research building may be obvious, but the need to humidify the grain milling test room may be hidden. Many powders can create explosive conditions in the presence of static discharges. Ask questions if there is any material handling in the building, including the storage or dispensing of chemicals, pharmaceuticals, or art supplies.
- Artifact storage. Materials such as paper, leather, cloth, wood, and wood adhesives are hydroscopic (moisture absorbing). This causes dimensional changes in the materials that will hasten their disintegration over time. Safe and effective artifact storage typically combines summer dehumidification with winter humidification.
- Food storage. Most owners of supermarkets and food warehouses are very aware of the negative impacts of low humidity on the weight and appearance of foods. But is this same knowledge present in the design of a culinary arts space for a community college or a school lunchroom? As the CA, be aware of new environments that may benefit from humidification.
When in doubt, ask questions. Make sure any need for special humidity control is defined. Then include that need as a specific rh requirement in the DID. As the CA, that document will become the standard for your acceptance testing of humidified spaces.
Check That SelectionDuring the design stage, the CA checks the drawings and specifications and develops the portions of the specifications related to commissioning. The design must include the equipment to humidify the spaces identified in the DID. Beyond that, the CA should confirm that the equipment is selected, sized, and designed to give the owner the best humidity control at the lowest cost and with the least maintenance.
The extent of design checking in the commissioning process is not well defined and is mostly a result of the owner's opinion at a particular time for a particular project. The owner should beware of giving the CA such a broad scope of checking as to duplicate the work of the design team. The design team was, after all, hired for its expertise. To do that would be cost-prohibitive as well as irrational.
So how much checking should be on the CA's plate? As for the selection of humidification equipment, the CA should be able to independently calculate the approximate humidification load required and confirm that the type of system specified is workable. For example, the humidification load can double or triple if special exhausts or economizer cycles are incorporated into the hvac system. This is the magnitude of error that should concern the CA, not a 10% difference in reading a psychrometric chart.
Once the CA knows whether the humidification load is 40 lb/hr or 400 lb/hr, the next consideration is the purity of the humidification. Using steam from heating boilers for general space conditioning is probably not acceptable due to the anti-corrosion and anti-scaling chemicals contained in the steam. Beyond that, how much purity is required? For labs and cleanrooms, this information is contained in the DID (see above). Is the equipment suitable?
Control is also an important issue. If humidity is supplied directly to a large auto-body workshop, the turndown of an individual spray head is probably not important. But if it is a computer room, accurate control may be important. What is the accuracy required? The flip side of control is neatness. Water blasting into media is hard to contain in an air-handling unit (ahu). It ends up on the floor of the machine room, providing a breeding ground for microbials. The CA might suggest a look at a compressed air vaporization system instead.
Humidifiers-Not Peas in a PodAlthough humidifiers started simple, they have steadily increased in their complexity. There has been a major leap in technology from the heated pan humidifier of yesteryear to the present-day microprocessor-controlled electrode unit. As the CA you should have the knowledge to judge what is best for the application.
Table 1 contains a listing of the most common types of humidifiers available today. There are certainly a few missing, but those listed illustrate the breadth of the selection. The CA should become familiar with the strong points of these pieces of equipment and be able to comment on their selection.
Disposable Electrode Steam Generators. The first unit in Table 1 is a compact, self-contained canister unit that makes steam with electrodes immersed directly in water. The steam may either be injected into an air supply duct or supplied directly to the humidified space. The electrodes and water are contained in a disposable canister that is discarded periodically along with the built-up minerals left behind when the water is boiled away. The new electrodes contained in the new canister ensure reliable performance.
Units come complete with controls and require only electrical, water, and drain connections and are good candidates for retrofits or spaces with small humidification loads. They will not work with the highly purified water that may be required for lab applications. The downside of electrode generators is its use of expensive electric energy. Using natural gas provides lower energy bills (about 1/2 to 1/4 the cost of electric power) but requires more installation labor and maintenance.
The bottom line is that electrode units go in fast, and demand minimal maintenance, but should be used for small loads only. The CA should question the use of these units for loads exceeding 50 to 100 lbs/hr, especially in areas of high electricity costs. The packaged gas-fired humidifier or dedicated boiler system is a more frugal substitute for large loads (see Figure 1). A steam-to-steam heat exchanger system should provide economical service if steam is available in the building.
Electric Resistance Steam Generators. This is similar to the self-contained electrode unit except it uses an electric resistance element to boil the water. This unit does not provide a disposable canister system to deal with mineral build-up and relies on an automatic flushing system to remove minerals from the boiling chamber. The resistance-heating element is of much more durable construction than its electrode counterpart.
Like the electrode steam generator, the resistance element unit is compact, and easy to retrofit. It has very good control characteristics. Unlike the electrode unit, it works well on deionized or reverse-osmosis treated water as is found in laboratory environments. Like the electrode unit, the resistance steam generator is an expensive solution for large loads.
Direct Steam Injection. Direct steam injection is the "big job" humidification system. Only the available steam supply and the size of the air supply ductwork limit the size of these systems. Capacities to 3,000 lb/hr are available off the shelf. The steam injection manifolds are typically duct-mounted and controlled by pneumatic or electric steam control valves. Steam may be provided by the building's heating boiler or by a dedicated boiler. Steam from the heating boiler probably contains chemical additives intended to extend the life of the boiler, piping, and related equipment. The CA should confirm that these chemicals are certified safe for human exposure and inhalation and safe for laboratory, computer, or material handling equipment in the space.
An alternative to using building steam is to provide a dedicated boiler for the steam injection system. This provides the best air quality but at the cost of an extra boiler. The CA should confirm that this boiler is specially designed for use on 100% make-up water and is equipped to withstand possible thermal shock and to deal with rapid mineral build-up. Operating cost depends mainly on the cost of steam. Steam is readily evaporated into a duct air stream and provides a reliable source of moisture requiring minimal absorption distance.
Steam-to-Steam Heat Exchanger. This steam injection system comes with its own steam-to-steam heat exchanger to isolate boiler chemicals from the injected steam. The chemically- treated steam is used to make pure steam that is injected into the ducted air stream. Built-up minerals are automatically flushed from the steam chamber. Unit is shipped complete with all required controls.
This steam injection system is another "big job" humidification package with capacities to about 1,500 lb/hr. The unit allows pure steam production without a special boiler but adds some complexity in its controls. It could be a good substitute for an electric resistance unit where a large amount of high-purity steam is required. A compressed-air atomization system provides comparable control and quality and also evaporative cooling.
Gas-Fired Steam Injection. This system combines direct steam injection and a special boiler all in one package. The boiler is built to withstand the thermal shock that results from 100% make-up water operation and is designed to constantly remove the minerals and impurities left behind as the steam is boiled off.
Capacity is limited to 300 lbs/hr. If units are combined to provide greater capacities, the CA should suggest consideration of the dedicated boiler/direct injection system described above. The larger boiler might be lower in maintenance than multiple gas-fired steam injection packages. The gas-fired system requires greater maintenance than the electric systems, but operates at a much lower energy cost. Like the other steam systems, the gas-fired system does not provide evaporative cooling.
Ultrasonic Vaporization. Ultrasonic vaporization takes place when a small piezoelectric element is vibrated at radio frequencies in a water bath. As the element contracts, the inertia of the water prevents it from filling in the void and the water boils due to the local low pressure.
This system is "adiabatic;" it vaporizes water without adding either latent or sensible heat to the room or air stream, making it especially useful for cooling applications. Ultrasonic systems are very energy efficient and produce a high-quality vapor of one-micron droplets. The electronic circuitry accurately controls the rate of vaporization.
Ultrasonic systems are small in capacity, reaching their limit at about 50 lb/hr. This makes them good candidates for computer rooms and one-room laboratories but too small for entire buildings. They must use demineralized water purified to 2-3 megohms or better. The CA should ask the owner to consider life-cycle cost in comparing these units to electrode or resistance-heating units. Do lower operating costs justify the higher first cost?
High-Pressure Spray Atomization. Spray atomizing nozzles force water through a small orifice at pressures of 300 to 1,000 psi. This causes localized turbulence that shears the water into droplets of approximately 15 to 20 microns in size.
The large droplets make this system difficult to apply to many buildings because the droplets do not readily evaporate in the space allowed in ahu's and must be taken out of the airstream by mist eliminators. Water spray systems are energy efficient and have nearly unlimited capacity with packages up to 5,000 lb/hr available off the shelf. These systems are well suited for direct dispersion into large, open areas such as paint shops.
When used as a duct injection system, this type of humidification inevitably results in liquid water in the ductwork. If this water is arrested by a mist eliminator and drained correctly there will be no adverse health impacts due to microbial growth. But if water gets past the mist eliminator into an unsealed or undrained portion of the duct, a dangerous microbial growth environment will result. This system must be used with a mist eliminator treated with an anti-bacterial agent. If this system is proposed for in-duct (or ahu) use the CA might suggest a compressed air system as an alternative (see below).
Compressed Air Atomization. This system uses compressed air to aid in the atomization of water in specially designed nozzles. It has an advantage over the "water-only" vaporizers because the compressed air causes additional turbulence in the nozzle that shears the droplets into smaller sizes. The smaller size droplets (about .3 to 10 microns) evaporate better into the airstream. The nozzles are arranged in arrays inside supply air ductwork or are available as packaged units for direct dispersion into a space.
Both compressed air and straight water atomizers are adiabatic systems that generate free cooling from low rh outside air. Not only does compressed air atomization generate water particles about one-tenth the size of the straight water nozzle, but also it will maintain this high level of performance over a wide range of flows (turndowns of 100:1 are claimed). Although mist eliminators should still be used with this system, it is much safer than water spray nozzles in avoiding water in ductwork. Operating costs are higher than the straight water system but still a fraction of the cost of electrically generated steam.
You Be The CAIn developing the DID and checking the design for the new building, keep the above guidelines in mind. Consider operating costs, installed costs, water and air purity, and control accuracy when evaluating the humidification design. Providing clear drawings and specifications that accurately reflect the needs of the building occupants builds a good foundation for the successful inspection and testing of the completed systems.ES
Take A Good Look At Humidification For Spaces And Operations Such As:Offices