VSDs shall be a minimum of 18 pulse. The harmonic voltages and currents can cause spurious operation of Potomac Electric Power Company (PEPCO) and NIH relays and controls, capacitor failures, motor and transformer overheating, and increased power system losses. These problems are usually compounded by the application of power-factor correction capacitors (especially on the NIH’s low-voltage system), which can create resonance conditions that magnify the harmonic distortion levels.

Several concerns associated with harmonic distortion levels need to be addressed in the project specification. This will avoid significant harmonic-related problems with both the VSD VFD equipment and the NIH operations controlled. These concerns include the following:

• Harmonic distortion on both the supply and motor side of the drive
• Equipment de-rating due to harmonic distortion produced by VSDs VFDs
• Audible noise caused by high-frequency (several kHz) components in the current and voltage
• Harmonic filter design and specifications

Where VFDs are appropriate for use, 18 pulse VFDs shall be provided for all motors 75 horsepower and above. For motors less than 75 horsepower, 6 or 12 pulse VFDs with harmonic filters (passive or active), phase multiplication sevices, or any other components required to mitigate harmonic voltage total harmonic distortion (THD) to 5% and current
THD to 3% at full load / 5% at any load maximum levels shall be an integral part of the variable frequency drive system. Compliance measurement shall be based on actual THD measurement at the VFD circuit breaker terminals during full load VFD operation. Designs which employ shunt tuned filters shall be designed to prevent the importation of outside harmonics which could cause system resonance or filter failure. Calculations supporting the design, including a system harmonic flow analysis, shall be provided as part of the submittal process for shunt tuned filters. Any filter designs which cause voltage rise at the VFD terminals shall include documentation in compliance with the total system voltage variation of plus or minus 10%. Documentation of Power Quality compliance shall be part of the commissioning required by the VFD supplier. Actual job site measurement testing shall be conducted at full load and documented in the operation and maintenance manuals. Harmonic measuring equipment utilized for certification shall carry a current calibration certificate. The final test report shall be reviewed and compliance certification stamped by a licensed professional engineer. Text and graphical data shall be supplied showing voltage and current waveforms, THD and individual harmonic spectrum analysis in compliance with the above standards.
Control wiring for VFDs shall NOT be run in same conduit as power conductors.

A three-phase VSD system consists of three components (rectifier, direct-current [DC] link, and inverter) and a control system. The rectifier converts the three-phase 60 Hz AC input to a DC signal. Depending on the system, an inductor, a capacitor, or a combination of these components smooths the DC signal (reduces the voltage ripple) in the direct current link. The inverter circuit converts the DC signal into a variable-frequency AC voltage to control the speed of the induction motor. Since for this application a voltage-source inverter (VSI) drive is considered, the concern regarding this particular device is outlined below. These drives (the most common types, up to 225 kW) use a large capacitor in the DC link to provide relatively consistent DC voltage to the inverter. The inverter then chops this DC voltage to provide a variable-frequency AC voltage for the motor. VSI drives can be purchased off -the shelf and employ PWM techniques to improve the quality of the output voltage waveform. However, there is a concern regarding nuisance tripping due to capacitor-switching transients. Small VSDs have a VSI rectifier (AC to DC) and use a PWM inverter (DC to AC) to supply the motor. This design requires a DC capacitor to smooth the DC link voltage. The controls for this type of drive have protection for DC overvoltages and undervoltages with narrow thresholds. It is not uncommon for the DC overvoltage control to cause tripping of the drive whenever the DC voltage exceeds 1.17 V per unit (for theparticular application, 760 V for a 480 V application). Since the DC capacitor is connected alternately across each of the three phases, drives of this type can be extremely sensitive to overvoltages on the AC power side. One event of particular concern is capacitor switching on the PEPCO system. PEPCO voltage-switching transients result in a surge of current into the DC link capacitor at a relatively low frequency (300-800 Hz). This current surge charges the DC link capacitor, causing an overvoltage to occur (through Ohm’s law). The overvoltage (not necessarily magnified) exceeds the voltage tolerance thresholds associated with the overvoltage protection, which most likely will trip the VSD out of service. This is called nuisance tripping because the situation can occur day after day, often at the same time. Several methods are available to ameliorate such tripping; some are simple, and some costly. Use of harmonic filters to reduce overvoltages, an expensive alternative, is effective in protecting drives from component failure but may not completely eliminate nuisance tripping of small drives. The most effective (and inexpensive) way to eliminate nuisance tripping of small drives is to isolate them from the power system with series inductors (chokes). With a concomitant voltage drop across the inductor, the series inductance of the choke(s) reduce(s) the current surge into the VSD, thereby limiting the DC overvoltage. The most important issue regarding this method is that the designer should determine the precise inductor size for each particular VSD; this requires a detailed transient simulation that takes into account capacitor size, transformer size, and so on. The choke size must be selected carefully. If the choke has too much impedance, it can increase harmonic distortion levels and notching transients at the drive terminals. Chokes for this application are commercially available in sizes from 1.5 to 5.0 percent of the VSD impedance at various kW ratings. A size of 3.0 percent is sufficient to avoid nuisance tripping due to capacitor-switching operations. Standard isolation transformers serve the same purpose.

The A/E for NIH buildings should specify a complete and comprehensive system
operation as part of a commissioning guide/plan developed for each project as needed that includes testing, adjusting, and balancing of environmental and other
systems to produce the design objectives.

The testing and balanced shall be done in coordination with commissioning as required by the NIH Model Commissioning Guide.

Many different fan types and arrangements exist in the marketplace from a large variety of manufacturers. The project engineer has the responsibility to select the fan and specify its requirements to meet the functional needs of the system while providing stable, efficient, and quiet operation. Fan selections should be based on the lowest reasonable speed while optimizing efficiency. Fan selections should consider longevity of components, especially bearing life at maximum design conditions.

Inlet vanes may be considered for use in varying air volume. The A/E shall evaluate the effects of low-frequency radiated noise on the system. During periods of normal building occupancy, most systems typically operate in the range of 50 to 80 percent design capacity. Therefore, the fan that has been selected on the basis of 100 percent design capacity will be functioning most often at a throttled or reduced capacity. As air volume is reduced, an increase in fan-generated noise results. An example is an application in which the pressure rise across the fan at design capacity may be 1 121 Pa, and the system static-pressure controller is set at approximately 374 Pa. Under these conditions, a commercial-quality, airfoil double-inlet double-width fan seldom achieves a static efficiency greater than about 60 percent even with the inlet valves set at full-open position. It is the presence of inlet vanes, even when in the full open position, that limits the achievable fan static efficiency. As the inlet vanes close to reduce air capacity from 100 percent to, for example, 65 percent of design, the operating fan efficiency drops dramatically. At 65 percent of the design, the air horsepower developed by the fan (proportional to air volume x total static pressure) is only about 40 percent of that produced at design capacity. However, by using inlet vanes to modulate flow, the reduction in brake horsepower is only about 12 percent. This condition is the result of the changes in the shape of the fan-characteristic curve that occurs with orientation of the inlet vanes. These changes result in a decrease in fan static efficiency from an initial value of about 60 percent to approximately 37 percent. This significant change in fan efficiency has an enormous effect on fan-generated noise. Rather than a reduction of 6 dB, which might have been expected before the lower air volume and total static pressure, an increase of about 6 dB will occur owing to the decreased fan efficiency. This increase in noise level typically appears in the low-frequency region of the spectrum and is perceived by the ear as an increase in the level of system rumble. Furthermore, at this reduced airflow condition, the masking level of diffuser-generated noise is typically about 10 dB lower than at maximum design airflow. Thus, with the beneficial mid- to highfrequency masking noise significantly reduced, the occupant’s perception of low-frequency rumble will increase.

All fans must be fully accessible for service and routine maintenance. Fan motors and drives should not be located within hazardous or contaminated exhaust airstreams. Fan bearings where possible should be serviceable outside hazardous or contaminated exhaust airstreams. Inline fans with motors or drive exposed to exhaust airstreams are not permitted.

Fan systems designed for parallel or manifold operation should be protected against backward rotation of fan wheels. Anti-rotation devices, motor brakes, or other approved methods should be considered for use on these systems. Solid fan shafts should be furnished whenever possible as an option.

Fans should have a certified sound and air rating based on tests performed in accordance with AMCA Bulletins 210, 211A, and 300. See AMCA Standard 99, Standard Handbook, for definitions of fan terminology. The arrangement, size, class, and capacity of all fans should be scheduled on the contract drawings for permanent records.

All fans should be statically and dynamically balanced by the manufacturer and should be provided with vibration isolation. Fans should not transmit vibration to the duct system or building structure. All fans 18.7 kW and larger should also be dynamically balanced in the field by the manufacturer after the installation is complete.

Diffuser cones and inlet bells are not permitted in rating a fan unless they are an integral part of the fan design. Inlets and outlets of fans not duct connected, including fans in plenum chamber or open to the weather, should have heavy, OSHA-approved guard screens to protect personnel. Guard screens should not impair fan performance and, when bolted to equipment, will permit their removal for fan service and cleaning.

Complete fan lubrication facilities should be provided, such as oil reservoirs, sight glasses, grease and relief fittings, fill and drain plugs, pipe connections, and so on. The facility should be placed in a readily and safely accessible location so that after installation they will perform the required function without requiring the dismantling of any parts or stopping equipment. For fans located within AHU casings, lubrication facilities should be piped to the exterior casing wall.

All parts of fans should be protected against corrosion prior to operation of the fan. Exhaust fans should be specifically addressed, as the airstream may contain excessive moisture, fumes, corrosive vapors, or contaminated or hazardous particles. Special consideration should be given to those fans handling explosive vapors or radioactive material.

Certified performance data including acoustical data should be submitted for each fan at maximum design conditions. Data should include published sound power levels based on actual tests on the fan sizes being furnished and conducted in accordance with current AMCA standards. Such data are to define sound power levels (PWL) (10-12 W for each of the eight frequency bands). The acoustical design of the fan system must conform to the space noise criteria. Fan curves should be submitted that will depict static pressure, total pressure, brake horsepower, and mechanical efficiency plotted against air volume. Fan curves should include estimate losses for field installation conditions, system effect, and actual installed drive components. All included losses should be defined on the fan curves. Data may also be submitted in tabular form, but tables are not a substitute for actual performance curves.

Each motor-driven fan should be equipped with a V-belt drive, except those that are direct drive by design. Where factory-designed and assembled belt drives that do not conform to the following are proposed to be furnished, such nonconformity must be noted on the shop drawing submittals and may be cause for rejection of the item. OSHA-approved mesh-type guards should be provided for all belt drives.

Each drive should be selected according to the rating and recommendations of the manufacturer for the service for which it will be used, giving proper allowance for sheave diameter, center distance, and arc of contact less than 82 °C. The motor drive should have a centrifugal fan, with forward curved blades and a nameplate rating of not less than 5 percent above the total of actual fan brake horsepower and drive loss at specified capacity.

Belts should be constructed of endless reinforced cords of long staple cotton, nylon, rayon, or other suitable textile fibers imbedded in rubber. The belt should have the correct crosssection to fit the sheave grooves properly. Belts should be matched carefully for each drive. Extended-horsepower belts are not acceptable.

Motor sheaves should be adjustable pitch type for 18.7 kW and less and selected so that the required fan rotational speed will be obtained with the motor sheave set approximately in mid-position and have the specified pitch diameter in that position. Fixed-pitch sheaves should be installed on fans 22.4 kW and larger. All multiplex belt drive assemblies regardless of horsepower should be fixed-pitch type. Variable-pitch drives should be used for all fans to accommodate initial fan balancing and converted to fixed-pitch where required when balancing is complete.

Fan motors should have the capacity needed to operate the equipment at the specified midposition operating condition. Where non-overloading motors are specified, the motor capacity rating at the most closed position of the motor sheave shall be selected. In no case should motors be a smaller size than that required to operate without overload. Fan sheaves shall not be smaller in diameter than 30 percent of the fan wheel diameter.

Sheaves should be constructed of cast iron or steel, bored to fit properly on the shafts, and secured with keyways of proper size (no setscrews). Keyways may be omitted for sheaves having 15 mm or smaller bores, where setscrews may be used. Fans should be furnished complete as a package with motors, drives, curves, bases, and inlet and outlet fittings. Detached vibration isolation devices may be provided separately.

Fans, both supply and exhaust, serving multiple zones, shall be equipment with either a VFD or a vortex damper or both for control of volumetric flow rate and duct static pressure. This is required for both variable and constant volume systems.

Fan systems designed for parallel or manifold operation shall have isolation dampers to prevent backward rotation of fan wheel. Isolation dampers shall have motors and shall be controlled by the BAS.

All fans on a manifold or in parallel shall be identical and have identical isolation dampers and volume/pressure controls.

All fans shall be fully accessible for service and routine maintenance. Fan motors and drives shall not be located within hazardous or contaminated exhaust airstreams. Fan bearings where possible shall be serviceable outside hazardous or contaminated exhaust airstreams. Inline fans with motors or drive exposed to exhaust airstreams are not permitted.

Fans shall have a certified sound and air rating based on tests performed in accordance with AMCA Bulletins 210, 211A, and 300. See AMCA Standard 99, Standard Handbook, for definitions of fan terminology. The arrangement, size, class, and capacity of all fans shall be scheduled on the contract drawings for permanent records.

Certified fan curves including power curves as well as acoustical data shall be submitted for each fan. All data shall be from test done in accordance with applicable AMCA standards. Data shall include published sound power levels based on actual tests on the fan sizes being furnished and shall define sound power levels (PWL) (10-12 W for each of the eight frequency bands).

Fan curves shall show volumetric flow rate of the fan as a function of total pressure, system flow rate as a function of total pressure, brake horsepower and efficiency. System curves shall include estimate losses for field installation conditions, system effect, and actual installed drive components. All losses shall be defined on the fan curves. Data may also be submitted in tabular form, but tables are not a substitute for actual performance curves.

All fans shall be statically and dynamically balanced by the manufacturer and shall be provided with vibration isolation. All fans 18.7 kW and larger shall also be dynamically balanced in the field by the manufacturer after the installation is complete.

All parts of fans shall be protected against corrosion prior to operation of the fan. Exhaust fans shall be specifically addressed, as the airstream may contain excessive moisture, fumes, corrosive vapors, or contaminated or hazardous particles. Special consideration shall be given to fans handling explosive vapors or radioactive material.

Each fan shall be equipped with a V-belt drive, except those that are direct drive by design. Belts shall be constructed of endless reinforced cords of long staple cotton, nylon, rayon, or other suitable textile fibers imbedded in rubber.

Variable-pitch sheaves shall be used to accommodate initial balancing and shall be replaced with fixed pitch when balancing is complete.

Sheaves shall be constructed of cast iron or steel, bored to fit properly on the shafts, and secured with keyways of proper size (no setscrews) except that for sheaves having 15 mm or smaller bores setscrews may be used.

Fans shall be furnished complete as a package with motors, drives, curves, bases, and inlet and outlet fittings.

Those duct systems requiring field pressure testing should be tested at 125 percent of the duct construction rating. Pressure testing shall conform to the SMACNA HVAC Air Duct Leakage Test Manual. The positive pressure side of any exhaust system installed within a building should be pressure tested to 150 percent of the duct construction rating.

Duct leakage shall be tested and shall not exceed 3% of the total volume in low pressure
systems or 0.5% in medium and high pressure systems. The duct leakage test shall be
done as part of the required air balance by subtracting the sum of flows at all air devices
from the total air flow measured at the air handler or exhaust fan. In addition the designer
may require a leakage test to be done in accordance with the SMACNA HVAC Air Leakage Test Manual. If the SMACNA test is done, the test pressure shall be the design working pressure of the system.

A wind analysis performed by a qualified wind consultant is recommended to
analyze and make recommendations on these factors.

The primary building exhaust stack locations and heights shall be in concurrence with air dispersion modeling. The bottom of all outdoor intakes should be located as high as
practical but not less than 1.8 m above ground level, sloped or vertical not horizontal.

A computational fluid dynamics (CFD) analysis must be performed to access reentrainment of exhaust air from one building entrained back into the supply duct of the same building or an adjacent building. The CFD model should be constructed to include the building under design, the buildings immediately surrounding the building under design, the exhaust and air intake vents of the buildings, and any other sources or obstacles that could affect the air intake of the building under design.

It should be noted that certain factors should be considered in the evaluation of external flow type scenarios.

The bottom of all outdoor intakes should be located as high as practical but not less than 1.8 m above ground level, sloped or vertical not horizontal.

Use of an expert consultant to do either wind tunnel or computational fluid dynamics (CFD) air dispersion modeling is highly recommended to analyze and make recommendations on these factors. Where this is done, it must assess the possibility of re-entrainment of any and all near-by exhausts into any and all near-by intakes. For example, where a new building is being designed, the CFD or wind tunnel analysis considers the impact of the new building as well as near-by existing buildings and other new and existing obstacles and considers new and existing exhaust relative to new and existing intakes.

When using CFD, certain factors should be considered in the evaluation of external flow type scenarios.

The heat exchanger fluid, consisting of ethylene glycol and water, is circulated through the system by a pump, removing heat from the hot air stream and transferring it to the cold air stream.

The heat exchanger fluid, consisting of propylene glycol and water, is circulated through the system by a pump, removing heat from the hot air stream and transferring it to the cold air stream.

Supplemental chillers may operate in winter months and therefore will have ethylene glycol automatic fill equipment.

Supplemental chillers may operate in winter months and therefore will have propylene glycol automatic fill equipment.

A minimum 0.30 m clear aisle space shall be maintained around laboratory benches and furniture.

A minimum 0.92 m clear aisle space shall be maintained around laboratory benches and furniture.

Drawthrough or blowover chilled water coils.

Draw-through chilled water coils.

Units may be either draw-through or blow through arrangements.

Units shall be in a draw-through arrangement.

In case of blow-over units with the drain pan on the discharge side of the fan, the trap height must be sufficient to account for the static pressure in the unit.

Where blow-over units are approved the drain pan on the discharge side of the fan, must have a sufficient trap height to account for the static pressure in the unit.

Fans may be arranged in either the blowthrough or draw-through position.

Fans shall be arranged in the drawthrough position.