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Design Generator rooms for optimum performance
Mechanical engineers should design generator set rooms so that the electrical system meets the design goals set by the owner and electrical engineer.
Michael Streich and Saahil Tumber, PE, HBDP, LEED AP
06/22/2015
Electrical power is essential to business continuity and life safety. Even a brief disruption in the electrical power supply can be costly. A backup generator set (genset) is an important line of defense for business owners that offers the ability to start and assume electrical load in a few seconds, providing power when the utility supply has failed.
Backup generator sets are available in a wide range of capacities (from kilowatts to megawatts, or kW to MW). They can be installed outdoors within specialty enclosures or within a building. Generator sets that are located indoors require careful attention to a multitude of factors to ensure optimal and reliable operation. A well-designed generator room will ensure that:
- Generator sets are accessible
- Manufacturer- and code-required clearances are maintained
- Major components can be removed and replaced
- Clean and relatively cool air can circulate around the generator set
- Ventilation airflow (room inlet airflow) is adequate to reject the heat produced during operation and support the engine combustion process
- Recirculation and bypass airflow is minimized; noise and vibration within and outside the building complies with code requirements, and ancillary components external to the generator set operate reliably.
Generator room ventilation 101
Proper ventilation of the generator room is necessary to support the engine combustion process, reject the parasitic heat generated during operation (engine heat, alternator heat, etc.), and purge odors and fumes. Generator-room temperature, ventilation airflow, ventilation air cleanliness, and air movement are critical design parameters that must be analyzed during the design process to ensure optimal and reliable operation of the generator set.
It is critical that an adequate amount of ventilation airflow be delivered to the generator room. For the same generator size, there can be a reasonable variation in required airflow between different manufacturers. Table 1 indicates the ventilation airflow requirements from different manufacturers for a 2-MW, standby-rated generator set with unit-mounted radiator. If the product specifications are nonrestrictive, the design should be based on the worst-case scenario to avoid wholesale revisions in the future.
Under fully loaded conditions, the temperature of flue exhaust from generator sets can be in excess of 900 F and the radiator (engine-driven or remote) discharge air temperature can be in excess of 160 F. Any recirculation of these high-temperature airstreams can cause the ventilation air temperature to exceed the ambient temperature. Recirculation is specifically influenced by the prevailing wind speed and direction—the two variables that cannot be controlled and are difficult to incorporate in design calculations. The thermal contamination of ventilation airflow should be eliminated or minimized. Generator-room temperatures in excess of 104 F typically require de-rating of the generator set and potential upsizing of components to support the design electrical load. The magnitude of de-rating varies with manufacturers, generator set capacity, engine fuel type, and more. Typical de-rating of 10% to 15% per 18 F rise over 104 F can be expected. De-rating becomes steeper for room temperatures above 122 F. High generator-room temperatures also necessitate de-rating of electrical equipment and components that typically are located within the generator room, such as transformers, switchgear, and electrical feeders. Assuming that the ventilation airflow temperature equals the ambient temperature can be a critical design flaw, and abatement methods can be costly.
Table 1: This outlines the ventilation airflow requirement for 2-MW generator sets (standby-rated) with unit-mounted radiators. All graphics courtesy: ESD
Wind-tunnel testing and CFD modeling
Once the proposed locations of flue exhaust, radiator discharge, and ventilation air intake have been identified, it is recommended that wind-tunnel testing or computational fluid dynamics (CFD) modeling be conducted to establish proof of concept. This is especially essential for gensets that are expected to operate at 100% rated power or serve critical applications such as data centers. Wind-tunnel testing involves the creation of a scale model of the generator-room building and other buildings and structures in its vicinity. The model is placed within a wind tunnel and tracer gases are released from radiator-discharge and flue-exhaust locations. The concentration of gas at the room’s ventilation air locations is measured by receptors for varying wind speed and direction. The data are correlated to local meteorological data to predict the degree of recirculation and peak ventilation air temperature anticipated at the generator room.
If wind-tunnel testing cannot be performed due to budgetary or schedule constraints, another option is to use CFD modeling. Programs are commercially available that are adept at performing outdoor CFD analysis. The trade-off is limited ability to incorporate wind effects and surrounding conditions.
Radiator discharge and flue exhaust can also impact the performance of equipment outside and in the vicinity of the generator room. For example, recirculation of radiator-discharge air or flue exhaust can impact performance of heat-rejection equipment such as air-cooled chillers, condensers, cooling towers, and dry coolers. Wind-tunnel testing or CFD modeling should include nearby equipment if deterioration in performance is anticipated. Also, flue-exhaust odors can be entrained into outside airstreams of air-handling equipment even if code-required clearances are maintained, thereby affecting the indoor air quality. Air-handling equipment in the vicinity of flue-exhaust locations should be included in the wind tunnel or CFD study, to ensure there is no recirculation.
No way out? Think outside the box
If elevated ventilation air temperatures cannot be avoided due to site constraints, one option is to use evaporative cooling technology to cool the air entering the generator room. This technology incorporates an evaporative media or mesh that is installed at the ventilation air source such as louvers. When the generator sets are operating and the room temperature approaches the maximum permitted value, water mist is sprayed over the media. As air flows over the wetted media, it gets cooled due to the evaporative process. The higher the ambient wet-bulb depression—the difference between dry- and wet-bulb temperature—the greater the potential of reducing the ventilation air dry-bulb temperature.
Key considerations for optimal performance
There are a number of design considerations that are key to maintaining optimal operation and equipment uptime over the life of the genset:
Room temperature during subfreezing conditions: Care should be exercised where subfreezing ambient temperatures are anticipated. In the absence of a temperature-control mechanism, prolonged operation during such conditions can potentially impact the performance of the generator set and associated components as the room temperature approaches the ambient temperature. For generator sets with unit-mounted radiators, one option is to install a motorized recirculation damper at the discharge plenum between the radiator and the louvers. The recirculation damper is modulated in tandem with the discharge damper, and a portion of the hot radiator-discharge airflow is recirculated back into the room where it mixes with the cold ventilation airflow to maintain acceptable room temperature. For generator sets using remote radiators, the exhaust fans serving the generator rooms can be provided with variable frequency drives to reduce ventilation airflow during conditions of genset operation during subfreezing weather. Maintaining the generator-room temperature above freezing is critical for installations involving multiple generator sets within the same room, as not all units are expected to be operating simultaneously. It ensures reliable start-up of the standby generator sets, prevents fuel oil clouding and the freezing of water pipes including the sprinkler system, and allows for environmental conditions that permit maintenance staff to occupy the room, if needed.
Air movement: Air movement within the generator room also is important for proper functioning and should be reviewed during the design phase. It directly impacts the effectiveness of heat removal from within the room. Preferably, the source of ventilation air should be as low as possible and the air should flow over the entire generator set, thereby cooling the alternator, engine block, and radiator (for sets with unit-mounted radiators) to remove the after-cooler and jacket-water heat. This strategy also ensures that air temperature at the engine turbocharger inlet is within limits. For generators with remote radiators, it is recommended that the exhaust air should be sourced as high as possible and directly above the generator sets. Significant bypass of ventilation airflow directly into the discharge airflow will lead to reduction in cooling effectiveness and elevated temperatures within the room.
Figure 3: Ineffective air movement around the generator set is shown in this diagram. Airflow bypasses the alternator, which could lead to potential overheating.
For example, Figure 2 shows a sample installation, which will result in effective air movement within the room and over the generator set. Figure 3 shows another installation, which could potentially lead to overheating of the alternator due to inadequate airflow around it. If it is anticipated that air movement within the generator room could be compromised, it is recommended that CFD modeling be conducted to establish proof of concept.
Figure 2: In this sample diagram, the installation depicts effective air movement around the generator set. Airflow adequately cools the alternator, engine, and radiator.
Air cleanliness: Air cleanliness also is important to minimize engine wear and tear. If the ventilation air is expected to be unreasonably contaminated with dust or other materials, it is recommended that the generator manufacturer be consulted and specialized heavy-duty engine filters, pre-filters, or dual-stage air filters be incorporated per their recommendation. In few instances, a means of filtration at the source of ventilation air might be necessary, e.g., installations affected by seasonal contaminants such as cottonwood. It should be ensured that specialized filtration does not impede airflow beyond acceptable limits.
Noise pollution: Noise-pollution control is an extremely important but often overlooked design element. An acoustical consultant should be involved early in the project to study the impact of generator sets on the nearby environment and provide recommendations to mitigate noise. It is equally important to be aware of applicable codes that establish criteria for permissible noise levels. An installation is frequently subjected to federal, state, and local codes, each with varying requirements.
For example, the Chicago Building Code stipulates that sound pressure levels from a mechanical stationary source shall not exceed 55 dB when measured from a distance of 100 ft from the source, or 70 dB when measured from a distance of 10 ft from the source. The noise limitation is applicable from 8 p.m. to 8 a.m. On the other hand, the Illinois Pollution Control Board is more stringent and stipulates allowable octave band pressure levels for daytime hours (7 a.m. to 10 p.m.) and nighttime hours (10 p.m. to 7 a.m.). For Class A land, the criteria equates to approximately 55-dB daytime limitation and 44-dB nighttime limitation. If the facility is expected to be occupied during generator set operation, then the effect of noise and vibration on indoor occupants also should be analyzed. An acoustical consultant can provide recommendations for sound attenuators and flue-exhaust silencers.
Fan static pressure capability: For gensets with unit-mounted radiators, the external static pressure (ESP) capability of the engine-driven fan should not be exceeded. The radiator fan has limited capacity to accommodate airflow restrictions in the range of 0.5- to 0.7-in wc ESP. Typically, this is adequate to support installations where the only restrictions are ventilation louvers, discharge louvers, and associated dampers. However, care should be exercised to support sound attenuators. Many installations require attenuation at paths of sound transmission such as radiator discharge and ventilation air intakes. For noise-sensitive applications, the intake and discharge attenuators can be as long as 10 ft to achieve the required level of attenuation. To reduce the static pressure drop at the attenuators, the air velocity through them needs to be kept within limits. An increase in ESP requirement beyond the generator set limit will lead to a reduction in airflow and will require de-rating of the generator.
Ancillary components: Attention also should be given to the ancillary components associated with the genset. For example, the actuators at the radiator-discharge dampers should be outside the airstream, as they are typically not rated for the expected discharge temperature. The actuators used at the ventilation dampers and radiator-discharge dampers should be quick-acting, as generator sets can reach rated speed within a few seconds after enabling. Actuators also should fail open as a safety measure. Barometric dampers are simple in operation, are inherently quick-acting, and minimize the control requirements. However, they tend to stick and impose a greater pressure drop compared to motorized dampers, so be careful to avoid this if possible.
Good design ensures proper genset operation
The ventilation system and overall layout of a generator room should be examined in detail during the design process. While a generator set is specified by the electrical engineer, the onus is on the mechanical engineer for an optimum design that maximizes the performance, longevity, and reliability of the genset. Failure to do so can lead to serious implications, resulting in system failures that are often unforeseen and can be potentially catastrophic. Post-construction modifications to mitigate the effects of a poorly engineered system can be extremely expensive and disruptive.
For example, a metropolitan-area data center was designed to be supported by two 1.5-MW generator sets with unit-mounted radiators. Due to site constraints, the generator sets had to be housed within the building near grade level. A simple solution was to locate ventilation air louvers, radiator-discharge louvers, and flue exhaust on the same side of the building. The ASHRAE annual ambient extreme temperature at the site was 99 F—just a few degrees below the de-rating threshold of 104 F. It seemed obvious that the effectiveness of the ventilation system would be compromised due to recirculation.
Therefore, a wind-tunnel study was commissioned to quantify the effect. The study predicted that ventilation air temperature will approach 127 F (i.e., 28 F above the ambient temperature) on a design summer day due to recirculation. Based on the de-rating data provided by the manufacturer, each generator set could only support 1.2 MW if subjected to the conditions predicted by the wind study. Reduction in genset output was not acceptable to meet the owner’s project requirements. The solution was to provide additional ventilation air openings at the opposite end of the building to mitigate recirculation. This approach increased the cost and complexity of the project, but was deemed necessary to ensure reliable operation of the generator sets.
Michael Streich is a lead mechanical engineer at ESD specializing in mission critical facilities and is responsible for the overall design of HVAC systems for data centers, trading areas, office build-outs, and other critical facilities requiring high availability. Saahil Tumber is a lead mechanical engineer at ESD, responsible for the overall design of HVAC systems for data centers, trading areas, and other mission critical facilities requiring high availability. His data center experience spans both enterprise and co-location projects.
