Engine Cooling for Emergency Power Generation Systems.
The typical diesel engine applied in emergency power generation is liquid-cooled, just like a diesel engine in an over-the-road truck. And just like the big rig, the engine's jacket water is cooled in a water-to-air heat exchanger called a radiator. The radiator for an engine generator can either be unit-mounted, or remotely installed.
The simplest radiator option in most cases is a local factory-mounted radiator. The radiator's cooling fan is typically mechanically connected to the drive shaft, so no external power source is needed, and the jacket water is usually pre-piped to and from the engine and radiator. Controls are extremely simple--if the engine is cranking, the radiator fan is rotating. The only field design required for the radiator is a source of intake or makeup air, and a duct enclosure from the radiator discharge to the building exterior. The primary disadvantage of the unit-mounted radiator is that, on large units, the airflow rate of the radiator's cooling fan can be exceptionally high--a difficult hurdle to overcome if the engine-generator package is to be located remote from a ready path to outdoors.
For very large radiators, or for engine-generator packages located where ample access to outdoor air is not available, remote radiators may be needed. Remote radiators can provide flexibility in terms of location, with the most common choices being on the building roof or on grade in a secure enclosure. Locating the radiator remotely has the disadvantage of requiring field jacket-water piping and often one or more jacket-water circulating pumps.
Engine generator packages with a unit-mounted radiator include factory-installed jacket water circulating pumps. Those pumps can still be provided even on packages with remote radiators, but their size and pressure capacity limits the distance to the radiator. Often, the engineer will need to specify separate external pumps, with a separate power connection. Since we are discussing emergency power generation, these external jacket water pumps and remote radiator fans will obviously also require a source of emergency power. Remind the electrical engineer to oversize the generation capacity because of the power that will need to be diverted from emergency use to power the jacket water pumps and remote radiator fans.
Design Tip: Today's engines often require two separate piping circuits for jacket cooling water-one each for the engine and turbocharger-and the radiators typically are coiled with the two circuits intertwined. A total of four pipe lines are required, two supply and two return, and cannot be combined due to their differing temperatures. Failure to recognize this may result in costly last-minute design changes and a lesson learned the hard way.
Design Tip: Selecting jacket water circulating pumps is not unlike selecting pumps for HVAC chilled or heating water. However, the jacket water is typically a glycol solution (in locations where winter temperatures can be expected to fall below freezing), and the operating temperatures are higher than most heating water selections, so density, specific heat, and other fluid properties must be properly applied. Do not use the common properties of water without appropriate adjustment.
Finally, radiator pressure ratings need to be reviewed. A typical radiator may be designed for close coupling with relatively low pressure ratings. If located far away from the engine, the pumping pressure requirements can exceed the radiator pressure rating. This may require addition of a secondary loop with an isolation heat exchanger.
Engine Exhaust Mufflers, Piping and Accessories
The engine exhaust must be ducted outdoors and discharged at an acceptable location. Common components of the exhaust system are flexible connectors, mufflers, anchors and guides, an end-cap or other termination, and pressure relief. Particulate filters can also be required in some pollution control districts. The piping itself can be standard-weight welded steel pipe. Insulation, typically Calcium Silicate, on that pipe is often needed for safety of personnel who may be working nearby, (2) as exhaust temperatures can approach 1,000[degrees]F (540[degrees]C).
The muffler is most often sized and selected by the engine generator manufacturer, and either critical-grade or super-critical grade are most often chosen. Physical styles include an inline cylindrical type, or space/layout considerations may indicate use of a "pancake" style that is wider but flatter, with bottom-in and side-out arrangement. Just as with the pipe, it may require insulation for safety reasons, or factory-insulated models may be specified. Engine generator manufacturers also frequently offer a pipe expansion fitting for use at the generator exhaust manifold connection, and a weather cap of some sort for the termination point.
Some engineers add a spring-loaded pressure relief device in the engine exhaust piping, because they believe it is required by NFPA 37-20103 Paragraph 8.1.4 "Exhaust systems shall be designed and constructed to withstand forces caused by the ignition of unburned fuel or shall have provisions to relieve those forces without damaging the exhaust system." However, the explanatory appendix material at the end of NFPA 37 Item A.8.1.4 reads "Normally, this provision is met by the built-in strength of the system." Building operators find the pressure relief devices to be a tremendous nuisance and the component manufacturers this author spoke to agree that the system is designed with sufficient robustness so as to avoid the pressure relief fittings.
The length of the exhaust run may be just a few yards/meters if located near an exterior wall or roof; but may be a great distance away if, for example, the exhaust must route vertically from a basement through a tall building. If lengthy, the engineer must consider the maximum exhaust static pressure available as published by the engine manufacturer - often on the order of 27-in. w.c. (6.7 kPa). Long runs also require consideration for additional expansion fittings as the piping will be subject to cycles between ambient temperature and the nearly 1,000[degrees]F (540[degrees]C) exhaust temperature. Supports, anchors, and guides need to be selected to allow for expansion and contraction.
Design Tip: Do not manifold the exhaust piping from multiple engine sets. Instead, route each engine's exhaust pipe to the outdoors separately. If mani-folded, the online machine will backfeed exhaust to an offline engine, causing portions such as the turbocharger to freewheel without the oiling system active.
Design Tip: In lieu of steel piping with field-applied insulation and applied expansion fittings, many engineers find it more convenient and cost-effective to specify a pre-engineered listed double-wall exhaust flue system offered by many of the same manufacturers that sell boiler exhaust flues. This type of product is already insulated, and the manufacturer includes the expansion fittings, guides and anchors, plus the technical expertise to properly select same.
Design Tip: Options for terminating the engine exhaust include the roof or sidewall. One trick features sidewall termination by poking through the same louver serving the radiator discharge. The radiator airflow helps carry away the exhaust fumes so the adjacent building walls don't stain.
The next challenge is to design and specify the ventilation, heating, combustion air, and radiator air cooling. Regardless of whether the radiator is unit-mounted or remote, the generator equipment room will need ventilation, combustion air, and (depending on climate) heating; but the radiator air cooling provisions described here are only applicable for unit-mounted radiators.
Combustion Air & Radiator Makeup-Air Design
For the case of the factory-mounted radiator, the airflow rate of the radiator's cooling fan can be quite high and its external pressure drop capability is quite low (e.g., 0.5 in.w.c [125 Pa]), requiring both a large discharge louver and a large intake louver. The intake louver should preferably be located on the wall directly opposite the radiator discharge louver. If located on an adjacent wall, the intake louver should be as remote as practical from the radiator discharge louver, so that outdoor air is drawn across the hot generator and engine during operation.
In locations where winter temperatures can be expected to fall below freezing, automatic motorized dampers should be used on both the intake and exhaust louvers so that they may be closed during long idle periods when the emergency generator is not in use. The dampers should be wired to open on the same signal that starts the engine generator. While the actuators can be simple two-position open-closed models, many engineers use a modulating damper actuator for room temperature control as explained in the subsequent subsection.
Design Tip: In locations where winter temperatures can be expected to fall below freezing, it is advantageous to install a separate smaller intake louver/damper sized for the larger of (a) the room's ventilation airflow rate or (b) the engine manufacturer's published combustion airflow demand. By keeping the ventilation and combustion air function separate from the large air requirement of the radiator, room heating during generator operation is facilitated. This feature can also be achieved using one large louver/damper with a separate actuator controlling only a few of the damper blades of the larger assembly.
Design Tip: To guard against risk of having the dampers fail to open when needed for emergency operation, specify these dampers to be actuated by motors with a spring return feature, which fail-safe in the position favorable to generator operation. If non-spring types are used, the actuators must be powered from the emergency power system.
Design Tip: Using a remote radiator does not completely remove the need for ventilation airflow across the engine/generator set during operation. If the radiator is remote from the engine/generator set, local ventilation of the engine/generator room is necessary for removal of heat from the engine itself, plus generator winding heat removal, which could be up to 5% of rated generator capacity. Manufacturer's literature provides the full-load rated heat losses to the room for the engine and generator that must be removed by ventilation.
Generator Room Heating & Ventilation
It would be uncommon for an emergency generator equipment room to be air-conditioned, but they certainly might be ventilated; and in many climates winter heating will be necessary as well. Ventilation can be as simple as an exhaust fan and makeup air intake louver, controlled either thermostatically, or based on occupancy, or both. Control the ventilation system to power on when room temperature in the summer exceeds a desired setpoint, and perhaps anytime the room is occupied.
Winter heat is usually provided via a unit heater; gas-fired, hot water, or steam are common sources; electric-resistance heat can also be used. While unit heaters or similar local heating units can be applied for heating the generator equipment room during long winter idle periods, don't attempt to heat the generator equipment room that way during actual engine operation if using a unit-mounted radiator. The amount of outdoor air induced during operation by a unit-mounted radiator fan is usually far too high to be heated in any practical way. To illustrate, a 200 kW engine generator package may require a 15.000 cfm (7500 L/s) radiator, and a 750 kW package may need 40.000 cfm (20 000 L/s). Even if you could heat this airflow, the energy penalty to do so is unacceptable.
But in many climates the room certainly will require heat to protect sprinkler or any other water-based piping systems from freezing during a long period of generator operation in cold weather. Therefore, use the waste heat from the radiator to your advantage.
Design Tip: Size the room's heating unit(s) for the winter heat loss through walls and roof, plus the heat needed to offset the day-to-day ventilation load. For room heating during generator operation, use an auxiliary damper on the discharge side of the radiator to recirculate warm radiator air back into the room (dampers MD-4 and MD-5 in Figure 1). A sheet metal duct of some practical length (often 3 to 6 ft [1 to 2 m]) is connected from the radiator to the exterior wall louver. The radiator discharge damper is placed near the louver, and a recirculation damper is placed on the side of the duct between the radiator and the discharge damper.
Upon winter startup of the engine-generator package, the intake louver (MD-2) and radiator discharge louver (MD-3) remain closed, while the recirculation damper (MD-4 & 5) and combustion air damper (MD1) are both open. This allows the radiator waste heat to keep the room warm. As the room temperature becomes too warm, the control system via a temperature sensor modulates the intake damper and radiator discharge damper in tandem but opposite to maintain acceptable room temperature.
Other Design Considerations
Since emergency generators are, by definition, intended for use in emergencies, we must keep in mind that all of the components discussed here need to be available in those same emergencies. Tornado, hurricane, or other extreme weather event can be responsible for loss of normal utility power, as can earthquakes or fires. So the design engineer must consider design features intended to minimize risk to any of the engine-generator system components in those scenarios. For that reason, indoor unit-mounted radiators are sometimes preferred over remote outdoor radiators, for example. If remote radiators are necessary, consider placing them in a secure courtyard protected as much as practical from damaging storms. If in a zone of higher seismicity, all components require seismic bracing and anchorage. If located in a flood-risk zone, locating accessories in safe dry areas away from potential flooding is necessary. And all components require access for maintenance.
Be kind to your neighbors! Diesel engines can be very loud when operating, and that noise often has a straight line of escape directly through the radiator, damper and louver. Sound attenuation for the radiator louver discharge and/or the outdoor air intake louver may be needed and even required by local codes, depending on distance to the property line, orientation, and type of neighborhood (residential, commercial, institutional, or industrial). Mitigating strategies may include sound attenuators or an exterior barrier wall. Space does not allow a full treatment of sound attenuation in this column, so see instead the topic in the ASHRAE Handbook. (4)
Design Tip: The engine-generator package manufacturer will publish allowable static resistance the radiator fan can overcome external to the radiator itself, and is often 0.5-in. w.c. (125 Pa), so the engineer must be mindful of this when selecting the louver, damper, and any sound attenuator.
The intent of this column was to review some of the necessary non-electrical design considerations for diesel engine emergency generator systems. The engineer or designer who is infrequently asked to design these systems may find useful these tips and suggestions on emergency generator auxiliaries such as exhaust mufflers and piping; local and remote radiators; combustion air and radiator makeup-air. The December 2015 "Engineer's Notebook" column included a related discussion of fuel oil storage tanks and tank accessories, fuel oil piping, valves, and accessories, leak detection and monitoring systems, return oil coolers, and fuel oil transfer pumps.
The author heartily thanks Jeffery M. Coffelt, P.E., Member ASHRAE, for his technical contributions to this column.
(1.) Duda, S. 2015. "Fuel oil systems." ASHRAE Journal 57 (12).
(2.) Duda, S. 2014. "Overlooked code requirements." ASHRAE Journal 56 (12).
(3.) NFPA 37-2010, Standardfor Installation and Use of Stationary Combustion Engines and Generators. Quincy, Mass.: National Fire Protection Association.
(4.) 2015ASHRAEHandbook--HVACApplications, Chapter 48.
STEPHEN W. DUDA, P.E., BEAP, HBDP, HFDP, FELLOW ASHRAE
Stephen W. Duda, P.E., is senior mechanical engineer at Ross & Baruzzini, Inc. in St. Louis.
Caption: FIGURE 1 Radiator with recirculating damper.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||COLUMN: ENGINEER'S NOTEBOOK|
|Author:||Duda, Stephen W.|
|Date:||Apr 1, 2016|
|Previous Article:||Fusing Form With Efficiency: HONORABLE MENTION: RESIDENTIAL (SINGLE AND MULTIFAMILY).|
|Next Article:||No Sweat: Condensation on Ductwork and Boots in Hot-Humid and Mixed-Humid Climates.|