The how and why of specifying resistive load banks

In specifying load banks as part of a backup power application, one should start with answering the “why” portion first.

11/02/2010


By design, diesel engines (which are the prime movers on a diesel generator set) need a minimum amount of load to insure proper operation. The amount of load varies per engine manufacturer, but the typical range is 40% to 50% of the kilowatt rating. It is also a proven fact that diesel engines operate more efficiently in the 70% to 80% range of rated output. Using a load bank allows the diesel engine to reach operating temperature more quickly and heats the engine correctly to avoid wet stacking. Wet stacking is a condition that is caused by running any engine in an unloaded condition (such as extended idle time) or continual use at less than the recommended minimum load level. When a diesel engine operates for a long time below 40%, it begins to wet stack (over fuel). Wet stacking happens because the fuel injection tip begins to carbonize and will eventually disrupt the fuel spray pattern. The disrupted fuel spray pattern results in unburned fuel, which collects in the emission system of the diesel engine and causes loss of horsepower and generator output.

Importance of emergency power

Consulting engineers that specify emergency power equipment also know that installations for mission critical facilities (like hospitals and data centers) are typically required to comply with NFPA 110 so that the facility can comply with the National Electrical Code. Section 7.13 of NFPA 110 details the type and length of load testing for generator sets in these mission critical applications. Remember, it is the specifying engineer’s responsibility to evaluate how each supplier’s equipment complies with NFPA 110.

As we have learned from Y2K and the 2003 blackout, properly operating emergency power is a critical lifesaving application. The good news is that today’s load bank technology offers cost-effective solutions to insure the diesel generator sets are properly loaded.

Specifying load banks

When specifying load banks, the first piece of required information is the voltage and kilowatt rating of the load bank. Many clients prefer to have the load bank the same kilowatt rating as the generator set. This is done primarily because many generator sets might be purposely oversized to accommodate for future growth. If the customer knows that it will be adding more equipment like computers, UPS systems, or drives, it might start with a higher capacity generator set at the initial stage. For example, a 1000 kW genset might be installed for a building that has only a 100 kW to 200 kW building load. This low load is bad for generator set performance, so customers will insist on a supplemental load (i.e., load bank) to bring genset loading to at least 40%.

The next step is to determine what type of load bank works best for the application. The three most common resistive load bank types are portable, permanent, and radiator mount.

Portable units

Portable units are designed to be used indoors, have integral controls/metering systems, and have the capability to load at multiple voltages. These portable units have casters so they can be moved to different locations. A properly designed portable load bank will be designed to easily fit through main doors and into freight elevators. If the application requires multiple generator sets at different locations, a single high kW portable load bank can be used for individual testing of these generator sets.

Permanent load banks

Permanent load banks are the preferred choice when the application allows for installation on a concrete pad or a rooftop. Permanent load banks are recommended when the client would like a 100% load on its generator set. Freestanding permanent load banks have their own integral cooling fans and do not add any static (back) pressure to the generator. Permanent load banks are designed to operate continuously in all weather conditions. Therefore, when specifying a permanent unit, the engineer should insure that it is designed for outdoor weatherproof construction and has some third-party certification like UL, Canadian UL, CSA, or CE. Permanent units typically use a remote control panel that can be located anywhere from 50 to 250 ft away from the load bank. As a consulting engineer, it is important to specify that the permanent load bank has an internal strip heater that operates from a dedicated voltage source (usually 120 V, 1-phase, 60 Hz). The strip heater is installed in the control area of the load bank and will activate when the temperature drops below 50 F. The heater will keep the control area from condensation buildup year-round.

Permanent load banks also come in two varieties based on the direction of exhaust air. The horizontal load bank has a low profile and directs hot exhaust through louvers that are angled downward (to avoid directing at personnel). The vertical load bank directs the hot air upward and away from any personnel. The vertical load bank has a smaller installation footprint than a comparable horizontal discharge unit. If multiple generator sets are being installed in a parallel bus installation, the engineer can specify a load bank that can interface with the parallel bus, and load either individual or multiple generator sets at a time. The basic criterion for the parallel bus load bank is that the load bank capacity needs to be equal to or greater than the genset capacity.

The user needs to be aware of the location and climate before specifying a permanent load bank. Since the load bank will be permanently installed at the same locations for years, a quick investigation of the environment will serve the client well. Outdoor rated load banks should be designed for continuous duty cycle operation with no limitations. The load bank should also be designed to operate in an ambient temperature of -20 to 120 F.

Finally, insure that the load bank has automatic load dump circuitry, which interfaces with the switchgear. The purpose of the load dump circuitry is to remove all loads when the switchgear transfers to the generator set for emergency power. In most cases, engineers do not want a large load on the generator set when it is providing power to a building in an emergency situation.

Radiator mounted

A radiator mounted load bank is the most cost-effective type of load bank as it does not have an integral cooling fan (or any of the associated fan controls). The radiator style load bank is designed to be installed on the radiator of the genset and uses the airflow through the radiator to cool its load elements. A properly designed radiator mounted unit should be approximately 13-in deep (or thick) to keep the backpressure on the radiator to a minimum. When specifying a radiator mounted load bank, the most critical point to understand is that a radiator mounted load bank is a supplemental load to the genset. A consulting engineer should not specify a 100% rated radiator mounted load bank as the increased backpressure could potentially overheat the generator set. The typical radiator mounted load bank is 50% to 70% of the generator set kW capacity. If a client requires 100% rated load bank, the safest option is to go with a freestanding permanent load bank as described above. Radiator mounted load banks are mounted in the hot exhaust of the radiator and need to be constructed of material that deflects the heat rather than absorbs it. The goal is to avoid having the load bank act like a heat sink for the radiator. Therefore, the engineer should specify load banks that are manufactured from aluminized steel, which reflects heat up to 1,250 F. The more common galvanized steel is not recommended for radiator mounted load banks as it starts to absorb heat at around 500 F. The radiator mounted load bank should also be specified with the features of the permanent load bank like automatic load dump, remote control panel, weatherproof construction, and third-party certification. With some of the new sound attenuated enclosures being provided for generators, a radiator style load bank should be available to mount on the enclosure either vertically (for horizontal radiator exhaust) or horizontally (for vertical radiator exhaust).

Conclusion

Once all the above information is gathered from the client, the engineer will have the tools to specify a resistive load bank that will serve the client’s needs and meet local/state and federal codes. Most major load bank manufactures have specifications that can be downloaded in a text format so the engineer can just “plug in” the required voltage, kilowatt, and any options the client might require.

Specifying a load bank for a client’s mission critical emergency power installation is necessary for proper operation of the diesel generator set and to comply with the various required electrical codes. The three most common types of load banks—portable, permanent, and radiator—all provide the means to meet these requirements in a cost-effective way. Remember, a backup power system is only good as its last load test. By properly specifying a load bank, the engineer is insuring his or her client will have reliable backup power.

Prevoznik is a product manager with Avtron LoadBank Inc. in Cleveland.



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