Mission-critical facilities are growing steadily as a niche market segment for MEP design firms. No longer limited to financial data processing or telecommunications, these critical facilities are now in high demand for a new breed of “in the cloud” data storage and “big data” applications. Whatever functions they support, one thing remains constant – these facilities have an absolute need for reliable electric power.
Dedicated and/or redundant utility feeds can improve the reliability of the utility’s electric service, but even then, the threat of an outage remains a real concern. To ease concerns, critical facilities have traditionally employed some form of distributed generation to provide a local source of backup power. Having better control over this “on-site” power system allows the owner to build a customized level of redundancy to meet a specific set of requirements. Diesel-driven generators and uninterruptible power supply (UPS) systems are key components of a distributed generation solution. But there are many other supporting components, one of which plays a particularly important part in ensuring the reliability of the on-site power system.
Testing with confidence
If you have experience with distributed power generation applications, you may have had some exposure to “test load banks”. Load banks are designed to simulate electrical loads, and are commonly used to test diesel-engine generators before the equipment is “accepted” and turned over to the owner. For mission-critical facilities, a load bank is considered a permanent installation, forming an integral part of the electrical system. A permanently-installed load bank allows the operator to test emergency generators and UPS systems without the concerns associated with portable equipment, temporary connections and outside vendor participation. Considering the sensitive nature of their operations, most facility managers prefer a permanent load bank as it will allow more regular and thorough testing with a lower threat of disruption to the business’ operations.
If you haven’t yet, I expect that you will soon be exposed to a project that requires a test load bank. While load banks are relatively simple devices, there are several important points that should be considered when applying them to mission-critical facilities.
Converting power to heat, lots of heat!
When preparing design specifications for test load banks, be sure to allow for proper air circulation. Load banks are devices that convert energy into heat, and forced-air-cooled load banks can produce a substantial blast of very hot air. Here is a basic formula to calculate the load bank’s exhaust air temperature rise:
Temp Rise (°F) = kW × 3000 ÷ cfm, where kW is the load bank rating in kilowatts and cfm is the estimated airflow, in cubic feet per minute.
As an example, a 3000kW, forced-air-cooled load bank produces exhaust airflow of approximately 70,000 cfm (you can request this value from your preferred load bank vendor). Its temperature rise can then be calculated as:
3,000 × 3,000 ÷ 70,000, or 128°F expected temperature rise over ambient temperature.
It is important to note that the exhaust temperature rise is a calculated value and, because the airflow is not laminar, hot spots as high as 500°F can be present around certain areas of the load bank’s exhaust outlet.
For obvious reasons, the installation site must provide for the free flow of cooling air to the load bank and the free exhaust of hot air to the atmosphere, without negatively affecting nearby buildings, pedestrian traffic, or landscaped areas. The exhaust airflow must also be kept from re-circulating to the load bank air intakes. When a load bank installation is required to be indoors, it must be equipped with an exhaust air duct, designed to route the hot exhaust to the outdoors.
Designed to last
Load banks should be designed to withstand the generated airflow and heat, as well as the surrounding environment. Here are some basic design elements, starting with mechanical components:
- The load bank’s fan area should be shrouded for efficiency, and fan blades should be made out of cast aluminum.
- Any outdoor enclosure should be NEMA Type 3R, and of double wall construction to provide a low temperature exterior shell.
- Depending on the surroundings, sound attenuation may be critical. Forced-air designs will produce a substantial amount of noise, but this can be mitigated by the enclosure’s design, in most cases limiting sound levels to less than 85dBA at 7-feet. When specifying sound attenuation, be sure to list not only the acceptable sound level, but also the distance at which is should be measured.
- Any exterior hardware and all door hinges should be built in stainless steel.
- Exhaust airflow should be directed upwards through rain/snow shedding louvers, and away from building openings.
The following electrical items should be addressed in any specification:
- Load elements, the heart of any load bank, should be derated to at least 60% of nominal values. These elements will experience wide temperature swings and should be selected to provide long life within their intended use.
- Load elements should be mechanically-supported over their entire length, to avoid any failed element from shorting to an adjacent conductor or to ground.
- Ease of service should be a key design feature for the load element assembly. Slide-out, removable trays are a common way to allow service without major disassembly of the load bank.
- Due to the operating temperatures, all power wiring should have 150°C insulation, and control wiring should have a minimum 105°C insulation. Control circuits should be fused with 200kAIC current limiting-type fuses.
- Any fan-cooled design should utilize a high-efficiency, totally-enclosed, fan-cooled (TEFC) electric motor.
Control and monitoring features are last but not least. These requirements can vary widely, depending on the specific needs of the facility manager. Be sure to have a conversation with the owner to understand how they will operate the load bank. Today’s load banks are controlled by sophisticated PLCs and many custom features can be easily programmed. If the owner would like to automate certain aspects of a load test sequence, check with your preferred vendor. It’s likely that most requirements can be accomodated. In any event, at a minimum, you should suggest these features:
- Automatic protection features should include “fan failure”, “high exhaust temperature” and “high intake air temperature”. Any of these failures should trigger an alarm and the immediate shutdown of the equipment.
- Standard load bank control and monitoring features are handled locally (on the load bank itself). While this is the norm, an additional remote control device is very common for facilities wishing to operate the equipment from a central control station. Be sure to specify how any remote control device is to communicate with the equipment (hardwired, TCP/IP, etc.).
- Data logging and test reporting software is normally specified to minimize the need for human intervention in the recording of test data.
- “Auto-load leveling” is a very desirable feature when automatic load bank operation is desired. With automatic load leveling, the load bank senses the building loads, and automatically adds or substracts blocks of load to maintain the total load at a constant level. A facility that is yet to be fully built-out might benefit from this feature. Whenever the generator is load tested (with insufficient building loads), the load bank applies sufficient artificial load to keep the total load at optimum levels for the generator(s).
Testing, by definition, requires that a piece of equipment be, from time to time, operated at its limit. Real-life test scenarios can be quite stressful, if not impossible to conduct, for business operations that must run 24 hours a day, 7 days a week. For these facilities, a test load bank can be a great asset, allowing for the development and simulation of sophisticated test scenarios without affecting ongoing business operations.
As you work to design future on-site power systems, be sure to introduce your client to the contributions that a load bank can make for the thorough and regular testing of generators and UPS systems. A recent study found that the average cost of a power outage incident to a data center is in exceess of $500,000. If your client can identify a problem area before a power outage makes it apparent, the load bank investment will have been recouped immediately, all while making you a hero in your client’s eyes. Nothing wrong with that!