Protecting Your Power

Advice on specifying outdoor generator enclosures

Virtually any size of generator set can be housed in an outdoor enclosure available from either the generator manufacturer or a third-party supplier. Overall cost and ease of installation will depend on the physical location of all elements of the system, including the generator set itself, the fuel tanks and accessories. Several types of enclosures are available, including weather-protective, sound-attenuating and walk-in versions.

Weather-protective: while all metal enclosures provide weather protection, these basic enclosures only secure the generator set and provide protection from the elements. They do not retain heat or hold temperatures above ambient in cold weather, nor do they provide cooling beyond the ventilation and airflow offered by their integral louvers or perforated panels. Sound attenuation is minimal and, owing to their tight fit, access panels or doors are required for maintenance and inspections.

Sound-attenuating: sound-attenuating enclosures may be specified when noise reduction is needed to meet local legislative requirements. These enclosures are generally larger and more costly than strictly weather-protective enclosures because of the design features required to reduce generator set noise.

Walk-in: this term encompasses a wide variety of enclosures that are custom- built for a specific application. Walk-in enclosures often include sound attenuation, as well as space for power switching and monitoring equipment, lighting, fuel tanks and other equipment. They can also accommodate insulating and heating capabilities. Walk-in enclosures may be treated as a building by local inspection officials.

When contemplating a new generator in an outdoor enclosure, the following factors should be considered.

Site selection

The location of an outdoor enclosure should be level, well drained and secure from flooding, fire, icing and vandalism. For reasons of reliability, the generator set should be located near the main electrical service but not so close that localized problems at the service entrance will interfere with the generator set. For example, a fire at the utility service entrance would be less likely to affect a standby generator set located some distance away.

Control panels and connections should have ample clearance for easy access, and the fuel tank should be located so that refuelling is convenient. In the US, the National Electrical Code (NEC) requires that all outdoor generators include an interior or exterior service disconnect, either at the generator or at the point where the generator feeder enters the building. The generator should also be located away from building openings, combustible materials or building ventilation inlets.

In multiple-generator applications there should be adequate spacing between the generators, and the units should be oriented so that the heat from one generator is not drawn into the others.

Sometimes, as a last resort, the only outdoor location for an enclosed generator set may be a rooftop. Provided the roof is properly designed or modified to take the extra weight, this can often be a solution in urban areas, although there are advantages and disadvantages. While a rooftop location may resolve space limitations, supplying fuel, load bank testing and maintenance may be more difficult.

All-weather protection

The amount of weather protection needed is often a matter of geography. In tropical climates the principal weather challenges are sun, heat, rain, salt spray, wind storms, blowing sand and lightning. Within 60 miles of the sea, aluminum enclosures resist corrosion from salt air. In more northern climates, ice storms, heavy snow and severe cold pose additional threats. It is recommended that any outdoor generator set enclosure has a corrosion-resistant finish (often aluminum or painted steel); stainless steel hinges, corrosion-resistant handles and other hardware; and rain shields on the cooling-air intakes and exhaust. In some cases, to protect against precipitation the cooling-air intakes and exhaust may need motorized louvers that open when the generator set is started and close automatically when the generator set is off.

In areas where ice and snow accumulate, it will be necessary to provide for regular removal of snow and ice around the doors, louvers and dampers. In cold climates, the enclosure may have to be fitted with an electric space heater in addition to a coolant heater on the generator engine. Also, diesel fuel may need to be heated to prevent gelling.

In storm-prone areas, an enclosure with added structural strength to withstand high wind loadings should be considered. Outdoor generator enclosures are available with enhanced strengths that can resist wind speeds of over 150 miles/h. In flood-prone areas, the generator and enclosure should be installed well above the highest expected water level, eg on an elevated platform or rooftop.

Controlling temperature

Generator sets can maintain their rated power output as long as the ambient air temperature flowing into the enclosure does not exceed the cooling system ‰ ambient temperature rating and static restriction. If this air temperature is exceeded, the output of the generator will have to be reduced to prevent overheating of the generator set.

Cooling requirements for an enclosed generator set can also be affected by site selection. Wherever possible, the enclosure should be located in an area where there is free airflow. Locations near walls or overhangs, which may restrict cooling airflow or require complex routing of the engine exhaust, should be avoided.

An enclosure with adequate flow-through ventilation to keep temperatures in the optimum operating range should be selected. This will allow the generator set to operate at its nameplate rating. The best enclosures incorporate advanced radiator, fan and louver designs, providing optimal airflow through the enclosure to control engine and generator operating temperatures even in the most severe environments. Cooling-air intakes and exhausts should be kept clear of obstacles.

Security

Vandalism is a common problem that can compromise a generator’s reliability. Lockable access points to connections, switches and valves will help to deter unauthorized persons from interfering with system operation, and access to the fuel tank and system controls should also be restricted.

Sound attenuation

The control of generator noise can be a major concern if the unit is located near neighbouring properties or in a crowded urban environment. Most locations have set limits on permissible sound levels at the property line. While exhaust silencers can greatly suppress exhaust noise, much of the noise from an enclosed generator set comes from the cooling air fan. This noise is harder to control but can be greatly reduced by careful design of the air intake and air exhaust plenums.

Sound attenuation is dependent on many factors, including the sound level produced by the generator engine, the design of the air intake and air exhaust plenums, the type of exhaust silencer and local conditions. Users should work with their generator set and enclosure supplier(s) to determine the level of noise suppression needed to comply with local regulations. Most generator set enclosure providers offer one or more levels of sound attenuation as options.

Additional issues regarding sound attenuation include:

• Critical sound-attenuation remedies will increase the overall footprint, complexity and cost of an installation.
• Sound-attenuating materials in the walls and ceiling of the enclosure not only trap sound, but also heat.
• If sound attenuation is required, non-hygroscopic and non-flammable material should be specified to prevent moisture build-up inside the enclosure.

Fuel

For increased security and to reduce the installation footprint, many generator sets can be ordered with an integrated high-capacity fuel tank. Integrated fuel tanks protect fuel lines and filler connections within the enclosure for better security and provide a very compact installation. Users should look for tanks that include dual-wall construction, fuel gauges and provisions for catching fuel leaks, ruptures and overflows. Sufficient clearance should be allowed around the enclosure to allow fuel trucks to access the enclosure.

Maintenance

In walk-in enclosures there should be sufficient room within the enclosure for easy maintenance access to key generator set components. Skin-tight enclosures should have large access doors and panels. Good interior and exterior lighting can make maintenance and troubleshooting tasks easier.

Conclusions and recommendations

Specifying the best generator enclosure is important because it can have an impact on both the short-term operation and long-term reliability of the power system. Users should begin with a thorough assessment of their power needs, installation location and environment, and work with their generator set and enclosure supplier(s) to design an installation that fits all the necessary specifications.

This article is based on a paper by James Iverson, senior applications engineer, Cummins Power Generation.