Enclosures for power sources must be more than just pretty wrapping.

By Paul Hull

Of course, you want the enclosures for your gensets or other power units to be attractive! They are often the most visible part of your installation and convey an image of your company to the ever-critical public. Aesthetic design is neither the only nor indeed the most important aspect of the enclosures. They must protect and shelter ... and still be acceptable in their environment. There are many designs, many variables available, but it is not difficult to decide what is best for your particular installations if you consider several practical aspects at the site.

If you divide the emphasis of your research into two sections—the site and the enclosure itself—you will find that simple logic and common sense will guide you through to a choice for the most appropriate protection or shelter equipment. In other words, it seems wiser to start from the question, “What do we have here that needs shelter?” than, “Look what we have in stock! Which components would you like?”—the approach proffered by some manufacturers and their representatives. Let’s start at the first of those: your site. For many users of onsite power equipment, there is little choice about site, and virtually all your decisions concerning shelter for the power equipment and its necessary peripherals will reflect the freedom or constraints of that site.

Your first concern may be your neighbors. How near are your neighbors? There are almost certainly local regulations (and “local” can mean federal, state, or community) concerning noise, and they must be observed. There could be rules, too, concerning the storage of fuel. Such basic needs would surely have been part of your initial investigations when you decided to install onsite power; their satisfaction can depend as much on the enclosures you choose as on the power sources themselves. The actual proximity of your neighbors could be significant from the aspects of environmental acceptability and of safety. There will be conduits. How will they access the enclosures?

The visibility of the equipment is a major concern—not just its attractiveness but the very fact that it can be seen at all. “The visibility will help you determine the materials of construction for your enclosures,” observes Michael Witkowski, vice president of sales at Pritchard Brown LLC, a leading designer and producer of enclosures for onsite power units. “Your level of visibility can also determine the level of vandal resistance you will need. If your enclosure is on the roof, perhaps next to chillers and behind a vision screen, it will have a different visibility and role than one that is located right outside the window of the chief executive offices on a landscaped campus. If there are local constraints for noise, security, or fuel storage at your site, these should be challenges addressed at the beginning of the enclosure design process.”

Your assessment of the external attributes of the installation should include everything that could have any impact on the expensive equipment inside. Security has been stressed frequently by government agencies, but it has tended to be concerned with attacks from professional terrorist organizations of foreign origin, while most damage and vandalism is probably caused by local persons who are members of no political organization and who have little or no knowledge of what they are attacking and attempting to destroy. Designers of enclosures have, in the last five years especially, given deserved emphasis to the ability of their equipment to resist assault by professional or amateur destroyers. There are also the natural vandals, the little creatures that love cable for breakfast or a comfortable lodging in which to nest and breed well away from the inclement wind, rain, and snow.

What Are You Sheltering?
What is inside the enclosure? More often than not, it will be the most expensive feature of the installation and the reason for an effective enclosure. Whatever it is, when you are considering the ideal enclosure, you will need to know its weight, its dimensions, and any environmental requirements that it has. Is it noisy, requiring sound attenuation? (In the past, how often did we hear an installation before we ever saw it or knew where it was housed? That’s changed.) “If the enclosure is expected to provide some sound attenuation, then the source fan and mechanical noise under full load should be calculated,” says Witkowski. “That noise will be in decibels. The information is available from manufacturers, but I would counsel that the enclosure designer should know at what distance the source noise readings were taken.” Not everything is estimated or reported in the same way. Measurements such as noise, airflow requirements and even physical dimensions may vary from manufacturer to manufacturer for a given electrical kilowatt rating, so the enclosure may have to be designed on figures that would state the worst scenario if you don’t know which genset supplier is being considered.

Heat is another top concern for enclosure configuration, especially if your application requires continuous operation. Whichever company you engage for the design and manufacture of your enclosure for such an installation should have successful experience in projects with similar heat and airflow challenges. “If there is a genset, you need to know how it is cooled and the particulars of all its cooling and combustion air requirements,” explains Witkowski. “Those are usually measured in cubic feet per minute. For many gensets there is an engine-driven ‘pusher fan type’ radiator mounted on the unit, and your enclosure should let the appropriate cooling and combustion air in and out, with a static drop that is acceptable.”

If the engine is cooled by remote radiator or by city water, the amount of heat rejected to the atmosphere of the location must be known. That heat (measured in Btus per minute) must be cooled by auxiliary cooling so that an acceptable temperature exists in the enclosure.

Fuel and its storage may be the most critical of your challenges when calling for a perfect enclosure for your power units. If your requirements are for what has been called a “daytank,” with a storage capacity of 100 gallons or thereabouts, you may opt to put the fuel tank inside the enclosure. That saves you and the daytank manufacturer the problem of weatherproofing. If, however, your fuel requirements are greater, you may consider having the tank as part of the enclosure base. It makes sense, especially from the space aspect. It is imperative, of course, that you check all local regulations about fuel storage.

We asked Witkowski what he thought of fuel-tank bases. “It saves space on the ground and can really simplify the piping scheme to and from the engine,” he comments. “Remember, however, that most communities have stringent requirements for onsite fuel storage, and your specifying engineers should address that issue at the outset of the project, because it is virtually impossible for packagers to maintain updated databases for every location.” Local regulations could require a separate tank rather than one in the base. The amount of fuel to be stored will decide the size of enclosures at the site. It is a consideration that should be given thorough thought at the earliest stage possible, then, because every manufacturer does not offer the same capacity.

The other common attribute of onsite power is electricity, and it deserves special attention in your enclosure planning, too. The physical dimensions of the equipment are crucial, and a concept that is particularly relevant to electrical installations is that of clearances. The clearances in front of transfer switches or breakers, for instance, must conform to standards. If there is rigging apparatus involved, you should allow enough clearance both overhead and in front. Watch those doors, too. Their swings should be accepted by the enclosure. “The cable entry and exit configuration is something to which the enclosure provider must give close attention,” advises Witkowski. “Even if the plan is not unusual, it should be coordinated with the manufacturer of the switchgear when design begins and confirmed before work on the enclosure starts. The most usual configuration when the enclosure contains freestanding distribution equipment is to cable overhead from the generator set into the top of the cabinet and have the field connections in the bottom. Conferring with the switchgear manufacturer will determine if any other arrangement is required at your particular site.” Heat also plays a part when electricity is onsite. How much heat will the electrical equipment send into the enclosure? Auxiliary cooling and heating may be required in certain weather conditions, and the design of the enclosure should help to avoid any accumulation of fumes from fueling, batteries, or maintenance. Check these aspects of the design with your enclosure manufacturer.

A Shelter Against Weather
To what extent do you wish to protect your valuable onsite power equipment against the weather? Each part of the country has its own seasonal weather challenges; it could be extreme heat here and wicked cold there. Do you want your enclosure to resist the weather or be proof against it? That’s an important difference and an important decision to make. If you are in a generally kind environment and all you need to do is to keep the rain—and perhaps occasional sleet and snow—away from the genset, you can achieve that with enclosures made by the manufacturers of the generator sets. Or you can find weather-resistant (or drip-proof) enclosures from an enclosure manufacturer. If, however, the conditions at your site demand protection against extreme weather conditions, you will need a weatherproof enclosure. Don’t just think of heavy rain. Your extreme weather could be rain, or it could include wind, seismic activity, or temperatures that are unusually hot or cold. Whatever the threat from the weather, the weatherproof enclosure will ensure that only the most negligible amounts of snow, rain, or sleet will sneak in, whether the generator set is operating or not. You can prescribe the levels against which you want to shelter your onsite power. Wind loading is usually given in miles per hour; roof loading is stated in pounds per square foot in those places where snow or ice accumulates; and rain penetration resistance is given in ounces or inches per hour.

We cannot stress too much the importance of the weatherproof nature of your enclosure, because our weather can be as hostile and destructive as any vandal, wherever we are located. There are certainly many locations in North America where one bad storm every five years could cause catastrophic damage, whether the storm is warm or cold, ice or tornado, or hurricane or torrential rain. An enclosure that can claim to be weatherproof will handle winds in excess of 115 miles per hour (that’s hurricane class) and a heavy fall of snow (more than 30 pounds per square foot) without being bent out of shape and rendered vulnerable. Some friendly words of warning here: The terms used by manufacturers vary in their details and coverage, so you should make sure that what you think of as weatherproof is what your chosen enclosure manufacturer thinks of as weatherproof. Underwriters Laboratories’ helpful listings refer to commercial building classifications or to the cover of the generator itself, so be quite clear about all the details and descriptions of what is offered for your enclosure.

This would be a good place to remind you that the best enclosures are engineered. They are not one-size-fits-all contraptions. The general description of a Pritchard Brown weatherproof genset enclosure (No. 2110) is brief: “The enclosure will consist of a roof, two sidewalls, two end walls, and an optional floor/underframe incorporating prepainted aluminum stressed-skin semi-monocoque construction.” On that description you cannot decide to take one next month and what color can we have? Following that brief description are several pages of details, and it’s the details in enclosure engineering that are important. To name a few for that same model, we could quote that it is a weatherproof, walk-in-style enclosure, rated to a wind load of 120 miles per hour, with a roof load equal to 40 pounds per square foot, a distributed floor load to 200 pounds per square foot, a rain test equal to 4 inches per hour, and a basic structure that meets all the seismic requirements of Zone 4 or equivalent. And there are still several more pages of engineering specifications so that supplier and customer get exactly what they want for the exact site under consideration. Your onsite power unit is one of a kind and should be respected as such.

Noise That Doesn’t Annoy
Noise is a complicated subject. It might sound simple enough when you hear it, but when you have to control it within legislated parameters it becomes a serious, complex problem. Decibels are scientific. The decibel is a logarithmic ratio and, as the requirement for attenuation of noise grows at your onsite power installation, the designer of the enclosure will see growth in the weight, size, and air-handling complexity—a growth that has been described as exponential by mathematicians who understand such terms. The cost will parallel that growth. Local authorities determine regulations about the greatest sound allowed in their communities, and they must be an early contact for everybody considering enclosures for onsite power sources. An interesting reflection of the feelings of consulting engineers concerning noise attenuation is that they are using the services of professional, expert acousticians more often. The acoustician can determine whether planned installations will meet regulations. “We only run this once a week for about an hour—unless there’s an outage,” comment some owners whose onsite power is chiefly for emergency use. That is the kind of situation than an acoustical expert can handle, along with questions about how much noise level may be reached at the property line and the effect that neighboring landscaping may have. Of eminent importance is that the decisions for sound attenuation should be based on accurate calculations regarding distance from the point source and not on formulae that are irrelevant to generator sets. Get an expert opinion.

“An aspect of choosing sound attenuation that is too often forgotten is that the enclosure will become larger as the attenuation increases,” advises Witkowski.

“The quieter we make the installation, the larger the enclosure must be,” he says. “This is more common when the kilowatt rating and airflow increase. You can see installations where there is as much enclosure space dedicated to air handling as to the equipment itself. It is imperative, therefore, to cooperate with an enclosure manufacturer to determine how the constraints onsite will affect sound attenuation and how much space you will need to achieve a specific level.”

One aspect of sound-attenuation is that the enclosure will become larger as the attenuation increases.

The enclosure can be more than an expensive box around a power source. Apart from the fundamental aspects, it may also be a shelter for electrical equipment, HVAC equipment, or air-handing devices.

Construction Materials
Steel and aluminum are the most popular materials for enclosure construction, but we should comment on different types and treatments of these metals. Stainless steel of the 300 series offers the best resistance to harsh chemical environments, and it does not need painting. But there is a high initial cost for 300-series stainless steel, and some potential users reject it for its weight and its appearance (which has been called too “industrial” for some locations). Four-hundred-series stainless has an acceptable initial cost, and it offers a long useful life. A perceived disadvantage is that this grade of stainless has a high carbon content and it requires painting to look its best and last a long time. The weight and longevity of painted steel are not the best, and it requires regular maintenance. It is, however, available at a low, initial cost, and it resists damage well at the site. It is also conducive to noise reduction.

When the steel is coated (such as with galvaneal, galvanized, or aluminum-coated) it is more durable than painted steel and does give a certain level of noise reduction. Its initial cost is more than painted steel but it offers advantages if the adhesion of paint is good. When the metal chosen is aluminum and prepainted at the mill, there is excellent longevity and an attractive glossy finish, and it offers a relatively low weight. Its initial cost is higher and it can require extra mass if you use insulation for noise control. One of its most attractive features is that it has proved a good surface in environments where salt is prevalent. When selecting the materials of construction for your enclosure, it is important to consider the life cycle of the equipment (in relation to the cost of both buying and owning) rather than just its initial cost.

The enclosure is more than a box around the expensive power source. Apart from the fundamental aspects already mentioned, the enclosure may also be a shelter for electrical equipment, HVAC equipment, or air-handling devices. There are always options. The feed from the building to the enclosure’s house panel should be post-automatic-transfer-switch (so that power is available at all times). Check the feed from the building for voltage, too. If a stepdown transformer is required, specify that, too. Your enclosure is also a good center for various fixtures and peripherals associated with power generators.

We could mention items like battery chargers, receptacles for lighting, and lighting for the maintenance crew at the enclosure. If you use fluorescent lighting, choose those with ballasts of low temperature, and all wiring and connections should be designed so that they can withstand the inevitable vibration.

There’s a lot to think of in enclosures, isn’t there? Compliance with local codes may be the most important consideration, whether you’re talking about the size, positioning, sound attenuation, or internal electrical items.

You can see why it is worthwhile working with an enclosure manufacturer who has experience in all these matters, wherever you are in the country, whatever the size of your onsite power installation.

A Huge Potential
Most enclosures for onsite power are not dramatic structures. Many of them are basically boxes that cover and protect. Now and then, however, a greater-than-usual challenge comes along, greater because of its sheer size, the sound attenuation required, or both. At an installation in Pennsylvania, the proposed enclosure for a 1,820-kW Spectrum Genset had to be big and quiet. The dimensions of the finished enclosure (from Pritchard Brown) show a length of 50 feet, a width of 15 feet, and a height of 30 feet. (The source noise of the power was 112 A-filtered decibels at 1 meter.)There were further complications for the designer/producer of the enclosure. There were strict dimensional constraints at the site; 100,000 cubic feet per minute of cooling air were required; the solution had to be factory tested. The latter is not a problem for most enclosures for good manufacturers, but can you imagine it for the dimensions mentioned above?

To many engineers, the sound attenuation would be the greatest challenge. “I’ve worked on projects like this for years, and am active in the acoustics community,” observes William R. Thornton, a consulting engineer in acoustics and vibrations at Cheswick, PA. “To my knowledge, this is by far the quietest 1.8-megawatt diesel installation anywhere!” Thornton told Pritchard Brown that measures were made at the property-line distance in the specification provided, and those measurements showed results that met or exceeded the requirements of the specifications. The specs stipulated noise levels at 45 decibels (A-filtered) or less at distances of 53 feet in all directions around the circumference of the standby generator. Measurements were also taken to compute the sound power levels of the unit. The purpose of the sound power level specification was to ensure that the property-line noise measurements were achieved (which they were). “This unit will comply with the noise ordinances of Pittsburgh when properly installed at its site,” concluded Thornton in his letter to the enclosure designers.

Among constituents in this huge onsite power enclosure were Quiet-Flo duct silencers, and WACO modified the factory fan to change the noise and airflow characteristics, coordinating the design with the genset manufacturer. There were four Silex combustion-exhaust silencers, and there was a 0.5-inch static pressure drop on the enclosure-handling portion. Of particular interest was the “dual box” design of this enclosure and the fact that it had to be packaged and tested before installation. A good reason for the testing being done at Pritchard Brown’s facility was because the ambient noise at the site was always higher than the required resultant level of 45 decibels. When ready, the specified unit was shipped as two modules so that there was minimal assembly required in the field. After studying the details of this installation, no one should think of an enclosure for an onsite power source as just a box, and the importance of having all the details accurate and clear before anything is fabricated or assembled becomes evident.

DE - May/June 2007