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The following article by Michael Witkowski, Pritchard Brown's Chief Operating Officer, is reprinted with permission from POWERLINE magazine:

What makes a good enclosure?

The past decade has seen unprecedented growth in the utilization of outdoor enclosures for standby power generation applications. As building costs have risen, the desire to dedicate large amounts of building square footage to generators, switchgear, etc. has diminished. At the same time, enclosure manufacturers have continued to improve sound attenuation capabilities, longevity and aesthetics. Further, more and more applications have arisen for remote installations in harsh environments. When coupled with the growing trend towards "turnkey" systems, the decision to package the generator set and its associated equipment in an enclosure and ship it directly to the job site seems to be a logical, cost-effective choice.

Shelters vary in size, complexity, air handling methods, materials of construction, and of course appearance. Despite these many variables, it is relatively easy to determine the best design criteria for a particular installation by analyzing a few basic parameters regarding the site requirements (and limitations) and the equipment you are enclosing.

The Site

First and foremost, one must have a clear understanding of the environment around the job site. Who and how close are the neighbors? What is the temperature range at this location? Are you near the ocean, subject to extremes of snow or wind, or perhaps in a desert environment with sand and heat to be addressed?

There will invariably be local ordinances regarding noise and fuel storage. Additionally, we have seen everything from building codes to community association aesthetics reviews affect standby power installation! By becoming familiar with potential obstacles early, you can often greatly simplify the approval process later on.

On the site itself, there are similar issues to address. How much space is available? What is the proximity to new or existing structures? How will conduits get to and from the enclosure- the most common method is from below, stubbed through the concrete pad and/or enclosure base, but sometimes they must enter over-head or coordinate with existing equipment, and this will affect the design.

Another aspect to consider is visibility. This will help determine the materials of construction and in some cases the level of vandal resistance required. An enclosure on the roof next to chillers and behind a vision screen takes on quite a different role from one situated right outside the chief executive officer's window on a groomed corporate campus!

The Equipment

With a clear picture of the world outside the installation, we can now focus on what will be inside the enclosure. Fundamentally, one must know three things about the equipment to be enclosed: Physical dimensions, weight, and environmental requirements. Where are the maintenance points and electrical or fuel connection points?

In the case of an engine generator, we further need to know how it is cooled and its total cooling and combustion air requirements, measured in cubic feet/min (CFM). Most commonly the genset has a unit-mounted engine-driven pusher fan type radiator, and the enclosure must allow the proper cooling and combustion air in and out with an acceptable static pressure drop. If the engine is remote-radiator cooled or city-water cooled, then we must evaluate the amount of heat rejected (measured in BTUs/min) to the ambient by all of the components and provide auxiliary cooling to maintain an acceptable temperature rise through the enclosure.

If the enclosure's purpose includes sound attenuation, then the source mechanical and fan noise under full load (in deciBels, Awtd, or dB(A)) must be ascertained. All of this data is routinely available from the generator set manufacturers. It is important to note at this point that dimensions, noise, and airflow requirements can vary greatly from manufacturer to manufacturer for a given electrical KW rating, so sizing the enclosure based on "worst case" specifications is often a good idea if more than one generator set supplier is being considered.

Another aspect of the system that should be considered during the early planning stages is fuel storage, as it can greatly affect the enclosure dimensions. If there is to be a daytank, up to about 100 gallons or so, it is often put inside the enclosure, thus eliminating the need for weatherproofing by the daytank manufacturer. "Slimline" or space saving options are available from many daytank suppliers, and can often simplify equipment layout. If, however, a larger capacity of fuel is to be stored above-ground at the site, then a tank built right into the enclosure base is a very logical choice. A fuel tank base conserves real estate and greatly simplifies the piping scheme to and from the engine. Specifying a containment basin and a U.L. listing for the tank is recommended, and certainly the fuel storage system you choose should meet any applicable local codes. Maximum capacity can vary between suppliers, and often the amount of fuel to be stored will dictate enclosure size, so these details should be addressed with the manufacturer(s).

For electrical control and distribution equipment, physical dimensions must again be known. In addition, clearance around this gear is equally important NEC clearances must be maintained in front of breakers, transfer switches, etc. Drawout type gear must be allowed enough clear space in front and overhead for any attendant rigging apparatus. Door swing requirements should also be included when laying out equipment cabinets within the enclosure In addition, the cable entry/exit must be coordinated with the enclosure scheme - the most common arrangement when free-standing distribution equipment is in the enclosure is to cable overhead from the generator set into the top of the cabinet, and have the field connections into the bottom. This should be coordinated with the switch gear manufacturer at the outset.

Finally, the required operating environment of the equipment must be considered, as well as how much heat the equipment will reject to the enclosure interior. Additional heating (coupled with motorized louvers or dampers) may be needed in very low ambient conditions to help maintain a reasonable temperature for equipment and maintenance personnel when the power generating system is in standby. Similarly, ancillary cooling may be required in high ambient environs to minimize heat buildup. This will also help prevent fume accumulation from batteries, fueling, and maintenance operations.

The Enclosure

After evaluating the intended site and equipment, we are now ready to address features about the enclosure itself and to select one that most closely, and cost-effectively, matches the customer's needs.

The most fundamental decision to be made concerns the level of weather protection required; that is, do we need weather-resistance or weather- proof? There are many instances where just keeping rain (and/or snow) off the generator set under anticipated weather conditions is all that needs to be accomplished. This can be done with "factory" enclosures supplied by the generator set manufacturer, or inexpensive "drip-proof" or "weather-resistant" enclosures from an enclosure manufacturer.

If, however, conditions at the site dictate that the enclosure withstand extremes of wind, precipitation, seismic activity, or temperature, it is important to insist on a "WEATHERPROOF" enclosure. "Weatherproof" also means that the entrance of rain, snow, sleet, etc. should be negligible during operation of the genset. Parameters such as effective wind loading (in mph), or roof loading (in Lbs/Ft2) in snow or ice-prone regions, and rain penetration resistance (in oz/in.rain/hr) can be used to compare potential manufacturers and to ensure a reliable system. A truly weatherproof enclosure will withstand hurricane-force winds (in excess of 115 mph) and a substantial amount of snow or ice (>3O lbs/ft2) without permanent deformation. As with many aspects of power generation equipment, the terms can vary from manufacturer to manufacturer, so simply specifying "weatherproof" won't guarantee compliance with the above criteria.

The next basic decision to make is whether or not sound attenuation is required, and if so, how much. We must determine this early on, as it will often dictate the enclosure size, air-handling choices, and even the materials of choice for construction.

It is helpful to remember that the fundamental unit of sound pressure measurement, the decibel (dB), is a logarithmic ratio. To the enclosure designer, this simply means that as the amount of attenuation required increases, the relative size, weight, air-handling complexity, and hence cost, tend to grow exponentially! In light of this, it is important to ascertain the true site noise requirements from the start. Most communities have ordinances regarding maximum permissible sound levels at the property line, but it is sometimes unclear how a standby generator set, which runs one hour a week for exercise or during the occasional utility power outage is defined as a noise source. A telephone call to the appropriate authorities can often shed some light on this question and affect the budget accordingly.

Of course, prime-power and co-generation applications have a greater noise impact on the environment because of their extended periods of operation. If a particular noise level at the property line must be achieved, then the enclosure manufacturer should be told what the level is and how far the nearest property line is from the enclosure location. Additionally, the manufacturer should know the layout of surrounding buildings and the general topography. If there is a large structure nearby, a grass-covered berm around the site, or an asphalt parking lot leading to the neighbor's yard, these can greatly influence the propagation of sound and hence the design.

A word of caution would be prudent at this point: because sound is a wave phenomenon, there is a mathematical rule, the inverse square law, which is often applied as a "rule-of-thumb" to determine the effects of distance on sound level. Without exploring all of the underlying theory, the inverse-square law simply predicts that for a point-source of sound under freefield conditions the sound level will decrease by 6 dB each time the distance from the source is doubled. For example, if we measure 100 dB(A) at 5 feet, then we would measure 94 dB(A) if we moved to 10 feet. Unfortunately, this rule does not work for generator sets unless we begin about 30 to 50 feet away. Again cutting conveniently past the theory, this is simply because until we get 30 to 50 feet away, a large internal-combustion engine is not a point source and we are not in the "free-field". Figure 1 shows a useful comparison of typical empirical results from site noise tests vs. predicted behavior. Beware of any manufacturer that uses this inverse-square-law within 30 feet or so of a generator set or generator set enclosure.

When there is no specific level to be met at a given distance, then it is common to specify only the amount of attenuation required by the enclosure itself. Many manufacturers therefore "standardize" on certain levels of attenuation at some predetermined distance, e.g. 10 dB(A) reduction @ 10 feet, or 25 dB(A) reduction @ 1 meter, and so on. These numbers usually represent an average of the reduction measured at various points around the enclosure. Verify that the design is such that there are no points at which the actual attenuation is more than 3-5 dB(A) less than the promised average. For example, if the radiator discharge air is not treated properly, the measured resultant sound level at that end of the enclosure may be unacceptably high, even though the average attenuation meets the design criteria.

Subjectively, any amount of sound attenuation will alter the engine noise and yield a more tolerable deadened sound and a characteristic rush of air, rather than the "rap" of an engine racing at 1800 rpm. By defining a performance specification, the type of insulation, air handling, etc. can be left to the enclosure manufacturer.

Perhaps the most overlooked aspect of choosing sound attenuation is that as the amount of attenuation increases, i.e. the quieter we make it, the larger the enclosure will necessarily become. This effect is more prevalent as KW rating, and hence airflow increase. It is not unusual to see as much (or more) enclosure space dedicated to air handling than to the equipment itself as the attenuation approaches the -4OdB(A) range, which is generally considered to be the maximum economically feasible reduction by a pre-fabricated enclosure. One should work closely with an enclosure manufacturer to determine the most attenuation available given site constraints or, conversely how much room will be required to achieve a particular level.

We have only scratched the surface of the often complex subject of sound attenuation - the reader should refer to the previously published series in Powerline, or chapters 22 & 23 of On-Site Power Generation: A Reference Book, 2nd edition, published by EGSA, for a more in-depth study.


Now that we know how sturdy and how quiet, it is time to consider what is the best material of construction. Table 1 lists the six most common enclosure materials, and their attendant attributes and deficiencies.

It is important to consider the balance between the initial cost for different materials and the long-term costs associated with maintenance, as well as the visibility of the installation and geographical location.

The Details

Addressing items such as shelter electrical equipment and distribution, air handling devices, and HVAC equipment will complete our exercise in enclosure design.Although these issues are less prominent than some previously discussed,they are a major contributor to the overall effectiveness of the system.

In the case of electrical distribution,there are a number of viable options.Note that the feed to the enclosure from the building 's electrical system should be from the load side of the A.T.S. so that power is available at all times.If the feed from the building is not 120/240 V or 120/208 V,it is also important to specify a stepdown transformer (with primary service disconnect). Within larger enclosures,it is common practice to provide lighting and convenience receptacles wired to a load center along with the generator set electrical accessories,namely jacket-water heaters and the battery charger.Although fluorescent lighting provides a better work environment for maintenance personnel,vaportight incandescent fixtures are often specified due to their inherent durability in the "industrial "environment of an equipment enclosure.Low tem perature ballasts should be used if fluorescent lighting is chosen.The wiring itself should be surface-mounted in E.M.T.or rigid galvanized conduit,and all connections to the generator set must be through flexible conduit,as it will vibrate during operation.Once again,care should be taken to ensure compliance with local codes;some jurisdictions require rigid galvanized conduit,for example.Battery backup emergency lighting or D.C.lights powered from the genset starting batteries are a good idea,as maintenance will almost surely be required if no A.C.power is available to the shelter!Use a timer type switch in the case of D.C.lights,thus preventing inadvertent draining of the all-important starting batteries.A general rule of thumb is to provide approximately 30 foot-candles of illumination at floor level for routine maintenance.If there is freestanding distribution equipment such as an output circuit breaker or an A.T.S.,it is routinely installed and prewired by the enclosure manufacturer.

A number of other miscellaneous items are often seen as part of the electrical package.These include,but are certainly not limited to:exterior lights with photocell control,horns and/or beacons to signify a fault or running condition, fire detection devices,intrusion alarm devices, battery heaters,generator strip heaters,and enclosure heaters,fans,or air conditioners.

Air handling and HVAC options are also varied, but fairly easy to define based on ambient conditions and equipment requirements.In very cold climates,the goal is usually to close off the enclosure,insulate the walls and roof,and provide auxiliary space heat to keep things warm under standby conditions.In tropical environments it is common to keep the enclosure ventilated and add an auxiliary fan with a thermostat. The right combination of air handling devices with or without auxiliary heating or cooling and enclosure insulation will yield an effective system regardless of ambient temperatures.

Air entry points are either fixed-open;as in the case of punched louvers,weather and/or acoustic hoods,acoustic baffle assemblies,or fixed louvers,or operable;such as motor actuated,pivot blade louvers or dampers.[By definition,a louver fully opens to less than 90 degrees and a damper opens to 90 degrees,providing greater airflow for a given area ].These devices can be combined if necessary.For example,one could have an acoustic hood with an internal damper or motor-operated louver with internal acoustic baffles,etc.

The important consideration is to keep the air velocity low enough to maintain approximately 0.25 "w.g.static pressure drop across the air intake.This will limit water aspiration and ensure that the generator set receives adequate cooling air.Because different devices have different aerodynamic properties,it is important to specify this desired static-pressure drop performance rather than actually size the air handling devices for the enclosure manufacturer.The devices supplied,whether manufactured by a third party or built by the enclosure manufacturer, should be A.M.C.A.(Air Movement &Control Association)certified.

Screening should be specified for the air intake devices to discourage vandals and vermin,and prohibit the entry of sizable debris.In areas with a large insect population,special "insect screen "should be used.

The air discharge can be treated somewhat differently,because air is being pushed out of the enclosure rather than pulled in.In addition to the options available for intake,"gravity "or "barometric "operated dampers are often a wise choice on the discharge -they close by their own weight under standby conditions,and are held open under very little resistance by the discharge airstream.Another choice that is available in weather-resistant applications is a simple screen in front of the radiator or auxiliary fan.In the case of a sound hood,we now have the option of discharging the air (and noise)upward -this improves acoustic performance. Again,the discharge air hand ling system should provide less than 0.25 " sta tic pressure drop so as not to overburden the cooling fan.


Despite the myriad of issues we have addressed, the custom nature of enclosures makes it difficult to touch on every aspect of a Po tential design in one art icle.Hopefully the reader has gained the tools necessary to work with an enclosure manufacturer to develop a system that is reliable, cost-effective,and ideally suited to a particular installation.The crucial site and equipment evaluation phase,though it may seem an obvious first step,is too often overlooked.If we consider all of the factors that affect our design early on,the various decisions during the design process will proceed much more smoothly.Finally,bear in mind that the enclosure,although usually not the most expensive element of the system,is the most visible part of an outdoor installation and should be chosen accordingly.