What to Consider When Selecting An Enclosure

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Enclosures can help protect your hardware from exposure to harsh environmental conditions such as extreme temperatures, dirt, dust, water, or soot. Unfortunately, choosing a sealed metal box and mounting the hardware in it can be difficult. You need to consider self-heating from the hardware, airflow, cable management, heating and cooling, connectivity in and out of the enclosure, and clearspace requirements to ensure the hardware performs to its specifications.

IP and NEMA Certifications

Enclosures are most often used because a DAQ system needs protection from the elements in its environment. NEMA in the United States and IP internationally are two standards that specify the amount of protection from dirt, dust, and water that an enclosure should provide. Choose an enclosure with the appropriate rating to defend your system against the environment you plan to deploy it in. Often standards specify a level of IP or NEMA protection required to operate electronics in certain environments. Table 1 summarizes the most common levels of NEMA and IP protection for enclosures.

 

NEMA Rating

IP Rating

Protection against large foreign objects and access to interior parts

1

20

Protection against large foreign parts and dripping water

2

22

Limited ingress of dust, and protected against water spray from any direction

12

54

Dustproof and resistant to water sprayed from a jet

4

65/66

Dustproof, resistant to water sprayed from a jet, and resistant to corrosive agents

4X

65/66

Dustproof and waterproof to 1 m under water

 

67

Table 1. Most Common Levels of NEMA and IP Protection for Enclosures

Temperature Range

Your CompactDAQ or CompactRIO system is rated and specified to ambient environment temperatures between either -40 ºC to 70 ºC, -20 ºC to 55 ºC, or 0 ºC to 55 ºC, depending on the chassis or controller. To maintain the performance and accuracy specifications of your system, you need to ensure that the air inside of an enclosure as measured 1 in. from the side and 1 in. from the rear cover of the chassis does not go beyond the specified temperature ranges.

If the self-heating of the components inside your enclosure or the air outside the enclosure cause the inside of the enclosure to exceed the specifications of your system, then you need to rely on a heating or cooling system to maintain a proper environment for the functionality of your system.

In especially cold environments, typically outdoors during the winter, you may need to provide heating for your enclosure. This is often as simple as putting an electrical strip heater at the bottom of the enclosure.

When enclosures are placed in the heat outdoors, directly in the sun, or even in unconditioned indoor environments, some amount of cooling is often required to maintain an appropriate internal air temperature. Sun shielding and reflective paints can help mitigate the amount of heat transferred to the enclosure, but often you need additional cooling using one of three methods: natural cooling, forced air convection, or closed-loop cooling.

 

Figure 1. Mitigate the amount of heat transferred to the enclosure using natural cooling, forced air convection, or closed-loop cooling.

 

Natural Cooling

Natural cooling uses the enclosure itself to dissipate heat with either conduction through the surface of the enclosure or with convection through the surrounding outside air. If the enclosure is made of a heat-conducting material and mounted to another conductive surface, a significant amount of heat can be conducted away from the enclosure itself. Additionally, if the enclosure does not need to be sealed, then you can use louvers or grilles to passively circulate cool, outside air into the enclosure.

Forced Air Convection

If natural cooling alone is not enough to adequately maintain the internal air temperature of your enclosure, then you may need to rely on forced air convection. This uses fans to force cool, outside air into the enclosure. Enclosures that use this method of cooling are by definition unsealed because they need to bring in air from the outside environment. It is crucial to calculate the amount of airflow, usually specified in cubic feet per minute (CFM), to maintain an appropriate air temperature inside the enclosure. You can use the following formula to accomplish this:

CFM = cubic feet per minute
Pwatts = power to be dissipated in watts
Delta T = maximum allowable internal enclosure temperature °F – maximum outside ambient temperature °F

CFM = (3.17 x Pwatts) /Delta T °F

Closed-Loop Cooling

If your system requires a sealed enclosure and you need to dissipate more heat than natural cooling alone can accomplish, then you should invest in a closed-loop cooling system, which is essentially air conditioning for your enclosure. A closed-loop cooling system is usually specified in BTU/hour (BTUH) and is sized based on the maximum specified power dissipation of the components inside the enclosure plus any heating that could occur from the outside environmental air being hotter than the air inside the enclosure. The following formula is generally used to determine the size of your cooling system:

BTUH = BTU per hour of the cooling system
Pwatts = power to be dissipated in watts
M = material constant (use 1.25 for metal and 0.62 for plastic)
SA = surface area of the enclosure exposed to outside air in square feet
Delta T = maximum outside ambient temperature °F – maximum allowable internal enclosure temperature °F

BTUH = (Pwatts x 3.412) + (M x SA x Delta T)

 

Enclosure Size and Clearspace Requirements

When choosing an enclosure, you need to ensure that all of your system components not only fit but also have the proper amount of clearspace around them. Most electrical products require some free space around them to allow air to circulate so they can properly dissipate heat. These requirements are usually specified in manuals or specifications documents.

Additionally, you should allow internal clearances for bulkhead connectors, backshell connectors, and wiring. Keep in mind that certain cabling, such as fiber optic, has a minimum allowable bend radius to maintain proper performance.

Finally, you should keep upgradability in mind when choosing an enclosure size. Are there any components you might need to add or upgrade in the future? Planning some extra space for expansion or upgrades now could save you significant headaches down the road.

 

Material

Most engineers automatically jump to using metal for their enclosures, but you can choose from several other materials, depending on your application requirements, that produce cheaper and more flexible enclosures.

Plastics

Plastics are lightweight and easy to carry while generally being corrosion resistant. They also can be transparent if you need to see inside your enclosure without opening it. In addition, you can easily modify, drill, and mount them without needing extra tools. The main disadvantage can be cost if you require a large or custom enclosure size because tooling and molds can quickly become expensive. Additionally, plastics can be flammable, and certain locations may require you to choose self-extinguishing plastic materials.

Fiberglass

Also a lightweight and corrosion-resistant material, fiberglass is generally stronger than plastics. This added strength does come at a higher price; fiberglass enclosures generally cost twice as much as plastics. Fiberglass is also very expensive if you need a custom or large enclosure because of the cost of tooling, but it can be an effective choice for a durable product, and it is still cheaper than steel.

Aluminum

Aluminum is a rugged and lightweight material option. It is easy to modify and wall mount, but it generally is not as attractive as some of the other material options because it scratches easily. Aluminum is also expensive to tool up for custom sizes, so you should try to fit your hardware in a standard enclosure size.

Steel

Steel is the most traditional enclosure material because of its durability. Unlike most other materials, it is fairly easy and cheap to make into custom-sized enclosures. Steel also offers several finishing options including stainless, which is not affected by rust or corrosive elements. Steel does tend to be more expensive than other enclosure materials, but for that price, you get a durable enclosure that can take a lot of physical abuse.

 

Passing Wires Through the Enclosure

Most systems are not entirely contained within the enclosure itself. Typically, they have to interface with the outside world, which means wires and cables need to pass through the walls of the enclosure. You can choose from two common ways to accomplish this: bulkhead connectors or glands.

Bulkhead Connectors

Bulkhead connectors are mounted to the wall of the enclosure. On the inside, they are wired to your system, and, on the outside, they feature a standard connector like BNC or LEMO that you can connect to a standard or custom cable. You can choose bulkhead connectors to maintain the IP or NEMA rating of the enclosure; most manufacturers offer dustproof and waterproof connector options. You generally need to know their purpose prior to purchase so you can select the appropriate number of pins for the connector.

Bulkhead connectors work well if you need to swap out cabling from your system regularly because they allow for easy connect and disconnect access.

Glands

A gland allows a cable or wire to pass through the wall of an enclosure uninterrupted, without having to use a connector. Glands offer passage to any type or number of cables through the wall of an enclosure, but this flexibility comes at the expense of connectivity ease. To switch the wires passing through a gland, you need to open and rewire the enclosure. This is in contrast to bulkhead connectors, which you can swap without opening the enclosure.

 

Conclusions

You need to consider many factors when choosing an enclosure for your DAQ or control system. The type of environment you are deploying your system in determines your NEMA ingress levels as well as your heating and/or cooling requirements. You also need to consider cost, weight, and durability when selecting your enclosure material. And, finally, you need to decide if you prefer the flexibility of glands or the easy connect/disconnect nature of bulkhead connectors to interact with the world outside your enclosure.

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