A Reader Writes:
As we pack more controller power, drives, I/O, even operator displays in our cabinet designs, we're getting reliability questions from potential customers. The ambient temperatures at some customer installations can be as much as 90° F. Fan/filter vendors tell us we're still OK with what we have. Vendors offering various types of heat-exchange tell us differently. We even hear that just changing to a stainless steel enclosure from plastic would make a big difference in heat dissipation. Any suggestions?
--from June 2003 Control Design
Non-Metallics and Heat Don't Mix
We have switched back to stainless steel enclosures after trying non-metallic enclosures. We experienced severe over-temperature problems. The non-metallic material did not conduct the heat load to the atmosphere as well as the stainless steel enclosure.
...Hoffman has a free software package for determining the size of enclosure based on the heat-producing components in the panel and also takes the environmental and mounting considerations in consideration. We have used this a baseline for our sizing and have not had any difficulties with overheating.
Tod Jester, Director of Engineering/Manufacturing
LMC International, Elmhurst, Ill.
Vortec Has Advantages
One way to cool your cabinet is to use a Vortec or similar design cooler. These coolers run off compressed air. It is positive ventilation, so it keeps your panel clean. Through some testing, we found filter fans allow contaminants into the panel. The Vortec unit is very low cost to operate, since it does not run off of electricity, but rather low air pressure. To increase the cooling you increase the air pressure.
...Even though it uses air, it does not create moisture. The unit is maintenance-free and does not have filters to replace. It is also easy on the eyes--it does not affect the appearance of the machine. The Vortec unit is small and easy to hide. Installation is also very easy.
Alexander Doods Co., Grand Rapids, Mich.
Use FEA and CFD
The right way to answer the question is by an analysis of the particular system for heat transfer, heat generation, and, if appropriate, air flow. The analysis could potentially include conduction, convection, and radiation considerations. It should include the specifics of geometry, orientation, load, and ambient conditions. It would then give the operating temperatures for the components in the cabinet.
...This type of analysis can be done by finite element analysis and by computational fluid dynamics (FEA and CFD). Creating a general model of the system would allow various case studies to be performed, such as the impact of ambient conditions, the impact of changing the power level of components, the impact of changing materials of construction, and the impact of various fan changes or other cooling methods. The calculations could be benchmarked against existing data and be used to reliably predict performance under other conditions.
Jack Kleinfeld, P.E.
Kleinfeld Technical Services, Bronx, N.Y.
Fan/Filter vs Heat Exchanger vs Air Conditioner
If the ambient temperature is as high as 90° F, the opportunity to use fans/filters is limited. We recommend avoiding temperatures above 95° F for electronics (PC boards, micro processors, etc.). Studies have shown product longevity declines and failures rise when operated at or above this temperature.
...With this said and the ambient temperature being 90°, that provides only 5° F (T) to work with. For very light loads, a fan/filter may work. The amount of air that will be needed to remove 5° depends on the amount of heat that is being produced. For a small heat load of 100 W, 66 CFM is needed.
...Air Flow/T/heat load relationship--The greater the T the less air flow you need to remove an equal amount of heat. Lower the heat load and you can lower the airflow for the same T. Lower the T, then you must increase the air flow for the same heat load.
...Fan/filters vs. heat exchangers--Fan/filters and heat exchangers can never cool below the ambient temperature. Both also follow the Air Flow/T/heat load relationship above. There is little difference in performance between the two. Heat exchangers are closed systems--no outside air is introduced to enclosed components.They are useful where ambient air will cause contamination to enclosed electronics, which may result in failure. Heat exchangers are typically 5-10 times more expensive than fan/filters, but they don't need as much regular maintenance.
...Heat exchangers increase the surface area of an enclosure. The air flow over this surface area provides a heat transfer from inside the enclosure to the ambient air. Heat exchangers can meet UL Type 4, 4X, 12, and 3R ratings.
...Fan/filters are open systems--ambient air flow is clean enough to be used as a means to pull heat out of the enclosure. Filters--requiring regular cleaning--are used to ensure the air is clean. Fan/filters are typically rated to UL Type 1 and in some cases Type 12. UL Type 12 fan/filters require even more maintenance, have significantly reduced air-flow performance, and have filters that are not reusable. If filters are not kept clean, air flow is reduced and in some cases eliminated. This ultimately leads to equipment failure.
...Air conditioners should be used with higher internal heat loads and where ambient temperatures are above 95° F. If the ambient temperature is above 95° F (or the max temperature the equipment can work at), air conditioning is the only alternative. Air conditioners are closed systems and can be used in UL type 12, 13, 4, 3R and in some cases 4X. They cost 10 times more than fan/filters. Air conditioners can cool the enclosure to below ambient temperature and dissipate far more heat than heat exchangers and fan/filters.
...Plastic vs. stainless steel--The heat that is generated inside the enclosure is dissipated radiantly. The components inside are typically mounted on a panel using stand-offs. This means that there's no conductive heat dissipation. The enclosure must absorb the heat via the surrounding air between. More important than the enclosure material is the surface finish and its ability to absorb heat. An inside surface finish that is flat and dark will absorb heat the best. Using a stainless steel enclosure with a shiny reflective surface could result in low heat absorption. The reflective nature of the interior surface and the resulting heat absorption characteristic is known as its emmissivity factor. Curiously, aluminum is a great conductor of heat, but is very poor at absorbing heat radiantly. Changing from a plastic enclosure to a stainless steel enclosure may result in increased temperatures due to the reflectivity of the inside surface finish of the stainless. The material make-up has less to do with heat dissipation than the inside surface finish and color.
Brian L. Mordick, Product Manager, DataCom and Thermal Products
Hoffman Enclosure Co., Anoka, Minn.
Simple, but Effective
Here are some alternatives:
1. Compressed air cooling if available.
2. Blow room air across the cabinet during hot weather.
3. There are many other alternatives. Cabinet coolers are a possible choice, but expensive.
Mike Goswami, PE, Plant Engineering
John Deere Engine Works, Waterloo, Iowa
Fans with filters and heat exchangers can be effective only if there is a temperature differential between the cooling medium and the air inside the cabinet. Changing the material of the cabinet to some sort of conductive steel to radiate the heat has the same problem. There is not a sufficient temperature differential between the ambient environment outside the cabinet and the atmosphere inside the enclosure.
...Cooling the enclosure with cold air is the best solution since ambient temperatures will be 90[deg] or higher. A maintenance free option our flashlight-sized cooler that mounts to the outside of the enclosure and operates by using a vortex tube to create a chilled air stream from an ordinary compressed air supply. These coolers have no moving parts, eliminating the high maintenance factor associated with the other cooling options. EXAIR Cabinet Coolers have been UL tested to meet NEMA 12, 4 and 4X environmental requirements and can operate continuously or on thermostat control depending on the customer's requirements in an application.
Neal Raker, Application Engineer
EXAIR Corp., Cincinnati
We Need Calibration Tools
Our after-sales support package is evolving to include responsibility for calibration of the instruments on our customer-located machines. It appears to be a lot of work for our technicians. We're thinking about computerized calibration software to help identify trends and establish calibration intervals, and "smart" devices that make better use of the machine network and provide information about their own health. We'd like some advice on how to establish basic guidelines.
Send us your comments, suggestions, or solutions for these problems. We'll include them in the February 2004 issue. Send visuals, too,a sketch is fine E-mail us at CDTheAnswer@putman.net or mail to The Answer to Your Problems, CONTROL DESIGN, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143. You can also fax to 630/467-1124. Please include your company, location, and title in the response. Have a problem you'd like to pose to the readers? Send it along, too.