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System designers frequently don't consider cooling / thermal design aspects in their new system designs. With today's ever more complex systems, thermal considerations are all important to ensure reliability. This page attempts to set the stage for successful thermal management of these new systems. Our thanks to Dr. Maurice J. Marongiu for contribution of the very interesting article featured on this page.

  Contents of This Page

Feature Article: Thermal Management of Outdoor Enclosures
Thermal Management Links
Reference Books on Thermal Management

Feature Article


Maurice J. Marongiu, Ph.D., P.E.
MJM Engineering Co.
1163 E. Ogden Av., Suite 103-077
Naperville, IL 60563
Phone: +1 630/653-5452 FAX: +1 630/245-1218

originally posted 1997

1.0 Introduction: The Importance of Thermal Management

The pace of development in the electronic/electrical/telecommunications fields is accelerating in all aspects of the business. In particular, most processing equipment is continuously increasing in capabilities, and more importantly, heat dissipation. The problem starts at the sub-component level and grows to the system level. The equipment often must be housed in enclosures that more often than not will be placed outdoors.

Electronic/telecommunications equipment has traditionally been housed in large buildings, smaller buildings (sheds) and outdoor enclosures. The cooling of these facilities has been carried out by and large using traditional methods. However, in many new systems being developed and deployed in near future such as Broadband ISDN, cellular and/or cable, heat dissipation densities will increase substantially [1].

In the telecommunications fields, the introduction of the electronics, furthermore, into the outside plant has imposed serious constraints on enclosure design since temperature and humidity are the two major causes of electronics failure in the telecommunications industry. Since most telecommunications electronic systems do not include environmentally hardened designs, the enclosure then must provide an environment in which they can survive. This task, therefore, is undertaken by the telecommunications engineer [2].

To meet the demands of protecting all type of temperature/moisture sensitive equipment in the outdoor environment, designers have been pushing for sealed designs. These allow for little or no exchange of enclosure air with the outside air. This is accomplished with near airtight construction, high-pressure door closure designs and closed loop thermal systems.

The thermal management of the outdoor enclosure thus becomes an issue of paramount importance. It is not only necessary to remove the internal heat generation from the equipment but also the effects of the solar loading which can be substantial depending upon the size of the enclosure and its orientation towards the sun.

Another important design issue is cooling device selection and its impact on power, battery back up and maintenance. Full active systems (like air conditioners) require AC power, are impossible to back-up efficiently and require lots of maintenance. Assisted systems like (air-to-air heat exchangers) run off DC power, can be backed up, and typically demonstrate a longer maintenance interval. Passive systems, on the other hand, require no power and are maintenance free. The choice of thermal management systems therefore requires a balance/trade-off between heat load and cost.

2.0 Thermal Management Techniques

The design and development of thermal management systems for all electronics components and systems rely on using the three major approaches in thermal fields: analytical, experimental and computational. Analytical approaches involve the development of mathematical thermal models in which the design engineer can obtain answers to his/her cooling problem. Most often analysis will provide the starting point for the creation of the management system--it provides order of magnitude and general information. The experimental method is normally used to obtain either full thermal performance data on components (chips or boards) or systems (enclosures), or it may be used to obtain key empirical data that would be used either in an analytical model or in a computational simulation. Finally, the computational approach involves the solving the equations governing the thermal phenomenon using numerical techniques. This implies nowadays the use of highly sophisticated CFD (Computational Fluid Dynamic) software packages from commercial vendors. Increasingly, analysis that in the past was routinely carried out to size and match cooling system is now performed using computational techniques--especially as systems become more complex.

3.0 Thermal Management of Specific Telecommunications Components and Systems

In the telecommunications fields one may encounter the entire range of cooling/heating problems encountered in all the electronic/electric systems. At the subcomponent level, electronic devices are packaged in chips which then set boards that then form racks. These are then housed in smaller enclosures/chassis. These individual components such as signal filters, rectifiers etc. will be placed in larger enclosures. Depending on the application, these units can be placed indoors, but increasingly, they are being installed outdoors as self-contained units with battery backup.

Since space in this paper is limited, the discussion here will now be confined to the management of outdoor enclosure design and auxiliary equipment: air conditioners, heat exchangers and battery back-up units. One should understand that the thermal management of subcomponents such as chips, boards and racks is covered extensively in the literature and the reader is kindly referred to the work by Peterson and Ortega [3]

3.1 Thermal Management System Design

Outdoor enclosures are being designed to house various equipment configurations with dissipating heat rates raging from 500 up to 10000 W, depending on the size and type of equipment. These enclosures are being installed in various environmental conditions, and typically the enclosures, without major structural modifications, should be fitted with either air conditioning or air-to-air heat exchangers as needed.

The goal of the designer is to maintain the peak temperatures in the enclosures below a certain level that is normally prescribed by the electronic equipment manufacturer. Humidity levels are of concern, but since most enclosures are either sealed or its temperatures are much higher that the air's dewpoints, humidity is generally not a problem (after the transient effect of opening/closing the enclosure is eliminated.)

The designer should be aware that the air temperatures within the enclosures will be a function of [4]:
  1. Amount of heat generated by all the electronic equipment in the enclosure.
  2. Amount of heat generated by auxiliary and cooling equipment (fans, etc.)
  3. Ambient conditions (outdoor air), particularly temperature, solar radiation, wind speeds, etc.
  4. Objects surrounding the enclosure (shading, ground reflections, buildings, trees, etc.)
  5. Enclosure design (surface area, shape, paint's radiation characteristics, etc.)
  6. Air exchange with the outside air, either passive by infiltration, or active, by fans or blowers.

Typical environmental conditions:

External (ambient) temperature range: -40° C to 55° C
Equipment Chamber temperature range (if air-conditioned): 20° C to 30° C
Equipment Chamber temperature range (if air-to-air h-ex): 50 to 75 °C
Optimum battery temperature: 25° C

Often, in telecommunications systems, battery back-up units are stored within the enclosure but in separate compartments. The battery compartments must be vented so that harmful fumes can escape. Enclosure design must ensure an evenly distributed battery temperature and the batteries must be kept as much as possible at 25° C. Typically, batteries are not actively cooled (only heated); however, there are some enclosure makers that have begun installing active cooling systems for battery trays [2].

3.2 Cooling/Heating Load Calculations for Enclosures

We are now ready to begin the design of the thermal management system for a typical enclosure. Let us say that the enclosure to be designed is typical for the business, measures 2 m high and has a footprint of 3 m x 0.75 m. The enclosure has installed equipment that dissipates, say 1500 W, and must be kept at either the air-conditioning conditions or the air-to-air heat exchangers. The first step is always to realize that the design temperature is that temperature that the enclosure air will attain when there is heat balance, or in equation form:

[Editor's Note: Equation is missing.]

where, Qequipment is the electronics heat dissipation, Qsolarload is the solar heat load and Qcoolingsystem is the amount of heat removed by cooling system. The solar load is a complicated term because it includes contributions from all modes or heat transfer. For example:

[Editor's Note: Equation is missing.]

Normally, the value of Qradiated will always be positive (towards enclosure) but the other two can be either positive or negative, depending on the enclosure's temperature. Thus, if Qbalance is not zero, this means that the temperature inside the enclosure is either higher/lower than the set temperature and the enclosure is losing/gaining heat by convection and conduction.

There have been considerable discussions, within the telecommunications industry for example, as to the best way to account for and test the effect of the solar load. Bellcore [5] used to recommend applying 70 W/ft2 (754 W/m2) 100% to the top and 50% to the sides. Now it recommends utilizing the ASHRAE Sol-air method, which takes into account some convection and reradiation effects[6]. IEC recommends, on the other hand, using the sol-air temperature but with higher incident radiation values [7].

Furthermore, since incident solar radiation varies during the daylight hours, the designer must decide whether to conduct a steady state or transient analysis. Moreover since Qradiation is a very complex term that includes, among other effects, solar declination, latitude, time of year, solar azimuth, atmospheric absorption, atmospheric clearness, reradiation from other walls, buildings, ground etc., incident wall surface properties, some simplifying measures must be taken into account [8]. The result is that one can effectively double or triple the amount of heat flux being added into the enclosure depending on the calculation method. The calculation of the cooling load is carried out using two methods. These methods are the ASHRAE's cooling load calculation methods. Normally, when calculating cooling loads, one would include a) Space heat gain, b) Space cooling load, and c) Space heat extraction rate [6]. Space heat gain is the rate at which heat enters or is generated within the space at any given instant. This includes heat transferred into the conditioned space from the external walls and roof due to solar radiation, convection and temperature differential.

One normally includes instantaneous solar radiation effects and delayed effects. The delayed effects include the slow build-up of energy that the external walls accumulate as they absorb solar radiation. This happens because walls are normally thick and massive, making energy absorbed important. For telecommunication enclosures this is not included since its walls are thin (at the most 3 cm when insulation might be added). Another component of heat gain is latent heat due to moisture infiltration. For most sealed outdoor enclosures, the power electronics are kept in an airtight enclosure with negligible contribution.

One method, the Sol-air temperature, involves calculating heat loads using an external temperature that lumps radiation effects and sensible air temperature. The second method uses a modified Sol-air approach. Both methods provide satisfactory results. No attempt at taking into account solar inclination and radiation intensity variations during a day-cycle is made. One must understand that the enclosure's solar load is calculated for the worst condition.

The Sol-air Temperature method involves calculating heat loads using an external temperature that lumps radiation effects and sensible air temperature. This is expressed as [6]:

Te = Tout + a It/ho - e DR/ho

where 'a' is absorptance of solar radiation surface, 'It' is total solar radiation [W/m2], 'ho' is the coefficient of heat by long wave radiation and convection [W/K-m2], 'e' is hemispherical emittance, and 'DR' is a radiation correction factor [W/m2]. Some sample conditions are:

For roofs: DR =63 W/m2, for walls: DR = 0, for dark surfaces, a/It = 0.052, which is the maximum value for any surface. To calculate heat transfer into the conditioned space,

Qsolar-load = U A (Te - Tin),

where 'U' is the overall heat transfer coefficient for the wall and 'A' is the surface area for the wall. The term 'U' includes convective and radiation effects by the internal and external airflow (See AHSHRAE's Fenestration Chapter for more details [6]) and the wind outside, in addition to conduction through the walls. All walls are insulated, say by a 1 inch layer of polyurethane of thermal conductivity of 0.026 W/m-K. Thus this value will be U ~ 1 W/m2-K. The total solar load will then be around 500 W, and the total load that the cooling unit must dissipate will then be 2000 W (Qsolarload + Qequipment). The area here refers to the 3 surfaces that can be illuminated simultaneously, with the roof always included.

The second method calculates the heat load using the following expression:

Qsolarload = U A a It / ho + U A (Tout - Tin ) (for maximum heat in)

From which the amount of heat to be dissipated is again around 2000 W.

Calculations must be carried out for the two design conditions for which either an air conditioning unit (refrigeration) or a heat exchanger (air-to-air) is needed.

3.3 Typical Cooling Systems for Enclosures

  1. Fully Active: Air-Conditioning/Refrigeration

    Once the heat rate to be removed has been calculated, then a cooling system must be matched to the outdoor enclosure. If, for example, enclosure air temperatures must be kept below the maximum ambient (outside) conditions, the preferred method is the installation of air-conditioning units.

    Returning to our case, if we estimated the heat load to be around 2000 W, then an air conditioning unit rated at 2000 W should be installed. These calculations do not include capacity for cooldown, since the calculations were carried out for steady state operation. That is, the system does not include cooling capacity to bring the system to design inside temperature from 55° C or above starting conditions. Thus, transient effects are not included. This is a realistic assumption since normally highest conditions will occur for a very short time and it is unlikely that the system will be started during the hottest period. Typically air conditioners have outlet air temperature of around 15° C or below to achieve the cooling required.

  2. Assisted (Semi-Active): Air-to-Air Heat Exchangers, Water-to-Air Heat Exchangers

    As with fully assisted cooling systems, once the heat rate to be removed has been calculated, then a cooling system must be matched to the outdoor enclosure. If, for example, enclosure air temperatures do not have to be kept below the maximum ambient (outside) conditions and the load is not too high, an air-to-air heat exchanger is the preferred system. Heat exchangers still allow for sealed electronics compartments but have much lower operating and maintenance costs along with allowing for battery back-up service for short down-time periods [9].

    For the conditions presented, one would install an air-to-air heat exchanger that transfers around 1500 W with the following conditions: a) Maximum outside (cooling air side) temperature of 20° and b) Maximum enclosure air temperature of 30°C. Unfortunately, most ofte
    n, due to the reduced space availability in most enclosure chambers (unless placed outside), it is very difficult to design a heat exchanger that would meet all the specifications. The reader should keep in mind that, unlike air conditioners, heat exchangers' heat removal capabilities change as a function of cooling air and enclosure air values; in fact the heat removal rate is a function of the differential (Tcab-Tamb). This fact brings the problem that if off-design temperatures are encountered, either the enclosure overheats or overcools.

    Heat exchangers are therefore constrained by the dimensions and temperature differentials, and some, unfortunately, come slightly short of meeting specifications (say 5°C below maximum conditions). However, these calculations are carried out by assuming steady state conditions, and, obviously, the enclosure does not operate under these conditions continuously. The external conditions are a function of the day cycle, and one expects that there exist thermal inertia effects, albeit small. These effects might compensate for the 5° C shortfall. In order to investigate this effect, a full 24-hour simulation can be carried out using a CFD (Computational Fluid Dynamics) software package or actual prototype testing [10].

    A typical CFD study divides a typical day into 4 hour intervals. During these intervals outside temperature and flux is varied. This simulation allows for taking into account thermal inertia effects, since the two key parameters fluctuate during a 24-hour cycle. These are: the solar radiation and the ambient temperature. Of these two, the ambient temperature is of paramount importance due to the fact that it controls, in conjunction with the enclosure temperature, the heat removal rate of the heat exchanger and thus, its cooling effectiveness.

  3. Passive Means: Natural Convection

    Smaller outside enclosures (pedestals, etc.), in which relatively high temperatures can be tolerated, can be cooled by passive means. Passive methods include primarily natural (free) convection and phase-changing materials (PCM). Natural convection is the transport of heat by buoyancy-induced fluid flows. Hotter fluid, heated (for example) by a hot wall exposed to the sun, rises and displaces colder fluid. In an enclosure, the hotter fluid moves up along the heated wall, then travels to the colder wall, and descends as it loses heat to the wall. The fluid will then make a recirculating closed loop that effectively transport heat from the hot wall to the cold wall. The fluid in the middle remains relatively undisturbed.

    The situation becomes more complex as power electronics are added, but there are ways to let the heat generated by the equipment be carried away by natural convection. The body of scientific literature on natural convection within enclosures is vast and the reader is referred to Ostrach's [11] review for an overview of the field. However, the designer must always keep in mind that the overall goal is to transfer to the outside as such heat as possible by natural convection in order to keep internal temperatures low.

    Another passive method that has become popular is the use of phase change materials (PCM), sometimes in conjunction with thermosiphons. PCM's are substances that change phase, most often from solid to liquid, as they absorb heat. Typical PCM's are waxes, salts, paraffins, etc. for high temperature applications and water (ice) for low temperature applications. A variation is to use a substance that high heat capacity such as water that absorbs large amounts of heat without changing phase.

    These materials are kept inside the enclosures in appropriately sealed enclosures, and take advantage of thermal inertia and phase change effects. For example, in an enclosure with PCM's, solar heat will be absorbed by the PCM device during the daylight hours, and not allowed to heat up the enclosure's air [12]. At night, the enclosure will release the stored energy to the cooler environment. During this entire cycle, heat will continue to be transferred in/out through the enclosure walls.

    Some systems, in order to enhance cooling, incorporate a thermosiphon (or natural convection) loop. Thermosiphons use water or other liquids (they must have high heat capacity) and these fluids are not allowed to change phase in the loop [13, 14]. A close loop conduit is connected to a large reservoir, which may be inside or outside the enclosure, and water heated by solar radiation will flow up naturally by buoyancy forces. The hotter fluid moves up, then travels to the colder wall, and descends as it loses heat to the wall. The fluid will effectively transport heat from the hot wall to the cold wall, in addition to absorbing heat as thermal inertial storage. In some cases, the reservoir may contain a PCM that melts and begins to circulate in thermosiphon-fashion until it returns to the reservoir.

4.0 Conclusions

This article has attempted to provide a brief overview of key topics in the thermal management of telecommunication components and systems. Thermal performance will become increasingly important as components and systems become complex.

5.0 References

  1. McKay, J.R., "Coping with Very High Heat Loads in Electronic Telephone Systems of the Future," 10 th International Telecommunication Energy Conference (INTELEC), San Diego, CA, USA, Nov. 1998

  2. Cosley, M.R., Garcia, M.P., L. K. Grzesik, J. Webster, and Marongiu, M.J., "Thermal Development Of Modular Outdoor Enclosures," presented at the 17th International Telecommunication Energy Conference (INTELEC), The Hague, Holland, Oct 29-Nov. 1, 1995.

  3. Peterson, G.P. and Ortega, A., "Thermal Control of Electronic Equipment and Devices," in Advances in Heat Transfer, Pergammon Press, Oxford, England, 1990, pp. 181-279.

  4. McKay, J.R., "The Effect of Solar Radiation and Wind Speed on Air Temperature Rise in Outdoor Enclosures Containing Telephone Equipment," 10th International Telecommunication Energy Conference (INTELEC), San Diego, CA, USA, Nov. 1998

  5. Bellcore TR-NWT-000487, Issue 2

  6. ASHRAE Handbook of Fundamentals, American Society of Heating, Refrigeration and Air-Conditioning Engineers, Atlanta, GA, 1981, 1986

  7. IEC Publication 721-2-4, Geneva, 1987

  8. Kreider, J.F. and Kreith, F. , Solar Energy Handbook, McGraw-Hill Book Co., New York, 1981

  9. McKay, J.R., "Use of an Air-to-Air Heat Exchanger to Reduce Peak Temperatures in Outdoor Enclosures," 12th International Telecommunication Energy Conference (INTELEC), Florence, Italy, Nov. 1990

  10. Marongiu, M.J., "Some Issues In Experimental Testing And Methodologies In The Thermal Management Of Telecommunication Components, Systems And Enclosures," to be presented at 17th International Telecommunication Energy Conference (INTELEC), The Hague, Holland, Oct 29-Nov. 1, 1995

  11. Ostrach, S., "Natural Convection in Enclosures," Advances in Heat Transfer, vol. 8, pp. 161-227

  12. Prudhoe, R. K., and Doukas, L., "The Use of Phase Change Materials (PCM's) and Vacuum Panel Heat Exchangers for Energy Conservation and Thermal Stability of Electronic Equipment Enclosures," 12th International Telecommunication Energy Conference (INTELEC), Florence, Italy, 1990

  13. Ghiraldi, A., "Passive Conditioning Systems for Temperature Control in Telecommunications Equipment Enclosures," 10th International Telecommunication Energy Conference (INTELEC), San Diego, CA, USA, Nov. 1988.

  14. McKay, J.R., Estes, R.C., and Kimsey, R., "Use of a Water Plenum and Thermosiphon to Control Peak Temperature in Outdoor Enclosures," 14th International Telecommunication Energy Conference (INTELEC), Washington DC, USA, Oct 1992
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Thermal Management Links

Enertron's thermal management library - lots of great papers on various techniques and design consideratioins.

2001 CommsDesign article, "Some Like It Cool..." by Maurice J. Marongiu. Remote comm equipment now resides in harsh environments, creating heat dissipation problems. Designers must rethink thermal management to beat the elements.

IERC Thermal Management Resource Center - products, design techniques, articles, links, and more.

Electronics Cooling Magazine Online - a great Resource for Practitioners in the Field of Electronics Thermal Management

The Cooling Zone's Library of technical articles. — linked directory of Thermal Management product sources. Includes news releases from the THERMAL MANAGEMENT pages of OEM Technology Magazine.

Outdoor Thermal Solutions - thermal management consultants.

PCM Thermal Solutions - custom Phase Change Material (PCM) Heat Sinks for Electronics Packaging Applications

Wakefield Engineering - Manufacturer of heat sinks and cooling fans for the electronics industry.

Thermacore - Thermal management solutions for the electronics industry

European Thermodynamics - information on the use of heat pipes for cooling electronics.

FLOTHERM - thermal analysis software for the electronics industry, many application examples, thermal model libraries, and published technical papers.

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Books on Thermal management

Click on a Title Below for a Direct Link to Purchase


Hot Air Rises and Heat Sinks : Everything You Know About Cooling Electronics Is Wrong , by Tony Kordyban. Paperback: 250 pages (July 1998).


More Hot Air, by Tony Kordyban. Paperback: 291 pages pages (January 2005).


Thermal Management of Electronic Systems III - Eurotherm 58, by J. P. Bardon, E. Beyne, J. B. Saulnier, J.B. Saulnier. Paperback: 276 pages (September 1998).


Cooling Techniques for Electronic Equipment, 2nd Edition , by Dave S. Steinberg. Hardcover: 512 pages (October 8, 1991).


Preventing Thermal Cycling and Vibration Failures in Electronic Equipment , by Dave S. Steinberg. Hardcover: 304 pages (June 22, 2001).


Thermal Management Handbook : For Electronic Assemblies (Electronic Packaging and Interconnection Series), by Al Krum (Contributor), Imaps, Jerry E. Sergent. Hardcover - 650 pages (April 1998).


An Introduction to Heat Pipes : Modeling, Testing, and Applications (Wiley Series in Thermal Management of Microelectronic & Electronic Systems) , by G. P. Peterson. Paperback - 356 pages (September 1994).


Liquid Cooling of Electronic Devices by Single-Phase Convection (Wiley Series in Thermal Management of Microelectronic & Electronic Systems.) , by Frank P. Incropera. Hardcover (May 1999).

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