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.
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]:
Amount of heat generated by all the electronic equipment in the enclosure.
Amount of heat generated by auxiliary and cooling equipment (fans, etc.)
Ambient conditions (outdoor air), particularly temperature, solar radiation, wind speeds, etc.
Objects surrounding the enclosure (shading, ground reflections, buildings, trees, etc.)
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
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.
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.
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
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
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.
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.
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
Bellcore TR-NWT-000487, Issue 2
ASHRAE Handbook of Fundamentals, American Society of Heating,
Refrigeration and Air-Conditioning Engineers, Atlanta, GA, 1981, 1986
IEC Publication 721-2-4, Geneva, 1987
Kreider, J.F. and Kreith, F. , Solar Energy Handbook, McGraw-Hill Book
Co., New York, 1981
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
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
Ostrach, S., "Natural Convection in Enclosures," Advances in Heat
Transfer, vol. 8, pp. 161-227
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
Ghiraldi, A., "Passive Conditioning Systems for Temperature Control in
Telecommunications Equipment Enclosures," 10th International
Telecommunication Energy Conference (INTELEC), San Diego, CA, USA, Nov. 1988.
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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