fig. 1 - thermal analog model of an osp enclosure.
over the past 20 years, telecommunications electronics have become increasingly decentralized - moving ever further from the controlled and protected environment of the central office into the outside plant (osp). the osp environment contains rain and humidity, dust and pollutants, significant daily and annual temperature swings, wide solar heat-load variations, and physical abuse. to protect electronics from this hostile environment, they are housed in a variety of osp enclosures, which can range from small buildings (30 m3) to small boxes (3x10-3 m3) that are mounted on the homes of customers.
heat densities within osp enclosures can be large, often rivaling those of electronics designed for controlled indoor environments. the combination of osp environment and high heat density means that the telecommunications osp thermal designer must be equally familiar with broad hvac (heating, ventilation and air conditioning) techniques and detailed electronic thermal-management technologies.
in addition, because the enclosures are located in business and residential neighborhoods remote from central maintenance facilities, the designer must take into account community restrictions on size, noise (e.g., from ventilation fans) and "aesthetics" and human factors that affect the accessibility and maintainability of the enclosure equipment.
figure 1, a much simplified analog model of an osp enclosure, shows the major factors that control the internal temperature ti of the enclosure. these include the enclosure heat sources, heat sinks, cooling method and the thermal parameters of the enclosure:
|ti = qi [ri+ro] + qα [ro] + tair[ro/rconv] + tsky[ro/rrad]
|l/ro = l/rconv + l/rrad
heat sources in the enclosure are qi, the heat generated by the internal equipment, and qα, the solar energy ("insolation") that is absorbed and transmitted into the enclosure. the two primary heat sinks are the local ambient air at temperature tair and the remote "sky" at temperature tsky. as indicated in equation 1, heat is transferred to the local air primarily by convection and to the remote sky by radiation. the internal thermal resistance, ri, is determined primarily by the cooling method, while the thermal parameters affect the internal and external resistances, ri and ro, and the fraction of incident solar energy that is absorbed into the enclosure.
heat sources and sinks.
internal heat generation, qi: probably one of the moste effective actions that can be taken to reduce internal temperatures in an osp enclosure is to reduce its internal heat genereation. this is due, in part, to the direct relationship between these two quantities indicated in equation 1. in addition, osp electronics are typically powered remotely - and several redundant powering systems are employed to ensure uninterrupted telecommunications service. each powering system exhibits an inefficiency, and these inefficiencies translate into added heat load within the enclosure.
a typical example is an osp electronic system whose primary power is provided by the local electric utility system and whose secondary (backup) power is provided by batteries housed in the enclosure. both the power conditioning used to convert utility power to the low voltages needed by the electronics and the battery charging can add significant internal heat to the enclosure. one additional incentive for keeping internal heat generation low is that larger heat loads generally require more "active" cooling systems; the power and primary/redundant power inefficiencies needed to operate these systems add, once again, to the heat in the enclosure.
solar heat load, qα: a second source of heat in an osp enclosure is the solar energy that is absorbed and transmitted into the enclosure:
in equation 2, qsun is the total shortwave radiation from the sun (insolation) that falls on the enclosure and qsol is the solar (shortwave) absorptance of the enclosure surfaces. the total insolation includes direct shortwave radiation from the sun, diffuse shortwave radiation that has been scattered by the atmosphere, and any shortwave radiation that is reflected from the ground or nearby surfaces. it is a complicated function of the orientation of the enclosure with respect to the earth-sun line, the amount of moisture, dust and contaminants in the air, and the solar reflectivity and location of nearby surfaces (e.g., landscaping, fences and buildings). procedures for calculating qsun for an enclosure with a known location and orientation are well documented. (see, for example, references 1 and 2.)
bellcore specifies that osp enclosures must be designed for ubiquitous osp use and must handle the maximum solar load at the maximum ambient air temperature and the maximum internal heat load. determining maximum solar load is not simple when the enclosure location and orientation are unknown. figure 2 shows the total insolation incident on a 60cm (deep)x120cm (wide)x180cm (high) (2ftx4ftx6ft) enclosure located in a low-reflecting open field environment (i.e., no surrounding reflecting surfaces). the lower curve in figure 2a shows the total insolation for san jose, ca for june 21 for a specific enclosure orientation: front surface facing due south.
the upper curve shows the maximum insolation for the same location and date: imagine an enclosure that is continuously rotated to capture the maximum sun. figure 2b shows the maximum insolation at 3pm (the approximate time that the maximum air temperature occurs) for various north american latitudes. for this case, the maximum insolation is over 2600 watts at a latitude of 24°n.
figure 2a: total solar energy incident on a 60cm x 180cm osp enclosure.
most enclosures are not located in open-field environments, but in normal communities surrounded by landscaping, fences and buildings. often these surroundings will shade the enclosure and hence reduce the solar load. equally often, the surroundings will reflect additional sunshine onto the enclosure and increase the load. the maximum insolation on the 60cmx120cmx180cm enclosure increases by a factor of 1.3 when it is located near a large light-colored (reflecting) building and by 1.4 when it is located in a three-sided, reflecting fence alcove. as discussed below, either of these locations may be chosen to make the enclosure less obtrusive.
the second parameter in equation 2 that affects the solar load absorbed into the enclosure is qsol, the shortwave absorptance of the enclosure surface. table 1 shows representative values for this parameter for several common enclosure surface treatments. also shown are longwave emittances for the surfaces. both parameters must be considered in enclosure design: the shortwave absorptance controls the fraction of incident solar heat absorbed into the enclosure, and the longwave emittance controls the heat that is dissipated by radiation to the surroundings.
this is illustrated in an example discussed below:
table 1. radiative properties of typical osp enclosure surfaces.
local ambient air temperature, tair: the major heat sink for the heat dissipated from the enclosure is the local ambient air. the temperature of this sink varies widely with location, time of year, and time of day. for osp enclosures intended for use throughout north america, bellcore specifies an ambient air temperature range of -40°c to + 46°c (-40°f to + 115°f).
remote sky temperature, tsky: a secondary heat sink, which is often neglected in enclosure design, is the "remote sky". hot surfaces radiate heat continuously in proportion to the fourth power of their absolute temperature (stefan-boltzman law); the net heat radiated depends on the longwave emissivity of the surface ó and on the emissivities and temperatures of the surroundings. since the atmosphere is generally cooler than the enclosure and any nearby terrestrial surfaces, it can be a very effective heat sink.
for an enclosure in an open-field environment, the atmospheric can be modeled by an "apparent sky temperature", defined as the temperature of a black body (i.e., perfect radiator) that emits radiation at the same rate as the atmosphere; the sky temperature can be estimated from local weather conditions (dew point and dry bulb temperatures). sky temperature is always lower than ambient air temperature, especially when the air contains little water vapor: this is the reason that frost occurs on terrestrial surfaces even when the air temperature is well above freezing.
the atmosphere/remote sky is most effective as a radiant heat sink during the spring and fall, when dew point and dry-bulb temperatures are relatively low, and in dry dessert climates. it is not effective under overcast conditions or summer conditions of high dew point/dry bulb temperatures. on account of this, conservative design techniques should be used and the sky temperature should be assumed to be equal to the local ambient air temperature. (note: if an enclosure is tested during dry, clear climate conditions, the additional radiation heat loss to the sky should be factored out of the results to get a clearer picture of how the enclosure will perform under more normal osp conditions.)
a large variety of cooling techniques have been proposed and used to cool osp electronics equipment enclosures. these include conventional techniques, ranging from passive natural convection to the use of commercial air conditioners or heat pumps, and nov novel concepts using thermosyphon and pcm (phase change material) technologies. many of these techniques have been presented at annual intelec conferences (international telecommunications energy conference).
the most common cooling method used today in the osp is relatively simple. the enclosure is unventilated to protect the internal equipment from the rain, dust, and contaminants in the outside air. the internal heat is transferred primarily by convection to the inside surfaces of the enclosure, by conduction through the walls of the enclosure, and then by convection and radiation to the external heat sinks. reference 5 presents a first-level model for estimating internal temperatures in such enclosures, including methods for estimating the external resistances in equation 1: rconv, the wind-induced convective resistance to the ambient air, and rrad, the radiative resistance to local and remote surroundings.
the interior resistance, ri, depends on the type of convection used (free and/or fan-forced) and the interior electronics characteristics; normal electronic thermal-management techniques can be used to determine this. (see, for example, reference 6).
figure 3. contributions to enclosure internal temperature rise.
figure 3 shows typical internal temperature rises from several factors for an unventilated 60cmx120cmx180cm enclosure located in an open field environment. two enclosure surfaces are depicted: an unpainted aluminum and a white-painted surface. the factors in the figure may best be understood by rewriting equation 1 as:
ti - tair = riqi + roqi + roqα - [ro/rrad][tair - tsky]
the first factor, ri qi, is not shown in the figure: the cooling method will be dictated by the difference between equipment design constraints on ti and the temperature rises from the remaining factors. the exterior resistances, rconv and rrad, were estimated using the first-level models in reference 5, assuming low wind velocities and radiation parameters from table 1. enclosure insolation was assumed to be 2600 watts, as detailed in figure 2b; thus the solar load qα was 80 watts and 650 watts for the aluminum and white-painted surfaces, respectively. the sky correction factor, [ro/rrad][tair - tsky], was estimated using ashrae summer design conditions for san jose, ca: 29oc dry bulb and 19oc wet bulb. for these conditions, the remote sky temperature is 17oc-12oc lower than the local ambient air.
as shown in figure 3, there is a temperature rise in the enclosure from the solar load even when there is no internal load. here the advantages of minimizing shortwave absorptance follows intuition: reducing the amount of insolation absorbed reduces the solar temperature rise. as the internal heat generation increases, however, removing the heat from the enclosure becomes critical. here, the advantage of increasing longwave emittance, even at the expense of increasing solar load becomes apparent.
other issues: besides the normal issues (e.g., equipment temperature limitations and heat load) that must be addressed in selecting a cooling system, there are issues unique to the osp environment. these include power availability and the cooling system power requirements, required frequency and ease of maintaining the system, reliability of the system in the harsh osp environment (and the effect on other equipment if the cooling system fails), and community restrictions/guidelines on noise and aesthetics. all of these issues should be addressed in relation to equivalent issues with the other equipment in the enclosure. for example, filters that require frequent maintenance should not be used in remote osp enclosures - unless other equipment in the enclosure also requires frequent and regular maintenance; if it does, circulation of filtered ambient air may be an effective cooling method. two issues that particularly effect cooling design are increasingly stringent community restrictions on noise and aesthetics.
noise restrictions: several potential cooling methods generate noise, e.g., convection-enhancing fans. communities are beginning to recognize noise as a form of environmental hazard and to impose noise restrictions on equipment located in the communities. generally, these restrictions depend on the "land use zone", e.g., restrictions for hospital and library zones are generally stricter than for industrial parks. bellcore currently specifies a maximum noise level of 60 dba for enclosures. this level is probably too high, since the environmental protection agency has identified 55 ldn (average day/night noise level) as the desirable maximum level for outdoor noise.
aesthetics: measures are often taken to hide or to obscure an enclosure in order to gain neighborhood approval/acceptance and to reduce the likelihood of graffiti and vandalism. examples include planting and landscaping around the enclosure, painting it to match its surroundings, placing it near like-colored walls or fences, recessing it into fence/wall pockets or alcoves, and completely fencing it. each of these measures could increase the internal temperature (e.g., by increasing the solar load or reducing the effective heat transfer), and must be considered in the enclosure design.
||heat generated by equipment inside the enclosure
||total solar (shortwave) radiation incident on the enclosure
||solar heat absorbed into the enclosure q= sol qsun
||wind-induced convective thermal resistance from exterior surfaces of enclosure
||thermal resistance from interior to exterior surfaces of enclosure
||thermal resistance from enclosure exterior surfaces to external heat sinks, 1/ro=1/rconv+1/rrad
||radiative thermal resistance from exterior surfaces of enclosure
||temperature of local embient air
||temperature inside enclosure
||temperature of atmosphere above enclosure
||solar (shortwave) absorptance of enclosure surface
2410 camino ramon, suite 100
||ashrae handbook & product directory - 1977 fundamentals, atlanta, ga: ashrae, inc., 1977, ch 26.
||hvac systems and applications - 1987 ashrae handbook, atlanta, ga: ashrae, inc., 1987, ch 47.
||bellcore ta-nwt-000487, generic requirements for electronic equipment cabinets, issue 2, june 1993.
||intelec proceedings are available from the ieee, new york.
||coyne, j.c., "an approximate thermal model for outdoor electronics cabinets", the bell system technical journal, vol 61, february, 1982.
||kraus, allan d. and bar-cohen, avram, thermal analysis and control of electronic equipment, new york: hemisphere publishing, 1983.
||cowan, james p., handbook of environmental acoustics, new york, ny: van nostrand reinhold, 1994.