referring to fig2 , an illustrative embodiment of a cooling apparatus 200 according to the present invention uses the same reference numerals for like elements as the prior art apparatus 100 shown in fig1 . however , the apparatus 200 differs from the prior art apparatus 100 in at least two significant ways . first , on the downstream faces of each cabinet 110 , 112 , 114 , 116 , the present invention employs , in contrast to the prior art air plenums 126 , 122 , a series of air - to - water heat exchangers 210 , 212 , 214 , 216 . second , the present invention uses , in place of the prior art &# 39 ; s multiple s - shaped air paths 138 , a single , row - wise airflow path 218 that travels substantially in the − x direction , straight through an entire flow - through row 220 . the flow - through row 220 comprises the cabinets 110 , 112 , 114 , 116 ; the heat exchangers 210 , 212 , 214 , 216 , and optionally an intake plenum and an exhaust plenum such as a bottom - intake plenum 222 , and a bottom - exhaust plenum 224 , respectively . the heat exchangers 210 , 212 , 214 , 216 make possible the row - wise airflow path 218 . referring to the graph 244 of air temperature vs . horizontal coordinate x at the top of fig2 , the heat - producing electronics in cabinet 110 cause the temperature of the air circulating along air path 218 to rise from t 0 to t 1 as it traverses cabinet 110 from the cabinet &# 39 ; s upstream face 110 a at x = x 0 to the downstream face 110 b at x = x 1 . the air - to - water heat exchanger 210 is typically a tube - and - fin heat exchanger well known in the art , wherein warm air passes over the heat - exchanger &# 39 ; s fins and a cold liquid flows in the heat exchanger &# 39 ; s tubes , thereby allowing heat to be transferred from the air to the liquid . the liquid is supplied to each heat exchanger from an external liquid - chilling system via a supply pipe 240 , and is returned to the liquid - chilling system via a return pipe 242 . therefore , in traversing the heat exchanger 210 from x 1 to x 2 , the temperature of the air , being cooled by the externally chilled liquid , drops from t 1 to t 0 . thus , the combination of cabinet 110 and heat exchanger 210 is thermally neutral for the air . this air - temperature cycle is repeated for subsequent cabinets and heat exchangers : the air is warmed to temperature t 1 a second time while traversing cabinet 112 in the region x 2 to x 3 , is cooled a second time to temperature t 0 by the heat exchanger 212 in the region x 3 to x 4 , is warmed a third time to temperature t 1 while traversing cabinet 114 in the region x 4 to x 5 , is cooled a third time to temperature t 0 by heat exchanger 214 in the region x 5 to x 6 , is warmed a fourth time to temperature t 1 by cabinet 116 in the region x 6 to x 7 , and is finally cooled a fourth time to temperature t 0 by heat exchanger 216 in the region x 7 to x 8 . thus , the entire flow - through row 220 is thermally neutral for the air ; that is , the air returns to the under - floor space 150 at temperature t 0 , ready to repeat the cycle . because the air path 218 is closed , the temperatures t 0 and t 1 will automatically float to whatever values cause equilibrium to occur . thus , it is necessary to choose heat exchangers 210 , 212 , 214 , 216 and air - moving devices 136 such that acceptable temperatures are obtained for the worst - case heat dissipation of electronics 134 . heat exchanges 210 , 212 , 214 , 216 are described in u . s . patent application ser . no . 11 / 939 , 165 , filed nov . 13 , 2007 , now abandoned , the disclosure of which is hereby incorporated herein by reference in its entirety . temperature control of a cooling fluid is also discussed in copending u . s . patent application ser . no . 12 / 483 , 542 , filed jun . 12 , 2009 , the disclosure of which is hereby incorporated herein by reference in its entirety . again referring to fig2 , the row - wise airflow path 218 is now described in detail . air enters the first cabinet 110 from the under - floor space 150 , flowing upward through row - intake hole 226 in the raised - floor 142 , and through the perforated metal screen 228 , which may be necessary , depending on the nature of the electronics , to prevent the escape of electromagnetic radiation therefrom into the room 140 . the row - wise airflow path 218 moves upward through the bottom - intake plenum 222 to the first cabinet 110 of the flow - through row 220 . the air - moving devices 136 within the cabinets 110 , 112 , 114 , 116 encourage the row - wise airflow path 218 through each cabinet 110 , 112 , 114 , 116 , and thereby through the entire flow - through row 220 . an intake - end wall 230 of the bottom - intake plenum 222 may , if desired , slant inward toward the top of the first cabinet 110 , inasmuch as upper cross - sections of the intake plenum 222 handle far less airflow than lower cross - sections , and thus require less cross - sectional area . alternatively , the intake - end wall 230 may be substantially vertical , or removed altogether . in the latter case , the flow - through row 220 draws air from the room 140 rather than from the under - floor space 150 . the row - wise airflow path 218 exits the last cabinet 116 of the flow - through row 220 , flowing downward through a perforated - metal exhaust screen 232 whose function is similar to that of the perforated - metal intake screen 228 , downward through a row - exhaust hole 234 in the raised - floor 142 , and thereby into the under - floor space 150 . an exhaust - end wall 236 of the bottom - exhaust end plenum 224 may , if desired , slant outward toward the bottom of the last cabinet 116 , inasmuch as upper cross - sections of the bottom - exhaust plenum 224 handle far less airflow than lower cross - sections , and thus require less cross - sectional area . alternatively , the exhaust - end wall 230 may be substantially vertical , or removed altogether . in the latter case , the flow - through row 220 exhausts air to the room 140 rather than to the under - floor space 150 . referring to fig3 , another embodiment of the invention is a cooling apparatus 300 that includes a top - exhaust plenum 324 instead of the bottom - exhaust plenum 224 previously shown in fig2 . the top - exhaust plenum 324 is identical to bottom - exhaust plenum 224 except that it is rotated 180 degrees about the x axis , such that top - exhaust plenum 324 is wide at the top , by virtue of a sloping end wall 336 , thereby to accommodate greater airflow at upper cross sections than at lower cross sections in the cooling apparatus 300 , a row - wise airflow 318 behaves as in cooling apparatus 200 , except that in apparatus 300 , the airflow 318 exits the row 220 flowing upward through the top - exhaust end plenum 324 , which has an opening 334 at the top . a perforated metal exhaust screen 332 at the top of top - exhaust plenum 324 serves the same purpose as screen 232 in plenum 224 , as discussed previously . as with the apparatus 200 shown in fig2 , and also pertaining to the embodiments shown in fig4 and 7 , depending on the nature of the electronics 134 , it may not be necessary to include the perforated metal screen 332 to prevent the escape of electromagnetic radiation from the flow - through row 220 . referring to fig4 , another alternative embodiment of the invention is a cooling apparatus 400 , where no intake plenum is used . in this embodiment , airflow 418 enters the flow - through row of cabinets 220 directly from the room 140 . the airflow exits the apparatus 400 as in the apparatus 300 shown in fig3 . pertaining to this embodiment as well as to that shown on fig5 , to prevent the escape of electromagnetic radiation from the flow - through row 220 , it may be necessary , depending on the nature of the electronics 134 , to affix to the upstream surface 110 a of the first cabinet 110 a perforated metal screen 428 , through which air flows immediately prior to entering cabinet 110 . referring to fig5 , another alternative embodiment of the invention is a cooling apparatus 500 where no intake - end plenum or exhaust - end plenum is used . in this embodiment , airflow 518 exhausts from the last cabinet 116 directly to the room 140 . airflow 518 is otherwise identical to airflow 418 discussed with reference to fig4 . to prevent the escape of electromagnetic radiation from the flow - through row 220 , it may be necessary , depending on the nature of the electronics 134 , to affix to the downstream surface 116 b of the last cabinet 116 a perforated metal screen 532 . referring to fig6 , another alternative embodiment of the invention is cooling apparatus 600 , where a top - intake end plenum 622 and the top - exhaust end plenum 324 are used . the top - intake plenum 622 is identical to the bottom - intake plenum 222 , shown in fig2 , except that it is rotated 180 degrees about the x axis , such that the top - intake plenum 622 is wide at the top , by virtue of a sloping end wall 630 , thereby to accommodate greater airflow at upper cross sections than at lower cross sections . in this embodiment , an airflow 618 enters the flow - through row 220 downward through the top - intake end plenum 622 and exits the flow - through row 220 upward through the top - exhaust end plenum 324 . referring to fig7 , another embodiment of the invention is a cooling apparatus 700 , which is similar to the apparatus 200 shown in fig2 . however , in the apparatus 700 shown in fig7 , the heat - exchanger 210 is replaced by a heat - exchanger assembly 710 that comprises , in addition to the heat exchanger 210 , an array of air - moving devices 760 , such as axial - flow fans . likewise , the heat exchangers 212 , 214 , and 216 shown in fig2 are replaced , in apparatus 700 , by heat - exchanger assemblies 712 , 714 , 716 respectively , which comprise , in addition to heat exchangers 212 , 214 , and 216 respectively , air - moving devices 762 , 764 , and 766 respectively . thus , the cooling apparatus 700 includes air - moving devices 760 , 762 , 764 , 766 that supplement the air - moving devices 136 within the cabinets 110 , 112 , 114 , 116 . alternatively , depending , for example , on the cost and pressure - rise requirements of the cooling system and on the space required by the electronics , the air - moving devices 760 , 762 , 764 , 766 may replace the air - moving devices 136 contained within the cabinets 110 , 112 , 114 , 116 . the heat - exchanger assemblies 710 , 712 , 714 , 716 , although described above for use with the airflow arrangement of the cooling apparatus 200 shown in fig2 , may also be used with any of the other airflow arrangements , as shown in cooling apparatuses 300 , 400 , 500 , and 600 of fig3 - 6 , respectively . referring to fig8 , another embodiment of the invention is a cooling apparatus 800 , wherein each of the cabinets 110 , 112 , 114 , 116 is internally divided into a front portion 802 and a rear portion 804 . note that fig8 is a plan view , as specified by the orientation of the x , y , and z axes 102 , 104 , 106 respectively , whereas fig1 - 7 and 9 are front elevational views . in each cabinet , the portions 802 , 804 are separated from each other by a vertical cabinet barrier 806 that substantially prevents air flow across it . the barrier 806 lies substantially parallel to an xz plane spanned by the x and z axes . likewise , each of the heat - exchangers 210 , 212 , 214 , 216 comprises , in this embodiment , a vertical heat - exchanger barrier 808 that substantially prevents airflow across it . the cabinet barriers 806 and the heat - exchanger barriers 808 are substantially co - planar . a first closed - end plenum 810 is abutted to the upstream face 110 a of the first cabinet 110 , and a second closed - end plenum 812 is abutted to a downstream face 216 b of the heat exchanger 216 . front air - moving devices 814 in the front portion 802 of the cabinets 110 , 112 , 114 , 116 are configured to drive a closed - horizontal - loop air - stream 818 in the − x direction , while rear air - moving devices 816 in the rear portion 804 of the cabinets 110 , 112 , 114 , 116 are configured to drive the closed - horizontal - loop air stream 818 in the + x direction , such that the air stream 818 circulates in a closed loop about the vertical z axis 106 . that is , the closed - horizontal - loop air - stream 818 flows toward + x in the rear portion 804 of the cabinets 100 , 112 , 114 , 116 and heat exchangers 210 , 212 , 214 , 216 , then toward − y in the first closed - end plenum 810 , then toward − x in the front portion 802 of the cabinets and heat exchangers , and finally toward + y in the second closed - end plenum 812 , thus completing a closed loop . this closed - loop embodiment is advantageous because it imposes no air - handling burden on the room 140 , and because it provides very quiet operation of the air moving devices 814 , 816 , particularly when the cabinets 110 , 112 , 114 , 116 , heat - exchanger assemblies 210 , 212 , 214 , 216 , and closed - end plenums 810 , 812 are acoustically insulated , because people in the room 140 are shielded from the noise of air movers and flowing air . again referring to the apparatus 800 shown in fig8 , it should be noted that the closed - horizontal - loop air stream 818 , at its + x end , traverses two sets of heat - producing electronics 134 , in the rear portion 804 of the first cabinet 110 and in the front portion 802 of the first cabinet 110 , without any intervening heat exchanger to cool the air . if this causes the air to become unacceptably warm in the front portion 802 of cabinet 110 , so as to compromise cooling of the electronics 134 therein , then an additional heat exchanger identical to 210 may be abutted to the + x surface of the first cabinet 110 . referring to fig9 , another embodiment of the invention is a cooling apparatus 900 , wherein each of the cabinets 110 , 112 , 114 , 116 is internally divided into a lower portion 902 and an upper portion 904 . in each cabinet , the portions 902 , 904 are separated from each other by a horizontal cabinet barrier 906 that substantially prevents air flow across it . barrier 906 lies substantially parallel to an xy plane spanned by the x and y axes likewise , each of the heat - exchangers 210 , 212 , 214 , 216 comprises , in this embodiment , a horizontal heat - exchanger barrier 908 that substantially prevents air flow across it . the cabinet barriers 906 and the heat - exchanger barriers 908 are substantially co - planar . a first closed - end plenum 910 is abutted to the upstream face 110 a of the first cabinet 110 , and a second closed - end plenum 912 is abutted to a downstream face 216 b of the heat exchanger 216 . lower air - moving devices 914 in the lower portion 902 of the cabinets 110 , 112 , 114 , 116 are configured to drive a closed - vertical - loop air - stream 918 in the − x direction , while upper air - moving devices 916 in the upper portion 904 of the cabinets 110 , 112 , 114 , 116 are configured to drive the closed - vertical - loop air stream 918 in the + x direction , such that the air stream 918 circulates in a closed loop about the horizontal y axis 104 . more specifically , the closed - horizontal - loop air - stream 918 flows toward + x in the upper portion 904 of the cabinets 100 , 112 , 114 , 116 and heat exchangers 210 , 212 , 214 , 216 , then toward − z in the first closed - end plenum 810 , then toward − x in the lower portion 902 of the cabinets and heat exchangers , and finally toward + y in the second closed - end plenum 812 , thus completing a closed loop . this closed - loop embodiment , shown in fig9 , is advantageous for the same acoustic reason described earlier in connection with apparatus 800 shown in fig8 . again referring to the apparatus 900 shown in fig9 , it should be noted that the closed - horizontal - loop air stream 918 , at its + x end , traverses two sets of heat - producing electronics 134 , in the upper portion 904 of the first cabinet 110 and in the lower portion 902 of the first cabinet 110 , without any intervening heat exchanger to cool the air . if this causes the air to become unacceptably warm in the lower portion 902 of cabinet 110 so as to compromise cooling of the electronics 134 therein , then an additional heat exchanger identical to 210 may be abutted to the + x surface of the first cabinet 110 . additionally , other embodiments and variations are possible keeping with the spirit and scope of the invention , for example , although the embodiments presented herein have included “ air - to - water heat exchangers ”, the heat exchangers may use other fluids . in another example , the water supply and return pipes 240 , 242 may enter the heat - exchangers 210 , 212 , 214 , 216 from the top rather than from the bottom . all the embodiments of the current invention , including those represented as cooling apparatuses 200 , 300 , 400 , 500 , 600 , 700 , 800 , and 900 , shown in fig2 - 9 , respectively , have a number of significant advantages over the prior - art apparatus 100 shown in fig1 , including those discussed hereinafter . a first advantage is that the total airflow required in the room 140 , and the associated acoustical noise , are greatly reduced by the invention vis - à - vis the prior art , leading to greater acoustical comfort for humans in the room 140 , and to less disruption of airflow if the room houses an existing installation of other equipment . quantitatively , if volumetric flow rate v of air is required to cool each cabinet , and there are n cabinets in a row , then the prior art requires a total flow rate of nv per row , whereas the present invention which requires only v per row . this is a factor of n improvement that allows installation of such cabinets in buildings unable to support large amounts of airflow , and also reduces the total amount of airflow noise . second , many fewer air - conditioning units 152 are required in the room 140 by the invention than by the prior art , leading to lower capital investment in air - conditioning units 152 and lower energy cost to drive air - moving devices therein . according to the invention , the heat load of electronics 134 is transferred from the air locally to water flowing in pipes 240 , 242 of heat exchangers 210 , 212 , 214 , 216 . therefore , the flow - through row 220 puts no thermal load on the room 140 , and thus requires only minimal air - conditioning for general dehumidification , and ancillary heat loads . in contrast , the prior - art row 108 dissipates all its heat load to the room , thus requiring , if the number of cabinets and the power dissipation therein is large , a great number of air - conditioning units 152 . third , the prior - art &# 39 ; s narrow airflow plenums 126 , shown in fig1 , are eliminated . such narrow plenums are required by the prior art to achieve compact packaging along the flow - through row 220 , and to insure that the holes 154 in the raised floor 142 match the periodicity p of the raised - floor tiles 146 . however , air velocity is high in the narrow airflow plenums 126 , typically much larger than in the cabinet itself , because the cross - sectional area normal to the airstream is much smaller in the plenum than in the cabinet . thus pressure drop in the airflow plenums 126 is large , and airflow rate through the prior - art electronics 134 is thereby restricted , increasing the temperature therein and reducing the lifetime and performance thereof . in the invention , this source of pressure drop is eliminated . some pressure loss occurs in the invention &# 39 ; s heat exchangers 210 , 212 , 214 , 216 , but because the cross - sectional area of the heat exchanger is large , air velocity is low , and therefore pressure drop is relatively small . fourth , flow non - uniformities that occur in the prior art are eliminated . specifically , the narrowness of the prior art &# 39 ; s airflow plenums 126 cause flow separation at locations near the upstream faces 110 a , 112 a , 114 a , 116 a of the cabinets wherever the airflow cannot negotiate a tight turn around a sharp edge . in the wake of such separation is a stagnation region of very - low - velocity airflow that causes very high temperatures of the electronics 134 therein . the tendency to separate may be minimized by widening the prior - art combined plenums 126 , but this is highly undesirable in the prior art , because of the desire to achieve a compact footprint of the row 108 of cabinets and plenums , and because of the aforementioned requirement to match the periodicity of the holes 154 with the pitch p of the removable tiles 146 . in contrast , embodiments 400 and 500 of the current invention require no air turn upstream of any electronics 134 , so the problem of flow separation is completely eliminated . all other embodiments require just one air turn per row 220 , upstream of the first cabinet 110 . because the invention has only one intake plenum per row 220 rather than one intake plenum per cabinet as in the prior art , beneficial widening of the intake plenum , mentioned above , has , for the invention , much less impact on the footprint of a row 220 than a similar widening would have for the prior - art row 108 . that is , widening each of the prior - art &# 39 ; s inlet plenums ( 118 and 128 ) by an amount d widens the prior - art cabinet row 108 by an amount nd , where n is the number of cabinets per row . in contrast , widening the invention &# 39 ; s intake end plenum ( 222 or 622 , depending on the embodiment ) by the same amount d widens the invention &# 39 ; s flow - through row 220 merely by d , a factor - of - n improvement over the prior art . fifth , the prior art &# 39 ; s need to turn the air twice in each cabinet 110 , 112 , 114 , 116 is eliminated by the invention . by replacing the prior - art &# 39 ; s s - shaped air - streams 138 , with the single , row - wise airflow path 218 most or all of the air turns are eliminated . specifically , instead of two 90 - degree turns per cabinet in the prior - art apparatus 100 , there are only four turns per row in apparatuses 200 , 700 , 800 , and 900 ; only two turns per row in apparatuses 300 and 600 ; only one turn per row in apparatus 400 ; and zero turns per row in apparatus 500 . fewer turns is desirable because turning air incurs pressure drop and thereby reduces airflow , raising the temperature , shortening the life and compromising the performance of the electronics 134 . sixth , compared to the prior art , the invention provides additional space for air - moving devices . as shown by apparatus 700 in fig7 , an air - to - water heat exchanger specified by this invention , such as 210 , need not occupy the entire space between the adjacent cabinets 110 and 112 ; instead , some of this space may be occupied by the array of air - moving devices 760 , which either supplement or replace the air - moving devices 136 internal to cabinet 110 . if air - moving devices 760 , 762 , 764 , 766 supplement air - moving devices 136 , then the pressure rise of the system ( and hence the air velocity ) is greatly increased , a benefit that may be used either to reduce the temperature of the electronics , or to cool more electronics or more powerful electronics . if , instead , the air - moving devices 760 , 762 , 764 , 766 replace air - moving devices 136 , then the space vacated by 136 may beneficially be used to house more electronics 134 in cabinet 110 . seventh , the periodic , large airflow holes 154 in the raised floor 142 of the prior - art apparatus 100 are eliminated by this invention , thereby reducing the system &# 39 ; s dependence on the pitch p of removable tiles 146 of the raised floor 142 . for example , in apparatus 200 shown in fig2 , pitch c of cabinets along a row , defined as c η x 8 - x 6 η x 6 - x 4 η x 4 - x 2 η x 2 - x 0 , is substantially unconstrained by the pitch p of the raised - floor tiles 95 , because the only holes therein are small holes for the supply and return pipes 240 and 242 . however , in the prior art , the holes 154 are large , and thus it is more important that the cabinet pitch c and the tile pitch p be more closely synchronized , to avoid interfering with struts that support the raised floor 142 . toward this end , in the prior art , c and p are preferably related by a simple proportion such as mc = np where m and n are small integer such as ( m , n )=( 1 , 2 ) or ( m , n )=( 2 , 3 ). no such restriction applies to the invention . eighth , redundancy of the air - moving devices 136 is improved by the invention vis - à - vis the prior art . specifically , along a flow - through row of cabinets 220 , air - moving devices 136 sharing a common streamline back each other up , such that failure of a single air - moving device 136 is much less significant than for the prior - art &# 39 ; s separate , s - shaped airstreams 138 , wherein failure of an air - moving device can cause the temperature of nearby electronics to rise . for apparatus 700 , similar redundancy is achieved for the supplementary , or alternative , series of air movers 762 , 764 , 766 , 768 . ninth , the invention improves cabinet - packing density vis - à - vis the prior art , thereby saving valuable floor space and also improving electrical - signaling performance between cabinets by allowing shorter cables . specifically , the stream - wise ( x ) dimension of one of the heat exchangers assemblies 210 , 212 , 214 , 216 is typically far smaller than the x dimension of one of the prior art &# 39 ; s combined plenum units 126 , because the heat - exchanger &# 39 ; s x dimension need only be large enough to accommodate tubes and fins to transfer heat from air to water , whereas the combined plenum unit &# 39 ; s x dimension must be large enough to accommodate , through the intake plenum 128 and the exhaust plenum 130 , the large volumetric flow - rate of air , denoted v , that is needed to cool electronics 134 . for example , in the ibm ® bluegene / p ® supercomputer , which comprises electronics 134 in each cabinet dissipating as much as 40 kw , and whose ( x , y , z ) cabinet dimensions are ( 70 cm , 89 cm , 180 cm ), the x dimension of one of the heat exchangers 210 , 212 , 214 , 216 need only be 10 cm , whereas the x dimension of the combined plenum unit 126 must be 52 cm in order to accommodate v = 2 . 35 m3 / s ( 5000 cfm ). thus , cooling bluegene / p according to the current invention saves about 42 cm of width per cabinet , which is about 47 % of the width of the cabinet itself . thereby , the present invention clearly is advantageous for at least the reasons above in use with a supercomputer requiring rows of cabinets such as ibm ®&# 39 ; s bluegene ®, by the single stream of air flowing through a row of cabinets , passing alternately through cabinets and heat exchangers , instead of flowing air separately through each cabinet . while the present invention has been particularly shown and described with respect to preferred embodiments thereof , it will be understood by those skilled in the art that changes in forms and details may be made without departing from the spirit and scope of the present application . it is therefore intended that the present invention not be limited to the exact forms and details described and illustrated herein , but falls within the scope of the appended claims .