sure drop which each pump meets is halved. In the so-lution with connected units it is also necessary, howe- ver, to consider the pressure drop which is due to the interconnected collectors. Provisionally, these pressure drops can be estimated at being about 20% of the total pressure drop and with the following result:

TABLE MANUFACTURER 1

E

NERGY SAVING IN TERMS OF MW

/

H AND ECONOMIC SAVING

%

FOR ANNUAL OPERATION BETWEEN AN INTELLIGENT FREE

-

COOLING SYSTEM AND TRADITIONAL FREE

-

COOLING

Energy saving [kWh][%]Frankfurt 345605% Rome 315873% PC

(2+1)

= ~

PC

2

+ 20% (PC 2

2

) Milan 291324% The decrease in the pressure drop implies an increase in the water flow and it is therefore necessary to com- pare the water flow in both cases. Manchester 460086% Paris 369544% Amsterdam 425587% Stockholm 281675%

Water capacity

Madrid 367434% Given that the water flow at nominal conditions is 70 m³/h, and considering the reduction in the pressure drop (which changes from 24 m w.c.m to 16.8 w.c.m), it is possible to obtain a new resistive curve and con-sequently the new water flow with the connected uni-ts, which is about 80 - 81m³/h. The water flow for each group of free-cooling coils is: (81 x 2)/3 = 54 m³/h.There is, therefore, a reduction in the flow per group of free-cooling coils from 70 m³/h to 54 m³/h and therefore a reduction of about 23%. Berlin 315254% London 460186% Copenhagen 380776%

Heat exchange capacity

It is now necessary to analyse how the water flow va-ries the heat transfer capacity of the free-cooling coils. By using analytical procedures, which for reasons of brevity are not shown here, it is possible to demon- strate how when considering the water flow in the two cases:1) for single units: water flow = 70000 l/h2) for connected units: water flow = 70000 l/h - 23% = 53900 l/hit is possible to establish the thermal exchange co-effi-cient value which, by interconnecting the free-cooling exchangers, can theoretically increase by up to 40%.The heat exchanged in the free-cooling coils will now be examined. This is given by: Q = K • S •

in

= 10°C • 11 l/h + 15°C • 70 l/h = 14,3°C 81 l/h Therefore, the relationship between the heat which is effectively exchanged in the two cases is: Q

FIGURE 6. C

OMPARISON EXAMI

-

NING WATER FLOW IN EACH OF THE
(2+1) TWO CASES

. S

INCE CONNECTING THE
(2)

(K =(K

(2+1) •

S

(2+1)

)•( Δ T

(acqua/aria)

)

(2+1) 2 •

S

2

) ( Δ T

(acqua/aria)

)

2

=1,43•0,93 = 1,33 Q

FREE COOLING COILS TO EACH OTHER INCREASES THE CROSS SECTION OF

Δ T

Q

(2+1)

= 1,33 • Q

2 WATER FLOW BY BY

1/5

FOR EACH PUMP COMPARED TO THAT FOR A SIN

-

Therefore, by interconnecting the free-cooling exchan-gers, the heat exchanged can increase by up to 33%.

GLE UNIT

,

THE PRESSURE DROP WHICH EACH PUMP MEETS IS HALVED

.

acqua/aria

It can be observed, therefore, that the heat exchanged depends not only on the capacity of the K•S exchange, but also by the difference between the temperature of the inlet water and the ambient air. Since the system is sized according to the water flow at nominal condi-tions, if this value increases, a by-pass situation will be created. Given that the nominal water flow is 70000 l/h, and that the system is sized according to this flow, if this flow increases, a certain amount of this flow will be re-cir-culated, as shown in figure 7.If an external temperature of 5°C (total free-cooling temperature) and a water temperature of 15°C SingleUnitConnected unitsPC (Δ T = 5 with set-point: 10°C) are hypothesized, the temperatu-re before the free-cooling pump is calculated propor- tionally: T

2

PC

(2+1)

FIGURE 6 4