5. Analysis of results

The diameters of the hot and cold channels are representative of the size of the whole network; the peripheral surface of the ducts stands as indicative parameter for the weight of the network and therefore of its cost. As an index of the overall dimension of the single trunk k, the

> Fk<sup>¼</sup> Gk, max Gk

It is defined as the ratio between the airflow rate Gk that flows through the kth trunk and

<sup>4</sup> vk <sup>D</sup><sup>2</sup>

which represents the flow rate that the trunk, already dimensioned, could carry if the air

The Fk factor takes values between 1 and 2; it approaches 1 when only one of the two ducts actually carries the entire flow rate of the trunk, while the other duct carries only a small correction. The factor tends instead to 2 when the both hot and cold ducts can both carry the entire flow rate of the trunk. With regard to the whole network, the Fnet factor can be defined as the weighted average of Fk, where the weights are the products between the lengths and the

hkþD<sup>2</sup> ck

� � (27)

Gk, max<sup>¼</sup> <sup>π</sup>

Network BA Network BB Network BC

k lk½ � <sup>m</sup> vmax½ � <sup>m</sup>=<sup>s</sup> Qk <sup>m</sup><sup>3</sup> ½ � <sup>=</sup><sup>h</sup> lk½ � <sup>m</sup> vmax½ � <sup>m</sup>=<sup>s</sup> Qk <sup>m</sup><sup>3</sup> ½ � <sup>=</sup><sup>h</sup> lk½ � <sup>m</sup> vmax½ � <sup>m</sup>=<sup>s</sup> Qk <sup>m</sup><sup>3</sup> ½ � <sup>=</sup><sup>h</sup> 1 3.00 17.50 12551.76 3.00 17.50 14691.15 3.00 17.50 13593.78 2 2.50 12.50 1774.82 2.50 12.50 2143.52 2.50 12.50 2089.40 3 5.00 17.50 10776.94 5.00 17.50 12547.63 5.00 17.50 11504.38 4 3.00 17.50 8573.20 3.00 17.50 10790.94 3.00 17.50 9563.88 5 2.50 12.50 2562.82 2.50 12.50 2586.83 2.50 12.50 2043.60 6 8.00 15.00 6010.38 8.00 15.00 8204.11 8.00 15.00 7520.28 7 3.00 15.00 4166.51 3.00 15.00 5965.41 3.00 15.00 5385.96 8 2.50 12.50 1982.66 2.50 12.50 3402.44 2.50 12.50 3463.73 9 6.00 12.50 2183.85 6.00 12.50 2562.98 6.00 12.50 1922.23 10 2.50 12.50 1843.87 2.50 12.50 2238.69 2.50 12.50 2134.32 11 2.50 12.50 2203.74 2.50 12.50 1756.69 2.50 12.50 1940.50

Fnet¼

P k P

FkGklk <sup>k</sup>Gklk

(26)

(28)

covering factor Fk was also introduced, according to [16, 19]:

Table 4. Data of the networks BA, BB, and BC of building B.

Gk, max

40 HVAC System

flowed at the maximum set speed.

flow rates of the trunks:

The results of the dimensioning of the networks with the first approach (method 1) refer to the networks AA, AB and BA, BB, and BC in the case of air temperature tc in the cold duct taken equal to the minimum required (minus 1�C) and of the air temperature th in the hot duct taken as equal to 40�C. Results obtained with the second approach (method 2) refer to the same networks, in the case of air temperature th in the hot duct taken equal to the maximum required (plus 1�C) and of air temperature tc in the cold duct taken as equal to the dew point value (for building A, 13�C in summer and 10�C in winter, for building B, 13�C in summer and 7�C in winter).

In order to make a comparison between the results obtained with the traditional sizing method, the savings are briefly presented in Tables 5 and 6, respectively, for building A and building B, as they are obtained for the five networks, in terms of the peripheral surface of the channels and of the evaluated values of the network factor Fnet.

In Table 7 we report the savings of side surface and the network factor by considering the whole building A and the whole building B.

For the building A, the maximum value obtained for Fnet is 1.33, and it occurs for the network AA when the hot duct temperature th varies (method 2); it decreases to 1.15, for the networks AB when we use the method 1. For the building B, the maximum is obtained for the network BC when method 2 is used; the minimum occurs for the network BA with method 2 again. The achieved values of Fnet with the proposed methods are always smaller than those obtained by using the traditional design criteria.


For method 1: th <sup>¼</sup> <sup>40</sup>� C. For method 2: tc, sum <sup>¼</sup> <sup>13</sup>� <sup>C</sup>, tc, win <sup>¼</sup> <sup>10</sup>� C:

Method 1 Method 2 Traditional method Saved surface (%)-Fnet Fnet Network BA 18–1.41 14–1.283 1.65 Network BB 25–1.34 18–1.36 1.70 Network BC 19–1.43 15–1.47 1.80 For method 1: th <sup>¼</sup> <sup>40</sup>� C. For method 2: tc, sum <sup>¼</sup> <sup>13</sup>� C, tc, win ¼ 7 � C.

Table 5. Savings' percentage of the total side surface and network factors for building A.

Table 6. Savings' percentage of the total side surface and network factors for building B.


Table 7. Savings' percentage of the total side surface and network factors for buildings A and B.

The results prove that the new methods allow a substantial reduction in the overall dimensions. This reduction is largely shared by the whole network. For all trunks, and with regard to method 1, a substantial equivalence, or a weak increase of the diameters of the cold duct (compared to those obtained with the traditional methods), is counteracted by a significant reduction of the diameters of the hot duct. Vice versa, with regard to method 2, a weak increase of diameters of the hot duct is counteracted by a substantial reduction of diameter of the cold duct. The effect on the overall dimensions can be represented in terms of sum of the diameters of cold and hot ducts. The comparison between the methods is reported in Figure 1 for each trunk of a network. Figure 1 refers to the case of the building A (network AA). Similar behaviors occur for the other networks; in terms of reduction of overall dimensions, method 1 seems to be more efficient.

The surface fraction, saved by using both the new methods compared to the traditional one, presents not negligible values. With reference to the traditional method, the savings range

between 14 and 27% for the building A (located in Lecce), when methods 2 and 1 are used,

Air Conditioning Systems with Dual Ducts: Innovative Approaches for the Design of the Transport Network…

http://dx.doi.org/10.5772/intechopen.80093

43

Therefore, both methods imply a reduction of side surface (and cost). For the building B (located in Rome), for which the room temperature in winter is lower, the savings obtained in terms of surface, by using the two methods, differ only for 4%. For the building A (in Lecce), for which the room temperature in winter is higher, the savings obtained with method 2 is almost half of that obtained with method 1. Method 2 works better for the building B, where room temperature is higher.

Both methods imply lower network factors, with respect to the traditional method, but it is preferable to use one method rather than another according to the summer and winter design temperatures. In our study, a lower winter design temperature implies an increase in savings

If we consider the absolute maximum and minimum values of the supply temperature (for all hours, all year round, and regardless of the zone), we can see that the network factors tend to increase with the difference between these maximum and minimum values, for both methods

In general in healthcare facilities, and in any case in many critical environments contained therein, the indoor air quality (IAQ) plays a significant role. For the health of patients, particularly immunosuppressed patients, it is necessary to maintain at the lowest possible levels the concentration of particulate matter, which may also be a support for the formation of colonies

In these cases (particularly in operating rooms, intensive care units, or departments for immunosuppressed patients), the air conditioning systems generally used are all-air systems with (outdoor) constant flow (CAV), since the high number of air changes per hour (ACH) must be guaranteed (sometimes values up to 50 are achieved). The leading value of the flow rate is that related to ventilation, rather than to summer or winter loads, and all-air systems with variable

The dual duct system ensures excellent IAQ and good control of the thermo-hygrometric conditions and allows temperature adjustment in each zone, up to individual environments (rooms). In this chapter, an innovative approach is presented for the channel dimensioning, based on the choice of not constant values for the temperatures of hot and cold duct. More specifically,

For the first approach, the cold duct carries air at a not constant temperature, equal to or slightly lower than the minimum supply air temperature, among those required hour by hour by the different zones; the hot duct carries air at a constant temperature, higher than the absolute maximum value of the zone supply temperature. For the second one, the hot duct transports air at a temperature value slightly higher than the maximum inlet temperature

1 and 2. In general, method 1 allows to obtain lower network factors.

of microorganisms, and the concentration of chemical pollutants.

respectively, and between 17 and 21% for the building B.

by using method 2.

6. Conclusions

airflow (VAV) are to be excluded.

two approaches are described.

Figure 1. Overall dimensions for each trunk of the network: sum of diameters of cold and hot ducts.

between 14 and 27% for the building A (located in Lecce), when methods 2 and 1 are used, respectively, and between 17 and 21% for the building B.

Therefore, both methods imply a reduction of side surface (and cost). For the building B (located in Rome), for which the room temperature in winter is lower, the savings obtained in terms of surface, by using the two methods, differ only for 4%. For the building A (in Lecce), for which the room temperature in winter is higher, the savings obtained with method 2 is almost half of that obtained with method 1. Method 2 works better for the building B, where room temperature is higher.

Both methods imply lower network factors, with respect to the traditional method, but it is preferable to use one method rather than another according to the summer and winter design temperatures. In our study, a lower winter design temperature implies an increase in savings by using method 2.

If we consider the absolute maximum and minimum values of the supply temperature (for all hours, all year round, and regardless of the zone), we can see that the network factors tend to increase with the difference between these maximum and minimum values, for both methods 1 and 2. In general, method 1 allows to obtain lower network factors.
