6. Conclusions

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

Saved surface (%)-Fnet Fnet

Building A 27–1.21 14–1.28 1.7 Building B 21–1.35 17–1.37 1.72

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

Method 1 Method 2 Traditional method

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

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

seems to be more efficient.

42 HVAC System

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 of microorganisms, and the concentration of chemical pollutants.

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 airflow (VAV) are to be excluded.

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, two approaches are described.

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 (variable with time) required by the zones, while the cold duct delivers air at a constant temperature, lower than the absolute minimum value of the zone supply temperature.

IAQ indoor air quality

TB tuberculosis

Symbols

Subscripts

SARS severe acute respiratory syndrome

D diameter of the duct (m)

( �C)

G air mass flow rate (kg/h)

lk length of the kth trunk (m) Q volume airflow rate (m3

Φ τð Þ sensible thermal loads (J/h)

r refers the variables to the room conditions

i refers the variables to the ith environment

j refers the variables to the jth time interval

sum refers the variables to the summer conditions win refers the variables to the winter conditions

k refers the variables to the kth trunk n refers the variables to the nth zone

in refers the variables to the supply (inlet) conditions

h refers the variables to the hot duct

t temperature (�C)

Fk covering factor of the kth trunk

Fnet covering factor of the whole network

cp specific heat at constant pressure of moist air (J/kg �C)

Δth,in ¼ thð Þ� τ tinð Þτ temperature difference between hot duct and inlet air conditions

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

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

45

Δtc,in ¼ tinð Þ� τ tcð Þτ temperature difference between inlet conditions and cold duct (�C)

Gk,max mass flow rate that the k-trunk could carry if the air flowed at the

/h)

maximum set speed (kg/h)

tin,min<sup>j</sup> minimum inlet temperature at the time j (�C)

vmax maximum air speed in the ducts (m/s)

The new approach implies reduced overall dimensions and cost of the channel network.

The method has been applied to some networks of channels, and results were compared with those obtained, on the same networks, using the traditional sizing criteria.

The comparison was carried out in terms of diameters, network factors, and total side surfaces of the network. It shows that the overall dimensions of the networks decrease compared to the traditional sizing methods; the factor Fnet varies between 1.21 and 1.37, while in traditional sizing, it assumes values around 1.7. As it has been seen in previous works [19], the decrease of network factor (and of side surfaces) is more significant for higher temperature of the hot duct (method 1) and lower temperature of cold duct (method 2). The constraint on the maximum speed of the air in the various trunks of the network is always respected, while it does not always occur with traditional criteria. The saving in terms of side surface varies between 27 and 21% with reference to the traditional approach for method 1 and between 14 and 17% for method 2. 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.

These new methods of sizing a dual duct network allow for a lighter network, with obvious economic advantages, and are easier to place. The network is able to guarantee the thermohygrometric comfort conditions for all conditions of thermal load, even in the medium periods; in addition, it allows a control of the air speed, always remaining below the maximum allowed values (this does not always happen with the traditional method).

It must be remembered that, unlike the traditional method that plans fixed temperature values for the hot and cold ducts, the proposed methods require a more complex control system that, starting from the value of the minimum or maximum supply temperatures of the different zones (which vary over time), can vary the temperature of the cold or hot duct.

For method 1, which guarantees better results compared to method 2, a further postheating battery dedicated to the cold duct must be provided.
