1. Introduction

The control of the thermo-hygrometric conditions and indoor air quality (IAQ) in rooms generally requires extensive networks for the transport of heat transfer fluids, most commonly water and air. A network of air distribution, due to the low density of the air, is considerably bulky. In general in healthcare facilities, and in any case in many critical environments contained therein,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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. This is achieved with a significant dilution of the contaminants, by means of the introduction into the environment of considerable airflows. Italian law [1–3] establishes that the air is drawn exclusively from outside (therefore recirculation is forbidden) and subjected to various filtration stages.

their installation, high values, depending on the flow rate, are set for velocities through the ducts. At values between 12.5 and 30 m/s, the arising aerodynamic noise may be greatly disturbing, and this noise problem is only partially solved by properly designing the channel configuration [18]. Nevertheless, the overall dimensions of the dual-channel networks still

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

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

33

The choice of constant temperature values for the cold duct and for the hot duct, respectively, lower and higher than the room temperature, implies that the hot and the cold flow rates, arising from the calculations, have high values, significantly close to the total flow rate that must be carried by the trunks. In fact, while the supply air temperature varies over time, tending to tc in summer and to th in winter, the air temperatures of cold and hot ducts are generally kept almost constant. When the zones require cold (presumably in summer), almost entire flow rates of the zones come from the cold duct; if the zones require heat, presumably in winter, the most part of the total airflow rates comes from the hot duct. Then, both the cold and the hot duct should each be able to carry more than 80—90% of the flow

In this chapter, an innovative approach is presented for the dimensioning of the channels, based on the choice of not constant values for the temperatures of hot and cold duct. The cold channel carries air at a temperature equal or slightly lower than the minimum supply air temperature, among those required by the different zones, variable with time. The hot duct delivers air at a constant temperature, higher than the absolute maximum value of the zone supply temperature [19]. As alternative, the hot channel transports air at a temperature value slightly higher (1 ÷ 2�C) 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

The method has been applied to some reference buildings, a day center for dialysis located in the Italian city of Lecce and a private hospital located in the Italian city of Rome. A comparison

The network of channels has a tree structure with the root representing the AHU and the

The different supply air conditions determine the different thermal zones. The calculation proceeds by considering the sensible thermal load Φið Þτ (of the ith environment), variable over time, and evaluating the constant flow rate Gi able to compensate these loads in each room. The calculation is also based on the usual constraint related to the temperature difference between air supply and room and takes into account the flow rate required for ventilation. The mixing box serving the zone, where the ith room is included, will supply the computed airflow rate required by the zone; into the mixing box will enter hot and cold airflow rates

is produced with the results obtained from the design criteria traditionally used.

remain high.

rate of the zone.

minimum value of the zone supply temperature.

leaves the terminal units of each zone.

2. Dimensioning criteria for the channel network

coming from the terminal trunks of the dual duct system.

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) have to be excluded.

The constraint on the airflow rate is very strict in all those cases in which the protection of persons from the propagation of pathogens and/or the action of chemical pollutants must be guaranteed, even outside strictly health-related environments. In fact, new problems always emerge relative to air quality: the resurgence of diseases originally eradicated, not least tuberculosis (TB); the onset of respiratory syndromes of acute type, some resistant to drugs and therefore potentially pandemics, as SARS [4–8]; and the threats of chemical and biological terrorism.

If it is necessary to remain in the context of the constant flow rate systems, one of the options is that of the dual duct system. It ensures a local control of temperature conditions, up to the individual environments (rooms), even if some require "hot" while others require "cold", and an excellent level of air quality.

On the other hand, this type of system implies expensive and bulky air distribution networks, requiring spaces for the installation of the duct systems not easily available in the building sector, and high energy consumption [9–14], conceivably due to the remarkable availability of cheap energy at the time when this type of system has been developed (1940s–1960s of the last century).

As it is known, two parallel branched networks transport the air from the air handling unit (AHU) to the terminal units. Air flows through the so-called hot duct at higher temperature, compared to the required temperature in the room [15–17], whereas it flows at lower temperature (than the room temperature) through the so-called cold duct. A mixing box, thermostatically controlled, draws air from the hot and cold ducts and supplies each zone with the required flow rate, at the appropriate supply air conditions.

By virtue of these supply air conditions, more specifically the supply air temperature, widely variable during daytime and along the course of the year, the conditions of comfort into the room can be ensured, for which the mean radiant temperature is not taken into account.

The mixing box limits the noise levels as well. Indeed, in order to avoid huge overall dimensions of ducts that may become excessive with respect to the spaces normally available for their installation, high values, depending on the flow rate, are set for velocities through the ducts. At values between 12.5 and 30 m/s, the arising aerodynamic noise may be greatly disturbing, and this noise problem is only partially solved by properly designing the channel configuration [18]. Nevertheless, the overall dimensions of the dual-channel networks still remain high.

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. This is achieved with a significant dilution of the contaminants, by means of the introduction into the environment of considerable airflows. Italian law [1–3] establishes that the air is drawn exclusively from outside (therefore

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

The constraint on the airflow rate is very strict in all those cases in which the protection of persons from the propagation of pathogens and/or the action of chemical pollutants must be guaranteed, even outside strictly health-related environments. In fact, new problems always emerge relative to air quality: the resurgence of diseases originally eradicated, not least tuberculosis (TB); the onset of respiratory syndromes of acute type, some resistant to drugs and therefore potentially pandemics, as SARS [4–8]; and the threats of chemical and biological

If it is necessary to remain in the context of the constant flow rate systems, one of the options is that of the dual duct system. It ensures a local control of temperature conditions, up to the individual environments (rooms), even if some require "hot" while others require "cold", and

On the other hand, this type of system implies expensive and bulky air distribution networks, requiring spaces for the installation of the duct systems not easily available in the building sector, and high energy consumption [9–14], conceivably due to the remarkable availability of cheap energy at the time when this type of system has been developed (1940s–1960s of the last

As it is known, two parallel branched networks transport the air from the air handling unit (AHU) to the terminal units. Air flows through the so-called hot duct at higher temperature, compared to the required temperature in the room [15–17], whereas it flows at lower temperature (than the room temperature) through the so-called cold duct. A mixing box, thermostatically controlled, draws air from the hot and cold ducts and supplies each zone with the

By virtue of these supply air conditions, more specifically the supply air temperature, widely variable during daytime and along the course of the year, the conditions of comfort into the room can be ensured, for which the mean radiant temperature is not taken into account.

The mixing box limits the noise levels as well. Indeed, in order to avoid huge overall dimensions of ducts that may become excessive with respect to the spaces normally available for

recirculation is forbidden) and subjected to various filtration stages.

with variable airflow (VAV) have to be excluded.

required flow rate, at the appropriate supply air conditions.

terrorism.

32 HVAC System

century).

an excellent level of air quality.

The choice of constant temperature values for the cold duct and for the hot duct, respectively, lower and higher than the room temperature, implies that the hot and the cold flow rates, arising from the calculations, have high values, significantly close to the total flow rate that must be carried by the trunks. In fact, while the supply air temperature varies over time, tending to tc in summer and to th in winter, the air temperatures of cold and hot ducts are generally kept almost constant. When the zones require cold (presumably in summer), almost entire flow rates of the zones come from the cold duct; if the zones require heat, presumably in winter, the most part of the total airflow rates comes from the hot duct. Then, both the cold and the hot duct should each be able to carry more than 80—90% of the flow rate of the zone.

In this chapter, an innovative approach is presented for the dimensioning of the channels, based on the choice of not constant values for the temperatures of hot and cold duct. The cold channel carries air at a temperature equal or slightly lower than the minimum supply air temperature, among those required by the different zones, variable with time. The hot duct delivers air at a constant temperature, higher than the absolute maximum value of the zone supply temperature [19]. As alternative, the hot channel transports air at a temperature value slightly higher (1 ÷ 2�C) 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.

The method has been applied to some reference buildings, a day center for dialysis located in the Italian city of Lecce and a private hospital located in the Italian city of Rome. A comparison is produced with the results obtained from the design criteria traditionally used.
