Abstract:
An HVAC system has two variable-air-volume (VAV) systems, a first VAV system for moving fresh air and a second VAV system for moving recycled air. Both the fresh air and recycled air are cooled and dehumidified by a single heat-exchanging coil. The fresh and recycled air streams travel independently in separate ducts until the air streams reach a remotely located mixing box where the air streams mix and ventilate into a room or zone of a building. The HVAC system uses carbon dioxide and temperature sensors in order to regulate the flow of fresh and recycled air to various mixing boxes throughout the building.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The following application claims the benefit of U.S. Provisional Patent Application, Ser. No. 60/334,062 which was filed, Nov. 30, 2001, entitled, SINGLE COIL TWIN VARIABLE AIR VOLUME (VAV) SYSTEM FOR ENERGY EFFICIENT CONDITIONING OF INDEPENDENT FRESH AND RETURN AIR STREAMS and which has the same inventors. 

   FIELD OF THE INVENTION 
   This invention relates to heating, ventilation and air conditioning (HVAC) and, more particularly, to energy-efficient cooling and ventilation systems. 
   BACKGROUND OF THE INVENTION 
   Air-conditioning, especially in tropical climates, is dictated by the stringent requirements of cooling and dehumidification. The HVAC engineer must seek an optimal design, balancing energy consumption and cost, thermal comfort, and indoor air quality (IAQ), the latter criterion being a function of how well the building is ventilated. Although health and thermal comfort issues can be usually addressed by attempting to eliminate the various sources of indoor pollutants, it is almost impossible to totally eliminate them. It is thus inevitable that ventilation with fresh air should play an important role in the eventual quality of the air in the indoor environment. The high energy-penalty of cooling and dehumidification associated with ventilation in the tropical context is apparent from the large enthalpy difference that exists between the outside air and the indoor air conditions. 
   Inadequate ventilation almost invariably leads to a deterioration of the indoor air quality due to a build-up of indoor pollutants such as carbon dioxide. Adequate ventilation with poor dehumidifying performance of the cooling coil would be even worse as it is likely to result in elevated humidity levels in the air distribution systems and the occupied zones. This phenomenon could be considered in the context of humidity as an “indoor” pollutant, which propagates the origin and sustenance of microbial contamination. The issue of high indoor humidity levels in tropical buildings originates from the high ambient humidity levels in the ventilation air (fresh air) and the need to provide adequate ventilation to avoid “Sick Building Syndrome”. 
   The basic requirements of any air-conditioning system are to provide comfortable temperature and humidity conditions for the occupants of a building. In a steady-state environment, this is a relatively simple task. However, in a dynamic indoor environment, where cooling and dehumidifying requirements fluctuate with changes in weather and occupancy, designing an energy-efficient air-conditioning is anything but a simple task. Optimizing the design of an air-conditioning system is an even more arduous task in hot and humid climates, where the fresh air contains extremely high humidity levels at all times of the year. 
   In typical cooling and dehumidifying coil designs, an air stream having an intake temperature and humidity flows over the tubes and fins of a coil while chilled water flows through the tubes of the coil. The chilled water enters the coil at a supply temperature and leaves at a return temperature. The psychrometric performance of the cooling coil determines the cooling and dehumidifying of the air flowing past the coil. In the event of two different air streams being conditioned, two separate coils would typically be employed with either a parallel or a sequential feed of chilled water. 
   DESCRIPTION OF THE PRIOR ART 
   Efforts have been directed in the past to achieve energy-efficient dehumidifying performance by addressing the air velocity across the coil, the chilled water velocity through the tubes of the coil and various configurations of the physical geometry of the coil. These designs are exemplified by, for instance, the following patents: U.S. Pat. No. 4,876,858 (Shaw), EP00415747A2 (Shaw), U.S. Pat. No. 5,461,877 (Luminis), KR09302466B1 (Luminis) and WO09220973A1 (Luminis). Some recent enhancements are aimed at collecting the water from the first coil in a manifold and then feeding this water (at the temperature of the chilled water return of the first coil) in a sequential mode to the tubes of the second coil. In essence, this still amounts to two different heat-exchanging coils both on the air side as well as the chilled water side. 
   However, certain drawbacks are evident from this two-coil design. The primary drawback is that two-coil systems are not very energy-efficient. Secondly, two-coil systems are costly to build, as they require more components. Thirdly, twin-coil systems occupy more space in a building, thus diminishing the usable floor space in building layouts. 
   Thus, there is a need in the art for an improved ventilation and air conditioning system that overcomes the foregoing deficiencies. 
   SUMMARY OF THE INVENTION 
   It is thus an object of the present invention to provide a single-coil twin-fan HVAC system for energy-efficient conditioning of independent fresh and recycled air streams. 
   As embodied and broadly described herein, the present invention provides a ventilation and air-conditioning system for energy-efficient conditioning of independent fresh and recycled air streams, said system comprising a first intake for drawing in a volume of fresh air; a second intake for drawing in a volume of recycled air; a single-coil heat exchanger for conditioning both the volume of fresh air and the volume of recycled air, thereby producing a supply of conditioned fresh air and a supply of conditioned recycled air; a first fan for moving said supply of conditioned fresh air downstream in a first duct; a second fan for moving said supply of conditioned recycled air downstream in a second duct; a mixing box connected downstream to said first duct and said second duct, said mixing box being an enclosure in which said supply of conditioned fresh air and said supply of conditioned recycled air may mix prior to being ventilated into a room or zone; and a control system for sensing a zonal temperature and a zonal ventilation requirement and for regulating said supply of conditioned fresh air in accordance with said zonal ventilation requirement and for regulating said supply of conditioned recycled air in accordance with said zonal temperature. 
   With such a system, the advantages are remarkable. A single coil enables the simultaneous conditioning of two separate air streams without mixing in the downstream section of the coil. By treating the fresh and return air streams separately (using separate fans which are independently and separately controlled), the system achieves optimal dehumidifying performance on the fresh air stream (which is predominantly latent cooling in the tropics) and optimal sensible cooling performance on the recycled air stream (which is predominantly sensible cooling). Another key advantage is the ability to achieve the desired cooling and dehumidifying performance in a rather independent manner using a single coil, both on the air and the chilled water sides. A single coil can be installed in a single air-handling unit (AHU) with suitable modifications, which essentially involves installing a thermally insulated sheet-metal barrier to distinctly separate the fresh and the return air streams. By using a single coil with a single chilled water feed, there is no longer any need for intermediate return and supply manifolds between the first (fresh air) and the second (return air) coils. Consequently, the single-coil twin-fan system not only occupies less floor area but it also costs less to build, install and operate. 
   In other words, the present invention is a single-coil twin-fan system for cooling and dehumidifying two separate air streams. Preferably, the fresh and return air streams are moved by variable-air-volume systems (one for each air stream). The fresh air and recycled air travel in separate ducts but through a single, common coil that cools and dehumidifies both air streams. The fresh air stream and the recycled air stream do not mix when the two streams flow through the coil. The fresh air and recycled air only mix downstream at a mixing box just prior to being ventilated into the room or zone to be cooled. The present invention diminishes humidity levels in the occupied zones of a building in direct response to occupant density. It is to be emphasized that occupant density alone governs the dehumidifying performance of the system in a distinct manner and all other thermal loads govern the cooling performance of the system separately. 
   This new air conditioning system is capable of distributing two streams of air (one stream of conditioned fresh air and the other stream of conditioned recycled air) to various mixing boxes located throughout a building. By feedback signals, the present invention optimizes energy-efficiency by tailoring the ventilation and cooling requirements in various zones of a building. This system thus provides enhanced indoor air quality (IAQ) with improved energy efficiency. 
   The significant advantage of this invention over existing state-of-the-art cooling and dehumidifying coils is the relative ease of operation of the coil, which uses a single feed of coolant (e.g. chilled water). The dynamic psychrometric performance of the coil is achieved by modulating the rate of coolant flow as a function of changing cooling and ventilation requirements. 
   Other objects and features of the invention will become apparent by reference to the following description and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further described, by way of example only, with reference to the accompanying drawings wherein: 
       FIG. 1  is a schematic depicting an energy-efficient single-coil twin-fan HVAC system in accordance with the present invention. 
       FIG. 2  is a simplified schematic of the system depicted in  FIG. 1 . 
       FIG. 3  is an isometric view of the heat-exchanging coil of the system depicted in  FIG. 1 . 
       FIG. 4  is a psychrometric chart showing graphically the superior dehumidifying performance of the system depicted in  FIG. 1 . 
   

   In the drawings, preferred embodiments of the invention are illustrated by way of examples. It is to be expressly understood that the description and drawings are only for the purpose of illustration and are an aid for understanding. They are not intended to be a definition of the limits of the invention. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , an HVAC system denoted generally by the reference numeral  2  for ventilating and conditioning the air in a building  4  has an air-handling unit (AHU)  5  which comprises two separate variable-air-volume (VAV) systems  6 ,  8 . The first VAV system  6  draws in fresh air from outside through a fresh air intake  10 . The fresh air intake  10  has a diffuser  11  which expands and slows the fresh air for maximal heat transfer. The fresh air is first filtered through a fresh air filter  14  and then cooled and dehumidified by a coil  20 . The coil  20  is a heat-exchanging coil with a single feed of chilled water. A fresh air fan  22  moves the conditioned fresh air downstream through a fresh air duct  26  to a fresh air VAV box  30 . The fresh air VAV box  30  is connected to a mixing box  38  into which the fresh air may flow. A fresh air damper  31  regulates the flow of conditioned fresh air into a mixing chamber  39  which is a portion of the mixing box  38 . 
   The second VAV system  8  draws recycled air from inside the building through a recycled air intake  12 . The recycled air intake  12  has a diffuser  13  to expand and slow the recycled air for maximal heat transfer. The recycled air is first filtered through a recycled air filter  16  and then cooled and dehumidified by said coil  20 . The coil  20  is the same heat-exchanging coil that cools the fresh air, even though the fresh air and recycled air travel in separate ducts. A recycled air fan  24  moves the conditioned recycled air downstream through a recycled air duct  28  to a recycled air VAV box  32 . The recycled air VAV box  32  is connected to the mixing box  38  into which the recycled air may flow. A recycled air damper  33  regulates the flow of conditioned recycled air into the mixing chamber  39  of the mixing box  38 . 
   A feedback control system, which is managed by a central processing unit (CPU)  54 , controls the operation of the HVAC system  2 . The CPU  54  computes optimal fan speeds and damper openings based on temperature and carbon dioxide (CO 2 ) readings from sensors located at various places in the building  4 . 
   Referring to  FIG. 1 , a CO 2  sensor  34  is provided for measuring the carbon dioxide concentration in the air. The CO 2  sensor is located in a return air duct  46  near a return vent  44 . The return air duct  46  also contains a temperature sensor  36  for measuring the air temperature leaving the room. The carbon dioxide sensor  34  and the temperature sensor  36  are linked via wires  55  (or cables) to the central processing unit (CPU)  54  of the control system. The CPU  54  computes the optimal damper openings for the fresh air damper  31  and the recycled air damper  33  so that the optimal mixture of fresh and recycled air flows into a mixing chamber  39  in the mixing box  38 . The CPU  54  also regulates the fan velocity of each VAV system to vary the overall cooling and ventilation in accordance with total building demand. The CPU  54  controls the first VAV system  6  and the second VAV system  8 . 
   The AHU  5  comprises a fresh air temperature sensor  56  located between the coil  20  and the fresh air fan  22 , that measures the “off-coil” temperature of the fresh air stream. The AHU  5  also comprises a recycled air temperature sensor  58  located between the coil  20  and the recycled air fan  24 , that measures the “off-coil” temperature of the recycled air stream. A fresh air humidity sensor  57  is also located in the AHU  5  between the coil  20  and the fresh air fan  22 . There are mainly two control modes for the system. In normal operating conditions (i.e. when humidity is within acceptable limits), the CPU  54  uses the temperature signal from the recycled air temperature sensor to control the coolant circuit  21 . When the humidity level of the fresh air downstream of the coil exceeds the acceptable limit, the CPU  54  uses the temperature signal from the fresh air temperature sensor to control the flow rate of the coolant in the coolant circuit  21 . As long as the humidity downstream of the heat exchanger  20  is within acceptable set-point limits, the temperature of the recycled air downstream of the heat exchanger  20  will predominate in the control. 
   Referring still to  FIG. 1 , conditioned fresh air and conditioned recycled air are mixed in the mixing box  38  in accordance with the local cooling and ventilation requirements. The mixing box  38  preferably functions as a plenum chamber. A plenum chamber is an enclosed space in which the air pressure is greater than that in the outside atmosphere. Air is thus forced into the mixing box  38  (plenum chamber) for slow distribution through a localized diffuser vent  40 . 
   As noted above, the mixed air is vented through a diffuser vent  40  into a room  42  (or office or building zone). The room  42  also has the return vent  44 , which draws room air into the return air duct  46 . A return fan  48  is provided to assist the return flow of the recycled air back to the second VAV system  8 . The return air duct  46  bifurcates into an exhaust vent  50  and the recycled air intake  12 . A controllable damper  52  is located at the bifurcation of the return air duct  46 . The damper  52  can be controlled to divert a variable amount of recycled air back to the second VAV system  8 , i.e., into the recycled air intake  12 . The CPU  54  controls the damper  52  to alleviate air pressure build-up when large volumes of fresh air are ventilated into the system. 
   As depicted by the simplified schematic in  FIG. 2 , the HVAC system  2  has two variable-air-volume (VAV) systems which share the single, common coil  20  for cooling and dehumidifying both the fresh air stream and the recycled air stream. In other words, the fresh air is filtered, conditioned and moved downstream by the first VAV system  6  whereas the recycled air is filtered, conditioned and moved downstream by the second VAV system  8 . However, the first and second VAV systems  6 ,  8  share the same cooling and dehumidifying coil  20 . Each pass of chilled water through the tubes of the coil serves both the fresh air stream and the return air stream. The circuiting of the tubes is done in such a way so as to ensure as much of a counter-flow arrangement as possible, which is illustrated in  FIG. 3 . Also shown in  FIG. 3  are two coil sub-circuits  20   a  and  20   b . It is important to note that these form part of the single, integrated coil  20 . They are arranged into sub-circuits  20   a ,  20   b  in order to optimize counter-flow heat transfer. A thermally insulated sheet metal barrier (a “common wall”)  27  separates the fresh air stream from the recycled air stream. The barrier, or common wall,  27  should be suitably modified to interface with one of the plate fins of the heat exchanger to ensure that the air streams remain independent and unmixed as they flow through the heat exchanger  20 . 
   Alternative circuiting arrangements are, of course, possible.  FIG. 3  is a schematic diagram of what takes place in the single coil of the system. It illustrates a full-circuiting arrangement (with four passes, for example). A half-circuiting or one-third-circuiting arrangement could also be used. In all of the circuiting arrangements, there is but one, single coil  20  for cooling and dehumidifying both the fresh air and the recycled air. The optimal dehumidifying performance of the single-coil is achieved by the fact that there exists a high driving potential for dehumidification between the high humid conditions of the fresh air in the tropics and the cold surface of the coil. The leaving condition of the fresh after passing through the coil will be typically fully saturated. 
     FIG. 4  illustrates the psychrometric performance of the single-coil twin-fan HVAC system  2 . (A psychrometric chart shows the humidity of air as a function of temperature.) In order to ensure proper cooling and ventilation, the psychometric performance of the coil is controlled based on readings from the “off-coil” temperature of each air stream. The two “off-coil” temperatures play a crucial role in controlling the dynamic psychrometric performance of the coil to ensure acceptable thermal comfort and indoor air quality (IAQ) in the occupied zones. The chart shown in  FIG. 4  illustrates the overall dehumidifying performance of the HVAC system  2 . Outside air is represented on the graph as condition curve  60 . The first VAV system (the fresh air stream) has a condition curve  62  having an off-coil temperature and humidity plotted at point  64  and on-coil conditions plotted at point  66 . The second VAV system (the recycled air stream) has a condition curve  70  having an off-coil temperature and humidity plotted at point  74  and on-coil conditions plotted at point  76 . The conditions in the room are plotted at point  85 . 
   By de-coupling the ventilation and cooling functions, excellent energy-efficiency may be achieved. A change in the ventilation requirement will only modulate the fresh air fan  22  and will not change the coolant flow rate under normal operating characteristics, i.e., no unusual increase in humidity differences downstream of the first VAV system  6  of the heat exchanger  20 . A change in zonal thermal loads, characterized by a modulation of the recycled air fan  24 , will modulate the coolant flow rate in the coolant circuit  21 . For instance, when the occupancy of the room  42  diminishes, the ventilation requirement also diminishes. The fresh air fan speed is modulated accordingly, but the flow rate in the coolant circuit  21  remains unchanged. The temperature in the room  42  will therefore decrease because the air being cooled is mainly recycled air. The temperature sensor  36  in the return air duct  46  detects this temperature change and modulates the dampers and fan speed accordingly. When temperature and humidity levels begin to rise above the acceptable limits, the CPU  54  uses the signal from the fresh air temperature sensor in order to control the flow rate in the coolant circuit  21 . 
   Although variable-air-volume systems are preferred because they further reduce energy consumption, the single-coil twin-fan system may also use twin constant-air-volume (CAV) systems or a hybrid system comprising a VAV system and a CAV system. If the hybrid system were to be properly implemented, the best mode of operation would be to use a CAV system to drive the air stream whose load characteristics are fairly constant and a VAV system to drive the air stream who load characteristics follow a fluctuating pattern. This provides for maximum flexibility in design and also maximizes energy-saving potential during operation. 
   The superior performance of the HVAC system of the present invention was validated by various experiments that were conducted in the Indoor Air Quality (IAQ) Chamber of the Department of Building at the National University of Singapore. The seven experiments presented in Table 1 below represent different points of operation of a variable-air-volume fan in a steady-state condition. 
   
     
       
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Simulated experimental conditions in two chambers (Room 1 and Room 2) and 
             
             
               simulated fan operating characteristics 
             
           
        
         
             
               Experi- 
                 
               Fresh air 
               Recycled air 
               Thermal load 
               Ventilation load 
             
           
        
         
             
               ment 
               Steady state period 
               (F/A) fan 
               (R/A) fan 
               Room 1 
               Room 2 
               Room 1 
               Room 2 
             
             
                 
             
             
               1 
               11:40 am–12:15 p.m. 
               30% 
               30% 
               Base 
               Base 
               Base 
               Base 
             
             
               2a 
               12:15–12:35 p.m. 
               30% 
               30% 
               Base 
               Base 
               High 
               Base 
             
             
               2b 
               1:00–1:20 p.m. 
               100%  
               30% 
               Base 
               Base 
               High 
               Base 
             
             
               3a 
               2:00–2:15 p.m. 
               30% 
               30% 
               High 
               Base 
               Base 
               Base 
             
             
               3b 
               2:20–2:45 p.m. 
               30% 
               100%  
               High 
               Base 
               Base 
               Base 
             
             
               4a 
               3:15–3:40 p.m. 
               30% 
               30% 
               Base 
               Base 
               High 
               Base 
             
             
               4b 
               3:40–4:10 p.m. 
               100%  
               30% 
               Base 
               Base 
               High 
               Base 
             
             
                 
             
           
        
       
     
   
   Tabulated above in Table 1 are various experiments that were run with thermal and ventilation loads that are characterized as either base or high. Base loads correspond to vacant or nearly vacant conditions, for which only minimal cooling and ventilation are required. Typically, base loads require fans to be run at 30% of their maximum speed. High ventilation loads are simulated by having eight occupants in Room  1  and high thermal loads are simulated by additional heat-emitting lights in Room  1 . For the purposes of the performance validation tests, a single-coil twin-fan system was designed and fabricated having the characteristics and geometry presented in Table 2 below. 
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Coil configuration and physical geometry 
             
           
        
         
             
                 
               Fresh air 
               Recycled 
             
             
               Coil Characteristic 
               compartment 
               air compartment 
             
             
                 
             
           
        
         
             
               Height (mm) 
               330 
               330 
             
             
               Width (mm) 
               305 
               305 
             
             
               Face area (m 2 ) 
               0.1 
               0.1 
             
             
               Face velocity at 100% flow (m/s) 
               1.30 
               1.52 
             
             
               Fin density (fpi) 
               10 
               10 
             
             
               Number of rows 
               6 
               6 
             
             
                 
             
           
        
       
     
   
   Even though the single-coil twin-fan system uses a single coil, the heat transfer characteristics of the portion of the coil in the fresh air stream need not be identical to the heat transfer characteristics of the portion of the coil in the recycled air stream. For example, the portion of the coil in the fresh air stream may have a different effective total surface area than the portion of the coil in the recycled air stream. Alternatively, the portion of the coil in the fresh air stream could have a smaller fin density than the portion of the coil in the recycled air stream. (A smaller fin density is better for dehumidification whereas a larger fin density is better for sensible cooling.) The difference in the effective surface areas of the two portions of the single coil is a design criterion that should govern the overall dimensions of the single coil in relation to the total anticipated cooling capacity. It is thus possible to design such coils in modular sizes that are capable of addressing the varying combinations of sensible and latent cooling requirements that are typical of different climatic conditions as well as varying occupancy levels. 
   The psychrometric analysis of all the seven experiments performed is presented in Table 3 below. 
   
     
       
             
           
             
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               Psychrometric analyis 
             
           
        
         
             
                 
               Psychrometric 
                 
                 
                 
                 
                 
                 
                 
             
             
                 
               Parameters 
               Exp 1 
               Exp 2a 
               Exp 2b 
               Exp 3a 
               Exp 3b 
               Exp 4a 
               Exp 4b 
             
             
                 
                 
             
           
        
         
             
               Fresh Air 
               F/A ON coil: 
               28.5 
               27.5 
               27 
               28.2 
               28 
               28.3 
               28.4 
             
             
               (F/A) 
               DBT (° C.) 
             
             
                 
               F/A ON coil: 
               23.6 
               23.6 
               23.4 
               23.5 
               22.4 
               23 
               23.3 
             
             
                 
               DPT (° C.) 
             
             
                 
               F/A OFF coil: 
               11.1 
               11.3 
               12 
               11.3 
               12.6 
               11.2 
               12 
             
             
                 
               DBT (° C.) 
             
             
                 
               F/A OFF coil: 
               11.1 
               11.3 
               12 
               11.3 
               12.6 
               11.2 
               12 
             
             
                 
               DPT (° C.) 
             
             
                 
               F/A Enthalpy 
               43.6 
               42.1 
               39.3 
               42.7 
               36.5 
               41.8 
               40.6 
             
             
                 
               difference (kJ/kg) 
             
             
                 
               F/A Air flow rate 
               53.9 
               53.9 
               129.7 
               48.3 
               48.3 
               53.6 
               148.3 
             
             
                 
               (lps) 
             
             
                 
               F/A Capacity 
               2.78 
               2.69 
               6.04 
               2.44 
               2.09 
               2.65 
               7.13 
             
             
                 
               (kW) 
             
             
               Recycled 
               R/A ON coil: 
               24.3 
               24.4 
               23.8 
               24 
               24 
               24.3 
               24.1 
             
             
               Air (R/A) 
               DBT (° C.) 
             
             
                 
               R/A ON coil: 
               17.4 
               17.8 
               17.2 
               17.3 
               18.2 
               17.5 
               17.2 
             
             
                 
               DPT (° C.) 
             
             
                 
               R/A OFF coil: 
               10.6 
               11.3 
               10.3 
               10.9 
               12.5 
               10.7 
               11.7 
             
             
                 
               DBT (° C.) 
             
             
                 
               R/A OFF coil: 
               10.6 
               11.3 
               10.3 
               10.9 
               12.5 
               10.7 
               11.7 
             
             
                 
               DPT (° C.) 
             
             
                 
               R/A Enthalpy 
               25.2 
               24.7 
               25.3 
               24.6 
               22.4 
               25.4 
               22.5 
             
             
                 
               difference (kJ/kg) 
             
             
                 
               R/A Air flow rate 
               75.8 
               75.8 
               77.2 
               74.4 
               152.2 
               65.8 
               53.1 
             
             
                 
               (lps) 
             
             
                 
               R/A Capacity 
               2.26 
               2.22 
               2.31 
               2.17 
               4.04 
               1.98 
               1.41 
             
             
                 
               (kW) 
             
             
               F/A + R/A 
               Total capacity of 
               5.04 
               4.91 
               8.35 
               4.61 
               6.13 
               4.63 
               8.54 
             
             
               capacity 
               single coil with 
             
             
                 
               twin fans (kW) 
             
             
                 
             
           
        
       
     
   
   Table 3 shows that the single-coil system is dynamically responsive to the varying thermal and ventilation loads. Experiments 2b and 4b show the steady-state operating conditions of the coil when high ventilation is required in Room  1 . The on-coil conditions in the recycled air stream (designated “R/A ON coil” in Table 3) are representative of the environmental conditions in the two rooms. Room  1 , which requires high ventilation, has a relative humidity of 67% and 65% in Experiment 2b and Experiment 4b, respectively. The “off-coil” temperature of the recycled air is used in Experiment 2b and that of the fresh air is used in Experiment 4b. It is logical to use the fresh air “off-coil” temperature to control the chilled water flow rate through the coil in situations of high ventilation loads, as would be typical in the tropics. 
   A comparison of Experiment 3a and Experiment 3b shows that high thermal loads in Room  1  can be addressed with a high-volume air stream of recycled air which results in room humidity levels around 70% relative humidity. The occupancy in Room  1  and Room  2  is at the base level. The humidity level in Experiment 3b is achieved through chilled water modulation based on the recycled air “off-coil” temperature. The humidity can also be controlled based on the fresh air “off-coil” temperature. Based on these experimental results, the following control strategy is proposed. Chilled water modulation should be achieved by fresh air stream “off-coil” temperature feedback in climatic conditions requiring high ventilation. In contrast, chilled water modulation should be achieved by recycled air stream “off-coil” temperature feedback in climatic conditions with high thermal loads. It is also possible to switch from one control strategy to another, with ventilation predominating. As the chilled water modulation is achieved by either of the two “off-coil” temperatures, the chilled water flow rate through the compartment of the coil whose “off-coil” temperature is not in use could be used to alter the psychrometric performance of the system. For example, if the recycled air stream&#39;s “off-coil” temperature is used to control the chilled water flow rate, the air conditions of the fresh air stream could be dynamically varied. However, it is the mixture of the fresh air and the recycled air in the mixing box that determines the space conditions. An inherent buffer is built into the design of this coil. In these experiments, a buffer of 2° C. (between 10° C. and 12° C.) on the saturation line is observed for the supply air temperatures. 
   Table 3 shows the energy-efficiency of the single-coil twin-fan system (SCTF). Energy is conserved because the fresh air fan and the recycled air fan are controlled independently to optimize their respective airflow requirements based on the individual, localized demand for ventilation or cooling. The coil is thus not required to provide more cooling than is necessary as operating conditions change. For example, in Experiment 4b, the total amount of fresh and recycled air used is 725 cubic meters per hour (m 3 /h). The fresh air fan operated at 100% of its maximum speed whereas the recycled air fan only operated at 30% of its maximum speed. If similar room conditions were to be achieved using a prior art VAV system, the fan would have to operate at 65% of its maximum speed to provide the same amount of total airflow to the room. In other words, Experiment 4b could be seen as a 65% part-load condition. This would translate to an airflow rate of 1115 m 3 /h when the conventional VAV fan operates at 100% speed. By simple proportionality, the fresh air to be provided at 100% fan speed ought to be about 820 m 3 /h if the amount of fresh air at the part-load condition of 65% is to be equal to what is being provided by the SCTF system (534 m 3 /h). This implies that at 100% output, the additional energy consumed by a conventional, prior art coil in a conventional VAV system is equivalent to the additional amount of fresh air provided at 100% output, which in this case is about 53%. By contrast, in the case of an SCTF system, the total amount of airflow at 100% output is 1115 m 3 /h. This breaks down into a fresh airflow rate of 534 m 3 /h and a recycled airflow rate of 581 m 3 /h. 
   Based on the foregoing psychrometric analysis, the SCTF system of the present invention is estimated to be approximately 15% more energy efficient than prior art VAV systems that are currently in use. This estimate, however, does not even include the energy savings attendant in reduced fan use. Moreover, the SCTF system does not need to have a second water pump or a second chilled water loop. This also results in further energy savings. 
   The above description of preferred embodiments should not be interpreted in a limiting manner since other variations, modifications and refinements are possible within the spirit and scope of the present invention. The scope of the invention is defined in the appended claims and their equivalents.