OPERATE SUCH SYSTEM

TECHNICAL FIELD

The present subject matter in general relates to a Heating, Ventilation, Air-Conditioning (HVAC) system and a method to operate such an HVAC system.

BACKGROUND

Acronym HVAC stands for Heating, Ventilation, Air-Conditioning and as the name describes an HVAC system is used for heating, ventilation and air-conditioning purposes. HVAC systems are used to maintain parameters, such as temperature, relative humidity and pressure, for providing comforts to occupants, proper operation of machines and proper storage of products in open or closed places, for example, buildings, operation theatres, laboratories etc. HVAC systems are designed to work and maintain a suitable environment under extreme load conditions, for example, peak summer, peak winter, maximum humidity, maximum occupancy number etc. In such situations, various controls of the HVAC system regulate the output of the system to match the load conditions.

A conventional HVAC system includes a plant at a central location where a fluid, such as water or brine, depending upon the purpose, is cooled or heated, and one or more pumps to circulate the cooled/heated fluid to one or more heat exchangers. In the heat exchanger, the cooled/heated fluid exchanges heat energy with another fluid, such as air. The cooled/heated air is then circulated to spaces or regions to be conditioned.

In the conventional HVAC system, more fluid from the central plant flows through the heat exchanger which is closer to the pump or which offers less internal resistance. This causes an imbalance in the volume of fluid reaching various heat exchangers. Some heat exchangers may receive more than designed fluid flow while some may starve. This mainly occurs at full load conditions, i.e. when a control valve of the system is fully open, and results in disturbing the desirable conditions of the conditioned spaces. Also, an unbalanced operation of heat exchangers results in an inefficient operation of the plant due to an incomplete transfer of the heat energy. For efficient working of an HVAC system, an automatic controller is provided to actuate a control valve to control the heat transfer capacity of the heat exchanger, and a manual or automatic flow regulating valve is provided to limit maximum fluid flow through the heat exchanger. The flow regulating valve is typically known as balancing valve. The balancing valve equalizes resistance of the path followed by the cooled/heated fluid from the central plant into the multiple heat exchangers to balance the amount of fluid in all the heat exchangers. The balancing valve is complimented by the control valve to regulate the fluid flow through the heat exchangers to meet the load requirement. Use of the balancing valves in addition to the control valve is superfluous and expensive. The balancing valves are also prone to failures. Depending upon the type of balancing used in the HVAC system either it requires lot of manpower to balance a system or it adds to the pressure exerted in the system thereby increasing the power consumption of the system. Also in the conventional HVAC system the balancing valve is useful only at the time the control valve is fully open or can allow more flow through the heat exchanger than the designed maximum capacity. The balancing valves make the HVAC system complicated, costly and laborious to install and maintain.

SUMMARY

The subject matter disclosed herein describes a method comprising generating a first signal based on deviation of a parameter of at least one fluid of a heat exchanger from a predefined set-point value; generating a second signal based on deviation of at least one parameter of at least one fluid of the heat exchanger from corresponding predefined set-point values; selecting one signal from the first signal and the second signal; and actuating a control valve based on the selected signal.

The subject matter disclosed herein further describes a system comprising at least one heat exchanger with a first fluid and a second fluid flowing through the heat exchanger, where the first fluid exchanges heat energy with the second fluid. The system further comprises at least one load, where the second fluid is flowing through the load, a control valve positioned in a path of the first fluid, and a controller to actuate the control valve. The controller is configured to generate a first signal based on deviation of a temperature difference of the first fluid between a first inlet of the heat exchanger and a first outlet of the heat exchanger from a predefined set- point value; to generate a second signal based on deviation of at least one parameter of the second fluid of the heat exchanger from a corresponding predefined set-point value; to select one signal from the first signal and the second signal; and to actuate the control valve based on the selected signal.

These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF DRAWINGS

The novel features of the subject matter are set forth in the appended claims hereto. The subject matter itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein the same numbers are used throughout the drawings to reference like features, and wherein:

Figure 1 illustrates an HVAC system, according to an embodiment of the present subject matter.

Figure 2 illustrates details of a control unit of the HVAC system, according to an embodiment of the present subject matter.

Figures 3(a) and 3(b) show plots of the signals generated in the control unit of the HVAC system, according to an embodiment of the present subject matter.

Figure 4 shows the flow diagram of the method for operating the HVAC system, according to an embodiment of the present subject matter.

Figure 5 illustrates an HVAC system, according to another embodiment of the present subject matter.

Figure 6 illustrates details of a control unit of the HVAC system, according to another embodiment of the present subject matter. Figures 7(a) and 7(b) show plots of the signals generated in the control unit of the HVAC system, according to another embodiment of the present subject matter.

DETAILS DESCRIPTION

In a conventional Heating, Ventilation, Air-Conditioning (HVAC) system a first fluid, such as water or brine, is cooled or heated in a centralized plant depending upon the purpose, and pumped and circulated to heat exchangers, where the cooled/heated first fluid exchanges heat (energy) with a second fluid, such as air, without coming in contact with the second fluid. The cooled/heated second fluid is then circulated in spaces to be conditioned.

For efficient working of the HVAC system the following two conditions have to be maintained. The two conditions are:

- controlling the heat transfer capacity of the heat exchanger based on conditioned space load; and

- limiting the maximum flow of cooled/heated water (first fluid) through the heat exchanger.

The heat transfer capacity H of a heat exchanger can be defined as

H = k.Q.AT,

where Q is flow rate of the fluid through the heat exchanger, ΔΤ is temperature difference of the fluid between inlet and outlet of the heat exchanger, and k is a proportionality constant based on physical parameters of heat exchanger and fluid type. Conventionally, the heat transfer capacity H of a heat exchanger is controlled by actuating a control valve positioned in the path of the fluid flowing through the heat exchanger to control the flow rate Q such that parameters, such as temperature and relative humidity, of the conditioned space is maintained constant.

Further, conventionally, the maximum flow of cooled/heated water through the heat exchanger is limited using a balancing valve system. The cooled/heated water from the central plant is pumped using one or more pumps. Based on fluid dynamics principles, the cooled/heated heater water reaches in shorter time and in larger volume to the heat exchanger which is closer to the pump or which offers lower internal resistance. Typically the heat exchangers located at far distances are starved. Due to this principle, the distribution of the cooled/heated water across the heat exchangers gets disturbed which affects in maintaining the desirable conditions in the conditioned spaces. Also, excessive flow of the cooled/heated water in the heat exchangers results in an inefficient operation of the central plant. To prevent the imbalance in the flow of the cooled/heated water in the heat exchangers, a limit is set for the maximum flow through each heat exchanger using the balancing valves. The balancing valves balance the volumes of the cooled/heated water reaching and flowing through all the heat exchangers. The need of balancing is essential at a full load condition when the control valve is fully open. Typical types of balancing valves are: static balancing valves and dynamic balancing valves. The balancing valves are connected in series with the control valves controlling the heat transfer capacity of the heat exchangers. Use of the balancing valves in addition to the control valve is superfluous and expensive. The balancing valves are also prone to failures. Depending upon the type of balancing used in the HVAC system either it requires lot of manpower to balance the system or it adds to the pressure exerted in the system thereby increasing the power consumption of the system. Thus, there is a need of an HVAC system which works efficiently and maintains both the above mentioned conditions of the heat exchangers without using the complicated balancing valves.

The present subject matter relates to a system, in particular an HVAC system, in which maximum fluid flow through a heat exchanger is limited and heat transfer capacity of the heat exchanger is controlled through a single control valve, without using any balancing valve. The present subject matter also relates to a method of operating such HVAC system.

In an embodiment of the HVAC system, according to the present subject matter, the single control valve is actuated such that temperature difference ΔΤ of the fluid flowing across the heat exchanger is maintained constant. This controls the heat transfer capacity H of the heat exchanger (following the above mentioned equation of H) and also limits the maximum fluid flow through the heat exchanger. Further, in the HVAC system, parameters, such as temperature and relative humidity, of spaces to be conditioned is also monitored and maintained. In the larger interest of an efficient operation of the system, limiting of maximum fluid flow condition takes precedence over the condition of controlling the heat transfer capacity and/or the conditions of the conditioned spaces. Figure 1 illustrates an HVAC system 1 , according to an embodiment of the present subject matter. The HVAC system 1 includes a centralized plant 2 in which a first fluid Fl is cooled or heated depending upon whether the HVAC system 1 is used either for cooling or heating, respectively. In an embodiment, the first fluid Fl can be water or brine. The HVAC system 1 also includes one or more heat exchangers 3. The heat exchangers 3 are conventional heat exchangers and are also commonly called cooling/heating coils. The cooled/heated first fluid Fl is circulated to the heat exchangers 3 using one or more pumps 4, as shown in the figure. A control valve 5 is positioned in the path of the first fluid Fl flowing through the heat exchanger 3. One control valve is designated for one heat exchanger. In an embodiment, the control valve 5 can be placed in the path either before the first fluid Fl entering the heat exchanger 3 or after the first fluid Fl exiting the heat exchanger 3.

In the heat exchanger 3, the cooled/heated first fluid Fl exchanges heat (energy) with another fluid, i.e. a second fluid F2. The two fluids Fl and F2 interact in the heat exchanger 3 without coming in direct contact with each other. In an embodiment, the second fluid F2 can be air. The second fluid F2, after exchanging heat with first fluid Fl, is circulated to one or more loads 6. The load 6 is a space to be conditioned and hence is also termed 'conditioned space' 6 in the specification.

The HVAC system 1, according to the present subject matter, further includes one or more control units 7. The control unit 7 actuates the control valve 5 to limit the maximum flow of the first fluid Fl through the heat exchanger 3 and control the heat transfer capacity of the same heat exchanger 3.

The HVAC system 1, according to the present subject matter, further includes sensors PI , P2 and P3. The sensor PI measures temperature Tu of the first fluid Fl at a first inlet 8 of the heat exchanger 3 and a corresponding signal Tl is fed to the control unit 7. The sensor P2 measures temperature Ti ₀ of the first fluid Fl at a first outlet 9 of the heat exchanger 3 and a corresponding signal T2 is fed to the control unit 7. The sensor P3 measure temperature T ₂ i of the second fluid F2 at a second inlet 10 of the heat exchanger 3 and a corresponding signal T3 is fed to the control unit 7. The functioning of the control unit 7 of the HVAC system 1 , according to the present subject matter, with the three input signals Tl, T2 and T3 is described in details below.

Figure 2 illustrates inner architecture of main components of the control unit 7 of the HVAC system 1 , according to an embodiment of the present subject matter. The control unit 7 includes a first unit 1 1 to calculate difference ΔΤι between the signals Tl and T2 fed to the control unit 7 and compare the calculated difference ΔΤι with a first predefined set-point value SP1. In an embodiment, the first unit 1 1 can include a differentiator and a comparator. Based on the comparison the first unit 1 1 generates a first signal S 1. The control unit 7 also includes a second unit 12 to compare the signal T3, fed to the control unit 7, with a second predefined set- point value SP2. In an embodiment, the second unit 12 can include a comparator. Based on the comparison the second unit 12 generates a second signal S2. The generation of the first signal SI and the second signal S2 in the control unit 7 is described in figures 3(a) and 3(b), according to an embodiment of the present subject matter. In an embodiment, the first signal SI and the second signal S2 can be voltage signals. However, the signals S I and S2 are not restricted to voltage signals, and can be of any type known in the state-of-the-art.

In figure 3(a), x-axis represents ΔΤι and y-axis represents magnitude of the first signal S I . When ΔΤι is more than SP1, the first signal SI takes a constant value, for example, 10 V. When ΔΤι is below SP1 and within p-band, the first signal SI takes a value between 0 and 10 V, as shown. The relationship between ΔΤι and the first signal SI in the p-band is shown linear, according to an embodiment. In another embodiment, the relationship between ΔΤ) and the first signal S I in the p-band can be any. When ΔΤι is below SP1 and on the left of the p-band, the first signal SI takes a value of about 0 V, as shown. Similarly, in figure 3(b), x-axis represents T3 and y-axis represents magnitude of the second signal S2. When T3 is more than SP2 and on the right of the p-band, the second signal S2 takes a constant value, for example, 10 V. When T3 is above SP2 and within the p-band, the second signal S2 takes a value between 0 V and 10 V, as shown. The relationship between T3 and the second signal S2 in the p-band is shown linear, according to an embodiment. In another embodiment, the relationship between T3 and the second signal S2 in the p-band can be any. When T3 is below SP2, the first signal SI takes a value of about 0 V, as shown. The constant values of the first signal S 1 and the second signal S2, as shown in figures 3(a) and 3(b), are kept same for the purpose of comparing the two signals S 1 and S2. In an embodiment, the constant value can be any suitable positive value. During the working of the HVAC system 1 , according to an embodiment of the present subject matter, the first signal SI and the second signal S2 vary according to the plots shown in figures 3(a) and 3(b).

In the control unit 7, the first signal SI and the second signal S2 are fed to a signal selector 13. The signal selector 13 compares the magnitudes of the two signals SI and S2 that follow the plots shown in figures 3(a) and 3(b), respectively, and selects one signal out of the two signals SI and S2 based on the comparison of the magnitudes. The selected signal is a third signal S3 and is outputted from the control unit 7. In an embodiment, the signal selector 13 selects the signal with a lesser magnitude and thus, the third signal S3 takes the magnitude of the lesser of the two signals SI and S2.

The third signal S3 is fed to an actuator 14 to actuate the control valve 5. Depending. upon the magnitude of the third signal S3, the control valve 5 is actuated to maintain the temperature T ₂ j of the second fluid F2 at the second inlet 10 of the heat exchanger 3 and to limit the maximum flow of the first fluid Fl though the heat exchanger 3. Maintaining the temperature T ₂ j of the second fluid F2 at the second inlet 10 is correlated to controlling the heat transfer capacity H of the heat exchanger 3. Thus, using only the control valve 5 and the control unit 7 both the heat transfer capacity of the heat exchanger 3 and the maximum flow of the first fluid Fl though the heat exchanger 3 is controlled. The HVAC system 1 , according to the present subject matter, does not use any balancing valve, yet it works efficiently.

In an embodiment, the signal selector 13 selects the signal with a larger magnitude and thus, the third signal S3 takes the magnitude of the larger of the two signals SI and S2, and the actuator 14 actuates the control valve 5 to maintain the temperature T ₂ i of the second fluid F2 at the second inlet 10 of the heat exchanger 3 and to limit the maximum flow of the first fluid Fl though the heat exchanger 3.

Figure 4 shows the flow diagram of the method for operating the HVAC system 1, according to the present subject matter. The method includes the following steps: - generating a first signal SI based on deviation of a temperature difference ΔΤι between a first fluid Fl at a first inlet 8 of a heat exchanger 3 and at a first outlet 9 of the heat exchanger 3 from a predefined set-point value SP1 ;

- generating a second signal S2 based on deviation of a temperature T ₂ j of a second fluid F2 at a second inlet 10 of the heat exchanger 3 from a predefined set-point value SP2;

- selecting one signal S3 from the first signal SI and the second signal S2; and

- actuating a control valve 5 based on the selected signal S3 to maintain the temperature T ₂ j of the second fluid F2 and to limit a maximum flow of the first fluid Fl through the heat exchanger 3.

In the method, according to the present subject, the selecting the signal S3 includes comparing a magnitude of the first signal SI with a magnitude of the second signal S2, and the selection of the signal S3 is based on the comparing of the magnitudes of the first signal S 1 and the second signal S2.

The embodiment of the HVAC system 1 illustrated in the figures, the second unit 12 in the control unit 7 generates the second signal S2 based on the comparison of the temperature T ₂ i of the second fluid F2 at the second inlet 10 of the heat exchanger 3 with the predefined set-point vale SP2. In another embodiment, instead of the temperature T ₂ i, the second unit 12 of the control unit 7 can generate the second signal S2 based on comparison of temperature T ₂ L of the second fluid F2 at the load 6 with a corresponding predefined value SP3.

In another embodiment, instead, the second unit 12 of the control unit 7 can generate the second signal S2 based on comparison of relative humidity RH ₂ j of the second fluid F2 at the second inlet 10 of the heat exchanger 3 with a corresponding predefined value SP4.

In another embodiment, instead, the second unit 12 of the control unit 7 can generate the second signal S2 based on comparison of relative humidity RH ₂ L of the second fluid F2 at the load 6 with a corresponding predefined value SP5.

In another embodiment, instead, the second unit 12 of the control unit 7 can generate the second signal S2 based on the comparisons of temperature T ₂ , and relative humidity RH ₂ i of the second fluid F2 at the second inlet 10 of the heat exchanger 3 with corresponding predefined values SP2, SP4. In another embodiment, instead, the second unit 12 of the control unit 7 can generate the second signal S2 based on the comparisons of temperature T ₂ L and relative humidity RH ₂ L of the second fluid F2 at the load 6 with the corresponding predefined values SP3, SP5.

Further, in another embodiment, depending upon the magnitude of the third signal S3, the control valve 5 is actuated to maintain the temperature T ₂ i and/or relative humidity RH ₂ , of the second fluid F2 at the second inlet 10 of the heat exchanger 3 and to limit the maximum flow of the first fluid Fl though the heat exchanger 3.

Further, in another embodiment, depending upon the magnitude of the third signal S3, the control valve 5 is actuated to maintain temperature T _2O and/or relative humidity RH _2O of the second fluid F2 at a second outlet 9' of the heat exchanger 3 and to limit the maximum flow of the first fluid Fl though the heat exchanger 3.

Further, in another embodiment, depending upon the magnitude of the third signal S3, the control valve 5 is actuated to maintain the temperature TL and/or relative humidity RHL of the load 6 and to limit the maximum flow of the first fluid Fl though the heat exchanger 3.

The HVAC system 1, according to the present subject matter, is not restricted to any particular type of centralized plant 2, heat exchanger 3, pump 4, control valve 5 and actuator 14. These elements of the HVAC system 1 are similar to the conventional HVAC system. The HVAC system 1 , according to the present subject matter, can include a plurality of centralized plants 2, heat exchangers 3, pumps 4, control valves 5, loads 6, control units 7 and actuators 14.

The HVAC system 1 , according to the present subject matter, can be used for the purpose of cooling, heating and/or ventilation/dehumidification. The HVAC system 1 , in the specification, is described for the cooling mode. Similar principle can be applied when the HVAC system 1 is working in heating and/or ventilation/dehumidification modes.

Figure 5 illustrates another embodiment of the HVAC system Γ, according to the present subject matter, in which maximum fluid flow through the heat exchanger 3 is limited and heat transfer capacity of the heat exchanger 3 is controlled through the single control valve 5, without using any balancing valve. Common elements of figure 1 and figure 5 have the same reference numerals and are not described in detail again. In the embodiment shown in figure 5, enthalpy difference of the second fluid F2 flowing across the heat exchanger 3 is used to limit the maximum flow of the first fluid Fl through the heat exchanger 3. Further, the heat transfer capacity H of a heat exchanger 3 for the second fluid F2 can be defined as

H = k.Q.AH,

where Q is the flow rate of the second fluid F2, preferably air, through the heat exchanger 3, ΔΗ is the enthalpy difference of the second fluid F2 between the inlet 10 and an outlet 9' of the heat exchanger 3.

The HVAC system Γ according to the present subject matter, includes sensors ΡΓ and P2'. The sensor ΡΓ measures temperature T ₂ j and relative humidity RH ₂ i of the second fluid F2 at the second inlet 10 of the heat exchanger 3 and calculates enthalpy H ₂ j of the second fluid F2 at the second inlet 10. A corresponding signal HI , based on the enthalpy H ₂ j, is fed to a control unit 7'. The sensor P2' measures temperature T _2o and relative humidity RH _2o of the second fluid F2 at a second outlet 9' of the heat exchanger 3 and calculates enthalpy H _2o of the second fluid F2 at a second outlet 9'. A corresponding signal H2, based on the enthalpy H _2o , is fed to the control unit 7'. The functioning of the control unit 7' of the HVAC system Γ, according to the present subject matter, with the two input signals HI and H2 is described in details below.

Figure 6 illustrates inner architecture of main components of the control unit 7' of the HVAC system Γ, according to the present subject matter. The control unit T includes a first unit 1 Γ to calculate difference ΔΗ ₂ between the signals HI and H2 fed to the control unit 7' and compare the calculated difference ΔΗ ₂ with a first predefined set-point value SP1'. In an embodiment, the first unit 1 1 can include a differentiator and a comparator. Based on the comparison the first unit 1 1 ' generates a first signal SI '. The control unit T also includes a second unit 12' to compare the signal HI, fed to the control unit 7', with a second predefined set- point value SP2'. In an embodiment, the second unit 12' can include a comparator. Based on the comparison the second unit 12' generates a second signal S2'. The generation of the first signal S I ' and the second signal S2' in the control unit 7' is described in figures 7(a) and 7(b), according to an embodiment of the present subject matter. In an embodiment, the first signal SI ' and the second signal S2' can be voltage signals. However, the signals SI ' and S2' are not restricted to voltage signals, and can be of any type known in the state-of-the-art. The explanation of figures 7(a) and 7(b) can be done on the similar lines as done for figures 3(a) and 3(b).

In the control unit 7', the first signal SI ' and the second signal S2' are fed to a signal selector 13'. The signal selector 13' compares the magnitudes of the two signals SI ' and S2' and selects one signal out of the two signals SI ' and S2' based on the comparison of the magnitudes. The selected signal is a third signal S3' and is outputted from the control unit 7'. In an embodiment, the signal selector 13 ' selects the signal with a lesser magnitude and thus, the third signal S3' takes the magnitude of the lesser of the two signals SI ' and S2'.

The third signal S3' is fed to the actuator 14 to actuate the control valve 5. Depending upon the magnitude of the third signal S3', the control valve 5 is actuated to maintain the temperature T ₂ j and/or relative humidity RH ₂ j of the second fluid F2 at the second inlet 10 of the heat exchanger 3 and to limit the maximum flow of the first fluid F 1 though the heat exchanger 3. Maintaining the temperature T ₂ j and/or relative humidity RH ₂ j of the second fluid F2 at the second inlet 10 is correlated to controlling the heat transfer capacity H of the heat exchanger 3. Thus, using only the control valve 5 and the control unit 7' both the heat transfer capacity H of the heat exchanger 3 and the maximum flow of the first fluid Fl though the heat exchanger 3 is controlled. The HVAC system Γ, according to the present subject matter, does not use any balancing valve, yet it works efficiently. In another embodiment, the control valve 5 can be actuated to maintain the temperature T _2O and/or relative humidity RH _2O of the second fluid F2 at the second outlet 9' of the heat exchanger 3 and to limit the maximum flow of the first fluid Fl though the heat exchanger 3, or to maintain the temperature TL and/or relative humidity RHL of the load 6 and to limit the maximum flow of the first fluid Fl though the heat exchanger 3.

In an embodiment, the signal selector 13' selects the signal with a larger magnitude and thus, the third signal S3' takes the magnitude of the larger of the two signals SI ' and S2', and the actuator 14 actuates the control valve 5 to maintain the temperature T ₂ j and/or relative humidity RH ₂ j of the second fluid F2 at the second inlet 10 of the heat exchanger 3 and to limit the maximum flow of the first fluid Fl though the heat exchanger 3, or to maintain the temperature T _2O and/or relative humidity RH _2O of the second fluid F2 at the second outlet 9' of the heat exchanger 3 and to limit the maximum flow of the first fluid Fl though the heat exchanger 3, or to maintain the temperature TL and/or relative humidity RHL of the load 6 and to limit the maximum flow of the first fluid Fl though the heat exchanger 3.

The HVAC system, according to the present subject matter, has the following advantages:

- no use of expensive and failure prone balancing valves;

- reduces complicated laborious balancing process;

- reduces energy consumption;

- makes the HVAC system simple, economical to install and maintain.

Other advantages of the inventive system and method to operate such system will become better understood from the description and claims of an exemplary embodiment of such a system.

The inventive system and method to operate such system of the present subject matter is not restricted to the embodiments that are mentioned above in the description.

Although the subject matter has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the subject matter, will become apparent to persons skilled in the art upon reference to the description of the subject matter. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present subject matter as defined.