Patent Application
20120192589
The present disclosure deals generally with heat exchange devices, and more specifically with evaporative cooling devices.
Vapor compression refrigeration cycles are widely used in many cooling systems, including refrigerators, air-conditioning systems, industrial and commercial refrigeration systems and the like. The process of refrigeration refers to extracting heat from a space and rejecting it somewhere else, thus lowering the temperature of the space. Vapor compression refrigeration cycles use a refrigerant for this process. The cycle includes four basic components: a compressor, a condenser, an expansion valve and an evaporator. The circulating refrigerant enters the compressor in the form of saturated vapor and undergoes isentropic compression, thus increasing its pressure and temperature, and converting into a superheated vapor. The refrigerant in the superheated vapor form is now in a state where it can be condensed by cold water or air. Next, it enters the condenser, where it comes into thermal contact with the cold water or air and rejects heat to it. Here, the refrigerant gets converted from superheated vapor to saturated vapor by absorbing heat (sensible heat), and eventually from saturated vapor to saturated liquid by further absorbing the latent heat from the cold air or water.
At the exit from the condenser, the refrigerant is in a thermodynamic state of saturated liquid. Next, the saturated liquid refrigerant is routed through an expansion valve, where it expands and undergoes a reduction in pressure, undergoing partial flash evaporation. This process converts the refrigerant into a mixed liquid and vapor form and reduces its temperature to a level colder than the space to be refrigerated. The mixture then enters the evaporator, where it extracts latent heat from the space, completely vaporizing into a saturated vapor. That vapor reenters the compressor to complete the refrigeration cycle. This description pertains to an ideal vapor-compression refrigeration cycle that is assumed to be reversible, neglecting the practical real-world effects such as the frictional pressure drop in the system and the slight thermodynamic irreversibility.
In a conventional evaporator, the refrigerant absorbs latent heat of vaporization from the medium (generally air or water) flowing across the evaporator coils. With sufficient heat transfer, the refrigerant undergoes a phase change at a constant saturation pressure and temperature. Ensuring the complete vaporization of the refrigerant at the outlet of the evaporator of a cooling unit utilizing vapor compression refrigeration cycle presents a difficult task, as the process occurs at constant saturation temperature and pressure of the refrigerant. It is extremely important, however, to ascertain refrigerant enters the compressor in a completely vaporized state and not as a mixture of liquid and vapor. In situations where considerable variations exist in the temperature of the air at the inlet to the evaporator, sufficient latent heat of vaporization may not be provided to the refrigerant to completely vaporize it. This problem becomes more difficult if the cooling unit is installed to condition outdoor fresh air. Outdoor fresh air changes temperature considerably over the course of a day, and those variations must be taken into account in the system design. If the design allows a fraction of the refrigerant to enter the compressor in liquid form, that liquid may considerably affect compressor performance, and that condition may also lead to partial or complete damage of the compressor's mechanical parts.
Chillers alleviate this problem by employing water as an intermediate medium. The system circulates water into and out of the evaporator side of the air-conditioning unit, where it comes into contact and exchanges heat with the refrigerant. The water rejects sufficient latent heat to the refrigerant to ensure the refrigerant's vaporization, and eventually the water exits at a relatively lower temperature. Thereafter, the water is routed through a set of cooling coils and the air to be cooled is blown over and across these coils. Being at a relatively lower temperature than the air to be cooled, the water extracts heat from the air, conditioning it. Then, the water, now at a higher temperature, re-enters the evaporator and completes the flow circuit.
In brief, the refrigerant (the ultimate cooling medium) extracts heat from the water, and the water (the intermediate medium) extracts heat from the air. As outside air temperature changes during the day, the water loses or gains sensible heat, and those changes affect the ability of water to transfer heat to the refrigerant. Here, the system can adjust for such changes in temperature by varying the water's mass flow rate to a value that both cools the air to be conditioned to a desired temperature and provide sufficient heat to completely vaporize the refrigerant. However, in many cases this configuration requires huge chilled water piping networks in the chillers to circulate the intermediate medium (water), making the entire system considerably more expensive.
Accordingly, there is a need for an effective, compact, and economical solution to the requirement to supply sufficient heat to completely vaporize a refrigerant in a system where the inlet air temperature at the evaporator varies substantially over time. Such a solution would increase the performance level of cooling system compressors, as well as reduce the risk of damage to the compressor's mechanical components.
The present invention is directed to an efficient, compact and economical evaporator for a cooling unit utilizing vapor compression refrigeration cycle. In one aspect, an evaporator in a unit for cooling an ambient medium provides for heat transfer from ambient air to an intermediate medium, and also to a primary heat exchange medium. The apparatus includes a number of heat exchange tubes, each tube including an inner heat exchange tube adapted to carry a primary heat exchange medium and an outer heat exchange tube adapted to carry an intermediate heat exchange medium. The intermediate heat exchange medium is in thermal communication with the primary heat exchange medium. A direct heat exchange mechanism, which in one aspect can be a set of fins, is in simultaneous thermal communication with the ambient medium and with the intermediate heat exchange medium.
According to another aspect, also addressing an evaporator in a unit for cooling an ambient medium, the evaporator is made up of a set of heat exchange tubes. Each of those tubes is made up of an inner heat exchange tube and an outer heat exchange tube. The inner tube is adapted to carry a refrigerant, and it includes a refrigerant inlet and a refrigerant outlet. The outer tube is adapted to carry an intermediate heat exchange medium in thermal communication with the primary heat exchange medium. A set of fins is mounted on the outer heat exchange tubes in simultaneous thermal communication with the ambient medium and the intermediate heat exchange medium. The inner heat exchange tubes are carried at least partially coaxially within the outer heat exchange tubes.
In yet another aspect, a heat exchange tube in an evaporator is presented. That device includes two tubes, an inner heat exchange tube, which carries a refrigerant, and an outer heat exchange tube, which carries an intermediate heat exchange medium. The outer tube at least partially surrounds the inner tube, defining an annular space between the tubes. The intermediate medium is carried in the annular space. A set of fins, mounted on the outer heat exchange tube, is in simultaneous thermal communication with the ambient medium and the intermediate heat exchange medium.
A flow control means changes the mass flow rate of the refrigerant at the refrigerant inlet when any change in the temperature of the fluid at the fluid outlet is observed. Changing the mass flow rate of the refrigerant ensures that the rate of heat gained by the refrigerant complies with the rate of heat required by it for its complete vaporization.
Additional features and advantages of the invention will be made apparent from the following detailed description of illustrative embodiments that proceed with reference to the accompanying drawings.
The summary above, as well as the following detailed description of preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, exemplary constructions of the invention are shown in the drawings. The invention is not limited to the specific methods and instrumentalities disclosed however. Moreover, those in the art will understand that the drawings are not to scale. Where possible, like elements are indicated by identical numbers.
a) shows an assembled exemplary heat exchange tube used in the three-media evaporator of
c) and
e) and
The description below illustrates embodiments of the claimed invention to those of skill in the art. This description illustrates aspects of the invention but does not define or limit the invention, such definition and limitation being contained solely in the claims appended hereto. Those of skill in the art will understand that the invention can be implemented in a number of ways different from those set out here, in conjunction with other present or future technologies.
As used herein, the following terms carry the had indicated meanings: “Ambient medium” is the medium targeted for conditioning by the evaporator of the claimed invention. In a building air-conditioning system, for example, the ambient medium would be the air inside the building. “Primary heat exchange medium” designates the medium affect about the system's basic temperature-affecting process. In a building air-conditioning system, for example, the primary heat exchange medium is the refrigerant operated on in the refrigeration cycle. “Intermediate heat exchange medium” designates a medium employed for heat exchange with both the primary heat exchange medium and the ambient medium, as described more fully below.
At point 2, the refrigerant enters the condenser and flows through the condenser coils where it comes in thermal contact with the air or water flowing across the coils. It rejects sensible heat to the air or water, and gets converted from superheated vapor to saturated vapor, thus coming from point 2 to point 2′ shown in
At point 3, the refrigerant exits the condenser and enters the expansion valve. Here, the saturated liquid refrigerant expands and hence undergoes an abrupt decrease in pressure and reaches the point 4 corresponding to the exit from the expansion valve. This sudden decrease in pressure causes a corresponding drop in the temperature of the liquid refrigerant and some of the liquid refrigerant flashes off into vapor, thus leaving the refrigerant in mixed liquid and vapor form at the exit 4 of the expansion valve. The enthalpy of the refrigerant however remains constant from point 3 to 4, as shown in
At point 4, the refrigerant enters the evaporator where it comes into contact with the air to be cooled. This air flows across the evaporator coils and exchanges heat with the refrigerant flowing in the coils. By extracting the latent heat of vaporization from the air, the refrigerant is converted from a mixed state to saturated vapor at point 1, where it again enters the compressor and completes the refrigeration cycle.
The refrigerant should be completely vaporized before it enters the compressor at point 1. If the refrigerant does not gain sufficient latent heat to achieve vaporization, it may be left in a mixed liquid and gaseous state. Such a mixed state would correspond to a point lying somewhere on the line segment joining points 1 and 4 of
The claimed invention solves this problem by providing an evaporator for a refrigeration cycle cooling unit employing three different heat-exchange media within the evaporator. The refrigerant and fluid flow in parallel and in thermal contact within the heat exchange tubes of the evaporator. The disclosed system ensures complete vaporization of the refrigerant at the exit from the evaporator.
Evaporator 300 includes a casing 301 having an anterior surface 302 and a posterior surface 303. A set of heat exchange tubes 304 are disposed parallel to each other within the casing 301. The heat exchange tubes extend along the entire length of the casing 301 in a direction substantially perpendicular to the anterior and posterior surfaces. Further, the anterior and posterior surfaces have perforations through which the heat exchange tubes extend into and out of the casing. Fins 305, more clearly shown in succeeding figures, are disposed within the casing 301.
For economy of expression, the phrase “heat exchange tube” will be rendered as simply “tube” in subsequent references of all components in the text that follows. Similarly, the “primary heat exchange medium” will be referred to as the “refrigerant” and the “intermediate heat exchange medium” will be referred to as the “intermediate medium.”
The coaxial arrangement of inner tube 306 and outer tube 307 defines an outer passage 310, annular in form, between those two elements. Similarly, inner tube 306 defines an inner passage 309.
In operation, the refrigerant enters the heat exchange tubes 304 through refrigerant inlet 311, flows through the inner tubular passage 309, and finally emerges through a refrigerant outlet 312. Further, an intermediate medium enters the outer passage 310 through an intermediate inlet 313, flows through outer passage 310 in a direction parallel to and in thermal communication with the refrigerant flow, and finally emerges through an intermediate outlet 314. In a preferred embodiment, the inner tube 306 and the outer tube 307 are cylindrically shaped, having a generally circular cross section and are arranged coaxially as illustrated in
Those of skill in the art will recognize that the coaxial arrangement of inner tube 306 and outer tube 307 promotes maximum heat transfer, while also recognizing that arrangement of the tubes within the overall structure is governed by conventional manufacturing tolerances. Moreover, those of skill in the art will further recognize that the detailed illustrations of
Fins 305 constitute a direct exchange mechanism, as they are in direct contact both with the intermediate medium and with the ambient medium. To provide contact with the intermediate medium, the fins are mounted at regular intervals over the outer surface of the heat exchange tubes 304, arranged in parallel. Space between the fins 305 allows the ambient medium to flow around and over the fins 305, in thermal communication. The fins 305 are aligned in a configuration perpendicular to the longitudinal axis AAl of the heat exchange tubes. The number, exact arrangement, and sizing of the fins 305 lies within the skill of those in the art. It should be noted that the fins 305 can contain any convenient number of perforations, adapted to fit over any desired number of tubes 304 as required by particular application designs.
It will be noted that the two tubes 304 shown in
Any suitable refrigerant known to those in the art for accomplishing the heat-exchange purpose of the claimed invention can be used. Common examples include the well-known family of refrigerants denoted by the ‘R-number’ system including R-11, R-22 and others. Further, the fluid flowing in the outer tubular passage of the heat exchange tubes can be chilled water, brine, ethylene glycol or any other appropriate fluid.
a), 6(b) and 6(c) illustrate the continuous and parallel flow path of the refrigerant and the intermediate medium in the exemplary evaporator of
As seen in
In an alternative arrangement, a number of circuit tubes 604 could be arranged in parallel, having multiple refrigerant inlets 309 and refrigerant outlets 312. Those of skill in the art may structure specific embodiments by configuring multiple inner circuit tubes 606 as desired.
Referring to
Fluid communication between outer tubes 307 (a) and 307 (b) is accomplished through a connecting pipe 650, further welded/sealed to perforations 615 provided in the outer tube 307 (a) and 307 (b). Once connected, using suitable means, these elements together form one outer circuit tube 607. Further, a intermediate medium inlet 313 and a intermediate medium outlet 314 are provided on the outer tubular passage where the intermediate medium enters and exits respectively. After entering the annulus through the intermediate medium inlet, the intermediate medium strikes the closed end 630, rebounds, and is forced to flow into the connecting pipe 650 through perforations (or holes) 615 provided in the outer tube 307 (a), as seen in
As was described for circuit tubes 604, multiple outer circuit tubes 607 can be linked in series or parallel to form heat exchange tube arrays, as seen in
e) illustrates how the different heat exchange tubes are connected to each other to form a continuous heat exchange tube row, wherein the refrigerant and the intermediate medium flow substantially parallel to each other within each heat exchange tube. This drawing depicted a complete flow path for the intermediate fluid and the refrigerant flowing simultaneously through the heat exchange tubes 304 and exiting the evaporator. As shown, an intermediate medium inlet pipe 655 is connected to the intermediate medium inlet of the leftmost heat exchange tube in the series connection. The intermediate fluid enters through the intermediate medium inlet pipe and gets transmitted to the adjacent heat exchange tubes connected in series through the connecting pipes 650. An intermediate medium outlet pipe 660 is further provided, connected to the intermediate medium outlet of the last heat exchange tube in the tube row. After traversing through the heat exchange tubes, the intermediate medium emerges at the intermediate medium outlet pipe and is redirected into the series connection of heat exchange tubes by circulating it through a fluid pump 680.
A temperature detecting device 675 is disposed on the intermediate medium outlet pipe 660, proximal to an end of the series connection of heat exchange tubes in the tube row. The temperature detecting device measures the temperature of the intermediate medium emerging from the intermediate medium outlet. An exemplary temperature detecting device could be any one of the devices conventionally used for this purpose, including a thermocouple, a thermostat or an industrial thermometer etc. Further, the temperature detecting device can be mounted at any other location for the purpose of measuring the temperature of the intermediate medium emerging from the heat exchange tubes. For instance, this location can also be at one of the intermediate medium outlets 314 of the heat exchange tubes or any appropriate location within the outer tubular passage of the heat exchange tubes. In an aspect, a number of such temperature detecting devices may also be provided to measure the temperature of the intermediate medium exiting the heat exchange tubes at different locations, to obtain a more precise value of the temperature. The objective of providing the temperature detecting device is to measure the temperature of the intermediate medium as it emerges from the heat exchange tube(s) after exchanging heat with the refrigerant and the air to be conditioned.
f) illustrates multiple heat exchange tubes connected in series, to form multiple tube rows, and these tube rows are simultaneously connected to the intermediate medium inlet pipe 655 at an end, and to the intermediate medium outlet pipe 660 at the other end. Similarly, the tube rows are further connected to a refrigerant inlet pipe 665 at one end and to a refrigerant outlet pipe 670 at the other end. Both the intermediate medium inlet pipe 655 and the refrigerant inlet pipe 665 furcate into different branches to provide a continuous supply of the intermediate medium and the refrigerant simultaneously into all the tube rows connected in parallel within the evaporator 300. This ensures a continuous flow of the intermediate medium and the refrigerant within the heat exchange tubes of the evaporator.
For the recirculation of the intermediate medium into the evaporator, the fluid pump 680 connects the intermediate medium inlet pipe 655 and the intermediate medium outlet pipe 660. The temperature detecting device 675 is preferably installed at the intermediate medium outlet pipe 660.
A flow control means 685 is disposed at the refrigerant outlet pipe 670 of the evaporator 300. The flow control means 685 adjusts the mass flow rate of the refrigerant (mass of the refrigerant flowing per second) flowing in the inner tubular passage 309 of the heat exchange tubes 306. Further, the adjustment of the mass flow rate of the refrigerant by the flow control means 685 is based upon any changes in the temperature of the intermediate medium at the intermediate medium outlet of the heat exchange tubes. Further, flow control means 685 caninclude any suitable device known to those skilled in the art. For instance, a control valve could be employed, or alternatively one could install a flow restrictor having machined holes able to be partially or completely opened to control refrigerant flow. Further, the flow control means can be disposed at any appropriate location within the heat exchange tubes 304 for controlling the refrigerant flow. According to an embodiment, the flow control means may also be coupled to the inner tubular passage 309 of one or more inner tubes 306.
Continuing further, the refrigerant outlet pipe 670 is further connected to a compressor 690. The compressor 690 is connected to one end of a condenser 695. The other end of the condenser 695 is connected to an expansion valve 696 which is finally connected to the refrigerant inlet pipe 665 to complete the vapor compression refrigeration cycle for the refrigerant flow.
The hot air 810 flows through the evaporator and thus through and over a set of fins 305. These fins 305 constitute a direct heat exchange mechanism, and are maintained at a considerably lower temperature than the ambient medium. The direct heat exchange mechanism is not only in contact with the ambient medium, but because the fins 305 are mounted directly on the outer tubes 307, the fins 305 are also in thermal communication with the intermediate medium. In the illustrated embodiment, the intermediate medium is made up of chilled water 812, maintained at a temperature sufficient to provide the desired heat exchange, as will be understood by those in the art. Thus, heat flows from the ambient medium (hot air 810) into the direct heat exchange mechanism (the fins 305), through the outer wall of tube 307, and into the intermediate medium (the chilled water 812), as indicated by arrow A.
Further, the intermediate medium is in thermal communication with the primary medium, refrigerant 814, flowing in the inner tubular passage 309. Thus, heat flows from the intermediate medium (chilled water 812) to the primary medium (refrigerant 814), as shown by arrow B. Heat thus flows from the air to the intermediate medium and consequently from the intermediate medium to the refrigerant.
The refrigerant 814 enters the inner tubular passage 309, in a mixed liquid and vapor form. As heat flows into the refrigerant 814, it undergoes a phase change at constant saturation temperature and pressure by absorbing latent heat inside the heat exchange tubes. That heat loss increases the temperature gradient from the ambient medium to the primary medium, maintaining the heat flow from arrow A to B. This heat gained by the refrigerant provides the latent heat of vaporization. When sufficient time has elapsed after the operation of the evaporator, a steady state is achieved when all the heat gained by the chilled water from the air is transferred to the refrigerant. Hence, the water acts only as an intermediate transmission medium (heat carrier) between the air and the refrigerant. When this steady state is achieved, no net heat is gained or lost by the water and its temperatures at the intermediate medium inlet and the intermediate medium outlet are substantially same, as detected by the temperature detecting device.
Corresponding to a specific value of the mass flow rate of the refrigerant (the mass of the refrigerant flowing per second) flowing in the inner tubular passage, a specific amount of heat per second is required to be provided to the refrigerant for its complete vaporization.
Considering the chilled water as a control volume, if its temperature at the intermediate medium outlet 314 is greater than the temperature at the intermediate medium inlet 313, it experiences a net heat gain and it can be inferred that the water rejects less heat to the refrigerant than it gains from the air to be conditioned. Similarly, if the temperature of the water at the intermediate medium outlet 314 is less than its temperature at the intermediate medium inlet 313, it experiences a net heat loss, and hence rejects more heat to the refrigerant than it gains from the air.
Eventually, the mass flow rate of the refrigerant in the vapor compression refrigeration cycle is adjusted when the temperature detecting device 675 (shown in
If the temperature of the air to be conditioned fluctuates and suddenly decreases, it rejects comparably lesser heat to the intermediate medium (the chilled water) in thermal contact with it. However, the refrigerant flowing through the inner tubular passage continues to extract the same amount of heat from the intermediate medium until a steady state is resumed. Thus, the intermediate medium (chilled water) rejects more heat to the refrigerant than it gains from the air to be cooled, and experience a net heat loss. This heat loss is manifested as a decrease in temperature of the intermediate medium at the intermediate medium outlet of the heat exchange tubes. The decrease in temperature is observed by consistently measuring the temperature of the intermediate medium at the 314 at regular intervals through the temperature detecting device 675.
When the system detects a decrease in temperature of the intermediate medium at the outlet, the flow control means 685 (shown in
An analogous method is employed when the temperature of the air to be conditioned fluctuates and suddenly increases. In this case, the air has more heat-transferring potential to the intermediate medium (chilled water 812). The sudden increase in air temperature is manifested as an increase in the temperature of the intermediate medium at the intermediate medium outlet 314 which can be detected by the temperature detecting device 675. Whenever this detection is made, the flow control means 685 is used to increase the mass flow rate of the refrigerant in the inner tubular passage of the heat exchange tubes. This would increase the latent heat per second required by the refrigerant for its vaporization and the refrigerant would start extracting more heat per second from the intermediate medium in thermal contact with it. Eventually, the temperature of the intermediate medium at the intermediate medium outlet also starts decreasing synchronously with the increasing mass flow rate of the refrigerant. These two actions of measuring the temperature of the intermediate medium at the intermediate medium outlet and increasing the mass flow rate of the refrigerant through the inner tubular passage are continued simultaneously till the point when the temperature of the intermediate medium at the intermediate medium outlet regains its former value that occurred before the mass flow rate of refrigerant was adjusted. When this happens, it is again ensured that the intermediate medium extracts a certain amount of heat from the air and rejects the same amount of heat to the refrigerant.
Following this approach, a complete vaporization of the intermediate medium at the exit from the evaporator in accordance with the present invention is ensured.
After complete vaporization within the evaporator, the refrigerant enters a compressor 690 (shown in
Although the present invention has been described in considerable details with reference to certain preferred versions thereof, other versions are also possible.
The three-media evaporator as disclosed herein can be used in several circumstances where a refrigerating effect is desired. In an aspect, the evaporator can be used as an integral part of a usual air-conditioning systems utilized in homes or other buildings. As another example, several such evaporators can be simultaneously used in collaboration for commercial and industrial applications where large scale air-conditioning is required, including residential buildings, factories etc. As a further example, the three-media evaporator can also be used in conditioning the air in movie theatres, concert halls, restaurants, cafeteria etc. The appropriate method of use would be to install the three-media evaporator at a suitable location within the space where the refrigerating effect is desired such that it can extract heat from the space, condition the air and reject this heat elsewhere. These and other variations are well within the scope of those of ordinary skill in the art.
