US20070235161A1

US20070235161A1 - Refrigerant based heat exchange system with compensating heat pipe technology

Info

Publication number: US20070235161A1
Application number: US11/728,315
Authority: US
Inventors: Eric Barger; V.J. Patel; Larry Defauw
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-03-27
Filing date: 2007-03-23
Publication date: 2007-10-11

Abstract

The present invention comprises an improved refrigerant based heat exchange system useful in air conditioning and refrigeration applications. The refrigerant based heat exchange system detects the presence of a passive heat pipe phenomenon in the refrigeration circuit and responds by either temporarily shutting off the flow of refrigerant in the circuit, or by temporarily removing the evaporator coil of the air conditioning portion of the system from the path of a forced air flow.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from the earlier filed provisional application Ser. No. 60/786,201, filed Mar. 27, 2006, by the same inventors.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates generally to refrigerant based heat exchange systems employing a compressor, a condenser, an evaporator and associated fluid circuitry used for air conditioning, food storage refrigeration, and similar applications, and to improvements which result from the recognition of a heat pipe phenomenon in such systems.
2. Description of the Prior Art
In a typical commercially available mechanical air conditioning or refrigeration unit, a condensable refrigerant is circulated in a closed loop. Liquid refrigerant under high pressure flows from, for example, a liquid receiver to a pressure reducing valve and into an evaporator. In the evaporator, the pressure of the refrigerant is greatly reduced. Liquid refrigerant boils and absorbs heat from the evaporator. Now a vapor, the refrigerant flows back to the compressor and is compressed to high pressure. The temperature of the refrigerant is greatly increased and, in the condenser, heat is transferred to the surrounding air and the refrigerant cools, becoming liquid again. The refrigerant then flows back into the liquid receiver and cooling cycle is repeated. This typical cycle and variations thereof forms the basis for much of the commercial HVAC industry which exists today and a large body of patent art and technical literature exists with regard to such systems. It should be noted, however, that liquid receivers are optional, and their inclusion in an air conditioning or refrigeration unit is subject to equipment design.
A fairly extensive body of knowledge also exists with respect to what is referred to in the art as “Heat Pipe” technology. A “Heat Pipe” is a refrigerant based heat exchanger that is commonly used for passive heat transfer (i.e. there is no compressor or pump). A heat pipe is typically two heat exchangers piped together, charged with a refrigerant. The flow of refrigerant occurs due to pressure differentials from a hot side to a colder side. As the “hot side” absorbs heat, the heat flows through the refrigerant tubing to the other side of the heat pipe where it is exchanged to a heat exchange medium (typically air). This is the result of a natural tendency of refrigerant to reach equilibrium in an enclosed circuit.
One application of heat pipe technology is used in conjunction with a conventional evaporator coil in an air conditioning system to improve humidity control performance—condensing more water from the humid air by means of creating a stage in the air flow that is colder than would normally be achieved by the evaporator coil in an air conditioning system.
Another example of commercially available “Heat Pipe” technology is a device used to remove heat from microprocessors, such as those used in notebook computers. The heat pipe facilitates the movement of heat from the processor to a fin coil near the edge of the notebook package to be expunged to the environment. Heat pipes of this general type can be purchased at electronics retailers such as “Fry's Electronics.”
Exemplary patents dealing with the heat pipe phenomenon include U.S. Pat. No. 3,543,839, issued Dec. 1, 1970. This patent shows a temperature controllable heat pipe with a switching device between the evaporator and condenser sections of the device.
U.S. Pat. No. 4,974,667, issued Dec. 4, 1990, shows a thermally actuated and switchable heat pipe system in which a condenser is in contact with an automobile engine coolant and an evaporator is in thermal contact with the exhaust of the vehicle.
U.S. Pat. No. 4,494,595, issued Jan. 22, 1985, shows a temperature controllable heat valve which illustrates many of the basic principles of the heat pipe phenomenon. The patent discloses a means for interrupting and modulating the return of liquid condensate to the evaporator end of the heat pipe.
The above patent references are merely intended to be illustrative of the general state of the art of heat pipes and heat pipe systems. A number of references can also be found in the technical literature. See, for example, P. S. Dunn and D. A. Reay, “Heat Pipes”, 2^ndedition (Pergamon, N.Y., 1978).
The present invention does not deal with improvements in heat pipe systems per se, but rather to the application of certain basic heat pipe principles to traditional heating, ventilating, air conditioning, and refrigeration (HVAC-R) type systems.

SUMMARY OF THE INVENTION

Applicants' have discovered that in HVAC and refrigeration systems, there exists an undiscovered “heat pipe effect” when one or more refrigerant circuits in the system are in an idle state, i.e. the compressor(s) are off. This effect occurs, in one instance, because nothing is restricting refrigerant flow and there are temperature differentials between parts of the air conditioning or refrigeration circuit.
In one broad aspect, the present invention comprises a furnace heat exchanger and an evaporative heat exchanger in series, as in a typical HVAC system. The evaporative heat exchanger is connected in a refrigeration circuit in which a compressor circulates refrigerant between a condenser and an evaporator coil. Both the furnace heat exchanger and the evaporator coil include heat exchange surface areas and wherein air flow from a source of forced air is used to provide a heat exchange effect over the heat exchange surface areas. Means are provided for temporarily removing or shielding the heat exchange surface area of the evaporative heat exchanger in response to the detection of a passive heat pipe phenomenon in the air conditioning circuit.
The means for temporarily removing or shielding the heat exchange surface area of the evaporative heat exchanger can be a mechanical actuator which temporarily repositions the evaporator coil with respect to a source of forced air. In one embodiment of the invention, the system can include a pair of evaporator coils which are connected by flexible tubing and wherein the mechanical actuator pivots the coils between an open and closed position relative to the forced air flow. In the closed position, the air must go through the coils. In the open position, the air can proceed unimpeded. In one form, the mechanical actuator can comprise an electronically controlled servo motor.
In a second version, the system of the invention can also be configured so that the evaporator coil and furnace heat exchanger are located within a common duct or housing and are separated by a partition. In this case, the flow of forced air is controlled by means of a movable damper which selectively directs a flow of air over either the heat exchanger or over the evaporator coil.
In a third version of the system of the invention, a means is provided for temporarily blocking the flow of refrigerant in a refrigeration circuit in response to the detection of a passive heat pipe phenomenon in the refrigeration circuit. For example, the means for temporarily blocking the flow of refrigerant in the refrigeration circuit can comprise one or more valves in the refrigeration circuit which are switched between an open position or open state when the compressor is on and a closed position or closed state when the compressor is off and there are temperature differentials between portions of the refrigeration circuit.
Any of a number of convenient valving schemes can be employed. For example, a pair of solenoid valves can be located on either side of the refrigeration circuit between the condensing and evaporating heat exchangers. Alternatively, a check valve is located on one side of the refrigeration circuit between the condensing and evaporating heat exchangers and a solenoid valve is located on an opposite side of the refrigeration circuit.
In each of the above versions of the system of the invention, mechanical elements are introduced into the refrigerant based air conditioner or refrigeration circuit in order to compensate for the “heat pipe effect” which has been found to exist when the refrigerant circuit is in an idle state, refrigerant flow is unrestricted, and temperature differentials exist between parts of the refrigeration circuit.
Additional objects, features and advantages will be apparent in the written description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of one version of the improved HVAC heat exchange system of the invention in the cooling mode showing the closed position of the evaporative coils.
FIG. 1B is a view similar to FIG. 1A, but showing the evaporator coils in the open position.
FIG. 2A is a schematic representation of one version of a refrigerant circuit employing the principles of the present invention.
FIG. 2B is a schematic representation of an alternative refrigerant circuit, similar to FIG. 2A.
FIG. 3A is a schematic representation of another version of the heat exchange system of the invention, showing the system operating in the cooing mode.
FIG. 3B is a schematic representation, similar to FIG. 3A, but showing the system operating in the heating mode.
FIG. 4 is a schematic representation of another version of the system of the invention, showing a system with multiple refrigerant circuits.

DETAILED DESCRIPTION OF THE INVENTION

The improved refrigerant based heat exchange system of the invention can be utilized to improve the efficiency of a conventional refrigerant type air conditioning system where a refrigerant such as Freon is circulated by compressor between an evaporator section and a condenser section, wherein it is respectively changed between liquid and gaseous states to effect cooling in the evaporator unit.
As briefly discussed in the “Background” section of the application, the conventional Freon type air conditioning circuit includes a compressor, condenser, a metering device and an evaporator connected in series in a refrigerant circuit. The air conditioning circuit may also include an optional liquid receiver. The system is charged with refrigerant, which circulates through each of the components in order to remove heat from the evaporator and transfer heat to the condenser. The compressor compresses the refrigerant from a low pressure superheated vapor state to a high pressure superheated vapor state, thereby increasing the temperature and pressure of the refrigerant. The refrigerant medium leaves the compressor and enters the condenser as a vapor at an elevated pressure. The condenser condenses the refrigerant vapor at a higher pressure to a saturated liquid state as a result of the heat transfer in the condenser, typically accomplished with either cooling water or to ambient air. The refrigerant then leaves the condenser as a high pressure liquid. The pressure of the liquid is decreased as it flows through a metering device, such as an expansion type valve, causing the refrigerant to change to mixed liquid-vapor state. The remaining liquid, now at low pressure, is vaporized in the evaporator section of the system resulting in heat transfer from the space being cooled. This vapor then enters the compressor to complete the cycle.
The above described type vapor-compression refrigeration cycle has, for many years, been the pattern for the majority of commercially available air conditioning and refrigeration systems in the marketplace. The present invention is directed to improvements in such systems, particularly when the refrigerant circuit is in an idle state during a normal part of the operating cycle of the system.
Applicants' have discovered that, in HVAC and refrigeration systems, there exists an undiscovered “heat pipe effect” when one or more refrigerant circuits are in an idle state, i.e. the refrigerant circuit's compressor(s) are off. This effect occurs because there is nothing restricting refrigerant flow and there are temperature differentials between parts of the refrigeration circuit.
The “heat pipe effect” is exacerbated if the exchange medium, such as air, is actively circulated with a part of the refrigeration circuit. The degree of the “heat pipe effect” is installation specific and usually significant. The utility of reducing or eliminating the phenomenon of an idle refrigerant circuit acting as a “heat pipe” is central to the present inventive concept. By preventing or reducing the effects of this phenomenon, efficiency gains can be achieved in both existing and future heating and/or cooling systems. Examples #1-3 below are intended to illustrate the heat pipe phenomenon in heating and cooling systems.

EXAMPLE #1

Heating via a HVAC System

A typical HVAC system combines both a cooling circuit and a heating system to provide a solution that satisfies the demand for climate control in all expected climate conditions. This is usually implemented by placing the heating system in series with the evaporative coil portion of the air conditioning circuit. For example, in FIGS. 1A and 1B, the furnace heat exchanger 11 is located in series with the evaporator coil 13 of the air conditioning circuit. This series arrangement allows the use of a single air flow mechanism to facilitate air circulation for both cooling and heating operation. As a result, air flow from a forced air source 15 crosses both the heating element 11 and the evaporator coil 13, regardless of system operating mode.
In such HVAC systems, while operating in heating mode, the air conditioning circuit is in an idle state while the heated air flow passes through the air conditioning circuit's evaporator coil 13. The refrigerant in the evaporator coil 13 absorbs some of the heat energy from the supply air 15 and then passively moves it to another part of the refrigerant circuit and dissipates it. As a result, some of the heat energy introduced by the active heating circuit is removed by the idle cooling circuit.
The idle refrigerant circuit operates as a passive “heat pipe” between the evaporator coil 13 and the condensing unit, comprising the compressor and the condenser, of the air conditioning circuit. The refrigerant while in the evaporator coil 13 increases in temperature, and therefore pressure. This increase in pressure differs from the rest of the refrigerant circuit especially the cold outdoor condenser unit. Naturally, the pressure in the system will attempt to equalize, moving the heat absorbed by the evaporator coil outside. This phenomenon reduces the net heating efficiency of the overall HVAC system.

EXAMPLE #2

Cooling via a Food Storage Refrigeration System

Most, if not all evaporator coil units in refrigeration systems are implemented as a forced air heat exchanger designed to facilitate the absorption of heat energy from the controlled environment. The heat in such a situation is pumped and expunged to an environment external to the controlled environment.
In this type of system, the “heat pipe effect” occurs once the refrigeration circuit becomes idle. The condenser portion (17 in FIGS. 2A and 2B) of the circuit is relatively hot compared to the evaporative portion (19 in FIGS. 2A and 2B) which is located in the cool controlled environment. The temperature differential creates a pressure differential which causes the refrigerant to flow. The net effect is a passive “heat pipe” between the condenser 17 and the evaporator 19. This phenomenon introduces heat into the controlled environment which results in an increased demand for refrigeration time.
Also in this type of system, there is a need to keep air circulating in the controlled environment even when the refrigeration circuit is off. The additional circulation increases the heat exchange between the controlled environment and the “heat pipe” thereby increasing the net amount of heat introduced.

EXAMPLE #3

Systems with Multiple Refrigerant Circuits

Many roof-top packaged HVAC units above 5 ton air conditioning capacity achieve the designed capacity by implementing multiple refrigerant circuits in a single package. In doing so, the evaporator coil comprises multiple refrigerant circuits, one per compressor, in a single coil assembly. Such a system is typically controlled as a two stage system, allowing one or both compressors to run based on system demand.
FIG. 4 illustrates the heat pipe effect in such systems. There is shown a typical 10 ton package unit operating in cooling mode, implemented using two 5-ton compressors (51, 53) each having separate refrigerant circuits (55, 57) wherein each heat exchange coil 58 per circuit shares a common assembly 59 with the corresponding heat exchange coil 58 in the other circuit. The compressors 51, 53 and their corresponding refrigerant circuits 55, 57 shall be denoted as circuits “A” and “B,” respectively.
When the control system calls for 10 ton operation, both refrigerant circuits A and B are active and operate as expected to facilitate heat exchange. However, when the control system calls for 5 ton operation, one refrigerant circuit is active (i.e., compressor A is ON) while the other is in an idle state (i.e., compressor B is off).
In this mode, idle refrigerant circuit B acts as a heat pipe, introducing heat from condenser B and compressor B and introducing it into the indoor air stream 61 via evaporator B. The result is that heat added to the circulated indoor air stream 61 via evaporator B will partially offset the cooling effect intended by the heat absorption of evaporator A. Depending on the mechanical assembly that houses evaporators A and B, this may also have an adverse effect on removing humidity from the indoor air stream 61.
Additionally, the fact that condenser coils A and B share a common physical assembly and therefore are simultaneously exposed to the same air stream 61 intended to facilitate heat exchange from an active refrigerant circuit exacerbates the heat pipe effect. In similar fashion, the fact that evaporators A and B share a common physical assembly results in a reduction of cooling capacity and humidity control.
A recognition of the heat pipe phenomenon, as explained above, has allowed Applicants' to implement several solutions to the problem which act to increase the overall system efficiency. Generally speaking, the solutions which have been implemented to address the heat pipe phenomenon include combinations of the following concepts:
1. Move a selected heat exchanger out of the path of a selected exchange medium. This design change will reduce the efficiency of heat transfer from the exchange medium to the refrigerant thus limiting the overall effect of the “heat pipe.”
2. Stop the refrigerant from flowing. This will prevent a “heat pipe” from being set up but the refrigerant will still absorb some heat.
3. Redirect the exchange medium around heat exchanger. This will also reduce the efficiency of heat transfer from the exchange medium to the refrigerant thus limiting the overall effect of the “heat pipe.”
FIGS. 1A-3B illustrate the above solutions in partly schematic fashion as applied to the above discussed Example #1—Heating via HVAC System.
Design 1:
The first design is a mechanical apparatus that physically moves the evaporator coil to a configuration that significantly reduces or prevents the flow of air through the evaporator coil during heating mode. With reference to FIGS. 1A and 1B, the evaporator coil 13 is actually provided as a pair of coils connected by a flexible refrigerant hose 21. A means is provided for temporarily removing the heat exchange surface area presented by the evaporator coils from the path of the air flow in response to detection of a passive heat pipe phenomenon in the refrigerant circuit of the evaporator coil. In the example, this is accomplished by mounting a servo motor 23 within the furnace enclosure 25 which can be electronically actuated to pivot the evaporator coil halves between and open and a closed position (illustrated in FIGS. 1A and 1B, respectively).
Design 2:
This design utilizes valves to ensure that the refrigerant is prohibited from flowing passively when the refrigeration circuit (30 in FIGS. 2A and 2B) is not in the air conditioning mode. This may be implemented using two solenoid valves (27 and 29 in FIG. 2A), or one solenoid valve (31 and a check valve 33 in FIG. 2B). The solenoid valve in either case may be replaced by a pressure relief valve that opens only when pressures are high enough to indicate cooling mode (pressure introduced by the compressor in the refrigeration circuit).
Design 3:
In this design, dampers are utilized to direct air flow depending upon whether the system is operating in either the cooling or the heating mode. With reference to FIGS. 3A and 3B, the furnace heat exchanger 35 and the evaporator coil 37 of the air conditioning circuit are located within a common enclosure or duct wall 39 and are separated by an internal partition 41. A movable damper 43 is electronically controlled and selectively positioned to direct a flow of air over either the heat exchanger or over the evaporator coil, depending upon whether the unit is in the cooling mode or the heating mode.
It will be appreciated that any of the above three “Design” concepts can be implemented with the Example 3 above in which the system is comprised of multiple refrigerant circuits.
An invention has been provided with several advantages. The recognition of the heat pipe phenomenon in conventional HVAC and refrigeration systems allows simple mechanical solutions to be implemented which increase the efficiency of these systems, sometimes dramatically. The mechanical components added to the systems are simple in design and economical to manufacture and, as a result, do not add greatly to the manufacturing costs. The resulting systems effectively compensate for the heat pipe phenomenon which otherwise exists in such systems when any refrigerant circuit in the system is in an idle state.
While the invention has been shown in only three of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit thereof.

Claims

1. A refrigerant based heat exchange system, the system comprising:

a furnace heat exchanger;

an evaporating heat exchanger connected in a refrigeration circuit in which a compressor circulates refrigerant between a condenser and an evaporator coil;

wherein both the furnace heat exchanger and the evaporator coil include heat exchange surface areas and wherein air flow from a source of forced air is used to provide a heat exchange effect over the heat exchange surface areas; and

means for temporarily removing at least a selected one of the heat exchange surface areas in response to the detection of a passive heat pipe phenomenon in the refrigeration circuit.

2. The refrigerant based heat exchange system of claim 1, wherein the means for temporarily removing at least a selected one of the heat exchange surface areas is a mechanical actuator which temporarily repositions the evaporator coil with respect to the source of forced air.

3. The refrigerant based heat exchange system of claim 2, wherein a pair of evaporator coils are connected by flexible tubing and wherein the mechanical actuator pivots the coils between an open and closed positions relative to the air flow from the source of forced air.

4. The refrigerant based heat exchange system of claim 3, wherein the mechanical actuator is a servo motor.

5. The refrigerant based heat exchange system of claim 1, wherein an evaporator coil and a furnace heat exchanger are located within a common housing and are separated by a partition, and wherein the flow of forced air is controlled by means of a movable damper which selectively directs a flow of air over either the furnace heat exchanger or over the evaporator coil.

6. A refrigerant based heat exchange system, the system comprising:

a condensing heat exchanger;

an evaporating heat exchanger connected in a refrigerant circuit;

a compressor for circulating refrigerant between the condensing heat exchanger and the evaporating heat exchanger in the refrigeration circuit;

wherein both the condensing heat exchanger and the evaporating heat exchanger include heat exchange surface areas and wherein air flow from a source of forced air is used to provide a heat exchange effect over at least a selected one of the heat exchange surface areas; and

means for temporarily blocking the flow of refrigerant in the refrigeration circuit in response to the detection of a passive heat pipe phenomenon in the refrigeration circuit.

7. The refrigerant based heat exchange system of claim 5, wherein the means for temporarily blocking the flow of refrigerant in the refrigeration circuit comprises one or more valves in the refrigeration circuit which are switched between an open position when the compressor is on and a closed position when the compressor is off and there are temperature differentials between portions of the refrigeration circuit.

8. The refrigerant based heat exchange system of claim 6, wherein a pair of solenoid valves are located on either side of the refrigeration circuit between the condensing and evaporating heat exchangers.

9. The refrigerant based heat exchange system of claim 6, wherein a check valve is located on one side of the refrigeration circuit between the condensing and evaporating heat exchangers and a solenoid valve is located on an opposite side of the refrigeration circuit.