US20110100035A1

US20110100035A1 - Two-phase single circuit reheat cycle and method of operation

Info

Publication number: US20110100035A1
Application number: US12/902,200
Authority: US
Inventors: Michael F. Taras; Alexander Lifson; Michael L. Balistreri
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2009-11-03
Filing date: 2010-10-12
Publication date: 2011-05-05

Abstract

A refrigerant system has a refrigerant circuit comprising a compressor for compressing a refrigerant and delivering it downstream to a condenser. A bypass line is provided around the condenser for selectively allowing at least a portion of refrigerant to bypass the condenser. Valves are provided on a line leading to the condenser and on the bypass line to individually control the flow of refrigerant. An expansion device is located downstream of the condenser, and an evaporator is located downstream of the expansion device. A reheat cycle is incorporated into the system. The reheat cycle includes a valve for selectively delivering at least a portion of refrigerant through a reheat heat exchanger, which is positioned in the path of air downstream of the evaporator. A control is provided for the system to achieve a desired level of dehumidification and temperature control to air being delivered into the environment to be conditioned.

Description

RELATED APPLICATION

This application claims priority to U.S. Provisional Application No. 61/257,598, which was filed Nov. 3, 2009.

BACKGROUND OF THE INVENTION

This application relates to refrigerant system controls for providing a reheat function to accurately tailor environmental conditions to desired conditions.
Refrigerant systems are known, and typically employ a compressor which compresses a refrigerant and delivers it downstream to a heat rejection heat exchanger. Heat is removed from the refrigerant at the condenser, and the refrigerant then passes through an expansion device. From the expansion device, the refrigerant passes through an evaporator, where heat is typically added to the refrigerant. From the evaporator, the refrigerant returns to the compressor. For simplicity, the heat rejection heat exchanger may be referred to as a condenser, although it is understood that this term only applies to a sub-critical cycle, while it is replaced by a gas cooler term for a trans-critical cycle.
Many system features have been utilized in combination with the basic structure mentioned above. One feature is a so-called reheat cycle. In a reheat cycle, a heat exchanger is positioned in the path of air downstream of the evaporator. The air is cooled in the evaporator to a temperature below that desired for the environment to be conditioned. In this manner, additional humidity is removed from the air. The air then passes over the reheat heat exchanger where it is heated back toward the target temperature for the environment.
One feature that is provided in combination with the reheat circuit is a bypass of refrigerant around the condenser. In this manner, the thermodynamic state of the refrigerant being delivered into the reheat heat exchanger can be controlled.

SUMMARY OF THE INVENTION

A refrigerant system has a refrigerant circuit comprising a compressor for compressing a refrigerant and delivering it downstream to a condenser. A bypass line is provided around the condenser for selectively allowing at least a portion of refrigerant to bypass the condenser. Valves are provided on a refrigerant line leading to the condenser and on the bypass line to individually control the flow of refrigerant through the two branches. An expansion device is positioned downstream of the condenser, and an evaporator is located downstream of the expansion device. A reheat cycle is incorporated into the refrigerant system. The reheat cycle includes a three-way valve for selectively delivering refrigerant through a reheat heat exchanger, which is positioned in the path of air downstream of the evaporator. A control is provided for the system to achieve a desired level of dehumidification and temperature control to air being delivered into the environment to be conditioned at any ambient conditions as well as internal latent and sensible thermal load demands. The control is operable to initially open the valve on the bypass line, and to move the valve to a relatively open position to achieve additional dehumidification and reheat capacity control. The control next closes the valve on the refrigerant line leading to the condenser to achieve additional dehumidification and reheat capacity control.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an exemplary refrigerant system.

FIG. 1B shows a P-h graph of the refrigerant system operation.

FIG. 1C schematically shows a control feature of the invention.

FIG. 2 shows a main system control flowchart.

FIG. 3 shows a sub-routine operating in parallel with the FIG. 2 control.

FIG. 4 shows another sub-routine operating in parallel with the FIG. 2 control.

FIG. 5A shows another sub-routine operating in parallel with the FIG. 2 control.

FIG. 5B shows another sub-routine operating in parallel with the FIG. 2 control.

FIG. 6A shows another sub-routine operating in parallel with the FIG. 2 control.

FIG. 6B shows another sub-routine operating in parallel with the FIG. 2 control.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A refrigerant system 20 is illustrated in FIG. 1A incorporating a pair of circuits 22 and 23. Of course, more than two circuits can be integrated into the refrigerant system 20. Circuit 22 is provided with a compressor 24, a condenser 26, an expansion device 28 and an evaporator 30. Air flow 32 passing over the evaporator 30 then passes downstream as shown on path 34, over a reheat heat exchanger 40 of the circuit 23. Thus, the single reheat heat exchanger 40 in the circuit 23 provides a reheat function for both circuits 22 and 23. The refrigerant circuits 22 and 23 may provide similar cooing capacities or may have components of different sizes.
Circuit 23 is provided with its own compressor 24, expansion device 28, and evaporator 30. In addition, a condenser 38 in the circuit 23 has a modulating valve 52 controlling the flow of refrigerant through the condenser downstream of the compressor. A bypass line 46 allows bypass of at least a portion of refrigerant around the condenser 38. A modulating valve 48 controls the flow through the bypass line 46, and to a check valve 50 before being returned to a main flow line for the circuit 23. The condenser modulating valve 52 may be positioned downstream of the condenser 38. The check valve 50 allows for minimal refrigerant charge migration in and out of the bypass line, in case the bypass line modulating valve 48 is position further upstream on the bypass line.
A reheat circuit includes a three-way valve 42 which selectively diverts at least a portion of refrigerant downstream of the condenser 38, but upstream of the expansion device 28. This refrigerant passes through the reheat heat exchanger 40, and back through a return line and check valve 44 to the main refrigerant circuit at a location upstream of the expansion device 28. The three-way valve 42 can be replaced by a pair of conventional two-way valves. The three-way valve 42 and a pair the two-way valves can be of an on/off or adjustable type.
Temperature sensor T₁senses the air temperature downstream of the evaporator, and a temperature sensor T₂senses the temperature of the air downstream of the reheat heat exchanger 40.
A control 100 controls all of the components mentioned above. The controls set forth below are disclosed in a system with dual circuits 22/23. However, the control features extend to a single circuit system or a multi-circuit system having more than two refrigerant circuits and more than one refrigerant circuit equipped with the reheat capability.
By selectively controlling the amount of refrigerant passing through the valve 48 and the valve 52, a designer can achieve control such that the two-phase refrigerant being delivered to the reheat circuit is of a desired quality. Valves 48 and 52, for example, can be step motor valves. Of course, similar control logic can be utilized for a refrigerant system operating in a trans-critical regime (vs. a sub-critical regime). In this case, the temperature of single-phase refrigerant (rather than quality of two-phase refrigerant) will be a controlled parameter, while the condenser becomes a gas cooler.
As an example, if the percentage of bypass fluid compared to the percentage of fluid having passed through the condenser is increased, then the overall quality of the mixed refrigerant shifts into a higher vapor quality region inside the two-phase dome, as illustrated in FIG. 1B. This in turn enhances the reheat coil capacity. On the other hand, decreasing the bypass flow causes the opposite effect.
FIG. 1C schematically shows a feature of the present invention, wherein an air flow discharge temperature is sensed (T₁) downstream of the evaporator, and a supply air temperature is sensed (T₂) downstream of the reheat coil 40. A desired temperature is calculated for both locations, and the sensed temperatures are fed back to the control.
When dehumidification is desired, the control 100 will change a compressor cooling capacity upwardly or downwardly to maintain the evaporator exit air temperature at a dehumidification cooling set point. This set point is configured in software and will be set to a temperature low enough to meet the latent capacity needs in the conditioned space positioned downstream of the reheat heat exchanger 40. Alternatively, the dehumidification cooling set point could be dynamic and be reset automatically based on input from a return air temperature sensor and a relative humidity sensor. In this way, the dehumidification cooling set point could be continuously reset, such as to the dew point temperature minus an offset. Further, such controls can be used to control the amount of moisture removed per a specified time interval, such as an hour or a minute.
Once a dehumidification cooling set point is established, the control 100 stages the compressor to meet the set point based on an algorithm that is an adaptive PID style of control. The PID is programmed within the control. The capacity control algorithm uses a modified PID algorithm, with a self-adjusting gain which compensates for varying conditions, including changing flow rates across the evaporator coil. This control uses a “rise per percent capacity” technique in the calculation. For each jump, up or down, in capacity the control knows beforehand the exact capacity change brought on. As the compressors stage up and down to meet the dehumidification cooling set point, the refrigerant valves (46/52) modulate refrigerant flow to meet the required supply air temperature entering the conditioned space.
The valves operate to provide two distinct stages of reheat capacity. In the first stage, the condenser bypass valve 48 begins to open to increase supply air temperature. If the supply air temperature is still too low (T₂), once valve 48 reaches a particular relatively high open percentage (in an example, 100% open), then the valve 52 at the entrance to the condenser will provide a second stage of reheat capacity. Valve 52 begins to close, moving the mixing point even further into the high vapor region. Both valves operate through their full range of motion to meet the supply air temperature requirement. The valves will move in series from Stage 1 (condenser bypass valve) to Stage 2 (condenser entrance valve) and back down again as the unit control logic runs a PID loop to meet the required supply air temperature. This avoids the valves “fighting” with each other when adjusting refrigerant flows through different flow paths.
FIG. 2 shows these method steps in an ordered fashion. In situations where there is only a need for latent capacity removal (no sensible load), the valves 48/52 modulate refrigerant flow so that the air can be reheated to either the return air temperature minus a return air temperature offset (configurable) or to a pre-set reheat set point. In situations where there is a need for both sensible and latent capacity (cooling and dehumidification), the valves 48/52 modulate refrigerant flow so that the air can be reheated to the required supply air temperature for cooling (or heating). This allows the unit to bring the evaporator down to a lower temperature for enhanced dehumidification, while reheating the air to the required supply air temperature. As the valves modulate to meet the required supply air temperature, the latent capacity of the unit remains nearly constant. This dynamics allows the system to provide a variable sensible heat ratio (SHR) that can be matched to the thermal load in the space to be conditioned.
Several additional logic sub-routines are developed to ensure reliable system operation, given the valve operation on the bypass line and the line leading to the condenser.
In a case where the discharge refrigerant downstream of the compressor is all bypassed, the reheat heat exchanger effectively becomes a condenser. In some applications and conditions, the indoor air flow across the reheat heat exchanger can be reduced to a level where the resulting discharge pressure increases beyond the limits of a compressor operating envelope. In such situations, a head pressure control is desirable as the outdoor fans will no longer have any impact on the discharge pressure. An additional head pressure control sub-routine is then activated. As shown in FIG. 3, should the modulating valve position indicate the need for alternative head pressure control, then a discharge pressure is compared to a bypass upper limit. If the discharge pressure is greater than the upper limit, then refrigerant flow is adjusted through controlling valves 52 and 48 to reduce the discharge pressure. This will continue until the valve 48/52 positions have changed to indicate no need for alternative head pressure control, or until the discharge pressure drops below the upper limit.
An additional start-up sub-routine is disclosed in FIG. 4. The disclosed refrigerant system contains more refrigerant than a normal cooling circuit is required. This additional refrigerant is stored in the reheat coil during normal cooling mode of operation. Charge migration during an off-cycle may result in excess refrigerant in the main refrigerant circuit than required during normal cooling mode of operation. This can create an overcharge situation that can in turn lead to unit shutdown due to high discharge pressure. To ensure that refrigerant is in the correct part of the system at start-up, the sub-routine shown in FIG. 4 is provided. Generally, the bypass valve 48 is open, and the reheat three-way valve 42 is open. The compressor is then started. The bypass valve 48 is closed once the bypass line is purged. Then, the reheat valve is also closed after the time required to migrate the refrigerant from the main refrigerant circuit into the reheat coil is complete.
Reduction in the flow through the condenser during the reheat mode of operation can result in flow rates being too low to adequately carry oil through the condenser. Thus, a sub-routine as shown in FIGS. 5A and 5B is disclosed. In the FIGS. 5A and 5B sub-routine, several steps are disclosed for ensuring that the oil return to the compressor is adequate. Further, the FIGS. 5A and 5B sub-routine will also ensure adequate oil return from the reheat coil, if the dehumidification portion of the refrigerant circuit has not been in operation for a prolonged period of time. Generally, as shown in the FIGS. 5A and 5B flowchart, in a cooling mode, if the time elapsed since entering the cooling mode is greater than a maximum time, then the system moves into charge/oil return mode. The reheat valve 42 is opened, as is the bypass valve 48. This occurs through a control loop as shown in the FIGS. 5A and 5B, and for a preset period of time. In this way, not only adequate oil return to the compressor is assured, and the refrigerant charge is also properly re-balanced/re-distributed.
On the other hand (FIG. 5B), if the system is in a dehumidification or reheat mode already, then the position of the condenser valve 52 is compared to a minimum position for adequate oil return. If the condenser valve position is less than the minimum position, then the oil return flag is set. The condenser valve 52 position is changed to a more open position required to ensure adequate oil return. Again, the operation will then continue for a predetermined period of time. Further details are shown in the flowchart.
Finally, a sub-routine shown in FIGS. 6A and 6B is utilized when a transition to cooling occurs. There may be discharge pressure spikes, as the unit switches from a dehumidification mode to a cooling mode. The circuit thus includes a control logic routine in which the three-way valve 42 is repeatedly cycled to push additional refrigerant into the reheat coil, thus reducing the amount of refrigerant in the cooling circuit and effectively managing pressure spikes. Thus, as shown in the FIGS. 6A and 6B sub-routine, the three-way valve 42 is repeatedly cycled between open and closed positions.
In general, the flowcharts shown in FIGS. 2-6 include additional details that may enhance but not limit the methods as claimed in this application. The claims should be studied to determine the true scope of the coverage of this application relative to these methods. However, the methods and sub-routines as shown in the several flow charts are those which are most preferred at this time.
It should be pointed out that many different compressor types could be used in this invention. For example, scroll, screw, rotary, or reciprocating compressors can be employed. Also, rather than a single compressor, plural compressors including multi-stage or plural compressors in series could be used.
The refrigerant systems that utilize this invention can be used in many different applications, including, but not limited to, air conditioning systems, heat pump systems, marine container units, refrigeration truck-trailer units, and supermarket refrigeration systems.
Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.

Claims

1. A refrigerant system comprising:

at least one refrigerant circuit comprising at least one compressor for compressing a refrigerant and delivering it downstream to a heat rejection heat exchanger;

a bypass line provided around said heat rejection heat exchanger for selectively allowing at least a portion of refrigerant to bypass said heat rejection heat exchanger;

a first valve controlling flow to said heat rejection heat exchanger and a second valve on said bypass line to individually control the flow of refrigerant through said heat rejection heat exchanger and around said heat rejection heat exchanger;

an expansion device positioned downstream of said heat rejection heat exchanger, and an evaporator positioned downstream of said expansion device, refrigerant from said evaporator passing back to said at least one compressor;

a reheat cycle incorporated into said refrigerant system, including a third valve for selectively delivering at least a portion of refrigerant through a reheat heat exchanger, said reheat heat exchanger being positioned in the path of air downstream of said evaporator; and

a control for said system being operable in a dehumidification mode to achieve a desired level of dehumidification and temperature control to air being delivered over said evaporator and said reheat heat exchanger and into an environment to be conditioned, said control initially opening said first valve on said bypass line to a relatively open position to achieve additional reheat control, and said control then beginning to close said second valve on said line leading to said heat rejection heat exchanger to achieve additional reheat control.

2. The refrigerant system as set forth in claim 1, wherein said reheat cycle incorporates an inlet positioned between said heat rejection heat exchanger and said expansion device, and a return line downstream of said inlet, but upstream of said expansion device.

3. The refrigerant system as set forth in claim 1, wherein said first valve on said bypass line is initially substantially fully open, and said second valve leading to said heat rejection heat exchanger then begins to be closed.

4. The refrigerant system as set forth in claim 1, wherein said control changes a level of capacity provided by said compressor to achieve evaporator discharge air temperature control, in combination with changing the position of said first and second valves to achieve a desired level of reheat control.

5. The refrigerant system as set forth in claim 1, wherein there are at least a pair of refrigerant circuits within said refrigerant system, with a first of said refrigerant circuits incorporating said reheat heat exchanger, and said bypass around said heat rejection heat exchanger, and a second of said refrigerant circuits including airflow downstream of an evaporator passing over said reheat heat exchanger in said first of said refrigerant circuits.

6. The refrigerant system as set forth in claim 1, wherein said control provides head pressure control when said position of said at least one of first and second valves is such that head pressure control is deemed desirable.

7. The refrigerant system as set forth in claim 1, wherein at start-up, said control moves said second and third valves to at least a partially open position.

8. The refrigerant system as set forth in claim 1, wherein said control operates in oil return mode when at least one of said first and second valves is in a position to indicate a need for oil return.

9. The refrigerant system as set forth in claim 8, wherein said control enters said oil return mode if said first valve is closed below a minimum position, and said control opens said first valve when in said oil return mode.

10. The refrigerant system as set forth in claim 1, wherein at a transition to a cooling mode, said control cycling said third valve on and then off periodically.

11. The refrigerant system as set forth in claim 1, wherein a check valve is provided on said bypass line, and adjacent to a location where said bypass line re-enters a main flow line.

12. The refrigerant system as set forth in claim 1, wherein said second valve is positioned upstream of said heat rejection heat exchanger.

13. A method of operating a refrigerant system including the steps of:

operating a refrigerant circuit and incorporating a reheat cycle into said system; and

providing a desired level of dehumidification and temperature control to air being delivered over an evaporator and a reheat heat exchanger in a dehumidification mode by initially opening a first valve to bypass refrigerant around a heat rejection heat exchanger, and allowing the bypassed refrigerant to enter a reheat heat exchanger, and to move said valve to a relatively open position to achieve additional reheat control, and then beginning to close a second valve on a line leading to the heat rejection heat exchanger to achieve additional reheat control.

14. The method as set forth in claim 11, including the step of changing a level of capacity provided by a compressor to achieve evaporator discharge air temperature control, in combination with changing the position of said valves to achieve a desired level of reheat control.

15. The method as set forth in claim 11, wherein head pressure control is provided when the bypass valve is passing the majority of refrigerant around the heat rejection heat exchanger.

16. The method as set forth in claim 11, wherein said valve on said bypass line is moved to at least a partially open position at start-up.

17. The method as set forth in claim 11, wherein when at least one of said first and second valves is in a position to indicate a need for oil return, an oil return mode is provided.

18. The method as set forth in claim 17, wherein said oil return mode includes opening said first valve.

19. The method as set forth in claim 17, further including the steps of entering the oil return mode when the system is in a cooling mode, but not a dehumidification mode, after a period of time, and includes the steps of opening said first valve on said bypass line.

20. The method as set forth in claim 11, wherein during a transition to a cooling mode, cycling a reheat valve is cycled on and then off periodically to allow and then block flow of refrigerant to the reheat heat exchanger.