LOW AMBIENT COOLING SCHEME AND CONTROL
A system and method for controlling pressure during low ambient conditions when operating in a cooling mode are provided. One or more controllers may be operable to receive a measurement of a first head pressure at a refrigerant line and determine whether the first head pressure is below a pre-determined minimum head pressure threshold. The one or more controllers may be further operable to send an instruction to open a hot gas bypass valve in response to a determination that the first head pressure is below the pre-determined minimum head pressure threshold and a speed of one or more outdoor fan motors is less than or equal to a pre-determined minimum operating fan speed.
This application is directed, in general, to heating, ventilation, and air conditioning systems (HVAC) and, more specifically, to low ambient cooling scheme and control.
BACKGROUNDA variable refrigerant flow (“VRF”) system is an HVAC system that typically utilizes an outdoor condensing unit and multiple indoor fan-coil units. A VRF system can be adjusted to provide for individual heating and cooling needs for different spaces within a building. The system responds to indoor loading variation by adjusting the outdoor compressor speed and controls, which allows refrigerant to be delivered to the individual indoor fan coil units at variable rates depending on the individual heating and/or cooling needs of each space. Whereas in conventional HVAC systems the compressor cycles on and off, in VRF systems the compressor operates continuously at varying speeds under normal ambient conditions.
SUMMARYAccording to one embodiment, a system for controlling head pressure during low ambient conditions comprises a condenser, an outdoor expansion valve, an indoor expansion valve, a hot gas bypass valve, one or more outdoor fans, one or more outdoor fan motors, and one or more controllers. The condenser is operable to receive refrigerant from a discharge line, condense the refrigerant, and discharge the refrigerant to a first portion of a refrigerant line. The outdoor expansion valve is disposed between the first portion of the refrigerant line and a second portion of the refrigerant line, and the indoor expansion valve is disposed between the second portion of the refrigerant line and a third portion of the refrigerant line. The hot gas bypass valve is coupled to the discharge line and to the second portion of the refrigerant line such that, when open, refrigerant through the hot gas bypass valve bypasses the condenser and the outdoor expansion valve. The one or more outdoor fans are operable to cool the condenser, and the one or more outdoor fan motors are operable to control the fan speed of the one or more outdoor fans. The one or more outdoor fan motors have a pre-determined minimum operating fan speed. The one or more controllers are operable to receive a measurement of a first head pressure at the refrigerant line (e.g., at the first portion and/or second portion of the refrigerant line) and determine whether the first head pressure is below a pre-determined minimum head pressure threshold. The one or more processors are further operable to send an instruction to at least partially open the hot gas bypass valve in response to a determination that: the first head pressure is below the pre-determined minimum head pressure threshold, and a speed of the one or more outdoor fan motors is less than or equal to the pre-determined minimum operating fan speed.
According to another embodiment, a method for controlling head pressure in an HVAC system during low ambient conditions comprises receiving, by a processor, a measurement of a first head pressure at a refrigerant line and determining, by the processor, whether the first head pressure is below a pre-determined minimum head pressure threshold. The method further comprises sending, by the processor, an instruction to at least partially open a hot gas bypass valve in response to a determination that: the first head pressure is below the pre-determined minimum head pressure threshold, and a speed of one or more outdoor fans is less than or equal to a pre-determined minimum operating speed of the one or more outdoor fans.
Advantageously, aspects of the present disclosure may allow for the HVAC system to operate in low ambient conditions while maintaining smooth operation. Another advantageous aspect of the present disclosure may provide a method of smoothly operating a VRF or HVAC system beyond the lowest speed of outdoor fans. Further, aspects of the present disclosure may maintain the HVAC system operation at optimal conditions while the HVAC system operates in low ambient conditions. Aspects of the present disclosure may be utilized, therefore, to prevent unstable suction pressure and capacity output, thus providing smooth operation of the HVAC system during low ambient conditions.
For a more complete understanding of the present disclosure and the advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying drawings, in which:
In general, VRF systems may provide smooth operation and close response to indoor loading variation by adjusting the compressor speed and controls. In certain situations, it may be desirable to operate a VRF system in a cooling mode during low ambient conditions. For example, in the winter, the outdoor ambient temperature may be below freezing. However, it may be desirable to operate the VRF system in a cooling mode during such low ambient conditions, for example, if the indoor space conditioned by the VRF system houses a number of computers that generate heat within the conditioned space.
When in cooling mode, refrigerant may pass through an outdoor condenser that cools the refrigerant. Cooling the refrigerant generally causes the pressure of the refrigerant to decrease. In normal ambient conditions, the amount of cooling and thus the amount of pressure decrease can be controlled by adjusting the speed of one or more outdoor fans that circulate ambient air across the outdoor condenser. The fan speed may be increased for more cooling and decreased for less cooling. In low ambient conditions, one or more of the outdoor fans of a VRF system slows down to reduce the heat rejection and maintain proper head pressure. However, the fan motors used to drive the outdoor fans have a minimum speed to ensure reliable operation, and when the ambient is very cold, the head pressure cannot be maintained even when the fan speed is at a minimum. Traditionally, the fan will be cycled on and off based on predetermined liquid pressures when the lowest fan speed fails to maintain the head pressure. This may cause unstable suction pressure and capacity output.
Certain embodiments of the present disclosure may allow for smoothly operating a VRF system beyond the lowest speed of the outdoor fan. For example, in certain embodiments, if the head pressure is below a pre-determined threshold and the fan speed is at the lowest fan speed, a hot gas bypass valve may be at least partially opened so that at least part of the refrigerant bypasses the condenser. The refrigerant that bypasses the condenser maintains a high temperature and high pressure. The refrigerant that bypasses the condenser may be combined with cooler, lower pressure refrigerant that has passed through the condenser to obtain an appropriate temperature and pressure. Examples of systems and methods for low ambient cooling control are further described with respect to
Referring to
HVAC system 100 may be provided with a component configuration capable of cooling only, heating only, or both heating and cooling operation. For example, referring to the embodiment of
Referring to
Outdoor unit 110 of HVAC system 100, as illustrated in
As shown in
In an embodiment, controller 112 may receive a measurement of a head pressure and determine whether the head pressure is below a pre-determined minimum head pressure threshold. For example, controller 112 may determine that the received head pressure measurement is below a pre-determined minimum head pressure threshold. Any suitable pre-determined minimum head pressure threshold may be used depending on the system. In certain embodiments, the pre-determined minimum head pressure threshold may between 200 and 300 pounds per square inch gauge (“psig”), such as 250 psig, for example, when using R-410A as the refrigerant for certain embodiments of the HVAC system. Additionally, or alternatively, controller 112 may receive a measurement of a head pressure and determine whether the head pressure exceeds a pre-determined maximum head pressure threshold. Any suitable pre-determined maximum head pressure threshold may be used depending on the system. In certain embodiments, the pre-determined maximum head pressure threshold may between 400 and 500 psig, such as 450 psig.
Controller 112 may selectively energize, de-energize, or configure HVAC system 100 components. For example, in an embodiment, controller 112 may set the operating speed of a compressor, motor, or the like. In some embodiments, controller 112 may be operable to send an instruction to partially or fully open or close one or more valves of HVAC system 100. Additionally, or alternatively, controller 112 may be operable to increase or decrease the speed of a fan motor of HVAC system 100. For instance, controller 112 may send an instruction to increase the speed of fan motor 134 of HVAC system 100 in response to a determination that a head pressure at a refrigerant line exceeds a pre-determined maximum head pressure threshold.
Controller 112 may operably couple to HVAC system 100 components via wired or wireless connections. For instance, as shown in
As shown in the embodiment of
Compressor 120 may be of any suitable type, such as a rotary compressor, a scroll compressor, a reciprocating compressor, or any other type of compressor suitable for use in HVAC system applications. Compressor 120 may be configured for variable speed or single speed operation. Compressor 120 may operably couple to controller 112 via a wired or wireless connection and may be selectively energized, de-energized, or set to a desired operating speed by controller 112 to meet a demand on HVAC system 100.
As shown, a single compressor 120 may be included in HVAC system 100. For instance, HVAC system 100 may include a single variable speed compressor. In alternative embodiments, HVAC system 100 may include more than one compressor 120. For example, HVAC system 100 may include a variable speed compressor plus a fixed-speed compressor. As another example, HVAC system may include multiple variable speed compressors. In such embodiments, the compressors provided may be configured to operate as a tandem compressor group. The tandem compressors may each be incorporated within a single circuit of HVAC components configured for vapor compression cycle operation, with each tandem compressor operatively coupling to the discharge line 126 and the suction line.
HVAC system 100, as shown in
According to the embodiment shown in
HVAC system 100 may include outdoor fan 132 for inducing flow of ambient air over condenser 130. According to the embodiment shown, outdoor fan 132 may comprise a plurality of blades which may couple to, and be rotated about, a hub or shaft through actuation of fan motor 134. Energizing of fan motor 134 may cause rotation of outdoor fan 132 blades about the hub. Rotation of outdoor fan 132 blades may cause ambient air movement to induce a flow of ambient air over condenser 130. The ambient air flowing over condenser 130 may be at an ambient temperature.
Fan motor 134 may be an electric motor which may rotate in response to a received signal, or signals. Fan motor 134 may be configured to operate as a variable speed motor, whereby fan motor 134 may operate at a plurality of speeds, as measured in revolutions per minute (RPM). The speed of fan motor 134 may vary in response to changes to the signal, or signals, received. In such embodiments, fan motor 134 may be provided with a range of speed values within which fan motor 134 may be operated. The range may have a pre-determined minimum operating speed below which fan motor 134 may not operate. If commanded to operate at a speed outside of the operating range of fan motor 134, fan motor 134 may de-energize or may, alternatively, operate at a lowest speed setting. Similarly, the range of speed values within which fan motor 134 may be operated may have a pre-determined maximum operating speed.
In an embodiment, fan motor 134 may be an electric commutation (EC) type motor. The electrical input to fan motor 134 may be a direct current (DC) input or an alternating current (AC) input. The speed of fan motor 134 may be controlled using any known method of motor speed control. Fan motor 134 may operably connect to controller 112. Controller 112 may transmit control and/or power signals to fan motor 134 for varying the speed of fan motor 134. In an embodiment, in an embodiment, a controller may vary the control and/or power signal voltage to adjust the speed of fan motor 134. For example, controller 112 of HVAC system 100 may send an instruction to increase the speed of outdoor fan 134 if a head pressure at refrigerant line 155 exceeds a pre-determined maximum head pressure threshold. Alternatively, the controller may vary the pulse widths of a power and/or control signal transmitted to fan motor 134 to vary the speed of fan motor 134. During low ambient conditions, controller 112 may de-energize fan motor 134. Low ambient temperatures may comprise outdoor temperatures ranging from 0 degrees Fahrenheit to 23 degrees Fahrenheit. The controller may accordingly set the speed of fan motor 134 in response to conditions within HVAC system 100 in accordance with control logic executed by the controller.
HVAC system 100 may comprise outdoor expansion valve 140, as shown in
Refrigerant flow within HVAC system 100 may be directed through outdoor expansion valve 140 to indoor expansion valve 160. Outdoor expansion valve 140 may receive refrigerant from condenser 130 and deliver refrigerant to indoor expansion valve 160 via refrigerant line 155. Outdoor expansion valve 140 may be operably coupled to a controller via a wired or wireless connection. In an embodiment, a controller may be operable to send an instruction to at least partially close an outdoor expansion valve in response to a determination that a head pressure is below a pre-determined head pressure threshold. For example, controller 112 of HVAC system 100 may send an instruction to fully close outdoor expansion valve 140 in response to a determination that a head pressure is below a pre-determined head pressure threshold of 200 psig.
In the embodiment shown in
In an embodiment, hot gas bypass valve 150 may be operably coupled to a controller via a wired or wireless connection. The controller may be operable to at least partially open hot gas bypass valve 150 in response to certain determinations. For example, controller 112 may fully open hot gas bypass valve 150 when a received head pressure measurement is below a pre-determined head pressure threshold and outdoor fan motor 134 is operating at a speed less than or equal to a pre-determined operating fan speed. Similarly, in some embodiments, controller 112 may be operable to at least partially close hot gas bypass valve 150 in response to certain determinations.
Hot gas bypass valve 150 and outdoor expansion valve 140 may work in conjunction to maintain the head pressure in refrigerant line 155 within a pre-determined head pressure range. In an embodiment, controller 112 may partially or fully open or close hot gas bypass valve 150 and/or outdoor expansion valve 140 to maintain a head pressure within 250 psig and 450 psig at refrigerant line 155. In certain embodiments, a minimum head pressure threshold of HVAC system 100 may be a value between 200 and 300 psig, whereas a maximum head pressure threshold may be a value between 400 psig and 500 psig. However, those of ordinary skill in the art will appreciate that the pre-determined head pressure range of system 100 will vary depending on conditions such as the type of refrigerant used in the HVAC system and the HVAC system's configuration. In some embodiments, the minimum head pressure threshold may be based on the pressure drop required by indoor expansion valve 160 to meet the demands of evaporator 170's load.
As shown in
As shown in the illustrated embodiment, indoor expansion valve 160 may be disposed between outdoor expansion valve 140 and evaporator 170 of HVAC system 100, as part of a vapor compression cycle. According to HVAC system 100 embodiment shown, a single indoor expansion valve 160 is provided. In alternative embodiments of HVAC system 100, additional indoor expansion valves may be provided. For example, additional indoor expansion valves may be provided in an HVAC system configured to operate as a dual flow, heat pump, VRF, and/or other HVAC system type. Additionally, some embodiments may include multiple indoor expansion valves and no outdoor expansion valves. The operation of such indoor expansion valves is well known to those of ordinary skill in the art and is omitted from this description.
In alternative embodiments, HVAC system 100 may additionally, or alternatively, include one or more valves for controlling the direction and/or rate of refrigerant flow within HVAC system 100. For example, in an embodiment of HVAC system 100, HVAC system 100 may be additionally provided with a reversing valve as well as additional piping sections to accommodate bi-directional refrigerant flow capability within HVAC system 100. Those of ordinary skill in the art will appreciate that corresponding variations to the refrigerant piping configuration may be provided to accommodate the particular component configuration of HVAC system 100 embodiment provided.
As shown in the embodiment of
Evaporator 170 may couple with, and receive refrigerant via refrigerant line 155. Refrigerant line 155 may comprise a low pressure pipe. According to the component configuration and refrigerant flow directions shown in
HVAC system 100, as shown, may be a cooling-only unit or, alternatively, may be a heat pump unit operating in cooling mode. In alternative embodiments, HVAC system 100 may be configured to operate in heating mode as part of a heating only, or a heat pump unit, for example. In such embodiments, evaporator 170 may be configured to operate as a condenser as part of the vapor compression cycle, with the refrigerant flow directed in the opposite direction than that shown.
As shown in
Sensor 180 may be a remote sensing device which may connect with controller 112 via a wired or wireless connection for transmitting sensed, or measured, data to controller 112. Sensor 180 may transmit analog or pneumatic signals either directly, or indirectly, to controller 112. In such an embodiment, the signals transmitted by sensor 180 may be converted to digital signals prior to use by controller 112. Alternatively, in an embodiment, sensor 180 may transmit digital signals to controller 112. In such an embodiment, the digital signals transmitted by the sensors 180 may be processed prior to use by controller 112 to convert the signals to a different voltage, to remove interference from the circuits, to amplify the signals, or other similar forms of digital signal processing. For each alternative described, herein, the signals of sensor 180 may be transmitted to controller 112 directly or indirectly, such as through one or more intermediary devices.
In an embodiment, sensor 180 may be disposed within the outdoor unit 110 and may be configured to sense the outdoor ambient air temperature within the outdoor unit 110. In such an embodiment, sensor 180 may be a thermistor. Alternatively, in such an embodiment, sensor 180 may be a thermocouple, a resistance temperature detection sensor, pyrometric sensor, an infrared thermographic sensor, or some other sensor type for sensing temperature values of outdoor ambient air. The sensor may transmit the sensed ambient temperature to controller 112 for use by controller 112 as input to one or more control methods. For example, controller 112 may use ambient temperature data received from sensor 180 as an input to a control method for setting the speed of motor 134 during the operation of HVAC system 100 to control the rate of heat transfer to, or from, refrigerant within the outdoor unit 110.
In alternative embodiments, sensor 180 may be disposed within HVAC system 100 at a position different from that shown in the particular embodiment of
In alternative embodiments, HVAC system 100 may be provided with component configuration differing from that shown in the embodiment of
Referring now to
Controller 200 may, additionally, be implemented with processor 220 for executing stored instructions. Controller 200 may be responsive to or operable to execute instructions stored as part of software, hardware, integrated circuits, firmware, micro-code or the like. The functions, acts, methods or tasks performed by controller 200, as described herein, may be performed by processor 220 executing instructions stored in memory 210. The instructions are for implementing the processes, techniques, methods, or acts described herein. Controller processor 220 may be any known type of processor commonly used in HVAC systems. The processor may be a single device or a combination of devices, such as associated with a network or distributed processing. Controller 200 may operably couple to HVAC system 100 components via wired or wireless connections.
Controller 200 may receive data, which may comprise signals from one or more remote sensing devices. The data received by controller 200 may be received directly from one or more remote sensing devices, or, may be received indirectly through one or more intermediate devices such as a signal converter, a processor, an input/output interface (e.g. interface 230), an amplifier, a conditioning circuit, a connector, and the like. Controller 200 may operate HVAC system 100 components in response to received data from remote sensing devices. Additionally, controller 200 may operate HVAC system 100 components in response to user input, demands of the conditioned space, refrigerant and/or ambient air conditions, control logic, and the like.
Referring now to
In the illustrated embodiment of
As shown in
Referring to
Method 400 starts at step 402. At step 402, method 400 is in normal cooling mode operation. In certain embodiments, during normal cooling mode operation, the outdoor expansion valve (e.g., outdoor expansion valve 140 or outdoor expansion valve 340) is fully open and the hot gas bypass valve (e.g., hot gas bypass valve 150 or hot gas bypass valve 350) is fully closed. Thus, refrigerant passes through the condenser (e.g., condenser 130 or 330), and the condenser lowers the temperature and pressure of the refrigerant. During normal cooling mode operation, the fan speed of outdoor fans can be adjusted to control the amount of cooling in order to maintain a proper pressure. As the fan speed decreases, less ambient air is circulated across the condenser, which results in less cooling and less temperature reduction. In general, the required fan speed tends to be relatively low in low ambient conditions, such as when the outdoor ambient air is freezing, because exposure to the low ambient conditions causes some heat loss.
At step 404, the processor determines whether a fan speed of one or more outdoor fans (e.g., fan 132 and/or fan 232) is less than or equal to a pre-determined minimum operating fan speed. For example, the one or more outdoor fans may be operating at a pre-determined minimum operating fan speed during low ambient conditions, such as when the outdoor temperature is at or between 0 degrees Fahrenheit and 23 degrees Fahrenheit. If the processor determines that the speed of the one or more outdoor fans is greater than the pre-determined minimum operating fan speed, the method stays in normal cooling mode operation. The method may repeat step 404 periodically, for example, according to a pre-determined time period or in response to a change in the fan speed. If at step 404 the processor determines that the speed of the one or more outdoor fans is less than or equal to the pre-determined minimum operating fan speed, the method moves to step 406.
At step 406 of method 400, as illustrated in
At step 410 of
The method then moves to step 412, where the processor receives a measurement of a second head pressure. As shown in
From step 416, method 400 of
Alternatively, if at step 414 the processor determines that the second head pressure does not exceed the pre-determined maximum head pressure threshold, the method moves to step 420. At step 420, the processor determines whether the second head pressure is below the pre-determined minimum head pressure threshold. If the processor determines that the second head pressure is below the pre-determined minimum head pressure threshold, method 400 moves to step 422. At step 422, the processor sends an instruction to at least partially close the outdoor expansion valve. As an example, if the head pressure measurement is 190 psig and the minimum head pressure threshold is 200 psig, the processor may send an instruction to fully close the outdoor expansion valve. In certain embodiments, closing the outdoor expansion valve may cause the amount of refrigerant that passes through the condenser to decrease such that the amount of refrigerant that passes through the hot gas bypass valve increases. Thus, higher temperature, higher pressure refrigerant may be provided to the refrigerant line.
Alternatively, if at step 420 the processor determines that the second head pressure is not below the pre-determined minimum head pressure threshold, the method moves to step 412, where the processor receives a measurement of a third head pressure. The third measurement may be compared to a maximum head pressure threshold at step 414. The method may repeat the steps as necessary and may adjust the outdoor fan speed, the hot gas bypass valve, and the outdoor expansion valve to maintain a proper pressure in the refrigerant line.
In the preceding discussion, numerous specific details are set forth to provide a thorough understanding of the present disclosure. However, it will be understood that the present disclosure may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present disclosure in unnecessary detail. Additionally, for the most part, details concerning well-known features and elements have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present disclosure, and are considered to be within the understanding of persons of ordinary skill in the relevant art.
Having thus described the present disclosure by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present disclosure may be employed without a corresponding use of other features. Many such variations and modifications may be recognized based upon a review of the foregoing description of preferred embodiments.
Claims
1. An HVAC system for controlling head pressure during low ambient conditions, comprising:
- a condenser operable to receive refrigerant from a discharge line, condense the refrigerant, and discharge the refrigerant to a first portion of a refrigerant line;
- an outdoor expansion valve between the first portion of the refrigerant line and a second portion of the refrigerant line;
- an indoor expansion valve between the second portion of the refrigerant line and a third portion of the refrigerant line;
- a hot gas bypass valve coupled to the discharge line and the second portion of the refrigerant line such that, when open, refrigerant through the hot gas bypass valve bypasses the condenser and the outdoor expansion valve;
- one or more outdoor fans operable to cool the condenser;
- one or more outdoor fan motors operable to control the fan speed of the one or more outdoor fans, wherein the one or more outdoor fan motors has a pre-determined minimum operating fan speed; and
- one or more controllers operable to: receive a measurement of a first head pressure at the refrigerant line; determine whether the first head pressure is below a pre-determined minimum head pressure threshold; and send an instruction to at least partially open the hot gas bypass valve in response to a determination that: the first head pressure is below the pre-determined minimum head pressure threshold; and a speed of the one or more outdoor fan motors is less than or equal to the pre-determined minimum operating fan speed.
2. The system of claim 1, wherein the controller is further operable to:
- receive a measurement of a second head pressure at the refrigerant line after at least partially opening the hot gas bypass valve;
- determine whether the second head pressure exceeds a pre-determined maximum head pressure threshold; and
- send an instruction to increase the speed of the one or more outdoor fan motors in response to a determination that the second head pressure exceeds the pre-determined maximum head pressure threshold.
3. The system of claim 1, wherein the controller is further operable to:
- receive a measurement of a second head pressure at the refrigerant line after opening the hot gas bypass valve;
- determine whether the second head pressure is below the pre-determined minimum head pressure threshold; and
- send an instruction to at least partially close an outdoor expansion valve in response to a determination that the second head pressure is below the pre-determined minimum head pressure threshold.
4. The system of claim 3, wherein the controller is further operable to:
- receive a measurement of a third head pressure at the refrigerant line after at least partially closing the outdoor expansion valve;
- determine whether the third head pressure exceeds the pre-determined maximum head pressure threshold;
- send an instruction to increase the speed of the one or more outdoor fan motors in response to a determination that the third head pressure exceeds the pre-determined maximum head pressure threshold.
5. The system of claim 4, wherein the controller is further operable to determine that the increased speed of the one or more outdoor fan motors equals a pre-determined maximum operating speed of the one or more outdoor fan motors and, in response, send at least one of:
- an instruction to at least partially open the at least partially closed outdoor expansion valve; and
- an instruction to at least partially close the at least partially open hot gas bypass valve.
6. The system of claim 5, wherein at least partially opening the outdoor expansion valve comprises fully opening the outdoor expansion valve.
7. The system of claim 1, wherein at least partially opening the hot gas bypass valve comprises fully opening the hot gas bypass valve.
8. The system of claim 1, wherein the system's capacity ranges from 6 to 36 tons, the minimum head pressure threshold is a value between 200 and 300 pounds per square inch gauge (“psig”), and the maximum head pressure threshold is a value between 400 and 500 psig.
9. The system of claim 1, wherein the low ambient conditions comprise outdoor temperatures ranging from 0 degrees Fahrenheit to 23 degrees Fahrenheit.
10. The system of claim 1, wherein the HVAC system is a variable refrigerant flow system and the third portion of the refrigerant line is coupled to two or more indoor expansion valves, each indoor expansion valve associated with a distinct evaporator of a plurality of evaporators.
11. A method for controlling head pressure in an HVAC system during low ambient conditions, comprising:
- receiving, by a processor, a measurement of a first head pressure at a refrigerant line;
- determining, by the processor, whether the first head pressure is below a pre-determined minimum head pressure threshold; and
- sending, by the processor, an instruction to at least partially open a hot gas bypass valve in response to a determination that: the first head pressure is below the pre-determined minimum head pressure threshold; and a speed of one or more outdoor fans is less than or equal to a pre-determined minimum operating speed of the one or more outdoor fans.
12. The method of claim 11, further comprising:
- receiving, by the processor, a measurement of a second head pressure at the refrigerant line after at least partially opening the hot gas bypass valve;
- determining, by the processor, whether the second head pressure exceeds a pre-determined maximum head pressure threshold; and
- sending, by the processor, an instruction to increase the speed of the one or more outdoor fans in response to a determination that the second head pressure exceeds the pre-determined maximum head pressure threshold.
13. The method of claim 11, further comprising:
- receiving, by the processor, a measurement of a second head pressure at the refrigerant line after opening the hot gas bypass valve;
- determining, by the processor, whether the second head pressure is below the pre-determined minimum head pressure threshold; and
- sending, by the processor, an instruction to at least partially close an outdoor expansion valve in response to a determination that the second head pressure is below the pre-determined minimum head pressure threshold.
14. The method of claim 13, further comprising:
- receiving, by the processor, a measurement of a third head pressure at the refrigerant line after at least partially closing the outdoor expansion valve;
- determining, by the processor, whether the third head pressure exceeds the pre-determined maximum head pressure threshold;
- sending, by the processor, an instruction to increase the speed of the one or more outdoor fans in response to a determination that the third head pressure exceeds the pre-determined maximum head pressure threshold.
15. The method of claim 14, further comprising determining that the increased speed of the one or more outdoor fan motors equals a pre-determined maximum operating speed of the one or more outdoor fan motors and, in response, sending at least one of:
- an instruction to at least partially open the at least partially closed outdoor expansion valve; and
- an instruction to at least partially close the at least partially open hot gas bypass valve.
16. The method of claim 15, wherein at least partially opening the outdoor expansion valve comprises fully opening the outdoor expansion valve.
17. The method of claim 11, wherein at least partially opening the hot gas bypass valve comprises fully opening the hot gas bypass valve.
18. The method of claim 11, wherein the system's capacity ranges from 6 to 36 tons, the minimum head pressure threshold is a value between 200 and 300 pounds per square inch gauge (“psig”), and the maximum head pressure threshold is a value between 400 and 500 psig.
19. The method of claim 11, wherein the low ambient conditions comprise outdoor temperatures ranging from 0 degrees Fahrenheit to 23 degrees Fahrenheit.
20. A non-transitory computer readable medium comprising logic that, when executed by a processor, is operable to:
- receive a measurement of a first head pressure at a refrigerant line;
- determine whether the first head pressure is below a pre-determined minimum head pressure threshold; and
- send an instruction to at least partially open a hot gas bypass valve in response to a determination that: the first head pressure is below the pre-determined minimum head pressure threshold; and a speed of one or more outdoor fans is less than or equal to a pre-determined minimum operating speed of the one or more outdoor fans.
Type: Application
Filed: Jul 30, 2015
Publication Date: Feb 2, 2017
Inventor: Der-Kai Hung (Dallas, TX)
Application Number: 14/813,298