Low ambient cooling scheme and control

Abstract

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.

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.