Defrost system for a conditioned space

Abstract

A defrost system for a vapor-compression based refrigerator/freezer combines increased operational efficiency with a high likelihood of robust adoption by technical and business personnel. The system includes a controller programmed to monitors and reports several key performance indicators on each evaporator of the system, and to provide reliable, repeatable “initiate defrost” and “terminate defrost” signals which may prompt actions by an operator or automatically control system components. The controller is designed to provide accurate and efficient defrosting signals regardless of the style of evaporator used, the location of the evaporator and other conditions including pull-down, high traffic locations and operations, and low use periods such as weekends.

What is claimed is: 
     
         1 . A defrost system comprising:
 a vapor compression system including a compressor, a condenser, an expansion valve, an evaporator, and a quantity of refrigerant;   a defrost heater operably connected to the evaporator and configured to melt accumulated ice or frost from coils of the evaporator upon activation;   a plurality of sensors operable to gather signals indicative of a need for a defrost cycle, the sensors including an air temperature sensor in fluid communication with an airflow pathway over the coils, a surface temperature sensor operably coupled to the coils, an electrical current sensor, or a combination thereof; and   a controller programmed to:
 receive the signals from the plurality of sensors; 
 process the signals to determine whether a defrost cycle is needed; and 
 upon determination that the defrost cycle is needed, initiate the defrost cycle via control over at least one component of the vapor compression system. 
   
     
     
         2 . The defrost system of  claim 1 , wherein the controller is further programmed to:
 process the signals to determine whether the defrost cycle is complete; and   upon determination that the defrost cycle is complete, terminate the defrost cycle via control over the at least one component of the vapor compression system.   
     
     
         3 . The defrost system of  claim 1 , wherein initiating the defrost cycle comprises issuing a notification to a user to initiate the defrost cycle. 
     
     
         4 . The defrost system of  claim 1 , wherein initiating the defrost cycle comprises:
 deactivating the compressor; and   activating the defrost heater.   
     
     
         5 . The defrost system of  claim 1 , further comprising:
 an upstream temperature sensor operably connected to the controller and located at an air supply of the evaporator, upstream of an airflow path through the evaporator, and   a downstream temperature sensor operably connected to the controller and located at an air return of the evaporator, downstream of the airflow path through the evaporator,   the controller programmed to:
 determine a frost-free temperature differential between the upstream temperature sensor and the downstream temperature sensor when the evaporator is in a frost-free condition; 
 determine an operational temperature differential between the upstream temperature sensor and the downstream temperature sensor at a time later than the determination of the frost-free temperature differential; and 
 when the operational temperature differential reaches a programmed threshold level below the frost-free temperature differential, initiating the defrost cycle. 
   
     
     
         6 . The defrost system of  claim 5 , wherein when the frost-free temperature differential is between 6.5 degrees F. and 7.5 degrees F., the programmed threshold level is between 5 degrees F. and 6 degrees F. 
     
     
         7 . The defrost system of  claim 1 , further comprising a plurality of surface temperature sensors in direct thermal communication with coil tubing of the evaporator, each of the plurality of surface temperature sensors operably connected to the controller, the controller programmed to:
 determine a frost-free temperature as measured by the plurality of surface temperature sensors when the evaporator is in a frost-free condition;   determine an operational temperature differential as measured by the plurality of surface temperature sensors, at a time later than the determination of the frost-free temperature differential; and   when the operational temperature differential reaches a programmed threshold level below the frost-free temperature differential, initiating the defrost cycle.   
     
     
         8 . The defrost system of  claim 7 , wherein:
 a first one of the plurality of surface temperature sensors is at a first location downstream of a fluid inlet of the evaporator;   a second one of the plurality of surface temperature sensors is at a second location downstream of the fluid inlet of the evaporator and downstream of the first location;   the controller programmed to:
 determine a frost-free temperature curve; 
 determine an operational temperature curve; and 
 when the operational temperature curve reaches a programmed threshold level of flattening compared to the frost-free temperature curve, initiating the defrost cycle. 
   
     
     
         9 . The defrost system of  claim 1 , further comprising:
 an evaporator fan is positioned and oriented to blow over the coils of the evaporator, the fan including a fan motor and a plurality of fan blades.   
     
     
         10 . The defrost system of  claim 9 , wherein:
 the controller receives a signal from the fan motor indicative of the electrical current drawn by the fan motor;   the controller is programmed to:
 determine a frost-free current; 
 determine an operational current; and 
 when the operational current reaches a programmed threshold level above the frost-free current, initiating the defrost cycle. 
   
     
     
         11 . The defrost system of  claim 1 , further comprising:
 a fluid collector positioned under the evaporator;   a drain line extending from the fluid collector to a drain; and   a drain line heater operably connected to the controller, the drain line heater positioned to heat the drain line, whereby the drain line heater can be activated to melt accumulated ice within the drain line and ensure liquid passing through drain line is prevented from freezing,   wherein the controller is programmed to activate the drain line heater in concert with the initiation of the defrost cycle.   
     
     
         12 . The defrost system of  claim 11 , wherein the controller is programmed to activate the drain line heater upon activation of the defrost heater. 
     
     
         13 . The defrost system of  claim 11 , wherein the controller is programmed to activate the drain line heater at a predetermined time before activation of the defrost heater. 
     
     
         14 . The defrost system of  claim 11 , wherein the controller is programmed to:
 monitor electrical current to the drain line heater and inferring malfunction when the current shows a completely open or closed circuit; and   when the controller infers malfunction, the controller is programmed to delay initiation of the defrost cycle until the drain line heater draws a normal operating current.   
     
     
         15 . The defrost system of  claim 1 , further comprising:
 a suction-stop valve positioned downstream of the evaporator and upstream of the compressor, the suction-stop valve operably connected to the controller and activatable to slow or stop a flow of fluid through the vapor compression system, wherein:   wherein the controller initiates the defrost cycle comprises activating the suction-stop valve to halt circulation of the refrigerant.   
     
     
         16 . The defrost system of  claim 15 , further comprising:
 a liquid solenoid positioned downstream of the condenser and upstream of the expansion valve.   
     
     
         17 . The defrost system of  claim 16 , further comprising:
 an evaporator fan is positioned and oriented to blow over coils of the evaporator to induce efficient heat transfer therefrom;   a hot-gas solenoid configured and positioned to admit a flow of hot gasses into the evaporator from the defrost heater during defrost cycles,   wherein the controller is programmed to initiate a defrost cycle by:
 performing a pump-out step comprising closing the liquid solenoid, opening the suction-stop valve, and activating the evaporator fan; 
 after the pump-out step, performing a hot-gas step comprising closing the suction-stop valve and deactivating the evaporator fan, and then opening the hot-gas solenoid to allow the hot gasses to flow to the evaporator; 
 after the hot-gas step, performing a bleed step comprising maintaining the configuration of the vapor compression system from the pump-out step until a pressure difference between an inlet of the compressor and a pressure in the evaporator is reduced to a predetermined threshold; and 
 after the bleed step, performing a refreeze step comprising opening the suction-step valve and opening the liquid solenoid while keeping the fan deactivated, for a time sufficient to allow any residual moisture to refreeze on the coils of the evaporator. 
   
     
     
         18 . The defrost system of  claim 17 , wherein the hot-gas solenoid is positioned to receive the hot gasses from the discharge of the condenser, whereby waste heat from the condenser is used for the defrost cycle. 
     
     
         19 . The defrost system of  claim 15 , wherein the expansion valve is an electronic expansion valve configured to modulate its throughput based on a signal from the controller. 
     
     
         20 . A defrost system comprising:
 a vapor compression system including a compressor, a condenser, an expansion valve, an evaporator, and a quantity of refrigerant;   a defrost heater operably connected to the evaporator and configured to melt accumulated ice or frost from coils of the evaporator upon activation;   a vibration monitor operably connected to the evaporator and operable to measure and report vibration signals indicative of vibration at the evaporator; and   a controller programmed to:
 receive the vibration signals from the vibration monitor; 
 determine a frost-free vibration amplitude when the evaporator is in a frost-free condition; 
 determine an operational vibration amplitude at a time later than the determination of the frost-free vibration amplitude; and 
 when the operational vibration amplitude reaches a programmed threshold level above the frost-free vibration amplitude, initiate a defrost cycle via control over at least one component of the vapor compression system.