Defrost system for a conditioned space
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.
1 . A defrost system comprising:
a vapor compression system including a compressor, a condenser, an expansion valve, an evaporator having coils and an airflow pathway therethrough, and a quantity of refrigerant;
a defrost heater operably connected to the evaporator and configured to melt accumulated ice or frost from the coils of the evaporator upon activation;
a fan positioned and oriented to blow over the coils of the evaporator;
a controller;
an upstream pressure sensor operably connected to the controller and positioned downstream of the fan and upstream of the airflow pathway through the evaporator;
a downstream pressure sensor operably connected to the controller and positioned downstream of the airflow pathway through the evaporator; and
the controller programmed to:
process signals received from the upstream pressure sensor and the downstream pressure sensor 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 programmed to:
compute an air pressure differential across the coils of the evaporator from the signals;
determine a frost-free air pressure differential as measured by the upstream and downstream pressure sensors when the coils of the evaporator are in a frost-free condition; and
determine an operational air pressure differential as measured by upstream and downstream pressure sensors, at a time later than the determination of the frost-free air pressure differential;
wherein the controller is programmed to determine whether a defrost cycle is needed by comparing an increase in air pressure differential from the frost-free air pressure differential to the operational air pressure differential; and
wherein the controller is programmed to initiate the defrost cycle when the increase in air pressure differential reaches a threshold level.
3 . The defrost system of claim 2 , wherein the controller is programmed to determine that the threshold level is reached when the operational air pressure differential is at least 100 hPa higher than the frost-free air pressure differential.
4 . The defrost system of claim 2 , wherein the controller is programmed to determine that the threshold level is reached when the operational air pressure differential is at least 125 hPa higher than the frost-free air pressure differential.
5 . The defrost system of claim 2 , wherein the controller is programmed to determine that the threshold level is reached when the operational air pressure differential is at least 150 hPa higher than the frost-free air pressure differential.
6 . The defrost system of claim 2 , wherein the controller is programmed to determine that the threshold level is reached when the operational air pressure differential is at least 175 hPa higher than the frost-free air pressure differential.
7 . The defrost system of claim 2 , wherein the controller is programmed to determine that the threshold level is reached when the operational air pressure differential is at least 200 hPa higher than the frost-free air pressure differential.
8 . 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:
the controller initiates the defrost cycle comprises by activating the suction-stop valve to halt circulation of the refrigerant.
9 . The defrost system of claim 8 , further comprising:
a liquid solenoid positioned downstream of the condenser and upstream of the expansion valve.
10 . The defrost system of claim 9 , further comprising 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 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.
11 . The defrost system of claim 10 , 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.
12 . The defrost system of claim 1 , further comprising:
an upstream temperature sensor operably connected to the controller and located upstream of the airflow pathway through the evaporator; and
a downstream temperature sensor operably connected to the controller and located downstream of the airflow pathway through the evaporator,
the controller programmed to:
determine a frost-free temperature differential between the upstream temperature sensor and the downstream temperature sensor while 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
in response to the operational temperature differential reaching a programmed threshold level below the frost-free temperature differential, confirm initiation of the defrost cycle.
13 . The defrost system of claim 1 , further comprising:
an upstream combined remote sensor including the upstream pressure sensor and the upstream temperature sensor sharing a single upstream housing; and
a downstream combined remote sensor including the downstream pressure sensor and the downstream temperature sensor sharing a single downstream housing.
14 . A method of defrosting evaporator coils, the method comprising:
activating a fan to direct an airflow over the evaporator coils;
measuring an upstream air pressure at a location downstream of the fan and upstream of the evaporator coils;
measuring a downstream air pressure at a location downstream of the fan and downstream of the evaporator coils;
calculating an air pressure differential between the downstream and the upstream air pressure; and
when the calculated air pressure differential reaches a threshold indicative of frost buildup on the coils, heating the coils to melt the frost buildup.
15 . The method of claim 14 , wherein:
the step of calculating comprises:
calculating a frost-free air pressure differential when the coils of the evaporator are in a frost-free condition; and
calculating an operational air pressure differential, at a time later than the step of calculating the frost-free air pressure differential; and
the threshold is measured as a threshold increase in the operational air pressure differential as compared to the frost-free air pressure differential.
16 . The method of claim 14 , wherein the step of heating the coils comprises directing a hot gas into the coils.
17 . The method of claim 16 , wherein the step of directing the hot gas into the coils comprises directing gasses from the discharge of the condenser, such that waste heat from the condenser is used to defrost the coils.
18 . The method of claim 14 , further comprising, before the step of heating, deactivating a compressor in fluid communication with the evaporator as part of a vapor compression system.
19 . The method of claim 18 , further comprising, after the step of heating and upon a determination that the coils are frost-free, ceasing the heating and reactivating the compressor.