Field of the Invention

The present invention relates to a defrosting system for refrigeration systems to save the wastage of energy during defrosting.

Background of the Invention

Refrigeration systems are an indispensable need of the current era. The refrigeration systems are ubiquitous from food preservation in households and supermarkets to heating ventilation and cooling at a large scale for keeping the ambient temperature down during extreme weather. According to the International Institute of Refrigeration (HR, France 2019), the refrigeration systems collectively consume 20% of the total electricity produced in the world. Therefore, improving the energy efficiency of refrigeration systems is of huge importance.

Frost's deposition on the evaporator coil of the refrigeration system is one of the bottlenecks in improving the energy efficiency of refrigeration systems. The accumulation of frost on the evaporator coil acts as a thermal resistance during convective heat transfer. Frost deposition also clogs the evaporator coil that results in the drop of the airside pressure drop of the evaporator and thus decreases the cooling capacity of the refrigeration system. The amount of frost deposition on the surface of the evaporator is in direct proportion to the loss of energy. Therefore, to improve the efficiency of the refrigeration system, the removal of frost from the surface of the evaporator becomes inevitable.

There are different types of defrosting systems that have been reported in the literature and are being in use nowadays. For example, the reverse cycle defrosting, the hot gas bypass defrosting, and the electrically powered resistive heater defrosting. In reverse cycle defrosting, a solenoid control valve is used to reverse the flow of the refrigerant that converts the evaporator into the condenser to melt the frost. In hot gas bypass defrosting, a bypass valve is used to send the hot refrigerant directly into the evaporator rather than passing through the condenser. In electrically powered resistive heater defrosting, a heating element is attached to the evaporator that transmits the heat through conduction or radiation to melt the frost. Owing to the ease of retrofitting, simpler installation criterion, and the availability of different sizes, the electrically powered resistive heater defrosting is considered the most viable defrosting system by the refrigeration industry. In a typical refrigerator, the resistive heater is normally turned-on periodically once in 24 hours and tumed-off either periodically or based on the temperature sensor that monitors the surrounding air temperature during defrosting. However, periodically controlled defrosting systems are inefficient because the frosting is a random process, and triggering the defrosting system without estimating the amount of deposited frost would result in the waste of energy. Therefore, different frost detection sensors have been reported and currently are being in use. The frost detection sensor monitors the thickness of frost on the surface of the evaporator and turns on the defrosting heater once a frost thickness exceeds the threshold point.

The addition of a frost detection sensor along with an electrically powered resistive heater is a widely adopted system nowadays. The heater is activated on-demand based on the amount of frost and deactivated when all the deposited frost melts away.

The deposition of frost on the surface of the evaporator is not uniform. For instance, in a tube-fin type evaporator of a domestic refrigerator, the frost starts to deposit from the top tubing and gradually reaches the bottom tubes. This phenomenon occurs because working fluid enters the evaporator from the top at a very low temperature and exits near the bottom at a significantly different temperature due to heat transfer. Additionally, the frost deposition is mainly dependent on different parameters such as relative humidity, air velocity, the temperature of the surroundings, and the evaporator. Therefore, the thickness of deposited frost varies along the surface of the evaporator. Modem refrigeration systems currently employ a single electrically powered, resistive heater that triggers-on upon the detection of frost by the frost detection sensor and remained on unless the sensor detects the melting of frost on the whole surface of the evaporator. However, due to the varying thickness levels of the deposited frost, the melting is performed faster on the points where there is a thin layer of frost and slower where there is a thick layer of frost. The surface of the evaporator where the amount of frost is less becomes frost-free earlier as compared to the surface where the frost layer is thick. The heater continues to provide heat to all the surfaces of the evaporator unless all the deposited frost on the evaporator surface will be melted away. This would result in energy loss during defrosting. Moreover, the surface of the evaporator, where the frost melts earlier would absorb an excessive amount of heat that would increase the overall surrounding temperature, and the compressor has to consume extra energy in the next cooling cycle. Thus, there is a need for a system that is comprising multiple low power resistive heaters that can be mounted on different regions of the evaporator along with multiple frost detection sensors.

In the technical field, in most of the solutions reported in the literature and patent applications, a single defrosting system is employed to remove the frost from the evaporator. The non-uniform deposition of frost on the surface of the evaporator requires multiple heaters that can be triggered based on the amount of frost. In the patent and literature research conducted for the state of the art, no document directly addressing the specified problem was found. However, there are some patents where an optical sensor, a capacitive sensor, a resistive sensor, and other sensors have been proposed as a frost detection sensor such as:

A patent application numbered US2005189493A1 relates to an optical sensor to detect frost in refrigeration systems. Once a preset amount of frost is detected, the controller triggers the defrost system. In this patent application document, a single defrost system is used as a defrosting system.

A patent application numbered CN107514860A relates to a capacitive sensor for detecting frost in refrigeration systems. The sensor detects the presence of frost and measures its thickness, based on the change in total capacitance. The capacitance varies in direct proportion to the amount of frost. They proposed a capacitive sensor to triggers the defrosting system on demand.

A US patent document numbered US6415616 B l relates to a defrosting method to defrost the evaporator without wasting energy. The method adopted, the on/off switching scheme of the heater during the defrosting process, rather than continuous heating.

Summary of the Invention

An objective of the present invention is to provide a system proposing multiple low power heaters installation on the surface of the evaporator. The control of individual heaters is achieved through individual frost detection sensors that are mounted on the same location where a heater is installed. This localized installation of heaters and sensors would enable the detection and defrost without excessive energy wastage. The industrial appliances in the technical field are using a single defrosting heater that is controlled by either a direct frost detection sensor that detects the presence and measures the amount of frost or indirect sensing of frost by monitoring indirect parameters such as temperature and the pressure variations inside the refrigerator cabin. When the frost accumulation surpasses the preset threshold, the defrost system actuates and remains actuated for an extended period. This would result in inefficient defrosting. With the present invention, this inefficiency is eliminated.

Detailed Description of the Invention

"A DEFROSTING SYSTEM" developed to fulfill the objectives of the present invention is illustrated in the accompanying figures, in which:

Figure 1 shows the evaporator coil with multiple frost sensors and defrosting heaters. Figure 2 shows the working principle of a single frost detection sensor and the defrosting heater.

The components in the figures are each given reference numbers as follows:

D Defrosting system

E Evaporator

1 Frost detection sensor

2 Defrost Heater

3 Control unit

4 Transmitter

5 Receiver

The present invention relates to a defrosting system (D) suitable for refrigeration systems comprising evaporator (E) characterizing that the defrosting system (D) comprises

• Multiple frost detection sensors (1) on various positions of the surface of the evaporator (E) for detecting the presence and measuring the amount of frost,

• At least one defrost heater (2) at the same position with each frost detection sensor (1) to melt the frost,

• At least one control unit (3) for processing the frost detection sensors (1) data and generating control commands to drive the defrost heaters (2).

The present invention relates to a defrosting system (D) suitable for refrigeration systems. The defrosting system (D) according to the invention comprises multiple frost detection sensors (1), at least one defrost heater (2) at the same location of each frost detection sensor (1), and at least one control unit (3) for communication between frost detection sensor (1) and defrost heater (2). The frost detection sensor (1) detects and measures the amount of frost deposited on the surface of the evaporator (E) of the refrigeration system. For this purpose, the frost detection sensor (1) is positioned at various locations on the surface of the evaporator (E). The resistive defrost heater (2) generates heat with the help of the electrical power supplied. In this way, the heat generated is used to remove the deposited frost. The control unit (3) is responsible for the communication between the frost detection sensor (1) and the resistive defrost heater (2).

In one embodiment of the invention, the frost detection sensor (1) is preferably based on optical sensing principle where at least one infrared or ultraviolet or visible light transmitter (4) transmits light on to the surface of the evaporator (E) where frost accumulates and at least one infrared or ultraviolet or visible light receiver (5) receives the reflected light bouncing back from the frost accumulated surface. The output voltage of the receiver (5) changes in direct proportion to the intensity of the received light beam as the intensity of the light is dependent on the thickness of the frost. The difference between the output of the infrared receiver (5) during no-frost and frosting is used to detect the frost. The output of the receiver (5) is calibrated to the amount of frost on the evaporator (E).

In one embodiment of the invention, the frost detection sensor (1) is preferably based on the capacitive sensing principle where the capacitance change is measured in proportion to the amount of deposited frost. Frost has a dielectric constant different than the air or water therefore the frost deposition has a direct effect on the change in capacitance. The deposition of frost disturbs the electric field between the electrodes of the capacitor and would result in the capacitance change. This change in capacitance is in direct proportion to the amount of frost on the evaporator (E). The capacitance change is calibrated to the frost thickness. In the preferred embodiment of the invention, the Frost detection sensor (1) comprises at least one capacitive sensor for measuring the change in capacitance to the amount of frost. The capacitive sensor can be configured in a parallel electrode configuration or a comb shape electrode configuration.

In one embodiment of the invention, the frost detection sensor (1) comprises at least one resistive sensor. The resistive sensor can be configured in the parallel placed electrode configuration. The working principle of the resistive sensor is varying resistance based on the presence of frost on the sensor electrodes. The potential difference is applied between the electrodes. When the frost starts to deposit the potential difference changes because of the change in resistance between the electrodes. This change is a potential difference that can be used as a deciding factor to detect the presence of frost.

The defrost heater (2) comprises at least one conducting element having an electrical resistance. When the potential difference is applied across the conducting element, the current begins to flow through the conductor. As a result of the flowing current, heat is produced by the conductor. This process is known as Joule heating. The defrost heater (2) in the defrosting system (D) is a low-power electrical resistive defrost heater (2) for defrosting purposes. To defrost efficiently, multiple low power electrical resistive defrost heaters (2) are installed on various locations of the evaporator (E). Based on the thickness of frost in a certain region of the evaporator (E), the electrical resistive defrost heaters (2) installed in that region are triggered-on when the thickness of the frost exceeds the set point. The electrical resistive defrost heater (2) triggers-off when the frost in that region melts ways. The defrost heater (2) can be either a sheathed type or a glass tube type. Sheathed type heater is a type of electrically powered resistive heater. The resitive heating element is capsulated in a meatlic pipe. When powered, heat is generated by Joule’s effect. Glass tube type heater is different than resistive heater, as it emits infrared radiations that produce heat. However, both sheathed as well as glass tube heaters are an alternate to resitive type heaters that we are proposing in this invention. The current refrigeration systems in the technical field employ a single heater to completely remove the frost from the evaporator (E), thus consumes more power. On contrary, if multiple low-powered defrost heaters (2) are installed and controlled individually, the defrost heaters (2) will consume significantly less power as compared to a single high-powered heater. For example, a single heater consumes 150 Watts of power and we are replacing this single heater with multiple defrost heaters (2) (each consumes 12 Watts of power). If a single heater takes approximately 10 minutes to completely remove the frost from the evaporator (E) then it will consume [(150xl0)/60] Watt-Hour of energy whereas, in the case of multiple defrost heaters (2), multiple defrost heaters (2) will consume significantly lesser energy because of the early deactivation of those defrost heaters (2) where frost will be melted earlier. The control unit (3) receives the data from multiple frost detection sensors (1) and continuously compares it with the preset threshold values for different frost thickness levels. When the output of any of the frost detection sensors (1) crosses the preset threshold, the control unit (3) triggers the respective defrost heater (2) on that location to tum-on. In one embodiment of the invention, the control unit (3) comprises at least one microcontroller or microprocessor or field-programmable gate array for analyzing the frost detection sensor (1) data.

Several other frost detection sensors (1) can be used to detect the frost such as temperature sensor, pressure sensor, inductance sensor, an impedance sensor, laser displacement gauge sensor, fiber optic sensor, and piezoelectric sensor in the defrosting system (D).

The evaporator (E) is preferably a tube-fin type evaporator (E).

The frosting is a complex random process that depends on various parameters. Therefore, the thickness of frost on the surface of the evaporator (E) is not uniform. The thickness of frost on the top side of the evaporator (E) would be significantly different than the bottom of the evaporator (E). Therefore, the defrosting heater takes less time to melt the frost on the bottom side of the evaporator (E) as compared to the top side. In these conditions, choosing a single threshold value to trigger defrosting would result in energy loss. Hence, multiple threshold values for each defrost heater (2) is chosen, based on the frost detection sensor (1) response. This would result in triggering-on/off of only those defrost heaters (2) where the frost thickness value is beyond the threshold value.