When the refrigeration equipment is running, the surface of the evaporative coil is prone to frost. If the frost is too thick, it will affect the cooling effect, so it needs to be defrosted in time. For the defrosting operation of low temperature refrigeration equipment and medium temperature refrigeration equipment, due to the different temperature ranges, the corresponding control components are also different. The defrosting methods generally include shutdown defrosting, defrosting by self-generated heat, and defrosting by adding external devices.
For medium temperature refrigeration equipment, the operating temperature of the evaporative coil is generally lower than the freezing point temperature, and it is higher than the freezing point temperature during shutdown, so the shutdown defrosting method is generally used for medium temperature refrigeration equipment, such as refrigerated display cabinets. During operation, the temperature in the cabinet is about 1°C, and the temperature of the coil is generally about 10°C lower than that in the cabinet. When the machine is shut down, the air temperature in the cabinet is higher than the freezing point temperature, the fan on the evaporator continues to run, and the direct defrosting is realized by the air in the cabinet with a higher temperature. Defrost can also be done by timed or random. Timed defrosting is to force the compressor to stop running for a period of time. During this time, the air in the cabinet will defrost the coil. The defrosting time and the length of the defrosting period are controlled by the timer according to the set order. It is generally set to shut off the compressor when the freezer is at the lowest heat load. The defrost timer can set multiple defrost times within 24 hours.
For low-temperature refrigeration equipment, the operating temperature of the evaporator is lower than the freezing point temperature, and a timed defrosting method must be used. When the air temperature in the freezer is far below freezing, heat needs to be supplied to the evaporator for defrosting. The heat required for defrosting generally comes from the internal heat in the system and the external heat outside the system.
The method of defrosting with internal heat is generally called hot air defrosting. It uses the hot steam from the compressor to connect the exhaust pipe of the compressor to the inlet of the evaporator, and makes the hot steam flow fully until the frost layer on the evaporator is completely melted. This method is an economical and energy-saving method because the energy used for defrosting comes from the system itself.
If the evaporator is a single line and the expansion valve is a T-shaped line, the hot gas can be directly sucked into the evaporator for defrosting. If there are multiple pipelines, hot steam must be injected between the expansion valve and the refrigerant flow divider, so that the hot steam flows into each pipeline of the evaporator evenly, so as to achieve the purpose of balanced defrosting.
The defrosting operation is generally started by a timer. For different equipment or states, the timer is set at different time to prevent the increase of energy consumption or improper temperature of food due to excessive defrosting time.
The defrost termination can be determined by time or temperature. If the temperature is terminated, a temperature sensing device needs to be set up to determine whether the temperature of the evaporator is higher than the freezing point temperature. If the temperature sensing device detects that the temperature is higher than the freezing point temperature, the hot steam entering the evaporator should be cut off immediately to restore the system to normal operation. . In this case, a mechanical timer is usually installed at the same time, and the defrosting operation is terminated according to the electrical signal of the temperature sensing element. The basic process of the action of each component is: when the set defrosting temperature is reached, the timer contact is closed, the solenoid valve is opened, the fan stops running, the compressor continues to run, and the hot steam is sent to the evaporator. When the coil temperature rises to a certain value, the thermostat contacts are switched, the X terminal on the timer is disconnected, and the defrosting is terminated. When the coil temperature drops to a certain value, the thermostat contacts switch and the fan restarts.
During the hot steam defrosting operation, the timer needs to coordinate the operation of the following components at the same time:
1) The hot steam solenoid valve must be opened;
2) The evaporator fan stops running, otherwise the cold air cannot be effectively defrosted;
3) The compressor must run continuously;
4) When the defrosting termination switch cannot terminate defrosting, the timer must be set with the maximum defrosting time allowed;
5) Drain heater is energized.
Other refrigeration equipment uses an external heat source for defrosting, for example, installing an electric heating device near the coil. This defrosting method is also controlled by a timer. The ability to defrost is derived from an external device, so it is not as economical as hot air defrosting. However, if the pipeline distance is long, the efficiency of electric heating defrosting is relatively higher. When the hot vapor pipeline is long, the refrigerant is prone to condensation, resulting in a very slow defrosting speed, and even liquid refrigerant enters the compressor, causing liquid backflow, causing damage to the compressor. The thermal defrost timer needs to control the operation of the following elements:
1) In most cases, the evaporator fan stops running;
2) The compressor stops running;
3) The electric heater is energized;
4) Drain heater is energized.
The temperature sensor used in conjunction with the timer is generally a single-pole double-throw device with 3 lead wires, a hot contact and a cold contact. When the coil temperature rises, the hot contact terminal is energized, and when the coil temperature drops, the cold contact terminal is energized.
In order to avoid the defrost duration being too long or the compressor overloading after defrosting, a defrost termination switch, also called the fan delay switch, can be installed on the system. The temperature bulb of the defrost termination switch is generally set at the upper end of the evaporator. Once the ice layer on the coil is completely melted, the defrost termination controller’s discrete temperature sensor can detect the defrost heat, close the contacts on the controller, and energize the defrost termination solenoid valve. Return the system to cooling. At this time, the evaporator and the fan do not start immediately, but will start running after a delay to eliminate the heat still lingering on the coil and avoid overloading the compressor due to excessive suction pressure after defrosting. At the same time, avoid the fan blowing moist air onto the food in the cabinet.
Post time: Jan-24-2022