How refrigeration system and defrost works?

Refrigeration system (Vapor compression cycle)

We will talk about the vapor compression cycle refrigerator and freezer. It is commonly use system than any other systems. Manufacturing and operating cost of vapor compression system is much lower than the other electrically power system which make this system much favourable than other systems. The coefficients of performance of Vapor compression is approx. 1.7W/W for -18⁰C evaporation and 32⁰C condensing. The advantage of vapor compression system is that they are hermetically sealed. Because of this design there is no need of refrigerant or oil replenishment during the useful life of refrigerator or freezer. Compressors, use of aluminium and smaller motors, fans, new refrigerant and oil played important role to improve design of refrigerator and freezer.

 Refrigerating Circuit

Following figure shows a vapor compression refrigerating system. Through compressor and evaporator energy enters the system and thermal energy is rejected through condenser and compressor shell to the ambient. Let us see how.  Electrically driven compressor draws cold, low-pressure refrigerant vapor from evaporator and compress it. This compress vapor becomes high pressure and high temperature vapor. These vapor are pass through condenser. Heat is rejected to the ambient in the condenser and vapor become liquid. This liquid passes through expansion valve or pressure reducing capillary and enters in the evaporator. Pressure is lowered by throttling of the liquid. Evaporator absorb heat from its surrounding as low pressure and low temperature liquid refrigerant enters the evaporator coil. The liquid refrigerant become gas in the evaporator and again withdrawn by the compressor. To increase the capacity and efficiency, capillary is soldered to the suction line to form heat exchanger. Filter drier is used before capillary tube to arrest contaminants and moisture. Temperature is controlled by a thermostat. Capillary equalize the pressure when refrigeration cycle is off.

Material Selection

Selection of material is done on following basis.

1. Mechanical property.
2. Compatibility with the refrigerant from inside.
3. Compatibility with the oil from inside.
4. Resistance to oxidation from outside
5. Resistance to galvanic corrosion from outside

Bonded aluminium sheet or tubing are used for Evaporator. Integral extruded of extended surface fins are attached the tubing. Steel or copper tubing are used for condenser with extended surface fins attached to the tubing. Due to high pressure at discharge side of the system steel tubes are used. This side is normally dry. Copper is used at the lower pressures side of the system that is suction side where condensation bound to happen. For other piping copper is used like capillary and connection piping. Copper is ductile, ease of brazing and corrosion resistance makes him ideal for piping in refrigeration.

Defrosting

Defrosting in necessary process in any refrigeration system. Basically, in refrigerator and freezer. Moisture enters the cabinet from fresh fruits and vegetables (Food items). Moisture also enter through door openings and infiltration as ambient air enters in the cabinet. This moisture become frost on the evaporator surface over time. This frost reduces evaporator performance. Cabinet room temperature can not be maintained due to frost on the evaporator. So, it become necessary to remove the frost from evaporator.

Types of defrosting:

Manual Defrost.

In normal household or kitchen freezer evaporator is located at the top side of food compartment. This portion get frost and is defrosted by natural air convention over the evaporator by switching of the temperature controller.

Cycle Defrosting (Partial Automatic Defrost)

It is a combination of two cycles. Some of the freezer has two compartments with two evaporator coils for each compartment. Fresh food compartment and frozen food compartment. Fresh food compartment evaporator defrost happens during each compressor off cycle. Defrosting happens due to heat leakage into the fresh food compartment. Compressor on and off cycle is connected to the evaporator sensors who senses the temperature and send single to the compressor. Freezer evaporator needs manual defrosting.

Frost-Free Systems (Automatic Defrost)

In modern days freezer evaporator are equipped with inbuilt heater under it or in the slot or holes provided on the evaporator coil. Two types of heaters are used for defrosting. Electrical heaters or hot refrigerant gas heater. Fan is also fitted in front of evaporator for better air circulation. This fan takes the energy through the air passing over the evaporator. As evaporator is colder than freezer compartment, it collects all frost from freezer compartment and foods. Evaporator is defrosted automatically by heaters for short period of time to keep on control rise in food temperature. Water is drained to the exterior through drainpipe near the hot condenser compartment in the pan. The water evaporates due to hot condenser. This cycle is fixed. In some freezer defrost cycle starts after sensing the room temperature and evaporator coil temperature. Sensors are placed in room and evaporator. When defrost cycle turn on, compressor is off. Once cycle is completed compressor turn on after few minutes till system pressure stabilize. This safety features incorporate to avoid excess pressure on the compressor due to starting load.

Adaptive Defrost

In household refrigerator this microprocessor-based control systems are being used. This system monitors parameters and manage the period of defrost as per the actual condition of use. The advantage is it not only improve food preservation but also reduce the energy consumption.