History of the Refrigerator
Before the invention of refrigerators, people would use traditional methods of preserving foods. These methods included salting, smoking, and drying. During winter seasons, people would dig holes and store their foods underground. People knew that food remained edible for longer when kept cold, but they did not know why.
Iceboxes were the first items to be used as refrigerators in the 1860-1890s. Each icebox was manufactured from wood lined with zinc and had compartments for storing ice. Unfortunately, the ice would only last for one week before needing to be replaced by an iceman (yes, this occupation existed!). As there was no way to manufacture ice, icemen harvested it during the cold winter season and stored it in icehouses until it was needed.
The introduction of the first commercial refrigerator occurred in 1911. The original design used ammonia as the refrigerant, but it was found to have a bad smell if it leaked and is poisonous when inhaled. Further development continued throughout the 1920s and 1930s until the common household fridge design became available on the open market; this more modern design used various types of freon gas.
Modern Domestic Refrigerator
How Refrigerators Work
Refrigerators chill storage space for perishable items in order to slow the growth of bacteria and prevent spoilage or deterioration. They achieve this cooling effect by absorbing heat in one area and transferring it to another. The process forms a continuous cycle of evaporation and condensation, which are both latent heat processes.
Latent and Sensible Heat
Low Side and High Side
Half of a refrigeration system lowers the refrigerant pressure and temperature. This part of the system is often referred to as the low side and it focuses upon the process of evaporation (where liquid changes state to gas).
The other half of a refrigeration system raises the refrigerant pressure and temperature. This part of the system is often referred to as the high side and it focuses upon the process of condensation (where gas changes state to liquid).
A refrigeration system is a closed system and requires a refrigerant that will change state/phase repeatedly, in a safe and efficient manner. All refrigeration systems consist of five main components: an expansion valve, an evaporator, a compressor, a condenser, and a thermostat.
Refrigerator Parts
Refrigerator System Components
A logical way to look at a refrigerator system is to start at the point where the refrigerant is in a liquid state, then follow the refrigerant through the system as it changes state, pressure, and temperature. Keep in mind that the refrigerant will evaporate and condense during the refrigeration cycle and that this results in the refrigerant absorbing or releasing heat (evaporation and condensation respectively).
Refrigeration System
Expansion Valve or Capillary Tube
An expansion valve or capillary tube marks the beginning of the low side of the refrigeration system and it is the point where the refrigerant changes state from a liquid to a vapour. Small refrigeration systems use capillary tubes, whilst larger units use expansion valves.
Capillary tubes are formed by winding copper piping multiple times to form a spiral cylinder; their design is basic, so this article will focus upon the expansion valve and its working. Irrespective of which design is used, the purpose of capillary tubes and expansion valves is the same i.e. pressure and temperature reduction.
An expansion valve regulates how much refrigerant is discharged to the evaporator. Refrigerant that passes through the expansion valve experiences a significant drop in pressure and temperature. To achieve this, the valve has an inlet, an outlet, and the valve internals (trim) installed between.
- The inlet admits hot pressurised liquid refrigerant from the condenser.
- The valve’s purpose is to regulate (throttles) how much refrigerant flows through the valve. The volume of refrigerant that passes through the valve depends upon the balance of pressure acting upon a diaphragm within the valve. Typically, gas from a sensing bulb acting on top of the diaphragm pushes the valve open, whilst pressure from the evaporator, and an adjustable pressure spring at the bottom of the valve, acts on the lower side of the diaphragm and cause the valve to close. The sensing bulb forms a feedback loop, which auto-regulates flow through the valve based upon the system pressure after the evaporator. Note that the positions of the sensing bulb and spring within the valve may be reversed.
- The outlet releases a spray of cold, low pressure, boiling liquid-vapour refrigerant into the evaporator.
Thermostatic Expansion Valve
Most refrigerators units use a sensing bulb filled with gas to generate a signal concerning if more or less cooling is required. When the sensing bulb is hotter than the desired temperature, the gas within the bulb expands, which causes pressure to act upon the thermostatic expansion valve diaphragm, which causes the valve to open and more refrigerant to flow to the evaporator (thus achieving more cooling and a lower temperature).
Once the evaporator has carried away sufficient heat and re-established the desired temperature, the gas inside the capillary tube contracts, which causes the thermostatic expansion valve to move towards the closed position.
In summary, the expansion valve receives liquid refrigerant from the condenser then lowers its pressure which causes a resultant drop in temperature. The lower pressure causes the refrigerant to boil and become vapour. The refrigerant is at its coldest temperature as it leaves the expansion valve. A sensing bulb sends a signal to the thermostatic expansion valve and this signal is converted to linear movement of the diaphragm, which results in the valve regulating flow (throttling flow).
Evaporator
The evaporator is what makes the inside of a refrigerator’s freezer and refrigerator compartments cold. As the cold, low-pressure vapour from the expansion valve travels through the coils of the evaporator, the cold coils draw and absorb heat from the surrounding area. A fan is installed on larger refrigerators to pass air across the evaporator coils and thus increase the heat transfer rate between the coils and surround area.
Compressor
The compressor is considered the heart of a refrigerator’s cooling cycle and the start of the system’s high side. It is the expansion valve’s counterpart in that the refrigerant is at its coldest and lowest pressure when it leaves the expansion valve, and at its hottest and highest pressure when it leaves the compressor.
Compressors come in reciprocating (the most common), rotary, and centrifugal designs, but regardless of their design, the purpose of a compressor is to raise the pressure of the refrigerant. Compressors achieve this by sucking/drawing refrigerant from the evaporator, then increasing the pressure of the vapour and discharging it to a condenser. Compression of the vapour causes its temperature to increase. Flow through the refrigeration system is caused by the pressure difference (ΔP) created by the compressor.
Condenser
In small refrigerators, the condenser is usually a series of tubes or a coil, and the cooling agent is air. In larger chiller plants, the condenser is usually a shell and tube heat exchanger, with refrigerant flowing through the tubes (tube side fluid) and cooling water flowing around the tubes (shell side fluid); cooling water is the cooling agent.
Modern Industrial Refrigeration Chiller
The purpose of the condenser is to cool (remove heat) from the hot refrigerant vapour so that it changes state to a liquid. On small fridges and freezers, the condenser is typically installed on the back of the casing. The length and bend configurations of the condenser are designed to expel heat whilst ensuring the refrigerant has sufficient time to cool. Like evaporators, air cooled condensers may utilise an axial fan to increase the heat transfer rate between the air and refrigerant.
Liquid refrigerant from the condenser is discharged to the expansion valve(s), thus closing the refrigerant system cycle.
Thermostat
A thermostat is used to measure the actual temperature within the refrigerated space and compare it to the desired temperature. Thermostats may use mechanical or electronic signals depending upon how complex the refrigerator system is.
The main purpose of a thermostat is to ensure the desired temperature is maintained. It achieves this by sending a signal to the compressor to start or stop, if the temperature is too high or low, respectively.
Systems that rely only upon electronic signals, have associated relays that start and stop the compressor(s). Systems that utilise electromechanical signals, usually utilise bimetallic strips to start and stop the compressor.
Temperature, Pressure and Saturation Point
To tell whether a refrigeration system is working properly, temperature and pressure readings are used as key indicators of a refrigerant’s saturation level.
The saturation point is also known as the boiling point, it is the point at which the liquid can store no more thermal energy without beginning to change state. There is often some confusion concerning the saturation point, the following definitions should help you understand the topic further.
- The saturation temperature is the temperature at which liquid boils, and changes state to vapour.
- The saturation pressure is the pressure at which liquid boils, and changes state to vapour.
- Vapour is gas that contains some liquid droplets, further heating of the vapour will lead to the vapour becoming a gas.
As the system pressure increases, so too does the saturation temperature i.e. the point at which the liquid begins to transform into vapour. For example, fresh/sweet water will boil at a temperature below 100⁰C (212⁰F) if the pressure is lower than atmospheric pressure (1 bar, 14.5 psi). Likewise, if the system pressure is increased to 100 bar (145 psi), fresh water will only begin to boil at a temperature much higher than 100⁰C (212⁰F). Thus, by controlling the system pressure, it is possible to control the saturation temperature of the refrigerant and correspondingly its state at various points within the system.
Boiling Point Based on Pressure and Temperature
The saturation temperature and pressure readings are especially important for the evaporator and condenser because the refrigerant must approach the saturation temperature in order to evaporate or condense.
Temperature, pressure, and saturation temperature relationships are indicated in refrigeration temperature-pressure charts. As each type of refrigerant has different properties (different saturation temperature etc.), the correct chart must be chosen for the correct refrigerant.
3D Model Details
This 3D model represents a typical domestic refrigerator. The invention of refrigerators massively changed the course of human development. It is now possible to store food for months prior to consumption and this has freed humanity of the scarce and abundance cycles associated with the changing of the seasons. In addition, it is now possible to ship food over considerable distances prior to consumption; this has allowed huge population concentrations in areas that could not have traditionally supported such a large population. The 3D model shows:
- Compressor
- Evaporator
- Condenser
- Expansion Tube (Capillary Tube)
Each of the main refrigerator components has been highlighted to aid understanding.
References
https://www.achrnews.com/articles/94025-refrigerant-pressures-states-and-conditions
https://www.fda.gov/consumers/consumer-updates/are-you-storing-food-safely
https://www.youtube.com/watch?v=RdTG3gjZGpI
https://www.swtc.edu/Ag_Power/air_conditioning/lecture/expansion_valve.html
https://www.hvacrschool.com/flash-gas/
https://tigermechanical.net/blog/how-a-refrigerator-compressor-works/
https://www.danfoss.com/en/about-danfoss/our-businesses/cooling/the-fridge-how-it-works