Refrigeration cycle explained: it’s 5 essential parts:
Below is the refrigeration basic schematic diagram (click to expand images below) separated into :
- Thermostatic expansion valve (TXV or TEV) or Metering Device.
1. Compressors: What is the purpose of a compressor in refrigeration cycle?
The compressor in a Refrigeration system is to compress the low-pressure dry gas refrigerant from the evaporator and raise its pressure and temperature to that of the condenser, to produce flow around the system.
The most easily recognizable compressor is a positive displacement type, which is the reciprocating or screw compressor or scroll compressor.
Let’s take an example of a refrigerator when the refrigerant passes from the evaporator, it changes its state from liquid to vapor by taking away the heat from the refrigerated space.
Now to convert back the refrigerant from vapor to liquid state, the compressor comes into action.
The compressor increases the refrigerant temperature and pressure so that the refrigerant gets converted into a liquid state by circulating hot surrounding air over the condenser coils.
In the absence of a compressor, the heat may start to flow from the condenser to the outside air.
Like in hotter regions the outside air temperatures can go up to 42°C – 48 °C with this hot outside air, the refrigerant gets cooled back from vapor to the liquid state.
This is possible due to the refrigerant property which is the “critical temperature of a refrigerant”.
2. How a refrigeration Condenser works:
The refrigeration condenser coil accepts hot, high-pressure gas from the compressor and cools it to remove first the superheat and then the latent heat so that refrigerant condenses back to a liquid state.
Condenser cooling mediums are usually fan or water
The small condensing surface required by a domestic appliance such as a fridge/freezer uses outside metal skin of the body itself as a surface heat exchanger.
In such a construction, the condenser tubes are mechanically fixed in close contact with the skin, so that heat gets conducted through to the outside air by natural convection.
Types of condenser:
- AIR-COOLED CONDENSERS
- WATER-COOLED CONDENSERS
- EVAPORATIVE CONDENSERS
3. What is a thermostatic expansion valve (TXV or TEV) or Metering valve: Throttling device?
The function of the expansion valve is to control the refrigerant flow from the high-pressure condensing side of the system into the low-pressure evaporator side.
The unit also controls the flow through the evaporator so that the condition of the refrigerant leaving the evaporator is a slightly superheated gas.
The metering valve or expansion valve ensures that the full latent heat has been absorbed by the refrigerant which in turn reduces the mass flow rate required by the system.
A sensing bulb with a good thermal contact is fastened at the evaporator outlet to make sure the refrigerant gas leaving the evaporator outlet is superheated.
As the bulb temperature increases, the refrigerant within the bulb expands thereby opening the expansion valve allowing more gas to flow through the evaporator.
When the refrigerant flow becomes excessive, then the temperature will fall reducing the pressure within the bulb, hence closing the expansion valve.
The capillary tube is used almost exclusively in small refrigeration and air conditioning systems.
Another factor takes place across the Thermostatic expansion valve(TEV), ie. flashing effect. We know (Gay-Lussac’s Law) Pressure Temperature Law that pressure is directly proportional to temperature for a given amount of gas at constant volume.
Therefore as the pressure drops due to the throttling effect of the expansion valve, the liquid refrigerant temperature goes down by extracting heat from the remaining refrigerant at the condenser side.
Due to the loss of heat in the remaining refrigerant, it starts to evaporate or flash to vapor state. This process is called “flashing”.
As per the above thermostatic expansion valve diagram when the evaporator and TEV valve is located vertically above the receiver, vapor or flash gas forms in the liquid line as the liquid refrigerant has to overcome the vertical head.
The expansion valve is designed to handle liquid and not a mixture of vapor and liquid to avoid TEV malfunction.
Here, the thermostatic expansion valve (TEV) plays a vital role in controlling and metering the flow of liquid in the system.
By connecting a sensing bulb via a capillary tube to the evaporator outlet filled with the same system refrigerant.
Avoid a flash gas problem by providing enough subcooling to the liquid refrigerant in the condenser. Liquid filled inside the sensing bulb is the same as that of the system (R134a), and the reason is:
- To get the same expansion as that of the system refrigerant, ie. R134a.
- To avoid contamination if the diaphragm ruptures or leaks.
TEV operation and it’s working:
- Bulb pressure on one side of the diaphragm tends to open the valve.
- Evaporator pressure on the opposite side of the diaphragm tends to close the valve.
- Spring pressure applied to the pin carrier gets transmitted through the pushrods to the evaporator side of the diaphragm. This assists in closing the valve.
4. How does an Evaporator Coil work?
The purpose of the evaporator is to receive low-pressure, low-temperature fluid from the expansion valve and to bring it in close thermal contact with the load.
The refrigerant takes up its latent heat from the refrigerated space and leaves the evaporator as a dry gas.
This unit is fitted in the space which is to be cooled and absorbs the heat by being at a cooler temperature than space. To ensure that there is a high heat transfer from the space (load) to the evaporator, the Convection heat transfer needs to be improved.
Convection heat transfer is achieved by extending the surface area using fins and fans, to increase airspeed across the cold evaporator surface.
As evaporator is in an area of high humidity (the ability of air to absorb water reduces as the temperature decreases), the surface of the evaporator coil may form an ice coating, which reduces heat transfer rates.
Defrosting elements remove the ice by increasing the local temperature of the evaporator, and hence causing the ice to melt and drain away.
Control the room temperature by either compressor capacity control or by closing the refrigerant inlet solenoid valve.
Types of evaporators:
- AIR COOLING EVAPORATORS
- LIQUID COOLING EVAPORATORS
5. Why critical temperature of a refrigerant must be high?
The critical temperature of a refrigerant is the temperature above which vapour refrigerant remains in the vapour state and cannot be liquefied even after passing through the condenser or any cooling medium at any given pressure.
This happens only when the refrigerant temperature reaches beyond its critical temperature, i.e. when a refrigerant has a low critical temperature.
So, it’s better to select a refrigerant having a high critical temperature to be able to condense these gases into liquid form at higher ambient temperatures.
6. What is Heat?
Heat is a form of energy where every object on earth contains heat energy in both quantity and intensity. Heat intensity is measured by its temperature, commonly in either degree Fahrenheit (°F) or degrees Celsius (°C).
Removing all the heat from an object decreases its temperature to -459.6°F [-273.2°C].
This temperature is referred to as “absolute zero” and is the temperature at which all molecular activity stops. The quantity of heat contained in an object or substance is not the same as its intensity of heat.
For example, the hot sands of the desert contain a large quantity of heat, but a single burning candle has a higher intensity of heat.
Air-conditioning and refrigeration systems use the principles of heat transfer to produce cooling and heating. The three principles discussed in this topic are:
- Heat energy cannot be destroyed; it can only be transferred to another substance
- Heat energy flows from a higher temperature substance to a lower temperature substance
- Heat energy is transferred from one substance to another by one of three basic processes
To produce a cooling effect, remove heat from the substance by transferring it to another substance.
7. What is a ton of refrigeration?
It is the amount of heat in BTU (British thermal unit) required to melt one ton, i.e. 2000 lb (907.18 kg) of ice over a period of 24 hours.
Therefore, one ton (2000lb) of ice melts to absorbs 288,000 Btu of heat. Similarly, a one-ton air conditioner absorbs 288,000 Btu of heat in 24 hours.
8. What is the difference between subcooling and superheating?
Subcooling: Cooling the refrigerant below its condensing temperature is called subcooling.
Condensing temperature means all vapor fractions turned to zero ie. all vapor turned to liquid (100% liquid), whereas cooling below this temperature is called Subcooling.
please note-The advantage of subcooling:
- To reduce Flash gas formation during the process of expansion in the thermostatic expansion valve.
- To reduce the specific volume occupied by the vapor refrigerant and thereby increasing the evaporator cooling capacity.
Also, increased evaporator efficiency extracts more heat from the refrigerated room.
9. Why does refrigerant gas need to be superheated before entering into the compressor suction?
Superheat means adding more heat to vapor refrigerant after it gets converted from its liquid state to a vapor state inside the evaporator coil, as the refrigerant absorbs heat from the load.
Cooling is achieved by the recirculation of the room air over the evaporator coils.
The compressor used in many refrigeration system designs are positive displacement type, and if by any chance refrigerant enters in the liquid form in large quantity, then it can cause severe damage to the compressor parts.
As liquid is incompressible, refrigerant needs to enter in gas form; this is the reason why refrigerants must to be superheated before it enters into the compressor suction.
10. Refrigeration cycle PV diagram
- C → D: low-pressure vapor refrigerant flows into the compressor suction and discharges into compressed high-pressure vapor.
- D → A: pressurized refrigerant vapor condenses in the liquid state at constant pressure, rejection of heat to the environment.
- A → B: liquid refrigerant flows through the throttling expansion device at constant enthalpy, to a two-phase state flashing as the pressure drops into 25% vapor and 75% liquid. Flashing lowers the coolant temperature to a saturation temperature at the low-side pressure.
- B→ C: Low-temperature refrigerant receives heat from the environment and continues to evaporate as it flows at constant pressure through the evaporator.