What are the Evaporators in AC unit or Refrigeration System?
All articles are written in simple language to visualize Mechanical Engineering. Let’s explore!
What are the Evaporators in AC unit or Refrigeration System?
What are evaporators? Any idea! In this article, we will learn the basics of evaporators in AC unit and refrigeration system along with parts, working principles, evaporator coil concept, applications, etc.
Let’s try to understand the basics of evaporators! Refrigeration and air conditioning have become an important part of our daily life.
We all have refrigerators, and we daily use them without knowing the intricate details which are behind their design.
One such part is the evaporator and as the name suggests, you guessed it right, it evaporates the liquid. If the compressor is the heart of the refrigeration system, then the evaporator is its lung.
It is that device that achieves the cooling effect. People many types get confused with condensers and evaporators.
Condensers are the equipment that rejects heat to sink to cool the liquid while the evaporator absorbs heat from things to be cooled.
Evaporation is a surface phenomenon and hence in an evaporator, it is always tried to achieve maximum surface area to achieve maximum heat transfer.
If you have seen any AC repair shop, you can see all the parts of air conditioner and the coil portion is known as evaporator coil. A simple residential split AC is broken down in parts to see evaporator coil.
To understand evaporators, we first need to know some basics of the refrigeration process.
Evaporators in AC & Refrigeration System
Let’s see the evaporator in AC and Refrigeration and it’s function in the Refrigeration cycle.
In a refrigeration cycle, evaporator is one of the main important part. we have already learned that there are 4 important components, compressor, condenser, expansion device, and evaporator.
The compressor compresses refrigerant, in this process it gets heat up due to increasing pressure and it gets superheated, so it has to be cooled by passing through the condenser.
Condenser first de-superheats and changes its medium and then sub-cools it as per need. Then expansion device slowly cools the system by reducing its flow and lastly, the evaporator absorbs the outside heat or cooling load brings down the temperature of a system to be cooled.
1-2 processes show compression. While 2-3’-3-4 shows condensation. Process 4-5 is an expansion that is work of capillary tube process while 5-6 is outside heat is getting absorbed by the evaporator. While 6-1 is a process where refrigerant gets superheated before entering compressor.
Process 5-6 and 6-1 is important to understand the working of the evaporator. At point 5, refrigerant is already condensed is at the lowest point of temperature in system and is in a liquid state. This refrigerant is allowed to pass through the expansion valve slowly and to control mass flow rate. The temperature of refrigerant is still decreasing as it is seen in graph. This process is seen in 4-5.
At point 5, refrigerant comes in contact with the cooling load. Here it starts absorbing heat from the load and during this process its temperature gets increased. The absorption is of latent heat. Hence in a graph, we see a straight line, as the temperature is constant, but phase change is taking place, from liquid to vapor state. As latent heat is getting absorbed, it becomes important that refrigerant gets superheated. So different sensors are used to achieve this to either increase or decrease mass flow rate.
Refrigerant is in vapor state at point 6. Still, it continues to absorb heat and gets superheated, this can be seen in processes 6-1. This is not necessary to have refrigerant in a superheated state in every system, but it is generally preferred as the compressor’s moving parts get eroded and their life gets affected by refrigerant droplets. To remove such possibility refrigerant is preferred to be in a superheated state.
Purpose of Evaporators in AC & Refrigeration
Its purpose is to accept low pressure and low temperature refrigerant which has come from expansion valve and to bring it in contact with the substance which is to be cooled that is a load on the system.
It absorbs the latent heat from the load and refrigerant gets heats up and as a result, it leaves in form of vapor in outlet of the evaporator.
Its primary function is to cool liquid or gases which can be either secondary refrigerant or it may load itself.
To explain secondary refrigerant or medium let’s go through this example- In case of an air conditioner, the air is cooled first, and then that cold air passes from a room from where it absorbs heat and brings down temperature. Water chillers are also an example, where refrigerant cools water, and water, in turn cools the load.
Resistance to heat flow
Fouling on side of water
Fouling on side of refrigerant
Construction of tubes
Material used and its conductivity
Type of Evaporators for AC & Refrigeration
Shell and tube type evaporator
Shell and coil evaporator
Bare tube coil evaporator
Finned tube evaporator
Tube in tube evaporator
Dry expansion type evaporator
What is Shell and Tube Evaporator?
Shell and Tube Evaporators
Shell and tube are the most common one. As the name suggests, shell and tube is the most common type of heat exchangers. Mostly used as chillers. A primary aim is to achieve heat transfer in close circuit and recirculation mode.
They are very efficient, and
They are very compact,
Parts of Shell and Tube Evaporator
Main parts of a shell and tube evaporators are, as follows
Shell or the casing
Nozzles in Shell side
Nozzles in Tube side channel
ConstructionofShell and Tube Evaporator
A big shell is made of a material that doesn’t go corrosion and faces less fouling. Generally, aluminium or stainless steel is used. This shell has 4 ports.
Consider a shell in a horizontal position that has two inlets on the opposite side, which are refrigerant inlet and outlet.
While 2 ports are on one end of shell, but trick is one end of shell is strictly divided into two divisions.
From one parts liquid which is to be cooled comes in which then enters the shell.
The small division is created at the inlet and then it goes into tubes connected to division, same way on another side liquid comes from a tube and goes into that separate division and then goes out.
This separate division is created to avoid backpressure as the tube gets filled with impurities and affects the swift flow of liquid.
How DoesShell and Tube EvaporatorWork?
This whole system is bulky but very easy to design. When liquid travels from one division through tubes.
It passes from one end of the cylinder to another end, which is called the head.
Here all liquid from tubes comes out and gets intermixes and then from the head it enters into tubes from where the liquid goes into a separate division and then it comes out of shell.
While whole shell is flooded with refrigerant and is constantly in contact tubes, so conductivity of tube in terms of heat transfer is important.
Tubes get cold and latent heat from the liquid is absorbed by tubes and then refrigerant and it gets carried away.
This way bulk amount of cooling is achieved as many tubes are simultaneously in contact with cold refrigerant, refrigerant is then removed from shell, when refrigerant enter sit is in liquid state but when it leaves it is in a vapor state.
This whole circulation keeps going at decides mass flow rate to achieve cooling.
The shell and tube evaporator can be classified into two types, as follows,
DX type or Dry expansion tye: In this type of evporator, refrigerant is flowing through the tubes and the fluid which needs to be cooled pass through the shell side.
Flooded type: Flooded type evaporator means the refrigerant will be flooded in the shell side and the fluid which needs to be cooled shall pass through the tube side.
Here, we have covered flooded type, dry expansion is covered later on in this article.
Shell and Coil Evaporators
What is Shell and Coil Evaporator?
There is a shell which has 4 ports, 2 for inlet and outlet of refrigerant and 2 for water. Coils are multi-turn or single turn which depends on the application. Refrigerant enters coil from one end exists from another.
While water enters in a shell which has many baffles plate. These act as small compartments and they also guide and give direction to the flow of water.
Workingof Shell and coil evaporator
Generally, tubes are in U shape, so it allows fluid to flow back and forward across the total length, As many tubes from the suction port take refrigerant they go on absorbing heat from one end to another, and as they take U-turn hence again from another end to first end.
This refrigerant has absorbed most heat in the first section and amount of heat absorbed compared to the first section goes on decreasing till another end.
That way the same refrigerant is in contact with different liquids which is at different temperatures in each baffle.
As the liquid in the first part in the baffle will be hotter while in the last will be most cooled.
Liquid then goes out of shell from the last baffle stage of the shell.
These baffles are used to achieve multi pass effect.
In some cases where the heat load is not much then the shell which is used doesn’t have baffles plates.
Plate Type Evaporators
What is Plate type evaporator?
These are generally used in industrial applications for chilling liquid and sometimes near to zero degrees Celsius. Mostly where water is to be cooled there mainly this system is used.
They are more rigid in construction.
External plate provides protection.
Easy to clean to they are also easy to manufacture.
It can be converted into various shapes as per need.
It can be used in box shape close enclosure to create freezers as in case refrigerator.
Construction and working ofPlate type evaporator
Vertical evaporator plates are connected to and fluid to be cooled is passed between these plates. While refrigerant is passed from small tubes which are wrapped on these plates.
Whilst fluid to be cooled falls below between these plates it is in constant contact between plates and plates are cooled by coil from which refrigerant is passing.
The coil is generally made of copper or aluminium for maximum heat transfer.
Bare Tube Coil Evaporators
This is the primary surface evaporator. That means only one surface is used for cooling the load. Cleaning and defrosting are done with ease.
The surface contact area is also less and hence its applications are limited.
Construction and working
It is very simple in construction. A small tube from which is refrigerant is allowed to pass.
The tube is fixated in the structure that holds it.
Refrigerant takes up heat from the atmosphere and cools the surrounding.
Tubes are massed in multiple turns so as refrigerant is passed through multiple passes and remains in contact with a tube for a long time.
Normally amonia is used as refrigeran and it is used in liquid chilling.
Finned Tube Evaporators
It is an extended surface evaporator. The heat transferring surface is enlarged here so as to achieve maximum contact.
Its shape, size, spacing can be adapted for better heat transfer which gives it freedom to use in different applications.
It is a modified version of a bare tube evaporator as a lot of cooling effects were getting wasted there as less surface was in contact.
Fins can be of many types based on their shape –
Construction and working
Refrigerant is allowed to pass through a bare tube and fins are fastened over it. These fins act as an extended surface as explained earlier.
They act like a baffle as well as a heat transferring surface.
Because they not only guide the air but also absorb heat from the air and this heat gets absorbed by refrigerant as fins are connected to a bare tube.
The bare tube also has multiple turns so as to achieve maximum heat transfer.
Mostly used in air conditioning systems, almost in all types. Split, window, and central air conditioning system.
Tube in Tube Evaporators
It is also called a double tube evaporator. It is generally used to cool liquid only.
Construction and working
Two concentric pipes are there. From inner pipe liquid to be cooled is allowed to pass while from outer pipe refrigerant is passed.
Both fluids are generally passed in opposite directions to achieve the maximum amount of cooling. A parallel passage is also allowed as per application.
Dry Expansion Type Evaporators
It is also called a dx type evaporator. It is mostly used in devices for producing a refrigerating effect in vapor compression refrigeration systems.
It is used in a system which generally having a capacity below 150 tonnes of refrigeration.
Construction and working
It is a modified type of bare tube evaporator. The tube is used to pass refrigerant which has multiple turns. Refrigerant enters from below and exits at top.
At the inlet of the refrigerant, an expansion valve is provided.
While at the exit of the refrigerant tube feeler bulb is provided to sense the temperature of the refrigerant.
This whole system works in a close loop.
As the water in the air in form of dews gets collected on a tube, the cooling effect is reduced, and refrigerant does not get superheated.
So, in such cases continuing supply of refrigerant at the same rate will reduce the efficiency of the system.
To do the correction, feeler bulb acts as a sensor and sends signals to expansion valve which in turn controls the flow rate of refrigerant.
This way wastage is reduced, and efficiency is enhanced.
The refrigerant absorbs heat from outside and it is getting very superheated which means more refrigerant should be allowed to pass.
That signal is given by feller tube to expansion valve.
When refrigerant is not superheated then the expansion valve is allowed to reduce flow rate as per the signal given by the feeler bulb.
This closed-loop system operates to maximize the cooling effect and making sure that the refrigerant which enters the compressor is superheated, that way power needs of a compressor are reduced, and the health of the compressor is prolonged.
When it comes to important, this concept is important. If air to be cooled is below freezing point, then it will accumulate over the evaporator in the form of frost which has to be removed periodically or it will obstruct heat transfer.
This is done by brushing, scraping, or chipping.
But as these processes are very labour intensive and can also damage the equipment.
Auto defrost features are used like household refrigerators.
When the temperature of the refrigerant reaches 4 degrees, its flow is stopped to melt the gathered ice.
But this can only be used where service period can be interrupted.
This can also be used in air conditioners.
Other methods are
Electric resistance heaters – heating element near evaporator.
Hot gas – superheated refrigerant from compressor is allowed to pass in separate tube around evaporator which then melts gathered ice when needed.
Hence, we have understood the basics of evaporators in AC and refrigeration system.