The refrigeration cycle is the main basic cycle for all air conditioning and refrigeration equipment. In this chapter, we will discuss, basic of a refrigeration cycle, mainly vapour compression cycle, main concept, parts, components, working principle along with a real example etc.
Let’s get into Refrigeration cycle!
Refrigeration cycle is thermodynamic cycle to generate refrigerating effect with the use of evaporator, compressor, condenser & expansion valve.
This process is basically a thermodynamic process where working fluid absorbs heat from the surrounding at low temperature and reject the heat to the atmosphere at higher temperature.
There are a few concepts of refrigeration, which is necessary to know before discussing the refrigeration cycles.
Evaporation means a change of phase from liquid to gas. In this case, heat needs to be added for the change. The amount of heat required to evaporate a liquid is called heat of evaporation.
Normally, refrigerant has a very less temperature very less, for example, Refrigerant R410a has -51.7 deg. C. It means this refrigerant start evaporating at normal atmospheric conditions.
Now, what do you think how this evaporation happens? It requires heat to evaporation and this heat comes from adjacent space. So, adjacent space or air or even water loses heat and become cool.
It is nothing but the boiling point, basically it means the temperature at which water boils into water vapor.
It is nothing but the pressure at which water boils into water vapor corresponding to saturation temperature.
Superheat, as the name suggest, means when heating is done in little different manner, basically heats after vapor or saturation temperature.
Let’s take some water,
Heat the water,
Here, the increase of temperature after the boiling temperature is called as superheat.
Superheat = Temperature at current stage – Boiling Temperature
Superheat = 105 – 100 = 5 deg. C
We all feel cold in the winter season. But do you know why this cooling happens? What is the exact reason?
Let’s try to understand with an example.
Take a small amount of medicated spirit on your palm, and soon your palm will get cooled. How does the palm get cold?
Condensation means a change of phase from gas to liquid. In this case, heat needs to be rejected to change a gas to liquid.
If a refrigerant vapour rejects heat, at certain conditions, it will be changed into liquid.
Subcooling, as the name suggest, means when cooling is done in little different manner, basically cools below the temperature where the water vapor turns into liquid water.
Cool the water,
Here, the decrease of temperature after the saturation temperature is called as subcooling.
Subcooling = Saturation Temperature – Temperature at current stage
Subcooling = 0 – (-2) deg. C = 2 deg. C
Boiling a liquid at low pressure is easy. You will be wonder when you will see water at normal pressure boils at 100 deg. C but the same water at the mountain will be boiled at 80 deg. C.
Why is this happen? It is because the pressure is less at the higher elevation and water boils at low pressure.
If you take an iron rod and heat one side of it holding the other side, very soon you will feel hot. Why does it happen? Because of heat transfer!
Here, heat from the hotter section of the iron rod is transferred to the opposite side of the rod that was normal.
Yes, possible if we do some external work.
Three ways i.e., conduction, convection & radiation are the method of heat transfer.
All refrigerating equipment & air conditioners work based on refrigeration cycles. We are discussing mainly the vapor compression cycle in this article.
In another refrigeration cycle, namely vapor absorption is used in case of waste heat is available. It will be discussed in another article.
Let’s us learn, the basic description of the vapor compression type refrigeration cycle.
Refrigerant circulates through evaporator, compressor, condenser, expansion valve and continues this cycle.
Now, let’s understand the parts of the components of a basic refrigeration cycle. Have you seen any inside view of our home split air conditioner during maintenance? Or have you visited any air conditioners repair shop?
There are various kinds of refrigeration machines or air conditioners, but basic refrigeration cycle has a basic cycle.
Let’s try to explore the different components of a refrigeration cycle along with brief explanation as well as functions of each part.
There are four (4) major components in air conditioners:
Along with the main components, all other parts of refrigeration cycle are
Let’s start to learn the main parts of the refrigeration cycle,
Evaporator is one of the main equipment in refrigeration cycle. It is nothing but a cooling coil which works as a heat exchanger in refrigeration cycle.
The main functions of evaporator are,
The continuous phase change process occurs, and cooling effects are going on so that air conditioning to be maintained.
This evaporator is installed in indoor unit, or air handling unit or indoor side of packaged air conditioners. Capacity of refrigeration units or air conditioners is based on the nos. or sizes of evaporator coils. If the capacity is more, evaporator size will be more.
The compressor, the most important part of the refrigeration cycle. I mainly the heart of the refrigeration system.
The main functions of the compressor are:
After compressor, the refrigerant goes to the third important component condenser.
As the name suggest, condenser simply condenses the refrigerant. It is a heat exchanger and it rejects heat from the refrigerant to the atmosphere. In the condenser, vapour changes into liquid by undergoing a phase change.
The expansion valve is placed after the condenser and before evaporator. The main function shall be as follows,
Refrigerant is the main working fluid in the refrigeration cycle. There are a few main characteristics of refrigerant by which it is selected. Normally, the following refrigerants are widely used in various refrigeration industries:
The selection of refrigerant is based on the followings,
Let’s try to understand the working principle of refrigeration cycel.
Step#1: Heat is being absorbed in the evaporator.
We have already learned that refrigerant has very low boiling temperature. So, normally, liquid refrigerant starts evaporating in the evaporator coil at normal pressure and temperature.
Step#2: Compressor increases pressure & temperature
When the power is on, compressor is started to rotate. Due to this rotation, there will be a slight low pressure at the inlet of compressor. Low pressure helps the refrigerant vapour to enter the compressor.
In the same way, compressor compresses the vapour and increase pressure & temperature at the outlet. It creases a pressure gradient or pressure difference which forces refrigerant vapour to enter the compressor.
Step#3: Heat removal in the condenser
After compressor, refrigerant vapour with high pressure and temperature, pass through the condenser.
Here, the refrigerant vapour changes its phase from vapour to liquid and release the heat into the sink or atmosphere.
Step#4: Flow control
At the outlet of condenser, liquid refrigerant along with vapour comes out but it needs to be,
These things are done in the expansion valve. This valve has throttle device by which it controls the flow and due to the expansion effect pressure reduces. This reduction of pressure results a phase change of vapour into liquid.
After expansion valve the mixture of liquid refrigerant & vapour changed into liquid and it will enter to the evaporator.
This refrigeration cycle continues and, in this way, refrigeration system works.
It’s a very easy process and explained step by step to have a clear understanding.
A blower circulates air through the evaporator coil and cool air is transferred to the areas to be conditioned. In the same way, the condensing coil also cooled by means of condenser fans.
The refrigerant cycle is described by the T-S & p-h diagram.
The efficiency of the refrigeration cycle is described by COP or coefficient of performance.
h1 = Sp. enthalpy before compression at Point 1
h2 = Sp. enthalpy after compression at Point 2
h3 = Sp. of enthalpy after cooling at Point 3
h4 = Sp. enthalpy after expansion at Point 4
Hence, as per COP definition,
COP = Refrigerating effect/Work-done
= (h1-h4)/(h2-h1) = (h1-h3)/(h2-h1) [As h3 is equal to h4]
The actual refrigeration cycle is different from the standard refrigeration cycle due to different losses:
Let’s try to understand, actual refrigeration cycle or actual vapour compression cycle,
The portion 1-2-3 indicates the flow of the refrigerant in the evaporator at suction pressure and temperature.
This portion indicates a pressure drop as there is a resistance of the suction valve to the compressor.
This portion of refrigeration cycle indicates the heat addition from the cylinder wall to the refrigerant.
This portion indicates the actual process of compression of the refrigerant in the compressor.
The portion 6-7 indicates cooling of the refrigerant at the outlet of the compressor.
This part of the refrigeration cycle indicates a pressure drop due to discharge valve resistance.
In this part, the refrigerant is desuperheated to dry state. Latent heat is removed and subcooling of refrigerant happens.
This process indicates throttling of sub-cooled refrigerant and in this portion condenser pressure is changed into evaporator pressure.
A simple application of the refrigeration cycle, you can see in the refrigerator. Apart from the refrigerator, you can many air conditioning machines are working on the refrigeration cycle.
So, we have learned how does different air conditioner works, along with a few practical examples. Any doubt, don’t hesitate to write to us! Anyways, try to solve the quizzes to test the core of learning! Cheers!
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