Working Principles of Vapour Absorption Refrigerant System

May 8, 2019 | By | Reply More

Vapor absorption refrigeration is suitable where an inexpensive source of heat energy is easily available. In these cases using the heat directly as the operating energy is the common practice. The powerplants where heat from the co-generation process is available can use VARS. Steam turbine power plants can use the waste heat to run the refrigeration system as well. So the difference between Vapor compression and Vapor absorption Refrigeration system is easily understood.

Two most common combination of vapor absorption refrigeration system is aqua-ammonia and LiBr-H2O. Next, we  will discuss NH3-H20

You can see the refrigeration and air conditioning viva questions here.

Basic Principles of Vapour Absorption Refrigeration System (VARS)

You might ask what are the components of VARS that makes it different from VCRS. Well, the need for the compressor is eliminated in the vapor absorption system. The compressor is replaced by a generator, a pump, and an absorber. So the basic components of a basic Vapor absorption refrigeration are-

  • Generator
  • pump
  • absorber
  • condenser
  • evaporator
  • expander or throttling device

And some other components like rectifier, analyzer and heat exchanger.

Vapour Absorption Refrigeration System Working Principle explained

Vapour Absorption Refrigeration System Schematic diagram

Remember that in NH3-H2O – ammonia is the refrigerant and water is the absorber but the LiBr-H2O water is the refrigerant and LiBr is the absorber. Most of the time in the interview board you will be asked about the refrigeration and air conditioning interview questions.

In this case, we are demonstrating the Aqua-Ammonia Vapor absorption system. Here NH3 is the refrigerant and H2O is the absorbent. In VA refrigeration system the requirement of the compressor is fulfilled by using an arrangement consisting of an absorber, a pump, and a generator. However, more elements are added to improve the performance of the system. The elements condenser, evaporator and expansion valve are all same for VC and VA cycles.

First, evaporator passes the refrigerant vapor to the absorber (In this case ammonia vapor). NH3 has a property of mixing with the cool water promptly. So, in absorber ammonia mixed with water is known as strong NH3-Water solution. With the help of a pump, this strong solution moves towards the generator where the solution is heated and NH3 is liberated from the water and collected on the top of the generator with high pressure. In the VC system, the function of the compressor is to compress the refrigerant vapor and increase its pressure. Here in VARS, the generator is doing the same. Hence, the function of the compressor is already achieved. Likewise, the refrigerant vapor with high pressure goes to condenser and liberates heat and ultimately becomes liquid. The liquid is collected in a receiver tank and then moves through the expansion valve. Here it is expanded, moves towards the evaporator and collects heat and becomes vapor and the cycle continues.

The solution exchange in generator and absorber also runs in a cycle. Now, what happens in the absorber and generator. With the help of a pump, the strong solution is transferred to the generator. In the generator, heat is supplied and separated ammonia is ready for the next step. The solution in the generator is thus called a weak solution. This weak solution comes back to the generator with the help of a return duct. For example, the heat exchanger is used to cool the weak solution, otherwise, the water of the absorber will be heated.

This type of refrigeration cycle is used where exhaust heat is available. Extra or unused heat can be used to run the generator. For developing solar refrigeration system this type of arrangement is very useful. But you might be thinking that here electrical is used to run the pump. But still, the running the cost is less in Vapor absorption system than Vapor compression cycle because the energy used in pumps is very less in comparison to that of a compressor.

We have analyzed the T-s and P-h diagram of Vapor Compression Refrigeration Cycle with subcooling and superheating. You can check it here.

The function of other elements used in the Vapor Absorption Refrigeration system

The function of Heat Exchanger:

The heat exchanger is placed between the generator and the absorber. The strong solution pumped to the generator must be heated and the weak solution coming back from the generator to the absorber must be cooled. This heat exchanger facilitates both and thus reduce the cost of heating and cooling the solution.

The function of the analyzer in the vapor absorption refrigeration system:

The analyzer is a series of plated situated at the top of the generator. It traps the water vapor from entering the condenser. Ultimately it helps the pure ammonia vapor to enter the condenser. Actually, it helps to remove some unwanted water vapor particles to enter the condenser with NH3 vapor. If these water vapors are permitted in the condenser, they might freeze in the expansion valve and the pipe may get choked.

Function of rectifier

The rectifier is nothing but a water-cooled condenser. It condenses the water vapor and some of the ammonia vapor and sends back to the generator. Elimination of the water vapor takes place in the condenser.

Calculating the COP of vapor Absorption System

The actual formula for calculating the COP of Vapor absorption Refrigeration system is COP = R.E/(Win+Qg)

Here- R.E – refrigeration effect, Win – the work done in the pump, Qin – The heat supplied in the generator.

But for ideal case, the pump work is negligible so the formula becomes COP = R.E/Qg

And if you know the different ranges you can just use this formula to calculate the COP of vapor absorption refrigeration system COP = TE(TG-T1)/TG(T1-TE)

Here- TE – The temperature at which evaporator extracts heat

TG – The temperature at which heat is added to the generator

T1 – Cooling water temperature/ room temperature.

If You are a mechanical engineering student you must need to solve the Refrigeration and Airconditioning mathematical problems.

Where Vapour Absorption Refrigeration system is used?

  • Electricity is unreliable or load shedding is very frequent.
  • The places where noise from the compressor can create disturbances. In VARS there is no compressor so very less noise.
  • Where extra or exhaust heat is easily available. For example, the surplus heat from the power plants can be used or solar energy can be used.

Cooling Tower is another example of heat exchanging where the cooling tower is used to chill that is needed to be used in the condenser. Check this out Cooling Tower basics with the schematic diagram.

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Please don’t forget to comment if you are having any problem understanding the article. Thanks.

Category: Advanced Mechanical Engineering, Mechanical, Q & A, Refrigeration

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Basic Mechanical Engineering is a small endeavor for the mechanical engineering students and fresh graduates. All the articles are written by mechanical "rocking" engineers \m/

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