Vapour Absorption Refrigerant System
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 power plants 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.
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
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| Fig-VARS |
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.
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 reduces 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 Air-conditioning mathematical problems.
Qn. 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.
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