VAPOR ABSORPTION REFRIGERATION CLASS NOTES

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VAPOR ABSORPTION

REFRIGERATION CLASS NOTES  

As cooling requirements increase, volume

of refrigerant increase. It increases the size

of the compressor and the power

requirements. Volume reduction decreases

Power requirements. Here comes the role

of absorption. Water has extremely high

affinity for ammonia and Lithium Bromide

has high affinity for water. Water in the

absorber absorbs ammonia vapors from

the evaporator.  800 cc of vapor become

1 cc of liquid after absorption.  Pump

handles decreased volume. It requires

very small amount of power. Compressor

reduces Volume in vapor compression

system. Here means of volume reduction

are different. Thus absorption system

suits where there is a shortage of

electricity. In this system the moving

part is only a pump which handles

small quantity of liquid only.

Vapor absorption systems

There are three vapor absorption systems.

These are

(i) Ammonia Water System

(ii) Water Lithium Bromide System

(iii) Electrolux System

Use Ammonia water system for

refrigeration. Use Water lithium bromide

system for air conditioning.

Electrolux is the domestic refrigerator.

Ammonia-water vapor absorption refrigeration

Fig.1 Vapor Absorption Refrigeration (NH3 + H2O)

Refer Fig., and start from point 12, the entry to evaporator

12 to 1 = E=Evaporator,

1 to 2 =AB = Absorber,

2= exit of absorber,

3 =entry of pump,

3 to 4 = Pump

4 to 5 = Strong aqua from pump to aqua heat exchanger

5 to 6 Analyzer and generator bottom,

6 to 7 = Weak aqua from generator to aqua heat exchanger

8 = Vapors coming out of analyzer,

9 = Water condensed from ammonia vapor and coming to generator called DRIP

10 = exit of rectifier and entry of condenser

11 = exit of condenser

11=  exit of receiver

12 = exit of expansion valve

13 = Entry of heating medium into the generator

14 = Exit of heating medium from the generator

15–16 =cooling in the rectifier

17- cooling in the condenser

19–20 = cooling in the absorber

E = Evaporator, AB = Absorber,  HE = Aqua Heat Exchanger,  A =Analyzer

G = Generator,  R = Rectifier,  C = Condenser,  Re = Receiver

WORKING

  1. Start from the evaporator. Vapor goes to absorber. Water absorbs the ammonia vapors.

  2. This releases latent heat and heat of mixing.

  3. Cool water in the absorber to keep temperature low.

  4. Low temperature in the absorber increases absorbing capacity.

  5. From absorber, send strong aqua solution (Water+ ammonia) near the top of the analyzer. Pump does this. On the way, hot strong aqua passes  through an aqua heat exchanger.

  6. Aqua Solution falls down to the generator through the analyzer.

  7. Heating in the generator separates low boiling ammonia from the solution.

  8. Heating may be by steam/electric energy/solar energy etc…

  9. Ammonia vapor rise through the analyzer.

  10. Analyzer is an open cooler and is an integral part of the generator. It is a vertical tower with baffle plates to provide a zig- zag path. It analyses the vapor and removes any water associated with ammonia vapor coming from the generator.

  11.  Rectifier sends dry vapors to the condenser.

  12. Rectifier is a closed cooler. Water cools indirectly ammonia vapors.

  13. Rectifier removes any small traces of water associated. Water separated goes back to the generator. It is DRIP.

  14. Only anhydrous ammonia vapors go to the condenser.

  15. Liquid ammonia from the condenser goes to evaporator through an expansion valve for repeating the cycle.

  16. Hot weak aqua from the generator comes to absorber through the aqua heat exchanger and an expansion valve.

  17. This weak aqua again absorbs ammonia vapors. The cycle repeats.

  18. Aqua heat exchanger heats the strong aqua going to generator. It reduces the heating requirements in the generator. This heat exchanger cools the weak aqua going to the absorber.  This reduces the cooling requirement in the absorber. It greatly improves the economy of the system.

Water Lithium Bromide System

Fig. Water Lithium Vapor Absorption System

    ( CHILLING UNIT)

Water -lithium bromide absorption refrigeration system is for large capacity air conditioning systems. Refrigerant is water. Solution of lithium bromide is absorbent.  Water is refrigerant. It is unable to produce temperatures below 0oC. Use it for air conditioning applications. The analysis of this system is relatively easy.  The vapor generated in the generator is almost pure refrigerant (water). Steam operated absorption unit produce chilled water at 50C. These units are available in seven sizes having 115 to 700 tons of refrigerating capacities.

CONSTRUCTION

The vapor absorption refrigeration system consists of two cylinders.

(i) One cylinder contains evaporator and absorber

(ii)The other cylinder contains the generator and the condenser

Evaporator

The refrigerant (water) evaporates at around 5oC under a high vacuum in the evaporator. Evaporation of water produces chilled water. The evaporated refrigerant vapor changes into liquid in the absorber containing Lithium Bromide. The chilled water circulates for cooling the space or products. Hot water returns to the evaporator. Nozzles spray this hot water into a space under high vacuum. Some water evaporates and chills the remaining water.

Absorber

Lithium bromide solution placed in the absorber absorbs water vapor. This releases the latent heat and heats the solution. Cooling water removes the heat generated in the absorption process. The absorption maintains the vacuum inside the evaporator.

High Pressure Generator

Dilute lithium bromide solution absorb less refrigerant vapor. Concentrate  the diluted lithium bromide solution to keep the absorption process goings. With the help of a pump, send solution rich in refrigerant to the high pressure generator. The pump increases the pressure. Steam or hot water concentrates solution by evaporating water from the solution. The concentrated solution returns to the absorber via a heat exchanger. It absorbs refrigerant vapor again.

Condenser

Condenser condenses the evaporated refrigerant vapor. Send this liquid refrigerant to evaporator through an expansion valve. It reduces the pressure in the evaporator.

Aqua Heat Exchanger

Hot weak aqua heats the strong aqua going to generator. It reduces the heating requirements in the generator. Strong aqua cools the weak aqua in the aqua heat exchanger. This reduces the cooling requirements in the absorber. This heat exchanger is highly an economical unit.

Expansion valve

Fitted between the condenser and evaporator. It reduces the high pressure of water to the low pressure in the evaporator.

ELECTROLUX ABSORPTION SYSTEM

Fig. Electrolux Refrigerator

Electrolux is a  vapor absorption refrigeration system. It uses three fluids namely ammonia, water and hydrogen. Ammonia is the refrigerant. Water is the absorbent.  Hydrogen is the carrier for the flow of refrigerant in the refrigerator. This refrigerator does use any pump.

ELECTROLUX REFRIGERATOR

It has three names: Electrolux refrigerator, Three fluid absorption system and Domestic Refrigerator.
This unit uses three fluids
(i) Ammonia as refrigerant
(ii)Water as absorbent
(iii) Hydrogen gas as carrier
Hydrogen is to used to
(a) Reduce the partial pressure of the refrigerant in the evaporator
(b) Improves COP
(c)  Circulates the refrigerant
(d) Provide a vapor seal
WORKING
1    It is a vapor absorption refrigeration system.
2. Start from the generator.
3. Heating in the generator creates ammonia vapors.
4.  Both ammonia vapor and water go to separator through the capillary tube. 
5.  Capillary tube ends near the bottom of the separator.  
6. Weak hot aqua solution from separator goes to absorber.
7.  From separator, ammonia vapor rise and goes to condenser and condenses.
8. Liquid NH3 travels to the evaporator from the condenser. There is an U-bend at the end of condenser to stop back flow of ammonia vapor and hydrogen.
9.  Hydrogen in evaporator reduces partial pressure of ammonia in the evaporator. It causes ammonia to evaporate. Total pressure in the evaporator is the sum of partial pressures of ammonia vapors and hydrogen.
10. Thus, hydrogen carries Ammonia to absorber, where ammonia is absorbed & hydrogen returns to evaporator via a reservoir.
11.  Ammonia solution returns to generator. The process repeats.
12. Total pressure at every point in the system is same.

 

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