vapour absorption refregiration cycle

Vapour Absorption Refrigeration System

• Peculiar property of some substances to have more affinity for another substances at some temperature and pressure conditions while less affinity at other

conditions• Ferdinand Carre, a Frenchman, invented the absorption

system in 1860• The absorption cycle is similar in certain respects to the

vapour- compression cycle• In the absorption system also, refrigeration is produced

by the evaporation of a liquid refrigerant in the evaporator• The difference between the two systems lies in the principle of converting the refrigerant vapour back to the liquid

Absorption Refrigeration System

•Economically attractive - source of inexpensive heat energy at a temperature of 100 to 200 oC. Also used where there are severe limits to the electrical power available.

• The overall energy used is greater than with the compression cycle, so the COP is lower.

• Heat energy to the generator may be any form of low-grade energy such as hot oil, natural gas, hot water, steam, Solar radiation etc.

• NH3–H2O and H2O-LiBr absorption systems are common.

• Major advantage is that liquid is compressed instead of vapour.

• The COP of actual systems is usually much less than 1.


• A refrigeration cycle will operate with the condenser, expansion valve and evaporator as shown

• From the evaporator, the low pressure vapours are transformed into high pressure vapour and delivered to the condenser• The absorption system first absorbs the low pressure

vapour in an appropriate absorbing liquid• Since this process is similar to condensation, so heat

must be rejected during the process• In the next step, the pressure of the liquid is elevated

with a pump• In the final step, the vapours are again released from the

absorbing liquid by addition of heat


• Vapour compression ---- work operated cycle ---

compression requires work

• Absorption cycle ----- heat operated cycle --- operating

cost associated with provision of heat,

• Some work in absorption system also, to drive pump,

but amount of work for a given quantity of refrigeration

is minor compared with that needed in vapour-

compression cycle


Absorption Refrigeration SystemThe Basic Absorption Cycle - The condenser, expansion valve and the evaporator are similar as in a standard vapour-compression cycle

• The compression operation is now provided by the assembly in the left-half of the diagram• Low pressure vapour from the evaporator is absorbed

by the liquid solution in the absorber• As the refrigerant vapour enters into the solution in the

absorber, the temperature of the solution tend to rise• To resist the tendency, a cooling coil removes this heat

of solution• The solution in the absorber is called a strong solution

because it is rich in refrigerant


• The pump draws the strong solution from the absorber, elevates the pressure of the solution, and forces the

strong solution into the generator• In the generator the addition of heat raises the temperature, which drives off some of the refrigerant as a vapour at high pressure and temperature• Solar energy, waste heat from the process industry, exhaust gases from automobile, power plants, steel plants, gas power plants, etc. • As the refrigerant vapour leaves the solution in the generator, the solution becomes weak or have a low concentration of refrigerant• The weak solution flows back to the absorber through a

throttling valve whose purpose is to ???• From the generator the refrigerant proceeds through the

condenser, expansion valve and evaporator


• The pattern for the flow of heat to and from the four-heat exchange components in the absorption cycle is as

follows:-oHigh temperature heat enters the generator while

low temperature heat from the refrigerated space enters the evaporator

oThe heat rejection from the cycle occurs at the absorber and condenser at temperatures such

that the heat can be rejected to atmosphere



S.# Refrigerant Absorber Absorber State

1. Ammonia Water Liquid

2. Ammonia Sodiumthiocynate Solid

3. Ammonia Lithiumnitrate Solid

4. Ammonia Calcium chloride Solid

5. Water Lithium bromide Solid

6. Water Lithium chloride Solid

7. Methylene chloride

Dimethyl ether or tetra ethylene glycol

Liquid

Refrigerant-absorber pairs

Absorption Refrigeration SystemLithium Bromide (LiBr) Water Absorption Cycle

• LiBr is a solid salt crystal, in the presence of water vapour it will absorb the vapour and become a liquid solution

• If two vessels were connected as shown in the figure, one vessel containing LiBr-water solution and the other pure water, each liquid would exert a water-vapour pressure that is a function of the solution temperature and the concentration of the solution.

• At equilibrium the water-vapour pressure exerted by the two liquids would be equal

The refrigerant is absorbed by a transport medium and compressed in liquid form. The most widely used absorption refrigeration system is the ammonia-water (aqua-ammonia) system, where ammonia serves as the refrigerant and water as the transport medium. Dissolution of NH3 into water is exothermic and inversely proportional to temperature. The work input to the pump is usually very small, and the COP of absorption refrigeration systems is defined as


gen

L

inpgen

L

abs

Q

Q

WQ

Q

inputWork

effectCoolingCOP

,

Temperature-pressure concentration diagram for LiBr-water solutions• Concentration is the abscissa of the graph and water-vapour pressure could be considered as the ordinate on the vertical scale on the right• The saturation temperature of pure water corresponding to these vapour pressures is shown as the ordinate on the left• The chart applies to saturated conditions where the solution is in equilibrium with water vapour• If the temperature of pure water is 40 °C, the vapour pressure the liquid exerts is 7.38 kPa, which can be determined from the opposite side of vertical scale

o A LiBr solution with a concentration x of 59 % & temperature of 80°C also develops a water-vapour pressure of 7.38 kPa• o If the solution had a concentration x of 54% & temperature of 70°C the water-vapour pressure would again be 7.38 kPa


Absorption Refrigeration SystemCompute the rate flow of refrigerant (water) through the condenser and evaporator in the cycle shown in Figure below if the pump delivers 0.6 kg/s and the following temperatures prevail: generator, 100C; condenser, 40C; evaporator. 10C; and absorber, 30C


?43

mmskgm /6.01

6.0132 mmm

2211 xmxm

)664.0(2

m

skgm /452.02

3m

The basic LiBr water vapour cycle is shown in the figure

WhereThe two mass flow balances can be written around the generatorTotal Mass flow balance:-

LiBr balance:-

From chart of LiBr-water solutionSince the pressure in condenser & generator must be same, the pressure in condenser is equal to the 7.38 kPa corresponding to a saturation temperature of 40°CSimilarly, the pressure in evaporator & absorber must be same, the pressure in absorber is equal to 1.23 kPa corresponding to a saturation temperature of 10 °CSo from charts:- x1 = 50% & x2 = 66.4%

Therefore (2) → 0.6 (0.5) =

And (1) ----- 0.148 kg/s

(1)

(2)


Enthalpy of LiBr Solutions

• For thermal calculations on the absorption cycle, enthalpy data must be available for the working substances at all crucial positions in the cycle

• Water in liquid and vapour forms flows in and out of the condenser & evaporator, so enthalpies at these points can be determined from a table of properties of water

• In the generator and absorber, LiBr-water solutions exist for which enthalpy is a function of both - solution temperature and concentration

• Figure represents the enthalpy data for LiBr-water solution


112233 hmhmhm

4433 hmhm

115522 hmhmhm

4455 hmhm

For the absorption system of previous example, compute qg, qc, qa, qe

and the COP h1 = h at 30 °C and x of 50% = -168 kJ/kg

h2 = h at 100 °C and x of 66.4% = -52 kJ/kg

The enthalpies of water liquid & vapour are found from steam tables:-h3 = h for saturated vapour at 100 °C = 2676 kJ/kg

h4 = h for saturated liquid at 40 °C = 167.4 kJ/kg

h5 = h for saturated vapour at 10 °C = 2519.9 kJ/kg

qg = = 473.3 kW

=371.2 kW

= 450.3 kW

= 348.2 kW

COP = qe/qg = 348.2/473.3 = 0.736

qc =

qa =

qe =

Absorption Refrigeration SystemAbsorption cycle with heat exchanger

•The heat-exchanger transfers heat between the two streams of solutions•It heats the cool solution from the absorber on its way to the generator and cools the solution returning from the generator to the absorber•By addition of heat exchanger, the COP increases as well

Absorption Refrigeration SystemAqua-Ammonia SystemIn aqua-ammonia absorption system, water is used as an absorbent while ammonia is used as a refrigerantThe system consists of all the components i.e., generator, absorber, condenser, evaporator, and heat exchanger---- plus a rectifier & analyser

Absorption Refrigeration System• Additional components as refrigerant vapours released at generator contains water vapour as well

• Normally aqua-ammonia system operate at evaporating temperature below 0 °C

• If large amounts of water vapours are present in the evaporator, chance that they may get converted to ice & block the lines

• So to remove as much water vapour as possible, the vapours driven off from the generator first flows through the rectifier, which is a direct-cooled heat exchanger

• In the rectifier, the vapours from the generator first flow counter-current to the incommoding strong solution from the absorber

• Next the solution passes through the analyzer which is a water-cooled heat exchanger, condensing some water rich liquids which drains back to the rectifier

Absorption Refrigeration SystemProblem: A water-LiBr absorption refrigeration system is shown (see figure). The temperature at point 2 is 52 °C. The mass flow rate delivered by the solution pump is 0.6 kg/s. What are the rates of energy transfer at each of the components and the COP of this cycle? Also, what is the temperature at state 4?

vapour absorption refregiration cycle

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