Refrigerants MCQ Quiz in मल्याळम - Objective Question with Answer for Refrigerants - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Explanation:

Required Properties of Ideal Refrigerant:

1) The refrigerant should have low boiling point and low freezing point.

2) It must have low specific heat and high latent heat; because high specific heat decreases the refrigerating effect per kg of refrigerant and high latent heat at low temperature increases the refrigerating effect per kg of refrigerant.

3) It must have high critical pressure and temperature to avoid large power requirements.

4) The pressures required to be maintained in the evaporator and condenser should be low enough to reduce the material cost and must be positive to avoid leakage of air into the system.

5) It should have low specific volume to reduce the size of the compressor.

6) It must have high thermal conductivity to reduce the area of heat transfer in evaporator and condenser.

7) It should be non-flammable, non-explosive, non-toxic and non-corrosive.

8) It should not have any bad effects on the stored material or food, when any leak develops in the system.

9) It must have high miscibility with lubricating oil and it should not have reacting properly with lubricating oil in the temperature range of the system.

10) It should give high COP in the working temperature range. This is necessary to reduce the running cost of the system.

Explanation:

Refrigerant:

A substance capable of absorbing heat from another required substance(space) can be used as a refrigerant. Air was used as a refrigerant in the olden days in many refrigeration systems because of most the safest and cheapest refrigerant.

(1) Halocarbon compounds:

Contains one or more halogens, chlorine, and bromine invented and developed by Charles Kettering in 1928. Sold in the market under the name of Freon, Genetron, Isotron, Areton.

For example, CFCl₃ is called Freon-11, CF₂Cl₂ is called freon-12.
\(\mathop {\begin{array}{*{20}{c}} {\rm Cl}\\ |\\ {\rm Cl - C - Cl}\\ \begin{array}{l} \,\,|\\ \,\rm F \end{array} \end{array}}\limits_{\rm Feron - 11} \ \ \mathop {\begin{array}{*{20}{c}} {\rm F}\\ |\\ {\rm Cl - C - Cl}\\ \begin{array}{l} \,\,|\\ \,\rm F \end{array} \end{array}}\limits_{\rm Feron - 12} \)

Application: Domestic, commercial, and industrial purposes.

(2) Azeotropic mixtures:

Mixtures of refrigerants which behave as pure substance are known as an azeotropic mixture. For example, R-502.

(3) Inorganic refrigerants: 

This is the refrigerant which consists of inorganic compounds like ammonia (NH₃) and water (H₂O).

(4) Hydrocarbon refrigerants:

Hydrocarbonare comprised of only hydrogen and carbon atoms. None of the hydrogen atoms have been replaced with chlorine or fluorine. Two common hydrocarbon refrigerants are methane and ethane.

\(\mathop {\begin{array}{*{20}{c}} {\rm H}\\ |\\ {\rm H - C - H}\\ \begin{array}{l} \,\,|\\ \,\rm H \end{array} \end{array}}\limits_{\rm Methane} \)   \(\underset{\rm \ \ \ Ethane}{\begin{array}{*{20}{c}} {\rm H\, \ \,\,\, \ H}\\ {| \ \, \ \,\,\,\,\,\,|}\\ {\rm H - C - C - H}\\ \begin{array}{l} \,\,\,|\,\,\,\,\,\,\,\, \ |\\ \rm \ H\, \ \ \,\,\, {} H \end{array} \end{array}}\)

Explanation:

• Vapour Compression Refrigeration Cycle is the most widely used refrigeration system.
• In this system, the working fluid is a vapour. It readily evaporates and condenses or changes alternatively between the vapour and liquid phase without leaving the refrigerating plant.

R2:

• R 21 is a hydrochlorofluorocarbon which is also known as Dichlorofluoromethane or Freon 21.
• Its chemical formula is CHCl₂F. It is a colourless and odourless gas used as a refrigerant but due to its contribution to Ozone depletion, it has been phased out.

R134a:

• Its chemical name is  1,1,1,2-tetrafluoro-ethane (CF₃CH₂F) and is a non-flammable gas used primarily as a “high-temperature” refrigerant for domestic refrigeration & automobile air conditioners.

R22:

• Its chemical name is Chlorodifluoromethane or difluoromonochloromethane.
• It is commonly known as HCFC-22, or R-22 and is used in VCRS cycle for water cooler application.

R171:

• R717 (Ammonia) is refrigerant grade Ammonia (NH₃) used in low and medium temperature refrigeration.
• It is a colourless, pungent, highly toxic gas but is a very efficient refrigerant with zero Global Warming Potential (GWP).

Explanation:

where m = No. of Carbon atoms, n = No. of Hydrogen atoms, p = No. of Florine atoms, q = No. of Chlorine atoms.

If the refrigerant is Inorganic Compound

R – (Molecular Weight + 700)

Ammonia NH3 whose molecular weight is 17 and the chemical formula is R – 717.

Water H20 whose molecular weight is 18 and the chemical formula is R – 718.

Carbon dioxide CO2 whose molecular weight is 44 and the chemical formula is R – 744.

Explanation:

Reciprocating compressor

• A reciprocating compressor is the workhorse of the refrigeration and air conditioning industry.
• It is the most widely used compressor with cooling capacities ranging from a few watts to hundreds of kilowatts.
• Modern-day reciprocating compressors are high speed ( 3000 to 3600 rpm), single acting, single or multi-cylinder (up to 16 cylinders) type. 
• Reciprocating compressors consist of a piston moving back and forth in a cylinder, with suction and discharge valves to achieve suction and compression of the refrigerant vapor.
• Its construction and working are somewhat similar to a two-stroke engine, as suction and compression of the refrigerant vapor are completed in one revolution of the crank.
• The suction side of the compressor is connected to the exit of the evaporator, while the discharge side of the compressor is connected to the condenser inlet.
• The suction (inlet) and the discharge (outlet) valve open and close due to pressure differences between the cylinder and inlet or outlet manifolds respectively.
• The pressure in the inlet manifold is equal to or slightly less than the evaporator pressure. Similarly, the pressure in the outlet manifold is equal to or slightly greater than the condenser pressure.
• The purpose of the manifolds is to provide stable inlet and outlet pressures for the smooth operation of the valves and also provide a space for mounting the valves. 
• The valves used are of reed or plate type, which is either floating or clamped.
• Usually, backstops are provided to limit the valve displacement and springs may be provided for smooth return after opening or closing.
• The piston speed is decided by valve type.
• Too high a speed will give excessive vapor velocities that will decrease the volumetric efficiency and the throttling loss will decrease the compression efficiency. 

The figure shows the schematic of a reciprocating compressor. 

Explanation

• Electrolux principle works on a 3-fluid system.
• There is no solution circulation pump.
• The total pressure is the same throughout the system.
• The third fluid remains mainly in the evaporator thus reducing the partial pressure of the refrigerant to enable it to evaporate at low pressure and hence low temperature.
• Example: An Electrolux refrigerator working on NH3 – H2O system with H2 as the third fluid. Liquid NH3, evaporates in the evaporator in the presence of H2. Hydrogen is chosen as it is non-corrosive and insoluble in water

Concept:

R-12 or Freon 12 is used in domestic refrigerators and freezers, liquid chillers, dehumidifiers, ice makers, water coolers, water fountains and transport refrigeration. The wide range of applications of the refrigerant are due to its safe properties.

Properties of R12 and its Advantages

1) Safe properties: Refrigerant R12 is nontoxic, nonflammable, and non-explosive.

2) Suitable for a wide range of operating conditions: R12 has the boiling point of -29.8°C due to which it condenses at the moderate pressures at the atmospheric temperature. This means the discharge pressure of the compressor should be only moderate. This helps in using the compressor of a low compression ratio that has higher efficiency.

3) Miscibility with oil: Refrigerant R12 is miscible with the compressor oil under all the operating conditions. So, there is no problem of the oil return back to the compressor.

Explanation:

Primary Refrigerant - These are the refrigerants or working substance which are directly used for cooling purpose. These refrigerants undergo a cyclic process and produce lower temperature. There is a latent heat transformation for the refrigerants. For e.g. R-11, R-12, R-22, R-134a, R-1150, Ammonia etc.

Secondary Refrigerant - These are the working fluids or working substance which are first cooled by primary refrigerant and then used for cooling at the desired place.eg, H₂O , Brine, Ethylene glycol.

Uses of different refrigerants.

R-11 – Large central air conditioning plant

R-12- Domestic refrigerator, water cooler etc.

R-22- Window AC

NH3- Cold storage or Icing plants

CO2- Transportation of dry ice

Air- Air-craft refrigeration system

Brine- Milk chilling plants

Concept:

where m = No. of Carbon atoms, n = No. of Hydrogen atoms, p = No. of Florine atoms, q = No. of Chlorine atoms

Calculations:

Given:

R 22

By comparing with R - (m - 1) (n + 1) P

m – 1 = 0 ⇒ m = 1 i.e. 1 → Carbon atoms

n + 1 = 2 ⇒ n = 1 i.e. 1 → Hydrogen atoms

p = 2 i.e. 2 → Florine atom

Now

n + p + q = 2m + 2

⇒ 1 + 2 + q = 2 × 1 + 2

⇒ q = 1 i.e. 1 → Chlorine atoms

∴ CmHnFpClq ⇒ CHClF2

Unsaturated Hydrocarbon

R - 1150

By comparing with R - 1(m - 1) (n + 1) P

m – 1 = 1 ⇒ m = 2 i.e. 2 → Carbon atoms

n + 1 = 5 ⇒ n = 4 i.e. 4 → Hydrogen atoms

p = 0 i.e. 0 → Florine atom

n + p + q = 2m

⇒ 4 + 0 + q = 2 × 2

⇒ q = 0 i.e. 0 → Chlorine atoms

∴ CmHnFpClq ⇒ C2H4

If refrigerant is Inorganic Compound

R – (Molecular Weight + 700)

NH3 whose molecular weight is 17 and the chemical formula is R – 717

H20 whose molecular weight is 18 and the chemical formula is R – 718

CO2 whose molecular weight is 44 and the chemical formula is R – 744

Explanation:

Properties of Refrigerants:

• Low freezing point: When the refrigerant goes through the expansion valve in the refrigeration cycle, it experiences a tremendous pressure drop and hence a tremendous decrease in temperature as well. It is therefore important that the refrigerant has a low freezing point than the normal operating conditions.
• Low boiling point: Refrigerant should have a low boiling point.
• Low viscosity: For the easy flow of refrigerants.
• Good stability: It should be chemically stable for the operating ranges of temperature. Also, it should not react with the materials of the refrigeration system or with which it comes into contact.
• Low vapor pressure: Condenser pressure of the refrigerator should be moderate and the evaporator pressure of the refrigerator should be around atmospheric pressure.

Other Important Properties of Refrigerants: