Refrigerants MCQ Quiz - Objective Question with Answer for Refrigerants - Download Free PDF

Explanation:

Air Refrigerant Designation (R729)

• In refrigeration and air conditioning systems, refrigerants are substances used for heat transfer by undergoing phase changes (such as evaporation and condensation). Refrigerants are assigned unique identifiers, known as "R-numbers," standardized by ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers). These numbers help classify and identify refrigerants based on their chemical composition and properties. Air, as a refrigerant, is designated with the number R729.
• Air is a natural refrigerant that operates on the principle of reverse Brayton (or Joule) cycle, where compressed air is cooled and then expanded to produce refrigeration. In refrigeration systems using air, the cycle involves the following key processes:
  • Compression: Air is compressed, increasing its pressure and temperature.
  • Cooling: The compressed air is cooled using heat exchangers to remove heat.
  • Expansion: The cooled air is expanded through an expansion device, causing its temperature to drop further.
  • Heat Absorption: The low-temperature air absorbs heat from the space or substance to be cooled, completing the refrigeration cycle.

Applications: Air as a refrigerant is commonly used in applications where environmental impact is a critical factor, such as in aircraft refrigeration systems and cryogenic applications.

Explanation:

Cryogenics

• Cryogenics is the branch of physics that deals with the production and effects of very low temperatures (below 120 K).
• The term is derived from the Greek words "kryos," meaning cold, and "genes," meaning produced.
• Cryogenics involves the study of materials at temperatures far below those experienced in everyday life, often involving the liquefaction of gases.
• Cryogenics focuses on the behavior and production of materials at extremely low temperatures.
• The primary goal is to achieve temperatures where gases like nitrogen, oxygen, and helium liquefy.
• This is done using various methods, including Joule-Thomson effect, adiabatic demagnetization, and cryocoolers.
• At these low temperatures, materials exhibit unique properties such as superconductivity, superfluidity, and changes in electrical resistance.

Applications:

• Medical Field: Cryogenics is used in cryosurgery to remove tumors and in cryopreservation to preserve biological samples like blood, sperm, and embryos.
• Space Exploration: Cryogenic fuels, such as liquid hydrogen and liquid oxygen, are used as rocket propellants due to their high energy content.
• Physics Research: Cryogenics is essential in studying quantum mechanics and particle physics. For instance, particle accelerators use superconducting magnets cooled to cryogenic temperatures.
• Electronics: Superconducting materials, which exhibit zero electrical resistance at cryogenic temperatures, are used in high-performance computing and MRI machines.

Advantages:

• Enables the study and utilization of unique material properties at low temperatures.
• Facilitates advancements in various fields, including medicine, space exploration, and physics.
• Allows for the storage and preservation of biological materials for extended periods.​

Explanation:

Hydrofluoroolefin (HFO) Refrigerants

• Hydrofluoroolefins (HFOs) are a type of refrigerant characterized by their unsaturated chemical structure, which includes a double bond between carbon atoms. This feature distinguishes them from other refrigerants like Hydrofluorocarbons (HFCs) and Chlorofluorocarbons (CFCs). HFOs are considered to be more environmentally friendly because they have lower Global Warming Potential (GWP) compared to their HFC counterparts.

Chemical Composition:

• The chemical composition of HFOs typically includes carbon (C), fluorine (F), and hydrogen (H) atoms. The presence of a double bond (olefin) in their structure is a key characteristic. For example, the chemical formula CF₃CF=CH₂ represents an HFO with a specific arrangement of these atoms.

Working Principle:

• HFO refrigerants work by absorbing heat during the evaporation process and releasing it during condensation. When used in refrigeration or air conditioning systems, they cycle between liquid and gas phases, transferring heat from one area to another. The double bond in their molecular structure allows for a lower GWP, making them a preferred choice in efforts to reduce environmental impact.

Advantages:

• Lower Global Warming Potential (GWP) compared to HFCs, making them more environmentally friendly.
• Similar thermodynamic properties to HFCs, allowing for easier retrofitting in existing systems.
• Non-ozone depleting, contributing to the protection of the ozone layer.

Disadvantages:

• Potential flammability issues, requiring careful handling and system design.
• Higher production costs compared to some traditional refrigerants.

Applications: HFO refrigerants are used in a variety of applications including automotive air conditioning, commercial refrigeration, and residential air conditioning. They are seen as a sustainable alternative to HFCs and CFCs, helping to meet regulatory requirements and environmental standards.

Concept:

where

m = No. of Carbon atoms,

n = No. of Hydrogen atoms,

p = No. of Florine atoms,

q = No. of Chlorine atoms

Calculation:

Given:

R 134

By comparing with R (m 1) (n + 1) P

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

n + 1 = 3 ⇒ n = 2 i.e. 2 → Hydrogen atoms

p = 4 i.e. 4 → Florine atom

2m + 2 = (2 × 2 ) + 2 ⇒ 6

n + p + q = 2m + 2

2 + 4 + q = 6

∴ q = 0

CmHnFpClq ⇒ C2H2F4

Explanation:

Refrigerant Properties

A refrigerant is a substance used in a refrigeration cycle to absorb heat from a space or substance and release it to another. The choice of refrigerant is crucial for the efficiency and effectiveness of the refrigeration system. Several key properties are essential for an ideal refrigerant, and these properties significantly influence the performance of the refrigeration system.

Important Properties of a Refrigerant:

• Boiling Point: The refrigerant should have a low boiling point to ensure that it can absorb heat effectively at low temperatures.
• Latent Heat Value: A high latent heat value is desirable as it allows the refrigerant to absorb a significant amount of heat during the phase change from liquid to vapor.
• Freezing Point: The refrigerant should have a low freezing point to prevent it from freezing at low operational temperatures, which could cause blockages and system failures.

Explanation:

• Domestic refrigerators generally run on the vapor compression cycle. In this cycle, a circulating refrigerant such as R134a enters a compressor as low-pressure vapour at or slightly below the temperature of the refrigerator space.
• Ammonia is generally used in the vapour absorption cycle.
• Nitrogen and CO₂ are not used as refrigerants.

The R 134a is an eco-friendly refrigerant.

Explanation:

There are two types of refrigerants

1. Primary Refrigerant: Primary refrigerants are substances that undergo a cyclic process and produce lower temperatures. There is a latent heat transformation for the refrigerants. For e.g. R-11, R-12, R-22, R-134a, R-1150 etc.
2. Secondary Refrigerants: There are working substances that are first cooled by primary refrigerants and then used for cooling at desired places. e.g.H2O, Brine.

​Additional Information

• 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

​​​​​​Mistake Points

​​​Both water and Brine are secondary refrigerants, but here he asks above the 0°C, so water will be the correct answer. 

Explanation:

The characteristics of a good refrigerant should have the following properties

1. Low boiling point
2. High latent heat value
3. Low freezing point
4. Non-flammable and non-toxic
5. High di-electric strength
6. Not affected by moisture
7. Non-corrosive to metals
8. Mixes well with compressor oil

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 11

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

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

n + 1 = 1 ⇒ n = 0 i.e. 0 → Hydrogen atoms

p = 1 i.e. 1 → Florine atom

Now

n + p + q = 2m + 2

⇒ 0 + 1 + q = 2 × 1 + 2

⇒ q = 3 i.e. 3 → Chlorine atoms

∴ C_mH_nF_pCl_q ⇒ CFCl₃

CF₃Cl

m = 1, n = 0, p = 3 and q = 1

n + p + q = 2m + 2

0 + 3 + 1 = 2 × 1 + 2

4 = 4 satisfied

R - (m - 1) (n + 1) P ⇒ R - 13

CHFCl

m = 1, n = 1, p = 1 and q = 1

n + p + q = 2m + 2

1 + 1 + 1 = 2 × 1 + 2

3 = 4 not satisfied

Not a Saturated Hydrocarbon

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

∴ C_mH_nF_pCl_q ⇒ 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

In the vapour absorption system, the water is used as the refrigerant while lithium bromide (Li Br) is used as the absorbent.

Basic Vapour Absorption Refrigeration System

• The basic absorption cycle employs two fluids, the absorbate or refrigerant, and the absorbent
• The most common fluids are Water/Ammonia as the refrigerant and lithium bromide/ water as the absorbent
• These fluids are separated and recombined in the absorption cycle
• In the absorption cycle, the low-pressure refrigerant vapour is absorbed into the absorbent releasing a large amount of heat (Absorber pressure is equal to evaporator pressure)
• The liquid refrigerant/absorbent solution is pumped to a high-operating pressure generator using significantly less electricity than that for compressing the refrigerant for an electric chiller
• Heat is added at the high-pressure generator from a gas burner, steam, hot water or hot gases
• The added heat causes the refrigerant to desorb from the absorbent and vaporize
• The vapours flow to a condenser, where heat is rejected and condense to a high-pressure liquid
• The liquid is then throttled through an expansion valve to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling
• The remaining liquid absorbent, in the generator, passes through a valve, where its pressure is reduced, and then is recombined with the low-pressure refrigerant vapours returning from the evaporator so the cycle can be repeated

Concept:

For Inorganic compounds (refrigerants) the code is specified as R for refrigerant then 700 plus the molecular weight of the compound.

Calculation:

Now according to the question, we have to find the chemical formula of R717

∴ 717 = 700 + 17;

Now we have to only find from the options provided which compound is having a molecular weight of 17.

N₂ = 28; NH₃ = 17; H₂O = 18 ; CO₂ = 44        

Hence from above values, we can conclude that the correct answer is NH₃ i.e. option (b)

Concept:

To determine which of the given refrigerants is a CFC using the formula method, let's break down the molecular formulas for each refrigerant step by step. The formula to determine the molecular structure of refrigerants is:

\( C_mH_nF_pCl_q \)

Where:

• \( m \) is the number of Carbon atoms
• \( n \) is the number of Hydrogen atoms
• \( p \) is the number of Fluorine atoms
• \( q \) is the number of Chlorine atoms

Calculation:

We will apply this method to each refrigerant option.

Option 1: R744 (Carbon Dioxide - CO₂)

Known Data:

R744 is carbon dioxide (CO₂), which has 1 carbon atom and 2 oxygen atoms. Since R744 does not contain hydrogen (H), fluorine (F), or chlorine (Cl), it does not fit into the formula structure for CFC refrigerants.

Conclusion: R744 is not a CFC refrigerant.

Option 2: R290 (Propane - C₃H₈)

Using the formula method, we can solve for the molecular structure of R290:

Given:

\( m - 1 = 2 \rightarrow m = 3 \) (3 carbon atoms)

\( n + 1 = 9 \rightarrow n = 8 \) (8 hydrogen atoms)

\( p = 0 \) (No fluorine)

\( q = 0 \) (No chlorine)

Substitute into the molecular structure:

\( C_3H_8F_0Cl_0 \)

This corresponds to propane (C₃H₈), which is a hydrocarbon refrigerant and not a CFC.

Conclusion: R290 is not a CFC refrigerant.

Option 3: R502 (Blend of R22 and R115)

R502 is a blend of two refrigerants:

• R22 (CHClF₂): Chlorodifluoromethane
• R115 (CClF₅): Chloropentafluoroethane

Let's break these down:

R22:

\( m = 1 \) (1 carbon atom)

\( n = 1 \) (1 hydrogen atom)

\( p = 2 \) (2 fluorine atoms)

\( q = 1 \) (1 chlorine atom)

So, R22's molecular structure is:

\( C_1H_1F_2Cl_1 \)

R115:

\( m = 1 \) (1 carbon atom)

\( n = 0 \) (No hydrogen)

\( p = 5 \) (5 fluorine atoms)

\( q = 1 \) (1 chlorine atom)

So, R115's molecular structure is:

\( C_1H_0F_5Cl_1 \)

Conclusion: R502, being a blend of these two CFC components, contains chlorine (Cl) and fluorine (F), making it a CFC refrigerant.

Option 4: R718 (Water - H₂O)

R718 is simply water (H₂O). It does not contain carbon (C), fluorine (F), or chlorine (Cl), so it does not fit the formula for CFCs.

Conclusion: R718 is not a CFC refrigerant.

Final Answer:

By applying the formula method, the refrigerant that is a CFC is:

3) R502

Explanation:

Refrigerants:

The refrigerant is a heat-carrying medium which during their cycle (i.e. compression, condensation, expansion, and evaporation) in the refrigeration system absorb heat from a low-temperature system and discard the heat so absorbed to a high-temperature system.

Azeotropic mixture:

• The mixture of refrigerants which boils at a constant temperature like a pure liquid and posses the same composition of components in liquid as well as in vapour phase i.e., the two refrigerants are mixed in the proper proportions, and the mixture forms the third refrigerant.
• The azeotropes are also known as a constant boiling mixture because the whole of the azeotrope changes into vapour state at constant temperature and their components cannot be separated by the fractional distillation.

Explanation:

Lubricants are essential for the performance and longevity of a refrigeration system. The choice of lubricant depends on the system, the refrigerant, and the type of application.

There are several categories of oils:

• Mineral oils: They are particularly suited for use with CFC (R11, R12), HCFC (R22, R142b, R123) refrigerants, and ammonia.
• Alkyl Benzene Oils (AB): AB oils are especially recommended for refrigerants like R-22 and HCFC blends.
• Poly Alpha Olefins (PAO): It can be described as ‘mineral synthetic oils’. PAO oils can be used in refrigeration systems working in extreme conditions with R-22 or ammonia. It has low pour point and excellent thermal stability.
• Poly Alkylene Glycols (PAG): These were the first oils developed for use with HFC refrigerants and are perfectly miscible with these products.
• Polyol Ester Oils (POE): POE constitutes the second generation of lubricants that were developed for use with HFC’s (R134a, R152a). POE oils are excellent lubricants, less hygroscopic than PAG (Poly Alkylene Glycols) and chemically more stable than PAG when in the presence of water. POE oils are intended for all refrigeration and air conditioning applications.

HFCs cannot be used with mineral oils. Generally, POE is used. R134a is not miscible with mineral oil, therefore, a synthetic lubricant of polyester is used.

Explanation:

Desirable properties of an ideal refrigerant:

• Low boiling point
• High critical temperature
• High latent heat of vaporization
• Low specific heat of the liquid
• Low specific volume of vapour
• Non-corrosive to metal
• Non-flammable and non-explosive
• Non-toxic
• Low cost
• Easy to liquefy at moderate pressure and temperature
• Easy of locating leaks by odour or suitable indicator
• Mixes well with oil