৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]

⇒ In sieve tray columns, trays contain perforations (holes) through which vapor rises and contacts the liquid. 

⇒ However, if all the liquid simply drains through the downcomer without any retention, there would be minimal vapor-liquid contact time, reducing mass transfer efficiency.

⇒ The weir creates a liquid hold-up on the tray by acting as a barrier at the outlet of the tray before the downcomer.

⇒ This ensures proper froth formation, improving the number of theoretical stages.

⇒ Without a weir, dry trays or weeping would occur, decreasing column efficiency.

⇒ Structural support is provided by tray decks and support rings, not the weir.

The hole diameter and open area of a sieve tray are carefully selected based on vapor flow rate and pressure drop limits.

**If holes are too small, they plug easily and create a high-pressure drop, reducing tray capacity.

**If holes are too large, weeping occurs at low vapor rates (liquid leaks through holes instead of staying on the tray).

Thus, hole diameter and pitch are chosen to maintain the correct vapor velocity for proper frothing without excessive pressure loss.
Weir height and tray spacing influence liquid holdup and residence time, but the hole diameter is primarily controlled by vapor flow design

The downcomer is the channel through which liquid flows from the weir of an upper tray to the tray below. It provides a controlled liquid path while preventing vapor from bypassing the tray system.
For efficient operation:
**Downcomer area must be large enough to avoid backup and flooding.

**Too small a downcomer → liquid backs up into the tray above, causing flooding.

**Too large a downcomer → reduces active vapor-liquid contact area, lowering efficiency.

Chimney trays are special trays used for liquid draw-off or vapor redistribution, not for regular liquid downflow.

Weeping occurs when liquid leaks through the perforations instead of flowing across the tray, typically because upward vapor force is insufficient to hold the liquid on the tray. This usually happens at low vapor rates.

To prevent weeping:
1. Hole size and pitch are designed to ensure vapor velocity provides adequate supporting force.
2. Weir height maintains a minimum liquid level, so small fluctuations in vapor flow don’t cause immediate weeping.
3. Reducing tray spacing has little effect; proper design focuses on vapor-liquid balance.

Trays in plate columns are heavy, especially in large-diameter columns, and must withstand mechanical stress from:

Liquid load
Vapor flow
Vibrations during operation

Support rings (or beams) are welded inside the shell to support tray decks, downcomers, and weirs, preventing mechanical failure and column damage.
They do not influence vapor-liquid contact or pressure drop directly; their purpose is purely structural integrity.

Sieve trays are simpler in construction—just perforated plates with weirs and downcomers—making them cheaper to fabricate and install compared to bubble-cap trays, which require many intricate caps and risers.

Additionally:
1. Sieve trays have a lower pressure drop, which is crucial for vacuum distillation or energy-sensitive processes.
2. However, sieve trays are more prone to weeping at low vapor rates (unlike bubble caps, which can operate at very low vapor loads).
3. They are not maintenance-free, and their mechanical life is comparable to other tray types if properly designed.

Thus, the main reasons for preference are cost-effectiveness and lower pressure drop, not superior efficiency at all loads.

COP of a reversed Carnot refrigerator:
  COP=T_L/(T_H−T_L)

Where
T_L= evaporator temperature (in Kelvin), 
T_H= condenser temperature.
If evaporator temperature increases,
T_L rises, so:
The difference (T_H−T_L) decreases → denominator gets smaller

The numerator
T_L also increases
Overall effect: COP increases.

This is why raising evaporator temperature improves efficiency, but reduces cooling effect (since the space is warmer).

If isentropic expansion (via an expander) were used instead of throttling, the work recovered would be small compared to:

Cost and complexity of the turbine

Maintenance and reliability issues

Thus, a throttling valve (irreversible, h = const) is used even though it causes an entropy increase and lowers COP slightly.

Option (খ) is misleading because expansion work would be an output, not an input.

COP:

COP = Q_L/W = 5

So,           
W = COP/Q_L​​ = 100/5​ = 20 kJ

Heat rejected:
 Q_H​ = Q_L​ + W
= 100 + 20
= 120 kJ

Refrigeration effect depends on enthalpy difference between evaporator outlet and inlet (h₁ – h₄).
If evaporator pressure decreases, the saturation temperature drops → refrigerant boils at a much lower temperature → latent heat may increase slightly, but more importantly, the specific volume increases, leading to lower mass flow per unit displacement.

Net effect: capacity reduces, so refrigeration effect per kg decreases.

Options:

(খ) improves performance by reducing compressor work

(গ) slightly increases refrigeration effect (due to higher enthalpy before compression)

(ঘ) increases refrigeration effect

In conventional air-conditioning, both sensible heat (temperature) and latent heat (moisture) are removed by cooling the air below its dew point using cooling coils. This often causes overcooling, which then requires reheating to maintain comfort temperature—wasting energy.

Desiccant dehumidification works differently:
**It uses hygroscopic materials (like silica gel or lithium chloride) that absorb water vapor directly from the air.
**This removes latent heat (moisture) without significantly lowering the dry-bulb temperature, so no extra sensible cooling is required.
**The process is energy-efficient for humidity control in industries (pharma, electronics) and comfort air-conditioning in humid climates.

Other options:

(ক) Condenser subcooling is unrelated to desiccants.
(খ) Dew point control is a consequence, but the phrase "maintain above ambient" is misleading because humidity control is independent of ambient dew point.
(গ) Compressor lubrication has nothing to do with desiccants.

Air behaves as an ideal gas under normal conditions. For condensation to occur at room temperature (≈30°C), the pressure must be extremely high (hundreds of atmospheres) because air has a very low boiling point at atmospheric pressure.
Using such high pressures is:
Impractical (requires very strong and expensive equipment)
Unsafe (risk of explosion)
Inefficient (compressor work would be enormous)

That’s why air is not used as a refrigerant in domestic vapor-compression systems.
Instead, air refrigeration (Bell-Coleman cycle) is only used in aircraft cooling, where the system can work at low temperatures and avoid condensation, operating as a gas cycle.

Other Options:
(ক) Specific heat does not determine condensation feasibility.
(গ) & (ঘ) Air does not chemically react with oil and does not inherently cause lubrication failure.

Here,
COP_Carnot​=T_L/(T_H-T_L)

Where,
T_L = 273 K (evaporator temperature)
T_H = 303 K (condenser temperature)

Now,
T_H​ − T_{L ​}= 303−273 = 30K

So,       
COP = 273/30 = 9.1

In a vapor-compression refrigeration cycle, there are two main pressure levels:
      Low pressure in the evaporator (where refrigerant evaporates at low temperature)
     High pressure in the condenser (where refrigerant condenses at high temperature)

The compressor is the component that:
 1.  Takes in low-pressure vapor from the evaporator
 2.   Compresses it to a high-pressure, high-temperature vapor before sending it to the condenser

Thus, the pressure difference between evaporator and condenser is primarily created by the compressor.
Without the compressor, the refrigerant could not circulate or maintain the necessary phase-change conditions.

(ক) Throttle valve: Only drops pressure from condenser to evaporator; does not create the main pressure difference.
(খ) Condenser: Rejects heat at high pressure but does not create the pressure difference.
(গ) Evaporator: Absorbs heat at low pressure; again, does not cause the pressure gap.

In a vapor-compression refrigeration system, the expansion process occurs when high-pressure liquid refrigerant from the condenser is reduced to low pressure before entering the evaporator. Two theoretical options exist:
1. Expansion valve (throttle) – simple, causes an isenthalpic process (no work recovered, but very cheap and reliable).
2. Expansion turbine (expander) – would recover some work during expansion (isentropic), potentially increasing COP.

So why don’t we use turbines?
1. The work recovered in expansion is very small compared to the compressor input power.
2. For typical refrigeration systems, the gain in COP would be less than 2–3%, which does not justify the added cost, size, lubrication, and complexity of a turbine.
3. A throttle valve is inexpensive, maintenance-free, and perfectly adequate for most systems.

In an absorption refrigeration system, instead of a mechanical compressor, the cycle uses heat energy to separate and circulate refrigerant. The main components are:

Evaporator: Where refrigerant absorbs heat from the cooled space and evaporates at low pressure.

Absorber: Where the low-pressure refrigerant vapor is absorbed by a strong absorbent solution (e.g., water absorbs ammonia or LiBr absorbs water vapor).

Generator (or Desorber): The component where heat is supplied (from steam, gas burner, or waste heat) to the strong solution. This heat input boils off the refrigerant vapor from the absorbent.

Condenser: Where the refrigerant vapor rejects heat and condenses into a liquid.

So, the generator is the part that uses heat energy to separate the refrigerant from the absorbent, replacing the function of the compressor in vapor-compression systems

Absorption refrigeration systems use a refrigerant–absorbent pair instead of a single refrigerant like in vapor-compression systems. The choice of pair depends on the application:

NH₃–H₂O (Ammonia–Water):
Refrigerant = Ammonia (NH₃)
Absorbent = Water
Used for low-temperature refrigeration, such as ice plants and cold storage.
Cannot be used for comfort air-conditioning because ammonia is toxic and operates at high pressure.

LiBr–H₂O (Lithium Bromide–Water):
Refrigerant = Water
Absorbent = Lithium Bromide
Commonly used in large-scale air-conditioning systems, such as chilled water plants in buildings.
Works well because water (refrigerant) evaporates at low pressure to provide cooling, and LiBr strongly absorbs water vapor, maintaining vacuum conditions.

CO₂–NH₃:
Not used in absorption systems. CO₂ and NH₃ are both refrigerants, not a refrigerant–absorbent pair.

HFC–POE oil:
This is a refrigerant–lubricant combination in vapor-compression systems, not an absorption working pair.

In an absorption refrigeration system, the pump circulates a liquid solution (e.g., LiBr–H₂O or NH₃–H₂O) from low-pressure absorber to high-pressure generator. The work required to increase pressure for a liquid is very small. So, in absorption systems, pumping liquid requires only about 1% (or even less) of the work needed for compressing vapor in an equivalent vapor-compression system.

খ) Smaller mass flow rate – The solution flow may be higher than refrigerant flow, so this is not the main reason.

গ) Zero viscosity – No fluid has zero viscosity; this is irrelevant.

ঘ) Higher efficiency – Pump efficiency is high, but this is not the dominant reason. The main factor is liquid incompressibility.

The key advantage of an absorption refrigeration system is its ability to run primarily on heat energy instead of large amounts of electrical energy. Unlike vapor-compression systems that require significant mechanical work to drive the compressor, absorption systems:

1. Use a thermal energy source (steam, hot water, solar energy, waste heat from engines or processes) to separate the refrigerant from the absorbent in the generator.
2. Require very little mechanical work (just the pump work, which is negligible compared to compression work).

This makes absorption systems ideal where electricity is expensive or heat energy is abundantly available, such as in cogeneration plants, solar-powered cooling, and industrial waste heat recovery.

ক) Lower installation cost – Absorption systems are typically more expensive to install because of their large size, corrosion protection, and complexity.
খ) Higher COP under all conditions – Actually, their COP is usually lower (0.6–1.2) than vapor-compression systems (3–6).
গ) Lower risk of crystallization – Crystallization risk is a problem in LiBr–water systems, so this is not an advantage but a drawback.

In a LiBr–H₂O absorption refrigeration system, the refrigerant is water, and the absorbent is lithium bromide (LiBr). The reasoning is based on their thermodynamic properties:

1. Water can evaporate at very low pressures (vacuum), producing cooling in the evaporator at temperatures suitable for air-conditioning (around 5–7 °C).
2.LiBr is a non-volatile salt with an extremely low vapor pressure. It remains in the solution and does not evaporate, making it an ideal absorbent.
3. If LiBr were used as a refrigerant, it would never evaporate at the required conditions, as it is essentially non-volatile.

So, water is chosen as refrigerant because it can provide the cooling effect by evaporation under vacuum, while LiBr simply absorbs water vapor to maintain low pressure

ক) Because water has a higher freezing point than LiBr – This is irrelevant to the choice of refrigerant. Freezing point matters for operating conditions, but it’s not the reason for selection.
খ) Because LiBr is highly volatile at low pressure – This is false; LiBr is non-volatile, which is why it’s a good absorbent.
ঘ) Because water has higher viscosity – Higher viscosity would actually be a disadvantage for heat and mass transfer, so this is incorrect.

In an ammonia–water absorption refrigeration system:

Generator:
Heat is supplied to the ammonia–water solution.
Ammonia (NH₃) vapor is released from the strong solution, but this vapor usually contains some water vapor mixed with it.

Analyzer:
The ammonia vapor coming from the generator passes through the analyzer.
Its job is to partially separate water vapor from the ammonia vapor, so that mostly pure ammonia vapor continues upward.
This increases the efficiency of the system, because water in the refrigerant reduces performance.

Rectifier:
After the analyzer, the rectifier further condenses and removes any remaining water vapor from the ammonia vapor, ensuring that only pure ammonia enters the condenser.

Absorber:
This is where low-pressure ammonia vapor from the evaporator is absorbed into weak water solution, forming a strong solution to be pumped back to the generator.
 
So, the analyzer is the first component that separates ammonia vapor from water solution, before the rectifier finishes the job.

In an ammonia–water absorption refrigeration system:
The generator produces ammonia vapor by heating the strong ammonia–water solution.
However, this vapor is not pure — it carries some water vapor along with it.
If this water vapor enters the condenser, expansion valve, and evaporator, it will:
1. Reduce the refrigeration effect
2. Lower efficiency
3. Cause operational problems

The analyzer (also called dephlegmator) is used to:
Partially condense water vapor from the ammonia vapor.
Ensure that mostly pure ammonia vapor passes onward.
Increase the overall COP (coefficient of performance) and efficiency of the system.

So the analyzer’s role is purification of refrigerant vapor, not cooling, absorption, or pressure increase.

In the ammonia–water absorption refrigeration system:

Generator: Produces ammonia vapor, but it carries water vapor with it.

Analyzer: Removes a large portion of the water vapor (partial separation).

Rectifier: The final purification stage — it condenses and removes the remaining traces of water vapor, ensuring that only nearly pure ammonia vapor enters the condenser.

Absorber: Absorbs low-pressure ammonia vapor into water, forming strong solution.

Pump: Increases the pressure of the strong solution to the generator.

Throttle valve: Expands the high-pressure liquid ammonia to low pressure before the evaporator.

Therefore, the rectifier is the component that ensures pure ammonia vapor reaches the condenser.

In absorption refrigeration systems, the choice of refrigerant depends on the temperature range:

Water–LiBr system → Used for air-conditioning (above 0 °C), because water is the refrigerant. But water freezes at 0 °C, so it cannot be used for sub-zero applications.

Ammonia–water system → Used for refrigeration below 0 °C, because ammonia remains a vapor/liquid without freezing, even at very low temperatures.

Ammonia has:
A low freezing point (–77 °C) → suitable for ice plants, cold storages, and food preservation.

High latent heat of vaporization → makes it efficient.

Good miscibility with water (as an absorbent).

 Therefore, ammonia is preferred over water for refrigeration below 0 °C, because it does not freeze at those temperatures.

In a vapor-compression refrigeration system (VCRS):
The compressor compresses low-pressure refrigerant vapor to high pressure.
Compressing vapor requires large work input, because vapor is highly compressible and occupies a large volume.

In an ammonia–water absorption refrigeration system (AARS):
There is no compressor. Instead, a pump raises the pressure of the ammonia–water liquid solution from absorber pressure to generator pressure. Since liquids are nearly incompressible, only a tiny amount of energy is required to raise their pressure. Thus, pump work is negligible compared to compressor work in VCRS (typically less than 1–2% of equivalent compressor work).

This is one of the key advantages of absorption systems — they replace high-work compressors with low-work pumps, while using external heat as the main energy input.

Ammonia as a refrigerant has several advantages:

High latent heat of vaporization → efficient cooling
High solubility in water → ideal for absorption systems
Low molecular weight → favorable thermodynamic properties
Does not contribute to ozone depletion

But the major drawback is:

Toxicity → Ammonia is poisonous if inhaled in high concentration.
Pungent odor → even small leaks are easily detected, but the smell is unpleasant and dangerous at high levels.
This makes ammonia systems require careful safety measures, leak detection, and ventilation.
Therefore, while ammonia is efficient, its high toxicity and pungent odor are the main disadvantages in refrigeration use.

Centrifugal chillers are widely used in large HVAC systems (like high-rise buildings, malls, airports) because they handle large cooling loads efficiently.

The refrigerant choice depends on safety, efficiency, and environmental impact.

Option Analysis:

R-717 (Ammonia) → Highly efficient, but toxic and corrosive → rarely used in comfort HVAC (more common in industrial refrigeration like cold storages).

R-134a → Most commonly used refrigerant in centrifugal chillers, especially after the phaseout of R-12. It is non-toxic, non-flammable, and has good thermodynamic properties.

R-12 → Used in old systems, but phased out due to ozone depletion (CFC).

R-22 → Also used in older systems, but being phased out due to ozone depletion (HCFC).

Thus, R-134a is the standard refrigerant for centrifugal chillers in modern large-scale HVAC systems.

HFOs (Hydrofluoroolefins) are the next-generation refrigerants designed to replace HFCs and HCFCs.

Their main advantages:

Zero Ozone Depletion Potential (ODP = 0) → they do not harm the ozone layer.

Ultra-low Global Warming Potential (GWP ≈ 1–10) → much lower than HFCs like R-134a (GWP ≈ 1430).

Similar thermodynamic properties to HFCs → so they can be used in existing system designs with minor modifications.

Other points:

Some HFOs are mildly flammable (A2L classification), so safety precautions are needed.

They are not more toxic than HFCs — in fact, most have low toxicity.

They do not operate at “extremely low pressures”; their pressures are similar to HFCs.

Therefore, HFOs are considered next-generation refrigerants mainly because of their environmental friendliness (zero ODP, ultra-low GWP)

Ultra-low temperature applications (cryogenics, medical freezers, environmental chambers, etc.) require refrigerants that can evaporate at extremely low temperatures (–80 °C or below).

Option Analysis:

R-134a → Common in chillers & automotive AC, but suitable only for medium temperature (down to about –15 °C).

R-22 → Used in air-conditioning and medium/low temperature refrigeration, but not suitable for ultra-low temperatures (minimum ~ –40 °C).

R-23 (CHF₃) → Specifically designed for ultra-low temperature systems, often used in cascade refrigeration with R-404A or R-508B. It can reach temperatures below –80 °C, making it ideal for cryogenic applications.

R-717 (Ammonia) → Excellent efficiency in industrial refrigeration, but typically used down to about –50 °C. Below that, it is less practical.
 
Hence, R-23 is the refrigerant of choice for cryogenic and ultra-low temperature refrigeration systems.

In an absorption refrigeration system (ARS):

Unlike vapor-compression systems, there is no compressor → instead, absorption and heat input drive the cycle.

The refrigerant must be easily absorbed by the absorbent to maintain system efficiency.

Thus, the chemical compatibility and strong affinity between the refrigerant–absorbent pair is the primary factor.

Examples:

Ammonia–Water system

Refrigerant = Ammonia (NH₃)

Absorbent = Water (H₂O)

Works well for sub-zero refrigeration because water has high affinity for ammonia.

Water–LiBr system

Refrigerant = Water

Absorbent = Lithium Bromide (LiBr)

Used for air-conditioning (above 0 °C) since LiBr strongly absorbs water vapor.

Other factors like pressure, critical temperature, or compressor efficiency are more relevant to vapor-compression systems, not absorption systems.

In a sieve tray column:
  1.Sieve holes are small perforations in the tray.
  2.Their main function is to let vapor pass upward through the liquid layer on the tray.
  3.This generates bubbling and turbulence, which enhances gas–liquid mass transfer.

Other components:
ক) Support liquid load and prevent weeping → Partially related, but the holes themselves don’t prevent weeping; proper vapor velocity does.
গ) Distribute liquid uniformly → Done by feed and downcomer design, not the sieve holes.
ঘ) Act as downcomers → Downcomers are separate passages for liquid to flow to the tray below.

Weeping occurs in sieve tray columns when vapor velocity is too low to support the liquid on the tray, causing liquid to drip through the tray holes instead of proper bubbling.

1. Primary design feature to prevent weeping:
    Hole diameter: Smaller holes increase resistance to liquid dripping.
    Open area fraction (% of tray area occupied by holes): Lower open area reduces liquid leakage through the holes at low vapor flow.

2. Together, these control the minimum vapor velocity needed to keep the liquid on the tray and maintain effective gas–liquid contact.

Other options:
ক) Tray spacing → Affects flooding and tray efficiency, not directly weeping.
খ) Column diameter → Impacts vapor velocity overall, but not a direct design control for weeping.
ঘ) Downcomer height → Controls liquid flow to lower trays, not weeping through holes.

In a plate (tray) column, the downcomer is the channel through which liquid flows from one tray to the tray below.

Downcomer height affects:
  1.Hydraulic head of the liquid on the tray.
  2.Liquid flow rate between trays.
  3.Prevents flooding by ensuring excess liquid can exit the tray efficiently.

Other options:
ক) Controls vapor pressure drop across the tray → Vapor flows through tray holes, not downcomer.
খ) Sets tray spacing → Tray spacing is independent of downcomer height.
গ) Regulates sieve hole diameter → Hole size is fixed; downcomer has no effect.

Sieve trays vs valve trays:

​
​Sieve trays have fixed perforations, so their performance drops at very low or very high vapor rates, providing less flexibility.
Valve trays adjust automatically to vapor load, maintaining efficiency across a wider range.

Other options:
ক) Sieve trays have no moving parts.
খ) Hole diameters are smaller for low-pressure drops.
ঘ) Sieve trays are simpler and cheaper, not higher cost.

In a vapor-compression refrigeration cycle, the COP is defined as:
COP=QL/W_net

Where:
Q_L= heat absorbed in the evaporator
W_net= work input to the compressor

1.Effect of evaporator temperature:
   Higher evaporator temperature → higher refrigerant pressure → lower compression ratio → less work required for the same heat absorbed.
   Result → COP increases.
2.Effect of condenser temperature:
   Higher condenser temperature → higher discharge pressure → more work → COP decreases.
​
​Other options:
ক) Increasing condenser temperature reduces COP.
গ) Equal increase in both temperatures → smaller net effect; COP may not improve significantly.
ঘ) Increasing mass flow rate → increases refrigeration capacity but does not inherently increase COP.discharge pressure → more work → COP decreases.

In a vapor-compression refrigeration system, the expansion device (capillary tube, thermal expansion valve, or electronic expansion valve) serves to:
   1. Lower the refrigerant pressure from the condenser (high pressure) to the evaporator (low pressure).
   2. This pressure drop reduces the boiling point of the refrigerant, allowing it to vaporize and absorb heat from the refrigerated space in the evaporator.

Other options:
ক) Increases pressure → That is the role of the compressor, not the expansion device.
খ) Absorbs heat → The evaporator absorbs heat, not the expansion device.
ঘ) Compresses vapor → Done by the compressor.

The sensible heat ratio (SHR) is defined as:
​   SHR=Sensible heat/Total heat (sensible + latent)
 1. It indicates the proportion of heat removed from the air that is sensible (temperature change) versus latent (moisture removal).
 2. Primary factor affecting SHR: The ratio of sensible to latent loads in the space being conditioned.
        High sensible load → higher SHR
        High latent load → lower SHR

Other options:
ক) Air velocity → Affects coil heat transfer coefficient, but not the intrinsic SHR of the space.
খ) Coil surface area → Influences total capacity, not SHR.
গ) Refrigerant type → Affects coil performance but does not directly determine SHR.

For a reversed Carnot refrigeration cycle:

COP_R=T_L/(T_H−T_L)

Where:
T_L= evaporator temperature (K)
T_H= condenser temperature (K)

​Here,
​ T_L=0∘C=273 K
T_H=40∘C=313 K

So, COP_R = 273/(313−273) = 273/40 ≈ 6.825

The COP of a vapor-compression refrigeration cycle depends on how much work is required to remove a given amount of heat:

Compressor efficiency:
Affects the work input; lower isentropic efficiency → more work → lower COP.

Condenser temperature:
Higher condenser temperature → higher discharge pressure → more work → lower COP.

In a LiBr-water absorption refrigeration system, the COP depends on how efficiently the system transfers heat and circulates the working fluids.

Solution heat exchanger (SHE):
Preheats the weak solution from the absorber using the hot strong solution leaving the generator.

​Higher effectiveness → less external heat required in the generator → reduces energy input → higher COP.

Flash gas refers to vapor that forms when high-pressure liquid refrigerant expands through the expansion device:

Ideally, the expansion device should reduce pressure so that mostly liquid enters the evaporator.

If vapor is already present at the evaporator inlet (incomplete expansion or flash before the evaporator), it:
   1. Reduces the effective liquid refrigerant available for absorbing heat.
   2. Lowers the refrigeration capacity because vapor cannot absorb as much heat as liquid through evaporation.

In an absorption refrigeration cycle, the maximum achievable COP is strongly influenced by the temperature difference (lift) between the generator and absorber:

Generator temperature (T₁): Provides the heat input to drive the desorption of refrigerant.

Absorber temperature (T₂): Must be low enough to absorb the refrigerant efficiently.

Larger temperature lift → Greater thermodynamic irreversibility → lower COP.

In a water–ammonia absorption refrigeration system:
The generator boils off a mixture of ammonia and water vapor.

The rectifier column above the generator:
   1. Acts as a purification column, removing entrained water vapor.
   2. Ensures that the vapor leaving the generator is nearly pure ammonia, suitable for condensation and evaporation in the rest of the cycle.

In an ammonia–water absorption system, the absorber removes ammonia vapor from the evaporator and dissolves it into the absorbent (water).

Higher surface area → better gas–liquid contact → faster ammonia absorption.

Faster absorption reduces vapor pressure in the absorber, improving mass transfer and allowing the cycle to operate more efficiently.

This enhances the system COP by reducing the required generator heat input per unit refrigeration.

In an ammonia–water absorption system:
The refrigerant (ammonia) vapor is not mechanically compressed as in a vapor-compression system.
Instead, the liquid solution (ammonia absorbed in water) is pumped from the absorber to the generator.

Reason:
The solution is much denser than vapor, so pumping it requires much less work than compressing low-density ammonia vapor to high pressure.

In an ammonia–water absorption system:

Generator temperature determines how much ammonia is desorbed from the strong solution.

Higher generator temperature → more ammonia vapor is released → more refrigerant available for the evaporator → increased refrigeration effect.

As a result, the system COP generally increases, assuming the absorber and condenser can handle the higher vapor flow.

R-717 (Ammonia, NH₃) is a natural refrigerant with:

ODP = 0 → Does not deplete the ozone layer.

Very low GWP → Minimal contribution to global warming.

Other options:
R-22 → HCFC, ODP ≈ 0.05, moderate GWP → being phased out.
R-134a → HFC, ODP = 0, GWP ≈ 1300 → moderate environmental impact.
R-123 → HCFC, ODP ≈ 0.02 → still depletes ozone.

Absorption refrigeration systems rely on a refrigerant–absorbent pair instead of mechanical compression.

Lithium bromide–water (LiBr–H₂O) is commonly used because:
   1. Water is the refrigerant, LiBr is the absorbent.
   2. High solubility of water in LiBr ensures efficient absorption.
   3. High latent heat of water allows effective cooling per unit mass.

Other options:
ক) R-744 (CO₂) → Used in mechanical cycles, not typical for absorption.
খ) R-717 (Ammonia) → Can be used in absorption (NH₃–H₂O), but LiBr–water is more common for water-cooled chillers.
গ) R-134a → Used in vapor-compression, not absorption systems.

CO₂ (R-744) has a critical temperature of 31.1°C.

In many applications, especially air-conditioning and heat pumps:
   1. The condenser cannot condense CO₂ below the critical temperature.
   2. Instead, heat rejection occurs in a gas cooler above the critical point, making the system a transcritical cycle.
This is a unique feature of supercritical CO₂ systems compared to conventional refrigerants.

Other options:
ক) Sub-zero operation only → CO₂ can operate above ambient temperatures in transcritical systems.
গ) Extremely low pressures → Incorrect; CO₂ operates at very high pressures.
ঘ) Forms azeotropes with lubricants → Not true; CO₂ is miscible with some oils but does not form azeotropes.

Ammonia (R-717) is an excellent refrigerant:

High latent heat → efficient cooling.

Zero ozone depletion potential (ODP = 0) and negligible global warming potential (GWP ≈ 0).

Limitation: Its toxicity and flammability make it unsafe for domestic and small-scale applications, where leaks could pose serious health hazards.

Other options:
ক) High GWP → False; ammonia has very low GWP.
গ) Low latent heat → False; ammonia has high latent heat.
ঘ) High ODP → False; ODP = 0.