Vapour Compression (V-C) Cycle MCQ Quiz - Objective Question with Answer for Vapour Compression (V-C) Cycle - Download Free PDF

Concept:  

p - h and T-s diagram for ideal vapour compression refrigeration cycle are shown below,

The decrease in the evaporator pressure will not affect condensing temperature because the condensing temperature is a function of condensing pressure not of evaporator pressure. So, there is no change in the temperature of condenser. 

From the p-h diagram:

Let pE1 and p­E2 be the initial evaporator pressure and decreased evaporator pressure respectively.

At pE1,

W (Compressor work) = h2 – h­1

RE (Refrigeration Effect) = h1­ ­­– h4

At pE2,

W’ (Compressor work) = h2’ – h1

RE’ (Refrigeration Effect) = h1’ – h4’

From the p-h diagram,

W’ > W, Compressor work is increased

RE’ < RE, Refrigeration effect is decreased

There is an increase in compressor work and a decrease in the refrigeration effect.

The coefficient of performance of a vapor compression refrigeration system decreases with the decrease in evaporator temperature at a fixed condenser temperature.

Explanation:

Centrifugal Compressor:

• A centrifugal compressor is a dynamic compressor used in various refrigeration systems. It operates by converting the kinetic energy of a fluid into pressure energy via a rotating impeller. In a vapour compression refrigeration system, the centrifugal compressor is critical for compressing refrigerant vapour to a higher pressure and temperature, enabling heat rejection at the condenser.

Working Principle: The centrifugal compressor works by accelerating the refrigerant vapour using a high-speed impeller. The velocity of the refrigerant increases as it passes through the impeller, and this kinetic energy is then converted into pressure energy in the diffuser. The resulting pressurized refrigerant vapour is sent to the condenser for heat rejection.

The centrifugal compressor's performance in a vapour compression refrigeration system is highly sensitive to both the evaporator temperature and the condenser temperature. Here’s why:

• Evaporator Temperature: The evaporator temperature determines the pressure at which the refrigerant enters the centrifugal compressor. Lower evaporator temperatures correspond to lower suction pressures. If the suction pressure drops too much, the compressor may face issues such as surge, reduced efficiency, or stalling. Additionally, the refrigerant density at the inlet is affected, which influences the compressor’s capacity.
• Condenser Temperature: The condenser temperature determines the discharge pressure of the compressor. Higher condenser temperatures correspond to higher discharge pressures. If the discharge pressure is excessively high, the compressor requires more work, increasing power consumption and potentially causing mechanical stress or operational instability.

Thus, the centrifugal compressor's operation is intricately linked to both the evaporator and condenser temperatures, as these parameters significantly affect the suction and discharge pressures, refrigerant density, and overall compressor efficiency. A balanced and stable operation of the refrigeration system necessitates keeping both temperatures within optimal ranges to ensure efficient and reliable compressor performance.

Explanation:

The Flash Chamber in Vapour Compression Cycle (Single Stage)

Definition: In a vapour compression refrigeration system, the flash chamber is a component that is used to separate the liquid and vapour phases of the refrigerant after the expansion valve. It is typically employed to improve the system's performance by managing the refrigerant's phase before it enters the evaporator.

Function of the Flash Chamber: The primary function of the flash chamber is to separate the liquid refrigerant from the vapour refrigerant. The liquid refrigerant, which has the desired cooling effect, is directed towards the evaporator, while the vapour refrigerant is bypassed to the compressor. This separation ensures that only the liquid refrigerant flows into the evaporator, optimizing its performance and reducing the load on the compressor.

Importance of Incorporating a Flash Chamber:

• Improved Efficiency: By ensuring that only liquid refrigerant enters the evaporator, the flash chamber enhances the heat transfer efficiency and maximizes the refrigerating effect.
• Reduced Compressor Work: The flash chamber prevents vapour refrigerant from entering the evaporator, thereby reducing the load on the compressor and improving the system's overall efficiency.
• System Protection: It prevents vapour refrigerant from inadvertently entering the evaporator, which could lead to inefficiencies or damage to the system components.

Reduce the size of the evaporator

• The flash chamber in a vapour compression cycle helps reduce the size of the evaporator. By separating the liquid refrigerant from the vapour refrigerant and ensuring that only liquid enters the evaporator, the flash chamber allows for a more compact and efficient evaporator design. Since the heat transfer in the evaporator is optimized, the size of the evaporator can be reduced without compromising the system's performance.

Concept:

Vapor Compression Refrigeration System:

Processes in the vapor compression system:

1. Process 1-2: Isentropic compression of saturated vapor in the compressor.
2. Process 2-3: Isobaric heat rejection in the condenser.
3. Process 3-4: Isenthalpic expansion of the saturated liquid in the expansion device.
4. Process 4-1: Isobaric heat extraction in the.

• In a vapor compression system, the condition of refrigerant before entering the compressor is the dry saturated vapor as shown in the figure at point '1'.
• At the end of the compressor or entry point of the condenser, the state of refrigerant is Superheated vapor.
• At the end of the condenser or entry point of the throttling valve, the state of refrigerant is Saturated liquid.

Explanation:

COP of a Carnot refrigerator:

COP = T_L / (T_H - T_L)

Where:
T_L = Lower temperature (in Kelvin)
T_H = Higher temperature (in Kelvin)

First, convert the temperatures from Celsius to Kelvin:

• T_H = 35ºC = 35 + 273 = 308 K
• T_L = -15ºC = -15 + 273 = 258 K

Now, substitute the values into the COP formula:

COP = 258 / (308 - 258)

COP = 258 / 50

COP = 5.16

Concept:

• The capillary tube is one of the most commonly used throttling devices in the domestic refrigerators, deep freezers, water coolers, and air conditioners.
• Capillary tubes have very small internal diameters and very long length and they are coiled to several turns so that it would occupy less space (compact).
• They are easy to manufacture, cheap and compact.

Working:

• When the refrigerant leaves the condenser and enters the capillary tube, its high pressure drops down suddenly due to the very small diameter of the capillary tube and the long length gives more friction head and drops pressure further.
• The decrease in pressure leads to cooling of refrigerant and the low-temperature refrigerant can take the heat from the room.

Explanation:

Vapour Compression Refrigeration System:

The four processes in simple VCRS include:

1. Isentropic compression (1-2) in Compressor.
2. Constant Pressure heat removal (2-3) in Condensor.
3. Isenthalpic expansion (3-4) in a Throttling device.
4. Constant pressure heat removal (4-1) in Evaporator.

From the Above T-s diagram, we can conclude that the lowest temperature during the cycle in a vapour compression system occurs after expansion and during the evaporation process(4-1) in the evaporator.

In the Question the exact location of the lowest temperature has not been asked here they have asked after which process the lowest temperature will be obtained. The lowest temperature surely is obtained in the evaporator which comes after the expansion process hence the expansion is the correct answer. 

Explanation:

The vapor-compression cycle is a process used to extract heat from a box or a room that underlies most refrigeration and air conditioning techniques. It consists of four separate stages:

• Compression(1-2) Isentropic Compression
• Condensation(2-3) Heat rejection at constant pressure
• Expansion(3-4) Constant enthalpy expansion
• Evaporation (4-1) Constant pressure heat addition

​The T-s and P-h diagram is shown in the figure below. 

Referring to the above diagram, the P-H diagram of the vapor compression refrigeration cycle, the compression process is shown by a straight line with a positive slope

Additional Information 

The diagram given is the P-h diagram as it is similar to the P-h diagram drawn as per the process occurring from 1-2-3-4-1 in the cycle. 

Refrigeration effect = R.E = mas flow rate × (enthalpy at evaporator exit – enthalpy at condenser exit)

Refrigeration effect = ṁ × (h1 - h4)

and $COP=\frac{h_1-h_4}{h_2-h_1}$

Explanation:

• Process 1-2: Reversible adiabatic compression.
• Process 2-3: Constant pressure heat rejection.
• Process 3-4: Irreversible expansion (throttling).
• Process 4-1: Constant pressure heat addition.

Working in Vapour Compression Refrigeration System:

• The refrigerant in a liquid state absorbs heat from the evaporator and converted it into vapour form.
• The refrigerant in vapour form enters the compressor where temperature and pressure increased by compressing vapour.
• In Condensor due to temperature difference heat rejected to the atmosphere.
• In Expansion device due to throttling process temperature and pressure drops by maintaining constant enthalpy.

Explanation:

Vapour Compression Refrigeration Cycle:

Process 1-2: Isentropic Work Done by Compressor

Process 2-3: Constant Pressure Heat Removal by Condenser

Process 3-4: Isentropic Expansion by Throttling Valve

Process 4-1: Constant Pressure Heat Addition by Evaporator

From the Heat Balance Equation:

Energy transferred from Condenser = Energy Transferred to Evaporator + Work Input to the compressor 

Heat rejection ratio is given by:

$\mathrm{H}\mathrm{R}\mathrm{R}=1+\frac{1}{\mathrm{C}\mathrm{O}\mathrm{P}}=1+\frac{W_{input}}{\mathrm{R}\mathrm{E}}=\frac{{\mathrm{Q}}_{\mathrm{C}\mathrm{O}\mathrm{N}\mathrm{D}}}{\mathrm{R}\mathrm{E}}$

Or

$\mathrm{H}\mathrm{R}\mathrm{R}=\mathrm{H}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{j}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}=\frac{{\mathrm{Q}}_{\mathrm{C}\mathrm{O}\mathrm{N}\mathrm{D}}}{\mathrm{R}\mathrm{E}}$

Where,

Q_COND is heat rejected in the condenser,

RE  is the Refrigeration effect and W_input  is the work input to the Refrigeration System

Heat rejected in the condenser is always more than heat absorbed in the evaporator.

So, the heat rejection factor in vapour compression refrigeration system will be more than one.

Concept:

Here, VCC stands for Vapour Compression Cycle.

For refrigeration cycle:

$COP=\frac{Refrigerationcapacity}{Powersupplied}$

Calculation:

Given:

COP = 3.5, Refrigeration capacity = 1 Ton = 3.5 kW

$COP=\frac{Refrigerationcapacity}{Powersupplied}$

we can rearrange it as

$Powersupplied=\frac{Refrigerationcapacity}{COP}$$=\frac{3.5}{3.5}$ = 1 kW

Hence the minimum power needed to run this machine would be 1 kW.

Explanation:

Vapour Compression Cycle

The vapour-compression cycle is a process used to extract heat from a box or a room that underlies most refrigeration and air conditioning techniques. It consists of four separate stages:

• Compression(1-2)
• Condensation(2-3)
• Expansion(3-4)
• Evaporation (4-1)

And in the domestic refrigerator, the purpose is to extract the heat from the refrigerator hence it follows the vapour compression cycle

a) Basic vapour compression b) T-s diagram c) p-h diagram

refrigeration system

*Trick to remember –Most of you will get confused that whether it works on vapour compression cycle or the vapour absorption cycle. So here is a simple trick to remember. You might have noticed a compressor installed at the back of our domestic refrigerator that’s it because the cycle on which it works is vapour compression cycle.

Explanation:

• Sub cooling is the process of cooling the liquid refrigerant below the condensing temperature for a given pressure.
• The difference between the saturation temperature and the temperature of sub cooled liquid at that pressure is called Degree of sub cooling.
• Subcooling is beneficial as it increases the refrigeration effect by reducing the throttling loss at no additional specific work input.
• Also, subcooling ensures that only liquid enters the throttling device leading to its efficient operation.

Effect of Sub cooling:

• Increase the refrigeration effect
• Work input remains same
• Increase in COP

As the Subcooling increases the refrigeration effect, Hence to produce more refrigeration evect a larger size evaporator will be required. Hence subcooling of the refrigerant in condenser increases the evaporator size.

Concept:

Subcooling is the process of cooling the liquid refrigerant below the condensing temperature for a given pressure.

The difference between the saturation temperature and the temperature of the subcooled liquid at that pressure is called the Degree of subcooling.

Subcooling is beneficial as it increases the refrigeration effect by reducing the throttling loss at no additional specific work input.

Also, subcooling ensures that only liquid enters the throttling device leading to its efficient operation.

Effect of Subcooling:

• Increase the refrigeration effect
• Work input remains the same
• Increase in COP

Concept:

Units of refrigeration are commonly defined in terms of a ton of refrigeration.

One ton of refrigeration is defined as the capacity to freeze one tone of water at 0°C into ice in 24 hours.