Refrigeration Devices MCQ Quiz - Objective Question with Answer for Refrigeration Devices - Download Free PDF

Explanation:

COP of a Refrigerator:

• The Coefficient of Performance (COP) of a refrigerator is a measure of its efficiency and is defined as the ratio of the heat absorbed from the refrigerated space (Q_L) to the work input (W) required to transfer that heat. Mathematically, for a reverse Carnot cycle:

The Coefficient of Performance (COP) of a Carnot refrigerator is given by:

\( \text{COP}_{\text{Carnot}} = \frac{T_L}{T_H - T_L} \)

Where, \( T_L \) is the lower temperature and \( T_H \) is the higher temperature, both in Kelvin.

Analysis:

If \( T_H \) is increased and \( T_L \) is decreased:

• The denominator \( (T_H - T_L) \) increases
• The numerator \( T_L \) decreases

As a result, the COP value decreases.

Concept:

Minimum power required is calculated using the COP of a Carnot refrigerator:

\( \text{COP}_{\text{Carnot}} = \frac{T_L}{T_H - T_L} \)

Then, power input is calculated by: \( W = \frac{Q_L}{\text{COP}} \)

Calculation:

Given:

Cold storage temperature, \( T_L = -3^\circ C = 270~K \)

Ambient temperature, \( T_H = 27^\circ C = 300~K \)

Energy to be removed, \( Q_L = 5 \times 10^6~\text{kJ/h} = \frac{5 \times 10^6}{3600} = 1388.89~\text{kW} \)

Now,

\( \text{COP} = \frac{270}{300 - 270} = \frac{270}{30} = 9 \)

\( W = \frac{Q_L}{\text{COP}} = \frac{1388.89}{9} = 154.3~\text{kW}\)

Explanation:

High Condenser Pressure in Refrigeration Systems

• High condenser pressure in refrigeration systems can significantly impact system performance and efficiency. Understanding the causes of high condenser pressure is crucial for diagnosing and addressing the issue effectively. The correct answer to the question is Option 2: 1, 2, and 3 are correct. Let us analyze each of the given statements in detail.

Statement 1: The water flow rate is lower than the desired value.

• This statement is correct. A lower water flow rate in the condenser cooling system reduces the heat transfer capacity of the condenser. As a result, the refrigerant cannot reject heat efficiently, leading to an increase in condenser pressure. Proper water flow is essential for maintaining optimal heat rejection and ensuring the condenser operates within its designed pressure range.

Statement 2: Non-condensable gases are present in the system.

• This statement is also correct. Non-condensable gases, such as air, can enter the refrigeration system due to improper maintenance or leaks. These gases accumulate in the condenser and reduce its effective heat transfer area. Additionally, non-condensable gases increase the pressure inside the condenser because they occupy space and do not condense like the refrigerant. This results in higher condenser pressure and decreased system efficiency.

Statement 3: Accumulation of lubricating oil in the condenser.

• This statement is correct as well. Lubricating oil can accumulate in the condenser due to improper oil return mechanisms or design issues in the refrigeration system. When oil accumulates on the heat transfer surfaces of the condenser, it creates an insulating layer, reducing the heat transfer efficiency. This causes the refrigerant to retain more heat, leading to an increase in condenser pressure.

Statement 4: Low charge of refrigerant in the system.

• This statement is incorrect. A low charge of refrigerant in the system typically results in reduced system capacity and lower pressures, including condenser pressure. While an undercharged system can lead to other issues, such as reduced cooling performance and evaporator freezing, it does not cause high condenser pressure.

Explanation:

Refrigeration Cycle:

• In a refrigeration cycle, the primary goal is to transfer heat from the interior of the refrigerator to its exterior environment, thereby cooling the interior space. The latent heat of ice (fusion) plays a crucial role in the efficiency and functionality of this process. Let's delve into how this latent heat contributes to the refrigeration effect and why Option 1 is the correct answer.

Refrigeration Cycle:

The refrigeration cycle consists of four main processes: evaporation, compression, condensation, and expansion. Here's a brief overview of each process:

1. Evaporation: The refrigerant absorbs heat from the interior of the refrigerator, causing it to evaporate. This phase change from liquid to gas requires the refrigerant to absorb a significant amount of latent heat from its surroundings, which in turn cools the interior space.
2. Compression: The evaporated refrigerant is compressed by the compressor, raising its pressure and temperature.
3. Condensation: The high-pressure, high-temperature refrigerant releases its heat to the exterior environment as it condenses back into a liquid.
4. Expansion: The liquid refrigerant passes through an expansion valve, reducing its pressure and temperature, and it returns to the evaporator to start the cycle again.

Latent Heat of Ice (Fusion):

• The latent heat of fusion is the amount of heat required to change a substance from solid to liquid at its melting point without changing its temperature. For ice, this process occurs at 0°C (32°F). When ice melts, it absorbs a substantial amount of heat, known as the latent heat of fusion, which is approximately 334 kJ/kg for water.

In the context of a refrigeration cycle, the latent heat of fusion is essential for the following reasons:

• Heat Absorption during Evaporation: In the evaporator, the refrigerant absorbs heat from the interior of the refrigerator. This heat absorption includes the latent heat of fusion when ice is present. As the refrigerant evaporates, it takes in a significant amount of heat, including the heat required to melt the ice. This process effectively cools the interior space.
• Efficient Cooling: The absorption of the latent heat of fusion ensures efficient cooling of the refrigerator's interior. By causing the refrigerant to evaporate and absorb this latent heat, the refrigeration cycle can maintain a low temperature inside the refrigerator, keeping food and other items cold.

Explanation:

Hermetically sealed compressor:

• A hermetic or sealed compressor is one in which both compressor and motor are confined in a single outer welded steel shell.
• In this, both compressor and motor have a common shaft to operate to make it portable and handy.
• Hermetic compressors are ideal for small refrigeration systems, where continuous maintenance (replenishing refrigerant and oil charge etc) cannot be ensured.

Semi-sealed or semi-hermitic compressor:

• Semi-hermitic compressors are typically used for medium cooling capacities.
• They are similar to hermitic compressors except that the outer housing is normally bolted together so that the compressor is accessible for service.
• An advantage of this design is the ease of construction of multicylinder large compressors without the need for the expensive deep drawing manufacturing process.

Explanation:

Refrigeration system:

A refrigeration system contains a minimum of four key components i.e., compressor, condenser, expansion valve, and evaporator.

Expansion device:

• The purpose of the expansion device is to rapidly reduce the pressure of the refrigerant in the refrigeration cycle.
• This allows the refrigerant to rapidly cool before entering the evaporator.
• Expansion device located closer to the evaporator in order to minimize the heat gain.
• The most common devices are capillary tube, thermal expansion valve, electronic expansion valve

​

Explanation:

Capillary tube:

• A capillary tube is a long, narrow tube of constant diameter.
• The word “capillary” is a misnomer since surface tension is not important in the refrigeration application of capillary tubes.
• Typical tube diameters of refrigerant capillary tubes range from 0.5 mm to 3 mm and the length ranges from 1.0 m to 6 m.
• The main objective of the capillary tube is to reduce the pressure.
• The pressure reduction in a capillary tube occurs due to the following two factors:
  1. The refrigerant has to overcome the frictional resistance offered by tube walls. This leads to some pressure drop, and
  2. The liquid refrigerant flashes (evaporates) into a mixture of liquid and vapor as its pressure reduces. The density of vapor is less than that of the liquid. Hence, the average density of refrigerant decreases as it flows in the tube. The mass flow rate and tube diameter (hence area) being constant, the velocity of refrigerant increases since mass flow rate = ρVA. The increase in velocity or acceleration of the refrigerant also requires pressure drop. 

Concept:

Coefficient of performance of a refrigerator is given by:

\( {COP} = \frac{{Refrigeration~Effect}}{{Work~Input}} = \frac{{{Q_2}}}W\)

For reversible refrigerating machine:

\(COP=\frac{{{T_2}}}{{{T_1} - {T_2}}}\)

where Q1 = Heat rejected to the surrounding, Q2 = Heat absorbed from storage space

W = Work input = Q1 – Q2

Q1 = Q2 + W

Calculation:

Given:

T1 = 47 ºC = 320K, T2 = 7 ºC = 280 K, Q2 = 1 TR = 3.5 kW

The coefficient of performance of a reversible refrigerator is:

\(COP=\frac{{{T_2}}}{{{T_1} - {T_2}}}\)

\(COP = \frac{{280}}{{320 - 280}} = 7\)

Now COP is also defined as:

\( {COP} = \frac{{Refrigeration~Effect}}{{Work~Input}} = \frac{{{Q_2}}}W\)

\(7 = \frac{3.5}{W} \)

W = 0.5 kW

Q1 = Q2 + W = 3.5 + 0.5 = 4 kW

Concept:

• A ton of refrigeration (TR), also called a refrigeration ton (RT), is a unit of power used, especially to describe the heat-extraction capacity of refrigeration and air conditioning equipment.
• It is defined as the rate of heat transfer that results in the melting of 1 ton of pure ice at 0°C in 24 hours.
• The unit conversions is:
  • 1 TR = 210 kJ/min = 3.5 kW 
  • 1 kCal = 4.18 kJ
  • ∴ 1 kJ = 0.239 kCal
  • ∴ 210 kJ = 50.19 kCal
  • ∴ 1 TR = 50.19 kCal/min = 3011.52 kCal/hr

Concept:

In a domestic refrigerator, the coils/tubes at its back serve as the condenser made up of copper.

• The evaporator means the internal part of the refrigerator, where the foodstuff is placed for chilling.
• The refrigerant absorbs the heat from the food stuff and passed through the compressor to increase the pressure of the refrigerant.
• After the compressor, the refrigerant enters into the condenser where it rejects the heat to the atmosphere.
• It does not carry any chilled water.

Concept:

The volumetric efficiency of the compressor is:

\({η _v} = 1 + C - C{\left( {\frac{{{p_2}}}{{{p_1}}}} \right)^{\frac{1}{\gamma }}}\)

We know that, \(P_1V_1^\gamma= P_2V_2^\gamma\)

\(\therefore (\frac{P_2}{P_1})=(\frac{V_1}{V_2})^\gamma\)

\(\therefore (\frac{P_2}{P_1})^{\frac{1}{\gamma}}=(\frac{V_1}{V_2})\)

\({η _v} = 1 + C - C\frac{V_1}{V_2}\)

Calculation:

Given:

C = 0.06, v1 = 0.03 m3/kg, v2 = 0.01 m3/kg

\(\begin{array}{l} {η _v} = 1 + C - C{\left( {\frac{{{p_2}}}{{{p_1}}}} \right)^{\frac{1}{\gamma }}}\\ = 1 + C - C\frac{{{v_1}}}{{{v_2}}} = 1 + 0.06 - 0.06 \times \frac{{0.03}}{{0.01}} \end{array}\)

η_v = 0.88

∴ ηv = 0.88

Explanation:

A water-cooled condenser is a heat exchanger that removes heat from refrigerant vapor and transfers it to the water running through it. They exchange heat by removing heat from one fluid and transferring it to another fluid. 

Following are the types of water-cooled condensers:

• Shell and tube-type condenser
• Tube-in-tube condenser
• Shell and coil-type condenser

Natural convection – It is a phenomenon due to which fluid flows naturally or freely i.e. without any forced means e.g. (fans, pumps, etc ). It flows because some parts are heavier than others. The force behind it is the gravitational force.

Explanation:

Double tube type:

• In its most basic form, a double tube type condenser consists of two co-axial tubes.
• The inner tube is act as a conductive barrier through which one fluid is flowing and another fluid is flowing through the outer tube.
• It can sustain both high temperatures and high pressure.
• It is used for small-capacity systems (up-to 35kW).

(1 TR = 3.5 kW)

Thus, option (2) is correct answer.

Explanation:

The basic functions of an expansion device used in refrigeration systems are to:

1. Reduce pressure from condenser pressure to evaporator pressure, and

2. Regulate or control the refrigerant flow from the high-pressure liquid line into the evaporator at a rate equal to the evaporation rate in the evaporator.

Compressor: The compressor is located at the back of the refrigerator and in the bottom area. The compressor sucks the refrigerant from the evaporator and discharges it at high pressure and temperature. The compressor is driven by the electric motor and it is a major power-consuming device of the refrigerator.

Condenser: The condenser is the thin coil of copper tubing located at the back of the refrigerator. The refrigerant from the compressor enters the condenser where it is cooled by the atmospheric air thus losing heat absorbed by it in the evaporator and the compressor

Expansive valve or the capillary: The refrigerant leaving the condenser enters the expansion device, which is the capillary tube in case of the domestic refrigerators. The capillary is the thin copper tubing made up of the number of turns of the copper coil. When the refrigerant is passed through the capillary its pressure and temperature drop down suddenly.

Evaporator or chiller or freezer: The refrigerant at very low pressure and temperature enters the evaporator or the freezer. It is the coldest part of the refrigerator. The refrigerant absorbs the heat from the substance to be cooled in the evaporator, gets evaporated, and is then sucked by the compressor. This cycle keeps on repeating.

Receiver: The primary function of the receiver is to catch and hold any liquid refrigerant that didn’t boil off in the evaporator. Liquid refrigerant getting to the compressor can damage the pistons.

Concept:

Refrigeration:

• Refrigeration is defined as the science of maintaining the temperature of a particular space lower than the surrounding space.

It is mainly categorized into 2 parts: 

(1) Cyclic refrigeration (2) Non-cyclic refrigeration

• In the non-cyclic method of refrigeration there is no thermodynamic cycle followed for creating the cooling effect

Ice refrigeration:

• It is a non-cyclic type refrigeration system.
• In this method, ordinary ice is used for keeping the space at a temperature below the surrounding temperature.
• It can be used to maintain temperatures of about 5 to 10 ºC. To use the ice for refrigerating effect a closed and insulated chamber is required.
• On one side of the chamber ice is kept while on the other side, there is a space that is to be cooled where some material to be cooled can be placed.
• When the material is initially placed inside the chamber, temp. difference between ice and material is larger. Therefore, the cooling effect achieved by ice is also higher initially. 
• However, as time passes, the temp. difference between material and ice decrease. Therefore, the rate of refrigeration also decreases.