undefined | unit 1 basics of refrigeration and psychrometry

Unit – 1

Basics of Refrigeration and Psychrometry

Q1) Explain Reversed Carnot cycle.

A1)

Reversed Carnot Cycle

Reversing the Carnot cycle does reverse the directions of heat and work interactions.

A refrigerator or heat pump that operates on the reversed Carnot cycle is called a Carnot refrigerator or a Carnot heat pump.

The reversed Carnot cycle is the most efficient refrigeration cycle operating between two specified temperature levels.

It sets the highest theoretical COP.

(1-2) refrigerant absorbs heat isothermally from a low temperature source at TL in the amount of QL.

(2-3) refrigerant compressed to state 3 (temperature rises to TH).

(3-4) refrigerant reject heat to high temperature at TH in the amount QH. Refrigerant changes from saturated vapor state to a saturated liquid state in the condenser.

(4-1) refrigerant expands adiabatically to state 1(temperature drops to TL).

Reversed Carnot Cycle consists of 4 processes:

1. Adiabatic Compression

2. Isothermal Compression

3. Adiabatic Expansion

4. Isothermal Expansion

(1-2) Adiabatic compression of the working fluid with the aid of external work. The temperature of the fluid rises from T2 to T1.

(2-3) Isothermal compression of the working fluid during which heat is rejected at constant high temperature T1

(3-4) Adiabatic expansion of the working fluid. The temperature of the working fluid falls from T2 to T1

(4-1) Isothermal expansion of air where heat is absorbed at low temperature T2 from the space being cooled

Q2) Define following terms:

Unit of refrigeration

Compressor power

Coefficient of performance

A2)

Unit of refrigeration:

The unit of refrigeration is expressed in terms of ton of refrigeration (TR). One ton of refrigeration is defined as the amount of refrigeration effect (heat transfer rate) produced during uniform melting of one ton (100kg) of ice at 0°C to the water at the 0°C in 24 hours.

Compressor power

A machine that is used to compress the vapor refrigerant from the evaporator and to raise its pressure so that the corresponding saturation temperature is higher than that of cooling medium.

Coefficient of performance

The performance of refrigerators and heat pumps is expressed in terms of coefficient of Performance, defined as

COP = desired output / required input

COP = Cooling effect/ work input = QL/Wnet

Q3) Differentiate between VCC & VAC.

A3)

Comparison between VCC & VAC are as follows:

Sr No.	Vapour Compression Cycle (VCC)	Vapour Absorption Cycle (VAC)
1	Vapor compression has high C.O. P	It has low C.O. P
2	The charging of refrigerant is simple.	The charging of refrigerant is difficult.
3	A possibility of leakage of refrigerant is more.	A possibility of leakage of refrigerant is less.
4	Performance is adversely affected by part loads.	Reduced loads have no effect on its performance.
5	It cannot be located outside without shelter.	It can be located outside without shelter.
6	It is less bulky.	It is bulky.
7	Liquid traces in the suction line may damage the compressor.	Liquid traces in the refrigerant at the exit of the evaporator is not harmful to any component.
8	Wear and tear are high.	Wear and tear are less
9	It has a compressor and a motor. Therefore, It is more noise in operation.	It has a pump only a moving part. hence it is quiet in operation.
10	It uses high-grade work energy. It needs electrical energy for its operation.	It uses low-grade heat energy therefore, It can operate on exhaust from I.C engines or on Kerosene lamp or process heat.

Q4) Define Psychrometric and name the psychrometric properties.

A4)

Psychrometry:

It is the study of the properties of mixtures of air and water vapour. Atmospheric air is a mixture of many gases plus water vapour and a number of pollutants. The amount of water vapour and pollutants vary from place to place. The concentration of water vapour and pollutants decrease with altitude. Above an altitude of about 10 km, atmospheric air consists of only dry air. The pollutants have to be filtered out before processing the air. Hence, what we process is essentially a mixture of various gases that constitute air and water vapour. This mixture is known as moist air.

Psychrometric Properties:

Dew point temperature

Humidity ratio

Dry bulb temperature

Wet bulb temperature

Relative humidity

Specific humidity

Absolute humidity

Psychrometric ratio

Q5) Explain the psychrometric properties in detail.

A5)

Psychrometric properties:

Dew point temperature:

Dew point temperature is determined by moving from a state point horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation temperature boundary is reached.

Humidity ratio:

Actual weight of water in an air – water vapour mixture.

Amount of moisture per unit of dry air.

Can be defined as, W=m/G

Dry bulb temperature:

The dry-bulb temperature is the temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation. The term dry-bulb is customarily added to temperature to distinguish it from wet-bulb and dew point temperature.

Wet bulb temperature:

Wet bulb temperature is the temperature recorded by thermometer when the bulb is enveloped by cotton wick saturated with water.

The accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings.

Relative humidity:

Amount of moisture that a given amount of air is holding = Amount of moisture that a given amount of air can hold 50% RH 100% RH - Saturated (percentage)

Specific humidity:

Specific humidity is defined as the proportion of the mass of water vapour per unit mass of the moist air sample (dry air plus the water vapour); it is closely related to humidity ratio and always lower in value.

Absolute humidity:

The mass of water vapor per unit volume of air containing the water vapor. This quantity is also known as the water vapour density.

Psychrometric Ratio:

The psychrometric ratio is the ratio of the heat transfer coefficient to the product of mass transfer coefficient and humid heat at a wetted surface.

Q6) What is the psychrometric process. Explain it.

A6)

Psychrometric Processes

Sensible Heating:

It is addition of heat to moist air without the addition of moisture. It follows a constant humidity ratio line on the psychrometric chart.

Sensible Cooling:

It is the removal of heat from moist air without the removal of moisture. It also follows a constant ‘W’ on the psychrometric chart.

Dehumidification:

In cooling coils temp of air reduces and the saturation point (dew point) is reached. O Further cooling results in reduction of absolute humidity.

• Humidifying:

It is the addition of moisture to moist air without the addition of heat.

Adiabatic Cooling:

A psychrometric process which involves the cooling without heat loss or gain. Sensible heat lost by air is converted to latent heat in the added water vapor.

Adiabatic Mixing (Moist air & Water vapour)

A psychrometric process that involves no net heat loss or gain during the mixing of two air streams.

Q7) What is Refrigeration? What is the unit of refrigeration and which thermodynamic cycle is implied in the refrigeration system?

A7)

Refrigeration is the process where heat is transferred from low temperature to high-temperature medium with the help of external work. Its unit is ton of refrigeration.

COP = Coefficient of Performance. It is used to measure the performance of a refrigeration system. It is generally expressed by the ratio of evaporation work or refrigeration effect to the compressor work or network.

Reversed Carnot or Reversed Rankin Cycle.

Q8) Draw the T-S, P-h diagram of refrigeration system with sub-cooling and super-heating in vapor compression refrigeration system.

A8)

Fig. T-S with subcooling and superheating

Fig. P-h diagram with sub-cooling and superheating

The cycle can be described as follows:

7-1: Evaporation of the liquefied refrigerant (coming from the condenser) taking place at a constant temperature. The process is isothermal. (T1 = T7)

1-2: The vapor coming from the evaporator is superheated and gains the temperature T2 from T1 at constant pressure PL. The process is isobaric.

2-3: The superheated working fluid is compressed. Pressure rises from PLto Ph.

And the temperature rises from T2 to T3

3-4 The superheated vapor is cooled to the saturated temp T3.

4-5: Isothermal condensation of the saturated vapor at high-pressure PH. and T4=T5

5-6: The liquid refrigerant is subcooled to the temperature T6 from T5 at high-pressure PH.

6-7: The expansion of the refrigerant takes place at constant enthalpy.

Q9) Describe Dry and Wet Compression in Refrigeration System.

A9) The starting point of compression in the T-S diagram determines the final state of vapor after compression. If the final state of vapor is dry and saturated then the compression is known as wet compression and if the final state of vapor is super-heated then it is known as dry compression.

The refrigerant enters as a vapor in compressor from the evaporator. If the vapor draw into the compressor is drier than the initial condition on the same pressure ranges the vapor becomes superheated.

Q10) What is Undercooling or Subcooling is refrigeration system? How Subcooling or undercooling is done? What is advantage of vapor compression refrigeration system?

A10) The COP can be improved by undercooling or pre-cooling. It is the process where the liquid refrigerant is allowed to cool below the saturation temperature. Subcooling is done by circulating greater amount of cooling agent i.e., water through the condenser or by using water cooler than natural temperature water.

Advantages of Vapor compression refrigeration system:

Smaller size for a given refrigeration effect

Higher COP

Lower power consumption

Less Complicated in design and operation.

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