1.1 Refrigeration Reversed Carnot Cycle | unit 1 basics of refrigeration and psychrometry

Unit – 1

Basics of Refrigeration and Psychrometry

1.1 Refrigeration: 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).

Fig. Schematic of a Carnot refrigerator and T-S diagram for the reversed Carnot cycle.

Reversed Carnot Cycle consists of 4 processes:

1. Adiabatic Compression

2. Isothermal Compression

3. Adiabatic Expansion

4. Isothermal Expansion

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

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

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

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

1.2 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.

1.3 Simple Vapour Compression Cycle (VCC)

Its improved refrigeration cycle which uses fluid instead of air as working substance. These systems belong to the general class of vapor cycles, wherein the working refrigerant undergoes phase change at least during one process. The working substance is circulated in the system in which it alternatively evaporates and condenses; thus, it undergoes a phase change. Refrigeration is obtained as the refrigerant evaporates at low temperatures. The input to the system is in the form of mechanical energy required to run the compressor. Hence these systems are also called as mechanical refrigeration systems.

Assumptions for ideal vapor-compression cycle are:

Irreversibility within the evaporator,

Condenser and compressor are ignored.

No frictional pressure drops.

Refrigerant flows at constant pressure through the two heat exchangers (evaporator and condenser).

Heat losses to the surroundings are ignored.

Compression process is isentropic Main components are:

Compressor

Condenser

Expansion Devices

Evaporator

1.4 Refrigerating effect

Refrigeration is the process by which heat (thermal energy) is transferred from a low temperature body to a high temperature body and the heat that is removed from the low temperature body accounts for the refrigeration effect.

1.5 Compressor power & COP

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 (COP):

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,input

A rule of thumb is that the COP improves by 2 to 4 percent for each °C the evaporating temperature is raised or the condensing temperature is lowered.

1.6 Simple Vapour Absorption Cycle (VAC)

Ammonia vapour from evaporator is absorbed by water in absorber. Condensation heat released is absorbed by cooling water.

After being pumped to generator, heat is supplied which gives ammonia vapour. Weak sol. sent back to absorber.

High pressure vapour is condensed to high pressure liquid ammonia in condenser.

Liquid ammonia is throttled by expansion valve, and then it evaporates absorbing heat from evaporator.

Fig. Simple Vapour Absorption Cycle

1.7 Comparison between VCC & VAC

Sr No.	Vapour Compression Cycle (VCC)	Vapour AbsorptionCycle (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.

1.8 Psychrometry: Introduction

Psychrometry Or Psychrometry is the science dealing with the physical laws of air – water vapour mixtures.

When designing an air conditioning system, the temperature and moisture content of the air to be conditioned, and the same properties of the air needed to produce the desired air conditioning effect.

In other words, we can say that Psychrometry is the study of moist air or mixture of dry air and water vapour.

1.9 Psychrometric and Psychrometric Properties

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:

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.

1.10 Basic Terminologies & Psychrometric Relations

Dew Point Temperature: The temperature of moist air saturated at the same pressure and humidity ratio. Or more simply the temperature at which water vapor will begin to condense from a sample of air.

Dry-Bulb Temperature: The temperature of air read on a standard thermometer indicating its thermal state.

Enthalpy: The thermodynamic property defined as energy per unit mass of dry air commonly used to define the internal energy of moist air.

Humidity Ratio: The ratio of the mass of water vapor to the mass of dry air of a sample.

Relative Humidity: The ratio of mole fraction of water vapor in a given moist air sample to the mole fraction in a saturated air sample at the same temperature and pressure.

Specific Volume: The ratio of the total volume of air to the mass of dry air in a sample.

Wet-Bulb Temperature: The equilibrium temperature reached as water evaporates from a thoroughly wetted psychrometer wick into an airstream.

1.11 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.

Fig. Sensible Heating

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.

Fig. Sensible Cooling

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.

Fig. Dehumidification

Humidifying:

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

Fig. Humidifying

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.