Unit – 1 | unit 1 refrigeration

RAC

Unit – 1

Refrigeration

Q1) What is refrigeration and its purpose. What is the main purpose of refrigeration?

A1)

Refrigeration

Mechanical refrigeration is a process of lowering the temperature of a substance less than that of its surroundings.
Capacity of refrigeration is expressed in tone.
A tone of refrigeration is expressed in designed as the rate of heat removed from the surroundings equivalent to the heat required for melting one tone of ice in one day.
It is defined as the science of providing and maintaining temperature below that of surrounding atmosphere.
It means that continuously extraction of heat from a body whose temperature is already below the temperature of its surrounding.

The main purpose of refrigeration was to produce ice, which was used for cooling beverages, food preservation and refrigerated transport etc.

The major applications of refrigeration can be grouped into following four major equally important areas:

1. Food processing, preservation and distribution

2. Chemical and process industries

3. Special Applications

4. Comfort air-conditioning

Q2) Write the methods of refrigeration?

A2)

Methods of refrigeration

a) Natural Method: The natural method includes the utilization of ice or snow obtained naturally in cold climate. Ice melts at 00 C. So, when it is placed in space or system warmer than 00 C, heat is absorbed by the ice and the space is cooled. The ice then melts into water by absorbing its latent heat at the rate of 324 kJ/kg. But, now-a-days, refrigeration requirements have become so high that the natural methods are inadequate and therefore obsolete.

b) Mechanical or Artificial Refrigeration: Atmosphere () Refrigerated System () δQ1 Refrigerating System (R) δW δQ2 as shown in fig. Reversed Carnot engine A mechanical refrigeration system works on the principle of reversed Carnot.

Work δw is delivered to the refrigerating system, heat δQ2 from the body or system (at lower temperature Tcold) and to deliver it along with work, δw, to another body at higher temperature, Thot, so that, Qcold+ δw = Qhot. There can be two methods by which the temperature T2 < T3 may be attained within the refrigerating system:

(i) By lowering the temperature of the working substance in the refrigerating system to the level of T2. In this case, the heat will be absorbed due to temperature difference and T3 will decrease as heat δQ2 flows out.

(ii) By evaporating some fluid at an appropriate pressure.

Q3) Explain the Carnot Vapour Cycle.

A3)

Carnot Vapour Cycle:

In Carnot vapor cycle working fluid is vapor.
A Carnot cycle with two isothermal and two isentropic processes can be thought of as a vapor power cycle.
However, in practice, it is almost impossible to design a vapor power plant, based on Carnot cycle.
Below figure shows the schematic diagram of a simple steam power plant working on vapor power cycle and T-s diagram.

Fig. Schematic diagram of a simple steam power plant on vapor power cycle

Fig. T-s diagram of vapor power cycle

Saturated vapour leaves the boiler at state 3, enters the turbine leaves and expands to state 4.
The fluid then enters the condenser, where it is cooled to state 1, and then it is compressed to state 2 in the pump.
Practically, it is very difficult to, reject or heat to or from the working fluid at constant temperature.
But it is comparatively easy to add or reject heat to or form fluid at constant pressure.
Therefore, Carnot Vapour cycle is not used as idealized cycle in steam power plant.

Q4) Define the following:

Unit of refrigeration
Refrigeration effect
Coefficient of performance (COP)

A4)

Unit of refrigeration

Capacity of refrigeration unit is generally defined in ton of refrigeration. A ton of refrigeration is defined as the quantity of heat to be removed in order to form one ton (1000 kg) of ice at 0C in 24 hrs. From liquid water at 0C. This is equivalent to 3.5 kJ/s (3.5 kW) or 210 kJ/min.

Refrigeration effect:

It is the amount of heat energy removed per unit time from the space to be cooled by the refrigeration process. It is expressed in kw or kJ/s. It is also called capacity of a refrigerator.

Coefficient of performance (COP):

It is defined as the rate of removal of heat from a substance which is to be cooled.

C.O.P is a measure of efficiency of a refrigeration system and is defined as

C.O. P = Refrigeration effect upon input energy

Q5) Explain the Reversed Carnot cycle.

A5)

Reversed Carnot cycle employing a gas Reversed Carnot cycle is an ideal refrigeration cycle for constant temperature external heat source and heat sinks.

Figure 1 (a) shows the schematic of a reversed Carnot refrigeration system using a gas as the working fluid along with the cycle diagram on T-s and P-V coordinates.

As shown, the cycle consists of the following four processes:

Process 1-2: Reversible, adiabatic compression in a compressor
Process 2-3: Reversible, isothermal heat rejection in a compressor
Process 3-4: Reversible, adiabatic expansion in a turbine
Process 4-1: Reversible, isothermal heat absorption in a turbine

Fig.(a). Schematic of a reverse Carnot refrigeration system

Fig. (b). Reverse Carnot refrigeration system in P-v and T-s coordinates

The heat transferred during isothermal processes 2-3 and 4-1 are given by:

Applying first law of thermodynamics to the closed cycle,

The work of isentropic expansion, exactly matches the work of isentropic compression

The COP of Carnot system is given by:

Q6) Explain the Bell Coleman or Reversed Joule air refrigeration cycle.

A6)

Bell Coleman or Reversed Joule air refrigeration cycle

Assumptions:

(1) The compression and expansion processes are reversible adiabatic processes.

(2) There is a perfect inter-cooling in the heat exchanger.

(3) There are no pressure losses in the system.

The components of the air refrigeration system are shown in Fig. In this system, air is taken into the compressor from atmosphere and compressed. The hot compressed air is cooled in heat exchanger up to the atmospheric temperature (in ideal conditions). The cooled air is then expanded in an expander.
The temperature of the air coming out from the expander is below the atmospheric temperature due to isentropic expansion. The low temperature air coming out from the expander enters into the evaporator and absorbs the heat. The cycle is repeated again.
The working of air refrigeration cycle is represented on p-v and T-s diagrams in Fig.

Q7) Explain the Aircraft refrigeration system in detail.

A7)

Figure shows the schematic of a simple aircraft refrigeration system and the operating cycle on T-s diagram.

This is an open system. As shown in the T-s diagram, the outside low pressure and low temperature air (state 1) is compressed due to ram effect to ram pressure (state 2).

During this process its temperature increases from 1 to 2. This air is compressed in the main compressor to state 3, and is cooled to state 4 in the air cooler. Its pressure is reduced to cabin pressure in the turbine (state 5), as a result its temperature drops from 4 to 5. The cold air at state 5 is supplied to the cabin. It picks up heat as it flows through the cabin providing useful cooling effect. The power output of the turbine is used to drive the fan, which maintains the required air flow over the air cooler. This simple system is good for ground cooling (when the aircraft is not moving) as fan can continue to maintain airflow over the air cooler.

By applying steady flow energy equation to the ramming process, the temperature rise at the end of the ram effect can be shown to be:

Where ‘M’ is the Mach number, which is the velocity of the aircraft ‘C’ to the sonic velocity ‘a’.

Due to irreversibility, the actual pressure at the end of the ramming will be less than the pressure resulting from isentropic compression. The ratio of actual pressure rise to the isentropic pressure rise is called ram efficiency.

The refrigeration capacity of the simple aircraft cycle is ‘Q’ is given by:

Where, m is the mass flow rate of air through turbine.

Q8) Explain the Bootstrap air refrigeration system.

A8)

Boot strap refrigeration

Fig. Bootstrap air refrigeration system

Fig. T-s Diagram of Bootstrap System

The Bootstrap system shown in figure has two heat exchangers instead of one and the expansion turbine drives a compressor rather than a fan.
Thus, it cannot be used for ground cooling.
The primary purpose of Bootstrap system is to provide an additional cooling capacity when the primary source of air does not have a sufficiently high pressure to provide the amount of cooling required.
The turbine drives the secondary compressor to rise the pressure of primary air before it enters the turbine.
It is used for high-speed aircraft where in the velocity of the aircraft provides the necessary airflow for the heat exchangers, as a result a separate fan is not required.

Q9) Explain the Reduced Ambient Air Refrigeration System.

A9)

Reduced ambient

Fig. Reduced Ambient Air Refrigeration System

Fig. T-S Diagram of Reduced Ambient System

In the reduced ambient system, there are two expansion turbines-one in the cabin air stream and the other in the cooling air streams.
Both turbines are connected to the shaft driving the fan which absorbs all the power.
The turbine for the ram air operates from the pressure ratio made available by the ram air pressure.
The cooling turbine reduces the temperature of cooling air to level of static temperature of ambient air.
Thus, primary compressed air can be cooled to, say T4 below the stagnation temperature T2 and a little above the static temperature T1.

Q10) What do you mean by Dry air rated temperature (DART).

A10)

DART is the index used to compare different aircraft cooling system.
It is defined as the temperature of air at the exit of the cooling turbine in the absence of any moisture condensation.
Thus, the capacity of the machine giving m. Kg/sec of air at a DART of to maintain a cabin at temperature () is

= m ( – )

Q11) What do you mean by Heat Pump.

A 11)

Heat Pump:

A warmness pump is a mechanical compression cycle that may be reversed to both warmness and cool a managed space.

Working

A traditional warmness pump includes parts: an indoor unit referred to as an air handler and an outside unit much like an air output unit.
A compressor circulates a refrigerant that absorbs and releases warmness because it travels among those units.
Here, the operating fluid or the refrigerant (in its gaseous state) is pressurized via way of means of a compressor and circulated thru the system.
The technique of compression makes the fluid hotter.
The warm and pressurized vapor, on the release aspect of the compressor, is cooled in a warmness exchanger referred to as a condenser, till it condenses right into an excessive stress, mild temperature liquid.
The stress of the condensed fluid is decreased the usage of a stress-decreasing tool including a capillary tube or an enlargement valve.
The temperature of the low-stress liquid is expanded in a warmness exchanger and then the refrigerant is made to go back to the compressor and the cycle is repeated.

Q12) Explain Open and closed air refrigeration cycle.

A12)

Open system: The air used in the refrigerator is thrown into the atmosphere.

Closed system: Air used is recirculated:

To increase C.O.P., T2 should kept low. But cannot be reduced below 25ºC –Atmospheric Temp.
T1 should be kept high. But cannot be increased above 0ºC. It is the required temperature.

Advantages of Air –Refrigeration Systems:

1. As the air is easily available compared with the other refrigerant, it is cheap.

2. The air used is non-flammable, so there is no danger of fire as in NH3 machine.

3. The weight of the air refrigeration system / T.R is quite low compared with the other refrigeration systems which is one of the major causes selecting this system in air craft.

Q13) What is need of aircraft refrigeration.

A13)

Figure shows the schematic of a simple aircraft refrigeration system and the operating cycle on T-s diagram.

This is an open system. As shown in the T-s diagram, the outside low pressure and low temperature air (state 1) is compressed due to ram effect to ram pressure (state 2).

By applying steady flow energy equation to the ramming process, the temperature rise at the end of the ram effect can be shown to be:

Where ‘M’ is the Mach number, which is the velocity of the aircraft ‘C’ to the sonic velocity ‘a’.

The refrigeration capacity of the simple aircraft cycle is ‘Q’ is given by:

Where m is the mass flow rate of air through turbine.

Q14) What is Regenerative air refrigeration system.

A14)

Regenerative Air Refrigeration System:

Fig. Regenerative system

Fig. T-S diagram of Regenerative system

The regenerative system shown in figure also has two heat exchangers but does not required ram air for cooling the air in the second heat exchanger. It is a modification of the simple system with the addition of a secondary heat exchanger in which the air from the primary heat exchanger is further cooled with a portion of the refrigerated air bled after expansion in the turbine as shown in figure. It provides lower turbine discharge temperatures but at the expense of some weight and complications.

Q15) What is Reduced ambient.

A15)

Fig. Reduced Ambient Air Refrigeration System

Fig. T-S Diagram of Reduced Ambient System

In the reduced ambient system, there are two expansion turbines-one in the cabin air stream and the other in the cooling air streams.
Both turbines are connected to the shaft driving the fan which absorbs all the power.
The turbine for the ram air operates from the pressure ratio made available by the ram air pressure.
The cooling turbine reduces the temperature of cooling air to level of static temperature of ambient air.
Thus, primary compressed air can be cooled to, say T4 below the stagnation temperature T2 and a little above the static temperature T1.

Unit – 1

Refrigeration

Q1) What is refrigeration and its purpose. What is the main purpose of refrigeration?

A1)

Refrigeration

Mechanical refrigeration is a process of lowering the temperature of a substance less than that of its surroundings.
Capacity of refrigeration is expressed in tone.
A tone of refrigeration is expressed in designed as the rate of heat removed from the surroundings equivalent to the heat required for melting one tone of ice in one day.
It is defined as the science of providing and maintaining temperature below that of surrounding atmosphere.
It means that continuously extraction of heat from a body whose temperature is already below the temperature of its surrounding.

The main purpose of refrigeration was to produce ice, which was used for cooling beverages, food preservation and refrigerated transport etc.

The major applications of refrigeration can be grouped into following four major equally important areas:

1. Food processing, preservation and distribution

2. Chemical and process industries

3. Special Applications

4. Comfort air-conditioning

Q2) Write the methods of refrigeration?

A2)

Methods of refrigeration

(ii) By evaporating some fluid at an appropriate pressure.

Q3) Explain the Carnot Vapour Cycle.

A3)

Carnot Vapour Cycle:

In Carnot vapor cycle working fluid is vapor.
A Carnot cycle with two isothermal and two isentropic processes can be thought of as a vapor power cycle.
However, in practice, it is almost impossible to design a vapor power plant, based on Carnot cycle.
Below figure shows the schematic diagram of a simple steam power plant working on vapor power cycle and T-s diagram.

Fig. Schematic diagram of a simple steam power plant on vapor power cycle

Fig. T-s diagram of vapor power cycle

Saturated vapour leaves the boiler at state 3, enters the turbine leaves and expands to state 4.
The fluid then enters the condenser, where it is cooled to state 1, and then it is compressed to state 2 in the pump.
Practically, it is very difficult to, reject or heat to or from the working fluid at constant temperature.
But it is comparatively easy to add or reject heat to or form fluid at constant pressure.
Therefore, Carnot Vapour cycle is not used as idealized cycle in steam power plant.

Q4) Define the following:

Unit of refrigeration
Refrigeration effect
Coefficient of performance (COP)

A4)

Unit of refrigeration

Refrigeration effect:

It is the amount of heat energy removed per unit time from the space to be cooled by the refrigeration process. It is expressed in kw or kJ/s. It is also called capacity of a refrigerator.

Coefficient of performance (COP):

It is defined as the rate of removal of heat from a substance which is to be cooled.

C.O.P is a measure of efficiency of a refrigeration system and is defined as

C.O. P = Refrigeration effect upon input energy

Q5) Explain the Reversed Carnot cycle.

A5)

Reversed Carnot cycle employing a gas Reversed Carnot cycle is an ideal refrigeration cycle for constant temperature external heat source and heat sinks.

Figure 1 (a) shows the schematic of a reversed Carnot refrigeration system using a gas as the working fluid along with the cycle diagram on T-s and P-V coordinates.

As shown, the cycle consists of the following four processes:

Process 1-2: Reversible, adiabatic compression in a compressor
Process 2-3: Reversible, isothermal heat rejection in a compressor
Process 3-4: Reversible, adiabatic expansion in a turbine
Process 4-1: Reversible, isothermal heat absorption in a turbine

Fig.(a). Schematic of a reverse Carnot refrigeration system

Fig. (b). Reverse Carnot refrigeration system in P-v and T-s coordinates

The heat transferred during isothermal processes 2-3 and 4-1 are given by:

Applying first law of thermodynamics to the closed cycle,

The work of isentropic expansion, exactly matches the work of isentropic compression

The COP of Carnot system is given by:

Q6) Explain the Bell Coleman or Reversed Joule air refrigeration cycle.

A6)

Bell Coleman or Reversed Joule air refrigeration cycle

Assumptions:

(1) The compression and expansion processes are reversible adiabatic processes.

(2) There is a perfect inter-cooling in the heat exchanger.

(3) There are no pressure losses in the system.

The components of the air refrigeration system are shown in Fig. In this system, air is taken into the compressor from atmosphere and compressed. The hot compressed air is cooled in heat exchanger up to the atmospheric temperature (in ideal conditions). The cooled air is then expanded in an expander.
The temperature of the air coming out from the expander is below the atmospheric temperature due to isentropic expansion. The low temperature air coming out from the expander enters into the evaporator and absorbs the heat. The cycle is repeated again.
The working of air refrigeration cycle is represented on p-v and T-s diagrams in Fig.

Q7) Explain the Aircraft refrigeration system in detail.

A7)

Figure shows the schematic of a simple aircraft refrigeration system and the operating cycle on T-s diagram.

This is an open system. As shown in the T-s diagram, the outside low pressure and low temperature air (state 1) is compressed due to ram effect to ram pressure (state 2).

By applying steady flow energy equation to the ramming process, the temperature rise at the end of the ram effect can be shown to be:

Where ‘M’ is the Mach number, which is the velocity of the aircraft ‘C’ to the sonic velocity ‘a’.

The refrigeration capacity of the simple aircraft cycle is ‘Q’ is given by:

Where, m is the mass flow rate of air through turbine.

Q8) Explain the Bootstrap air refrigeration system.

A8)

Boot strap refrigeration

Fig. Bootstrap air refrigeration system

Fig. T-s Diagram of Bootstrap System

The Bootstrap system shown in figure has two heat exchangers instead of one and the expansion turbine drives a compressor rather than a fan.
Thus, it cannot be used for ground cooling.
The primary purpose of Bootstrap system is to provide an additional cooling capacity when the primary source of air does not have a sufficiently high pressure to provide the amount of cooling required.
The turbine drives the secondary compressor to rise the pressure of primary air before it enters the turbine.
It is used for high-speed aircraft where in the velocity of the aircraft provides the necessary airflow for the heat exchangers, as a result a separate fan is not required.

Q9) Explain the Reduced Ambient Air Refrigeration System.

A9)

Reduced ambient

Fig. Reduced Ambient Air Refrigeration System

Fig. T-S Diagram of Reduced Ambient System

In the reduced ambient system, there are two expansion turbines-one in the cabin air stream and the other in the cooling air streams.
Both turbines are connected to the shaft driving the fan which absorbs all the power.
The turbine for the ram air operates from the pressure ratio made available by the ram air pressure.
The cooling turbine reduces the temperature of cooling air to level of static temperature of ambient air.
Thus, primary compressed air can be cooled to, say T4 below the stagnation temperature T2 and a little above the static temperature T1.

Q10) What do you mean by Dry air rated temperature (DART).

A10)

DART is the index used to compare different aircraft cooling system.
It is defined as the temperature of air at the exit of the cooling turbine in the absence of any moisture condensation.
Thus, the capacity of the machine giving m. Kg/sec of air at a DART of to maintain a cabin at temperature () is

= m ( – )

Q11) What do you mean by Heat Pump.

A 11)

Heat Pump:

A warmness pump is a mechanical compression cycle that may be reversed to both warmness and cool a managed space.

Working

A traditional warmness pump includes parts: an indoor unit referred to as an air handler and an outside unit much like an air output unit.
A compressor circulates a refrigerant that absorbs and releases warmness because it travels among those units.
Here, the operating fluid or the refrigerant (in its gaseous state) is pressurized via way of means of a compressor and circulated thru the system.
The technique of compression makes the fluid hotter.
The warm and pressurized vapor, on the release aspect of the compressor, is cooled in a warmness exchanger referred to as a condenser, till it condenses right into an excessive stress, mild temperature liquid.
The stress of the condensed fluid is decreased the usage of a stress-decreasing tool including a capillary tube or an enlargement valve.
The temperature of the low-stress liquid is expanded in a warmness exchanger and then the refrigerant is made to go back to the compressor and the cycle is repeated.

Q12) Explain Open and closed air refrigeration cycle.

A12)

Open system: The air used in the refrigerator is thrown into the atmosphere.

Closed system: Air used is recirculated:

To increase C.O.P., T2 should kept low. But cannot be reduced below 25ºC –Atmospheric Temp.
T1 should be kept high. But cannot be increased above 0ºC. It is the required temperature.

Advantages of Air –Refrigeration Systems:

1. As the air is easily available compared with the other refrigerant, it is cheap.

2. The air used is non-flammable, so there is no danger of fire as in NH3 machine.

3. The weight of the air refrigeration system / T.R is quite low compared with the other refrigeration systems which is one of the major causes selecting this system in air craft.

Q13) What is need of aircraft refrigeration.

A13)

Figure shows the schematic of a simple aircraft refrigeration system and the operating cycle on T-s diagram.

This is an open system. As shown in the T-s diagram, the outside low pressure and low temperature air (state 1) is compressed due to ram effect to ram pressure (state 2).

By applying steady flow energy equation to the ramming process, the temperature rise at the end of the ram effect can be shown to be:

Where ‘M’ is the Mach number, which is the velocity of the aircraft ‘C’ to the sonic velocity ‘a’.

The refrigeration capacity of the simple aircraft cycle is ‘Q’ is given by:

Where m is the mass flow rate of air through turbine.

Q14) What is Regenerative air refrigeration system.

A14)

Regenerative Air Refrigeration System:

Fig. Regenerative system

Fig. T-S diagram of Regenerative system

Q15) What is Reduced ambient.

A15)

Fig. Reduced Ambient Air Refrigeration System

Fig. T-S Diagram of Reduced Ambient System

In the reduced ambient system, there are two expansion turbines-one in the cabin air stream and the other in the cooling air streams.
Both turbines are connected to the shaft driving the fan which absorbs all the power.
The turbine for the ram air operates from the pressure ratio made available by the ram air pressure.
The cooling turbine reduces the temperature of cooling air to level of static temperature of ambient air.
Thus, primary compressed air can be cooled to, say T4 below the stagnation temperature T2 and a little above the static temperature T1.

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