� Refrigeration and Air Conditioning�

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Introduction to refrigeration system

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If you were to place a hot cup of coffee on a table and leave it for a while, the heat in the coffee would be transferred to the materials in contact with the coffee, i.e. the cup, the table and the surrounding air. As the heat is transferred, the coffee in time cools. Using the same principle, refrigeration works by removing heat from a product and transferring that heat to the outside air.

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• Refrigeration System Components

There are five basic components of a refrigeration system, these are: - Evaporator - Compressor - Condenser - Expansion Valve - Refrigerant; to conduct the heat from the product In order for the refrigeration cycle to operate successfully each component must be present within the refrigeration system.

• 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.

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

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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.

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Refrigeration Cycle:

Heat flows in direction of decreasing temperature, i.e., from high-temperature to low temperature regions. The transfer of heat from a low-temperature to high-temperature requires a refrigerator and/or heat pump. Refrigerators and heat pumps are essentially the same device; they only differ in their objectives.

The performance of refrigerators and heat pumps is expressed in terms of coefficient of performance (COP):

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(COP)R=

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The 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.

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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.

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Air Refrigeration cycle:

1. Air is used as working fluid.
2. No change of phase through out.
3. Heat carrying capacity/kg of air is very small compared with other refrigerant systems. High pressure air readily available in the Aircraft .
4. Low equipment weight.

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Basic elements: 1. Compressor 2. Heat exchanger 3. Expander 4. Refrigerator

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

Closed system:

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Air used is recirculated

1-To increase C.O.P., T2 should kept low. But cannot be reduced below 25ºC –Atmospheric Temp.

2-T1 should be kept high. But cannot be increased above 0ºC. It is the required temperature.

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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.

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Air Refrigeration System And Bell-Coleman Cycle Or Reversed Brayton Cycle:

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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.

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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.

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.

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AIR Refrigeration System For Aircraft Cooling

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• Application Of Aircraft Refrigeration

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• External heat gain due to solar radiations.
• Heat released by the occupants.
• Internal heat gain due to electrical and mechanical equipment used.

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• Types Of Air Refrigeration System�

• Simple air refrigeration system.
• Bootstrap air refrigeration system.
• Regenerative air refrigeration system.
• Reduced ambient system

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Simple Air Refrigeration System

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T-S diagram of simple air refrigeration system

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It is used for ground cooling[when the aircraft is not moving]

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Simple Air Refrigeration System�

• In the simple system shown in figure the compressed air after cooling in air cooler is passed through a cooling turbine.
• The work of this turbine is to drive a fan which draws cooling air through the heat exchanger.
• The air is discharge from turbine at a pressure slightly above the cabin pressure.
• The fan is put on the down stream side thus avoid the additional temperature rise of the cooling air.
• This system is good for ground cooling since the fan driven by the turbine is a source of providing cooling air for the heat exchanger.
• However the turbine work is not available for the compressor.

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Bootstrap Air Refrigeration System��

Bootstrap air refrigeration system

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T-s Diagram of Bootstrap System

It is used in a high speed aircraft

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Bootstrap Air Refrigeration System

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• 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.

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Regenerative Air Refrigeration System

Regenerative system

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T-S Diagram of Regenerative System

It is used for ground cooling as well as high speed aircrafts

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Regenerative Air Refrigeration 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.

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Reduced Ambient System

Reduced Ambient Air Refrigeration System

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Reduced Ambient System

T-S Diagram of Reduced Ambient System

It is used in Supersonic aircraft and Rockets.

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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 T_1.

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Dry Air Rated Temperature(dart)�

• 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 t_o to maintain a cabin at temperature t_i is

Q^.₀ = m Cp( t_i – t₀)

Vapour compression refrigeration system:

Most common refrigeration cycle in use today

• There are four principal control volumes involving these components:
• Evaporator
• Compressor
• Condenser
• Expansion valve

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