Lab 1 Air Cycle Systems

8/11/2019 Lab 1 Air Cycle Systems

1/5

MM321 Refrigeration and Air Conditioning

Lab 1: Air Cycle Systems Page 1

The University of the South PacificSchool of Engineering and Physics

MM 321 Refrigeration and Air Conditioning

Lab 1: Air Cycle Systems

Name: Jowesh Avisheik Goundar

ID: S11076586

AIM

To understand the various aircraft refrigeration systems and the assumptions made in the

aircraft refrigeration systems; Simple Aircraft Refrigeration System, Bootstrap System.

EQUIPMENTo Video Presentation

INRODUCTIONAir cycle refrigeration systems belong to the general class of gas cycle refrigeration systems,

in which a gas is used as the working fluid. The gas does not undergo any phase change during

the cycle; consequently, all the internal heat transfer processes are sensible heat transfer

processes. Gas cycle refrigeration systems find applications in air craft cabin cooling and also

in the liquefaction of various gases. [1]

Air cycle refrigeration system analysis is considerably simplified by making following

assumptions:i. The working fluid is a fixed mass of air that behaves as an ideal gas

ii. The cycle is assumed to be a closed loop cycle with all inlets and exhaust processes

of open loop cycles being replaced by heat transfer processes to or from the

environment

iii. All the processes within the cycle are reversible, i.e., the cycle is internally

reversible

iv. The specific heat of air remains constant throughout the cycle

In this experiment, two air cycle systems are discussed [Simple Air Refrigeration System and

Bootstrap System].

In an aircraft, cooling systems are required to keep the cabin temperatures at a comfortable

level. Even though the outside temperatures are very low at high altitudes, still cooling of

cabin is required due to:

i. Large internal heat generation due to occupants, equipment etc.

ii. Heat generation due to skin friction caused by the fast moving aircraft

iii. Solar radiation


2/5



Simple Aircraft Cooling System

Figure 1: Schematic of a Simple Aircraft Cooling System[2]

A simple aircraft cooling system employs an air cooler, a cooling air fan and a cooling turbine

apart from part of compressed air it draws from the jet compressor. The ambient air first

undergoes ram compressiondue to deceleration at the engine inlet and is further compressedin the jet compressor. The compressed air, after cooling in air-cooler, is passed through a

cooling turbine. The work of this turbine is used to drive a fan which draws cooling air

through the heat exchanger (air cooler). Air is expanded in the turbine to a pressure that is

required to be maintained in the cabin. The fan is installed on the downstream side thus

avoiding any additional temperature rise of the cooling air. The fan driven by the turbine is a

source of cooling air for the heat exchanger. The fan power overcomes the drag of the air

cooler. However, the turbine work is not available for the compressor. The thermodynamic

cycle is shown in the second figure. The ram compression is shown by the process 1-2 and the

compression in the engine compressor by 2-3. The cooling in the air cooler is shown by the

process 3-4 during which there is a small drop in pressure (ideally, this process is isobaric).The expansion in the cooling turbine to cabin pressure is shown by 4-5. At state 5, the air is let

in to the cabin. Ti is the temperature maintained inside the cabin.[3]

Bootstrap Cooling System

Figure 2: Schematic of a Bootstrap Cooling System [4]

A boot-strap cooling system employs two heat exchangers and a cooling turbine drives a

secondary compressor instead of cooling fan. The air bled from the main compressor is first

cooled by the ram air in the first heat exchanger (air cooler). This cooled air, after

compression in the secondary compressor, is led to the second heat exchanger (after-cooler)


3/5



where it is again cooled by the ram air before passing to the cooling turbine. The primary

purpose of the 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 figure below shows the T-s diagram of the boot-strap system. The ram compression is

shown by the process 1-2 and the compression in the engine compressor by 2-3. The coolingin the air cooler is shown by the process 3-4 during which there is a small drop in pressure

(ideally, this process is isobaric). Compression process in the secondary compressor is shown

by the process 4-5; this is followed by the constant pressure cooling process 5-6 in the after-

cooler by the ram air. The expansion of the air in the cooling turbine is the process 6-7 at the

end of which the pressure is equal to the atmospheric pressure. Air is let in to the cabin at

state 7.

This type of cooing system is mostly employed in transport aircraft. [5]

RESULTSAND DISCUSSIONQuestions to Answer

1) DART is the temperature of the air at the exit of the cooling turbine. (True or False)

TRUE

2) A Simple system is adequate to handle high Mach numbers. (True or False)

FALSE

3) Specify few reasons why we need cooling in the airplane when the outside temperatures are

very low.

Even though the outside temperatures are very low at high altitudes, still cooling of

cabin is required due to:o Large internal heat generation due to occupants, equipment etc.

o Heat generation due to skin friction caused by the fast moving aircraft

o Solar radiation

4) Give few reasons why air-cycle refrigeration system is used in aircraft.

o Air is cheap, safe, non-toxic and non-flammable. Leakage of air is not a

problem

o Available in abundance

o Eliminates the need for low temperature heat exchanger for open systems

since cold air can directly be utilized for cooling thus reducing weight.

o

Eliminates the requirement for a separate compressor for cooling system,since the aircraft already consist a high speed turbo-compressor, which

reduces large amount of weight typically less than 50% compared to

equivalent vapour compression system.

o Cabin pressurization and air conditioning is combined into one operation.

o Maintenance required is less.

5) What is RAM effect?

As the air enters the duct of the engine, before the main compressor, its

temperature and pressure is increased due to the fast movement of the airplane and

thus its kinetic energy is converted to heat or enthalpy. This process is known as

ramming effect or ram effect.


4/5



6) Explain how simple aircraft cycle is different from bootstrap system.

Bootstrap system is a modification to the simple cooling system. This system

consists of two heat exchangers (air cooler and aftercooler), instead of one air

cooler of the simple system. It also incorporates a secondary compressor, which is

driven by the turbine of the cooling system. This system is suitable for high speed

aircraft, where the velocity of the aircraft provides the necessary airflow for theheat exchangers; as a result a separate fan is not required as in simple aircraft

cooling system.

7) A simple air cooled system is used for an aeroplane to take a load of 35.16 kW. Atmospheric

temperature and pressure is 25C and 91.19 kPa respectively. Due to ramming the pressure

of air is increased from 91.19kPa, to 101.3kPa. The pressure of air leaving the main

compressor is 354.6 kPa and its 50% heat is removed in the air-cooled heat exchanger and

then it is passed through a evaporator for future cooling. The temperature of air is reduced

by 10C in the evaporator. Lastly the air is passed through cooling turbine and is supplied to

the cooling cabin where the pressure is 104.36 kPa. Assuming isentropic efficiency of the

compressor and turbine are 75% and 70%, finda. Power required to take the load in the cooling cabin

b. COP of the system.

The temperature of air leaving the cabin should not exceed 25C.


5/5



DISCUSSIONCooling in aircraft is required essentially to maintain temperature inside the cabin at a

comfortable level as well as to maintain atmospheric pressure inside. Even though the outside

temperature is very low at such high altitudes, cooling is still required since there are several

factors which leads to an increase in temperature inside the cabin such as; large heat

generation of occupants, electronic devices, skin friction of the aircraft, heat dissipated from

the engines, and solar radiation.

Employing air as the working fluid indeed has lot of advantages for aircraft refrigeration

system. It is availability is abundance, cheap, non-toxic thus leakage does not has effect, as

well is non-flammable thus risk of fire is also eliminated. It is safe to be utilized as well

reduces the weight of the aircraft since separate compressor is not required for cooling as in

vapor compression cycle.

Bootstrap system is a modification to simple aircraft cooling system, and this is basically

utilized for high speed aircraft and those at high altitudes.

CONCLUSIONFrom this experiment, fundamentals of aircraft cooling system were learnt and what are

different cooling methods utilized. It was also learnt how to calculate COP of the system, and

the work required to fulfill the load capacity of the cabin. This experiment has taught the

importance of employing air as the working fluid for cooling the aircraft as well as the

difference between bootstrap and simple aircraft cooling system with their respective

importance was learnt.

REFERENCE

[1] Kharagpur, I. (2006, July 06).Air cycle refrigeration systems.Retrieved August 15, 2013,

from Refrigeration and Air Conditioning-Mechanical Engineering; IIT Kharagpur:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD8

QFjAE&url=http%3A%2F%2Fnptel.iitm.ac.in%2Fcourses%2FWebcourse-

contents%2FIIT%2520Kharagpur%2FRef%2520and%2520Air%2520Cond%2Fpdf%

2FRAC%2520Lecture%25209.pdf&ei=TU0SUsTVHZCTiAeC7oCYDw&u

[2] Ahmed, D. M. (2013). Simple Aircraft Cooling System; Schematic Diagram. In MM321

Refrigeration and Air Conditioning.

[3] Ahmed, D. M. (2013). Simple Aircraft Cooling System; Lecture Notes. In MM321

Refrigeration and Air Conditioning.

[4] Ahmed, D. M. (2013). Bootstrap System; Schematic Diagram. In MM321 Refrigeration and

Air Conditioning.

[5] Ahmed, D. M. (2013). Bootstrap System; Lecture Notes. In MM321 Refrigeration and Air

Conditioning.
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD8QFjAE&url=http%3A%2F%2Fnptel.iitm.ac.in%2Fcourses%2FWebcourse-contents%2FIIT%2520Kharagpur%2FRef%2520and%2520Air%2520Cond%2Fpdf%2FRAC%2520Lecture%25209.pdf&ei=TU0SUsTVHZCTiAeC7oCYDw&uhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD8QFjAE&url=http%3A%2F%2Fnptel.iitm.ac.in%2Fcourses%2FWebcourse-contents%2FIIT%2520Kharagpur%2FRef%2520and%2520Air%2520Cond%2Fpdf%2FRAC%2520Lecture%25209.pdf&ei=TU0SUsTVHZCTiAeC7oCYDw&uhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD8QFjAE&url=http%3A%2F%2Fnptel.iitm.ac.in%2Fcourses%2FWebcourse-contents%2FIIT%2520Kharagpur%2FRef%2520and%2520Air%2520Cond%2Fpdf%2FRAC%2520Lecture%25209.pdf&ei=TU0SUsTVHZCTiAeC7oCYDw&uhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD8QFjAE&url=http%3A%2F%2Fnptel.iitm.ac.in%2Fcourses%2FWebcourse-contents%2FIIT%2520Kharagpur%2FRef%2520and%2520Air%2520Cond%2Fpdf%2FRAC%2520Lecture%25209.pdf&ei=TU0SUsTVHZCTiAeC7oCYDw&uhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD8QFjAE&url=http%3A%2F%2Fnptel.iitm.ac.in%2Fcourses%2FWebcourse-contents%2FIIT%2520Kharagpur%2FRef%2520and%2520Air%2520Cond%2Fpdf%2FRAC%2520Lecture%25209.pdf&ei=TU0SUsTVHZCTiAeC7oCYDw&uhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD8QFjAE&url=http%3A%2F%2Fnptel.iitm.ac.in%2Fcourses%2FWebcourse-contents%2FIIT%2520Kharagpur%2FRef%2520and%2520Air%2520Cond%2Fpdf%2FRAC%2520Lecture%25209.pdf&ei=TU0SUsTVHZCTiAeC7oCYDw&uhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD8QFjAE&url=http%3A%2F%2Fnptel.iitm.ac.in%2Fcourses%2FWebcourse-contents%2FIIT%2520Kharagpur%2FRef%2520and%2520Air%2520Cond%2Fpdf%2FRAC%2520Lecture%25209.pdf&ei=TU0SUsTVHZCTiAeC7oCYDw&uhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD8QFjAE&url=http%3A%2F%2Fnptel.iitm.ac.in%2Fcourses%2FWebcourse-contents%2FIIT%2520Kharagpur%2FRef%2520and%2520Air%2520Cond%2Fpdf%2FRAC%2520Lecture%25209.pdf&ei=TU0SUsTVHZCTiAeC7oCYDw&uhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD8QFjAE&url=http%3A%2F%2Fnptel.iitm.ac.in%2Fcourses%2FWebcourse-contents%2FIIT%2520Kharagpur%2FRef%2520and%2520Air%2520Cond%2Fpdf%2FRAC%2520Lecture%25209.pdf&ei=TU0SUsTVHZCTiAeC7oCYDw&u

Documents

Lab 1 Air Cycle Systems