GAS REFRIGERATION CLASS NOTES FOR MECHANICAL ENGINEERING

GAS REFRIGERATION CLASS NOTES

FOR MECHANICAL ENGINEERING

Air refrigeration cools all types of

air-crafts in the world. air is the

refrigerant. It is safe and

cheap. Its COP is < 1. It gives

the cooling as sensible effect. Cooling effect is quiet less.

 Plant size increases. The main

function of a refrigeration cycle is absorb

heat at one location and rejects at some

other location.

AIR AS REFRIGERANT IN AIRCRAFT’S

Why to use Air  Refrigeration in Air crafts?

(i) The working fluid (air) is free.

(ii) environmentally friendly.

(iii) safe

(iv) nontoxic and non-inflammable.

(v) Air cycle equipment is extremely reliable

(vi) Reduced maintenance costs

(vii) Reduced system down-time

(viii)  Air cycle performance is not much affected much with change of operating conditions•

 Change of operating conditions badly affect the performance of vapor compression refrigeration system.

(ix) It produces very cold air.

(x) Air is cheap, non corrosive, inert and non-flammable.

(xi) Leakage of air is not a problem.

In the event of air leakage, it does not have any undesirable effect on occupants.

(xii) The turbines ends the cold air to cabin for cooling. It eliminates the low temperature heat exchanger (cooler) in open systems and leads to lower weight equipment.

(Xiii) The aircraft engine already consists of a high speed turbo-compressor. There is no need of a separate compressor for cooling.  Weight of equipment reduces.

(xiv) Low operating pressure require Simple design of the system.

(xv) Require less and simple maintenance.

(Xvi) Turbine produces cold air at around-120⁰C.

NECESSITY OF AIRCRAFT COOLING

 Aircraft requires cooling to keep the cabin temperatures at a comfortable level. The outside temperatures are low at high altitudes.   Requires cooling because

(i) Large internal heat generates from occupants, equipment’s, lights, hot food articles etc.

(ii) Heat generates from skin friction caused by the fast moving aircraft.

(iii)  At high altitudes, the outside pressure will be sub-atmospheric. Compress the low pressure air. Supply it to the cabin at pressures close to atmospheric with significant temperature increase. For example, pressure & temperature of outside air are 0.2 bar and 223 K (at 10000 m altitude). The cabin pressure is 0.8 bar and  the temperature is about 332 K. This effect is RAM EFFECT. Ram effect adds heat to the cabin.

(iv) Solar radiations

  Air cycle refrigeration COP is less because of sensible cooling.

BELL COLEMAN CYCLE 

Aircraft cooling uses this cycle. This consists of

two isentropic processes and two

isobaric processes. It is a reversed

Joule cycle. This 4-process refrigeration

cycle involves isentropic compression,

 isobaric heat rejection, isentropic expansion, and finally

isobaric heat intake. It is applicable to

refrigeration systems where working

substance is gas. There is no phase

change in the cycle. There is no condenser and no evaporator.

Cooler replaces condenser. Refrigerator

replaces the evaporator.

Fig. Line Diagram Bell Coleman Cycle

Fig. Bell Coleman Cycle on p-V Chart

Fig. Bell Coleman Cycle on T-s Chart

(i)Firstly  Point 1 is entry to compressor and exit of refrigerator

(ii) Secondly Point 2 is exit of  compressor and inlet of cooler

(iii) Thirdly Point 3 is exit of cooler and entry to turbine

(iv) Fourthly Point 4 is entry to refrigerator and exit of turbine

VARIOUS PROCESSES IN A BELL COLEMAN CYCLE

(1) Isentropic compression (1-2).

(2) Constant Pressure cooling process. (2-3)

(3) Isentropic Expansion Process. (3-4)

(4) Constant Pressure expansion process. (4-1)

Isentropic Compression (1-2)

Compressor draws cold air from the refrigerator.  The compressor compresses air isen-tropically. Pressure & temperature increases but specific volume decreases from condition 1 to 2.  No heat absorption or rejection during isentropic compression.

Constant Pressure cooling process. (2-3)

The warm air from the compressor goes to the cooler. Hot air cools  at constant pressure. Temperature reduces from t ₂ to t ₃.

Heat rejected during this process /kg is   q _2-3 = c _p (T ₂-T ₃).

Isentropic Expansion Process. (3-4)

After the cooler, air passes through the expander where isentropic expansion occurs. Pressure decreases from p ₃ to refrigerator pressure p ₄, which is equal to atmospheric pressure. The temperature of air during

expansion falls from t ₃ to t ₄. No heat absorption or rejection since it is an isentropic expansion.

Constant Pressure expansion process. (4-1)

The cold air from the expander goes to the refrigerator.  It sucks the heat from the area cooled. The temperature of air increases at constant pressure. Volume increases from  v ₄ to v ₁. In this way, cycle completes and cooling occurs.

ADVANTAGES OF BELL COLEMAN CYCLE

(i) Air is a refrigerant. It is easily available and cheap.

(ii) Since air is non inflammable, there is no fire risk. Hence it is safe.

(iii) Air is non-toxic, non-corrosive, stable and inert.

(iv) Weight of air refrigeration equipment per ton of refrigeration is much less for an aircraft than other refrigeration systems. Hence it is light. It is because of air compressor already available in the air-craft.

DISADVANTAGES OF BELL COLEMAN CYCLE

(i) Low COP

(ii) High running cost

(iii) Mass of air required per ton of refrigeration is large as compared to other systems of refrigeration. Hence size of the system is large.

TYPES OF AIR REFRIGERATION CYCLES

(i) Open air refrigeration cycle

Fig. Line Diagram Open Air Refrigeration Cycle

When cooled air from the turbine enters the cabin and comes in physical contact with the occupants. It is not much in use because of moisture added to air in the cabin.

(ii) Closed air refrigeration cycle

Fig. Closed Air Refrigeration Cycle

When cooled air from the turbine passes through the coil and a fan circulates and recirculates cabin air over it. The pressure of cooled air in such systems is much higher than in the open system. Because of high pressure, volume is less and hence density of air is high. It is therefore a dense system.  It reduces compression ratio and COP is high. There is no moisture problem too

Disadvantages of open cycle

(i)  Volume handled by compressor and turbine increase.  Thus it increases the size of the compressor and the cooling turbine.

(ii) Causes fog formation at the outlet of turbine because of moisture addition in the cabin. Therefore, use a drier before the turbine. But the size and capacity of the drier puts limitations on the use of this method.

Advantages of closed cycle

(i) Increases pressures in the cycle. Reduces volume to be handled both by the compressor & the turbine. The size of compressor and turbine reduces. Thus it makes the system compact and reduces capital cost.

(ii) Reduces compression ratio. Thus reduces work input. Increases COP. Operating cost reduces.

(iii) Eliminates a heat ex-changer and makes it compact and light.

(iv) combine Cabin pressurization and air conditioning into one operation.

Practically, therefore, use a closed air refrigeration cycle in actual practice.

SIGNIFICANT DEVIATIONS IN THE AIR REFRIGERATION CYCLE

(i) Significant pressure drops in the heat ex-changer

(ii) Irreversibility in compression–Losses are there in the compression

(iii) Irreversibility in expansion–Losses are there in the expansion

(iv) COP is less than 1.

ASSUMPTION USED IN THE ANALYSIS

 Use turbine work for partial compression of air

COOLING CAPACITY OF AIRCRAFTS

(i) Passenger aircrafts: 10 tons for passengers + 3 tons for electronic equipment equipment = 13 Tons of Refrigeration

(ii) Jet fighters: 10/ 20 tons of refrigeration.

POWER REQUIRED FOR REFRIGERATION AND PRESSURIZATION OF THE CABIN

There is  same power requirement at ground and height because

(i) At ground, it requires only refrigeration.

(ii) At higher altitudes, requires less refrigeration & more of cabin pressurization.

 HEAT REMOVAL FROM THE ELECTRONIC EQUIPMENT OF THE AIRCRAFT

By vaporization of water/CH₃Cl/NH₃ and the vapors discharge to the atmosphere.

Mass of water vapors used for electronic cooling = 3 x 211 x time of flight in minutes/LH of water

 DIFFERENT AIRCRAFT COOLING SYSTEMS

1. Simple Air Refrigeration System: Two systems

(i) Without evaporative cooling,

(ii) With evaporative cooling

2. Boot Strap Air Refrigeration System: Two systems

(i) Without evaporative cooling,

(ii) With evaporative cooling

3. Regenerative Air Refrigeration System

(i) Without evaporative cooling,

(ii) With evaporative cooling

4. Reduced Ambient Air Refrigeration System

(i) Without evaporative cooling,

(ii) With evaporative cooling

Comparison of various aircraft cooling systems                       

Simple System	Boot-strap System	Regenerative System	Reduced Ambient Temp System
Suits at ground  and at low speeds	More Suitable at high speeds	Suitable at high speeds	used in Jet air-crafts (very high speeds)

 Purpose of boot strap compressor

Bootstrap compressor is another compressor to increase the pressure. Of course, it will result in getting a lower temperature at the cooling turbine inlet and outlet.

Second compressor (boot compressor) increases the pressure of the refrigerant coming out of the first heat exchanger.  A second compressor compresses the air. Cool the compressed refrigerant in a second heat exchanger. Then, expanded in the turbine to get much lower temperature at the outlet of the turbine. It will supply further cooled air to the cabin at a much lower temperature. Thus, the cooling effect increases.

COP Of Air Refrigeration Cycle

COP = Cooling effect/ Net work input

Net work input = Work in compressor –Work from Turbine

Normally COP of Air Refrigeration Cycle is less than 1.

VARIOUS EXPRESSIONS FOR COP OF AIR REFRIGERATION

(i) Firstly compression and expansion are isentropic

COP = (T₁—T₄)/[(T₂—T₁)—(T₃—T₄)]

(ii) Secondly compression is isentropic and expansion is poly-tropic

COP = (T₁—T₄)/[(T₂—T₁)—(n/(n-1)((γ—1)/γ)(T₃—T₄)]

(iii)  Thirdly compression is poly-tropic and expansion is isentropic

COP = (T₁—T₄)/[ (n/(n-1)((γ—1)/γ)( (T₂—T₁)— T₃—T₄)]

(iv)  Fourthly both compression and expansion are poly-tropic

COP = (T₁—T₄)/[(n/(n-1)((γ—1)/γ)] [  (T₂—T₁)— (T₃—T₄)]

Normally it is less than 1.

 

 Differences between Air Refrigeration and Vapor Compression Refrigeration

Sr. No.	Air refrigeration	Vapor compression Refrigeration
1.	Refrigerant is gas (Air)	Refrigerant is vapor
2.	Cooling is by sensible effect only. COP is less and <1.	Latent heat causes cooling. COP is high and >>1.
3.	No phase change of refrigerant	There is a phase change of refrigerant
4.	System is bulky	System is compact
5.	More refrigerant used	Less refrigerant used
6.	Weight of equipment is small since a compressor and a turbine already exist in the aircraft	Weight of equipment per  ton of cooling is 2.5 times than for the air refrigeration
7.	There is work of expansion in turbine.	There is no turbine.

 

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