In this article you will learn about Dry Air Rated Temperature, I have written lot of articles related to refrigeration system.
But in this blog you will get all topics of Refrigeration and air conditioning with proper knowledge and information.
What is Dry Air Rated Temperature(DART)?
It is used to compare different type of aircraft refrigeration cycle. It is its key significance.
Dry Air Rated Temperature(DART) can be defined as the temperature of the air at the exit of the cooling turbine in the absence of moisture condensation.
For condensation not to occur during expansion in turbine, the dew point temperature and hence moisture content of the air should be very low, that means air should be very dry.
Aircraft refrigeration systems(ARS) are rated depend on the mass flow rate of air at the
design dry air rated temperature(DART).
Mathematical Representation of DART-
Cooling Capacity(Q) = m*(Tc – T[DART])
m* – Mass Flow Rate
Tc- Cabin temperature
T[DART]- Dry Air Rated Temperature
Lets know comparison between different aircraft refrigeration systems based on Dry Air Rated Temperature(DART) based on different Mach numbers-
a. DART increases monotonically with Mach number for all the systems except the reduced ambient system.
b. The simple system is adequate at low Mach numbers.
c. At high Mach numbers either regenerative system or bootstrap system should be used.
d. Reduced ambient temperature system is best suited for very high Mach number, supersonic aircrafts.
List of Aircraft Refrigeration Cycle-
Before proceeding with the refrigeration system used in an aircraft, it is necessary to understand that in most cases, an air refrigeration cycle is used instead of a vapor compression refrigeration system.
Do you know why?
There there are many reasons behind of this, but one big advantage of vapour compression refrigeration system over air refrigeration cycle.
Here are those reasons-
- Air refrigeration cycle perform better in high altitudes than vapour compression refrigeration cycle.
- Air is cheap and non toxic refrigeration, so it is quite safe.
- Air refrigeration cycle having less moving parts so less maintenance than vapour compression cycle.
- Air refrigeration cycle is compact and less weight than vapour compression cycle, so ideal for aircraft.
Simple aircraft refrigeration cycle-
Here are the basic steps of the simple aircraft refrigeration cycle:
- Compression: The cycle begins with a compressor, which compresses a refrigerant gas to a high pressure and temperature. Then, the system passes the compressed gas to the condenser.
- During condensation, the condenser cools and condenses the high-pressure refrigerant gas into a liquid state, releasing heat to the outside environment. The system then passes the condensed liquid to the evaporator
- Expansion: In the evaporator, the liquid refrigerant is expanded through a valve, causing it to evaporate and absorb heat from the air. This cools the air, which is then circulated throughout the aircraft.
- In compression, the compressor passes the low-pressure refrigerant vapor back to be compressed again, thus starting the cycle over.
Overall, the simple aircraft refrigeration cycle is an effective way to provide cooling in small aircraft air conditioning systems. However, it is less efficient than more advanced refrigeration cycles, and may not be suitable for larger or more complex aircraft systems.
Bootstrap System-
A bootstrap refrigeration system refers to a type of refrigeration cycle that uses the refrigerant vapour itself to drive the compressor.
The main basic principle behind a bootstrap refrigeration system is that it diverts a portion of the refrigerant vapor from the main refrigeration cycle and compresses it in a small compressor.
Compressed refrigerant is then fed back into the main refrigeration cycle which helps to drive the larger compressor.
Use of a bootstrap system in refrigeration can have several benefits like-
- Using the refrigerant vapor to drive the compressor
- System can operate more efficiently
- Lower energy consumption.
This can be particularly important in applications where energy efficiency is critical, such as in aircraft or other mobile refrigeration systems.
One potential drawback of a bootstrap refrigeration system is that it can be more complex and require more components than a traditional refrigeration system.
However, this additional complexity may be justified by the increased efficiency and performance of the system.
Overall, the use of a bootstrap system in refrigeration can be an effective way to improve the efficiency and performance of refrigeration systems in a wide range of applications.
Note- It is a modified version of simple aircraft refrigeration cycle.
Key Points to Remember for Air Refrigeration Cycle-
- In air refrigeration cycle air used as refrigerant.
- It is useful for low temperature purpose because of low molecular weight and low boiling point.
- Brayton cycle is an ideal air refrigeration cycle but in general modified version is used that is reversed Brayton cycle or Joule-Thomson cycle is used.
- Air refrigeration cycle is environment friendly.
- This cycle having compressor, condenser, evaporator and an expansion valve.
Lets learn some questions with answers asked in various competitive exams like SSC JE, GATE or RRB JE with solution.
MCQ’s of Air Refrigeration Cycle with Solution-
1. The refrigeration capacity of an air refrigeration cycle is ___________.
a) directly proportional to the specific heat of the refrigerant
b) inversely proportional to the specific heat of the refrigerant
c) directly proportional to the pressure ratio
d) inversely proportional to the pressure ratio
Answer- d) inversely proportional to the pressure ratio
2. The COP of an air refrigeration cycle is ___________.
a) directly proportional to the pressure ratio
b) inversely proportional to the pressure ratio
c) directly proportional to the specific heat of the refrigerant
d) inversely proportional to the specific heat of the refrigerant
Answer- b) inversely proportional to the pressure ratio
3. In an air refrigeration cycle, the refrigerant experiences ___________.
a) an isobaric cooling process
b) an isobaric heating process
c) an isentropic compression process
d) an isentropic expansion process
Answer- d) an isentropic expansion process
4. The pressure ratio in an air refrigeration cycle is ___________.
a) the ratio of high side pressure to low side pressure
b) the ratio of compressor exit pressure to evaporator inlet pressure
c) the ratio of compressor exit pressure to condenser exit pressure
d) the ratio of evaporator exit pressure to condenser exit pressure
Answer- b) the ratio of compressor exit pressure to evaporator inlet pressure
5. The capacity of an air refrigeration cycle is ___________.
a) directly proportional to the mass flow rate of the refrigerant
b) inversely proportional to the mass flow rate of the refrigerant
c) directly proportional to the specific heat of the refrigerant
d) inversely proportional to the specific heat of the refrigerant
Answer- a) directly proportional to the mass flow rate of the refrigerant
7. In an air refrigeration cycle, the refrigerant is cooled in a ___________.
a) condenser
b) compressor
c) evaporator
d) throttle valve
Answer- a) condenser
8. In an air refrigeration cycle, the refrigerant is compressed in a ___________.
a) condenser
b) compressor
c) evaporator
d) throttle valve
Answer- b) compressor
8. The refrigerant used in air refrigeration cycle is ___________.
a) a liquid
b) a gas
c) a mixture of liquids and gases
d) any of the above
Answer- b) a gas
9. The efficiency of an air refrigeration cycle is ___________.
a) the ratio of refrigeration effect to compressor work input
b) the ratio of compressor work input to refrigeration effect
c) the ratio of evaporator exit temperature to condenser exit temperature
d) the ratio of compressor exit pressure to evaporator inlet pressure
Answer- a) the ratio of refrigeration effect to compressor work input
10. The working fluid used in air refrigeration cycle is ___________.
a) air
b) water
c) ammonia
d) any of the above
Answer- a) air