Air Refrigeration system | air refrigeration depends on | bootstrap air refrigeration system | Dry Air Rated Temperature (DART)

Air Refrigeration

Air cycle refrigeration systems belong to the general class of gas cycle refrigeration systems, in which a gas/air 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. 

Ideal reverse Brayton cycle

This is an important cycle frequently employed in gas cycle refrigeration systems. This may be thought of as a modification of reversed Carnot cycle, as the two isothermal processes of Carnot cycle are replaced by two isobaric heat transfer processes. This cycle is also called as Joule or Bell-Coleman cycle. Figure 9.2(a) and (b) shows the schematic of a closed, reverse Brayton cycle and also the cycle on T-s diagram. As shown in the figure, the ideal cycle consists of the following four processes:

Process 1-2: Reversible, adiabatic compression in a compressor

Process 2-3: Reversible, isobaric heat rejection in a heat exchanger

Process 3-4: Reversible, adiabatic expansion in a turbine

Process 4-1: Reversible, isobaric heat absorption in a heat exchanger

Process 1-2: Gas at low pressure is compressed isentropically from state 1 to state 2. Applying steady flow energy equation and neglecting changes in kinetic and potential energy, we can write:

                                        w 1 − 2 = m( h 2 − h 1 ) = mc p ( T 2 − T 1 )

S 2 = S1

where rp = (P 2 /P 1 ) = pressure ratio

Process 2-3: Hot and high pressure gas flows through a heat exchanger and rejects heat sensibly and isobarically to a heat sink. The enthalpy and temperature of the gas drop during the process due to heat exchange, no work transfer takes place and the entropy of the gas decreases. Again applying steady flow energy equation and second T ds equation:

q 2 − 3 = m ( h 2 − h 3 ) = m c p ( T 2 − T 3 )

s 2 − s 3 = cp lnT 2/T 3

P 2 = P 3

Process 3-4: High pressure gas from the heat exchanger flows through a turbine, undergoes isentropic expansion and delivers net work output. The temperature of the gas drops during the process from T 3 to T 4 . From steady flow energy equation:

w 3 − 4 = m ( h 3 − h 4 ) = m c p ( T 3 − T 4 )

s 3 = s 4

where r p = (P 3 /P 4 ) = pressure ratio

Process 4-1: Cold and low pressure gas from turbine flows through the low temperature heat exchanger and extracts heat sensibly and isobarically from a heat source, providing a useful refrigeration effect. The enthalpy and temperature of the gas rise during the process due to heat exchange, no work transfer takes place and the entropy of the gas increases. Again applying steady flow energy equation and second T ds equation:

q 4 − 1 = m ( h 1 − h 4 ) = m c p ( T 1 − T 4 )

S4 − S1 = Cp lnT 4/T 1

P 4= P 1

From the above equations, it can be easily shown that:

( T2/T1 ) = ( T3/T4 )

Applying 1 st law of thermodynamics to the entire cycle:

( q4 − 1 −q2 − 3 ) = ( w 3 − 4 −w 1 − 2 ) = −wnet

The COP of the reverse Brayton cycle is given by:

COP = q4-1/ Wnet = ( T l − T 4 ) / ( T 2 − T 1 ) − ( T 3 − T 4)

COP = ( T4 / T3 -T4 )

Dry Air Rated Temperature (DART) 

The concept of Dry Air Rated Temperature is used to compare different aircraft refrigeration cycles. Dry Air Rated Temperature is 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, i.e., the air should be very dry. The aircraft refrigeration systems are rated based on the mass flow rate of air at the design DART. The cooling capacity is then given by:

Q = m c p ( T i − T DART )

where m is the mass flow rate of air, T DART and T i are the dry air rated temperature and cabin temperature, respectively.

A comparison between different aircraft refrigeration systems based on DART at different Mach numbers shows that:

• DART increases monotonically with Mach number for all the systems except the reduced ambient system
• The simple system is adequate at low Mach numbers
• At high Mach numbers either bootstrap system or regenerative system should be used 
• Reduced ambient temperature system is best suited for very high Mach number, supersonic aircrafts