Chu trình lạnh refrigeration cycles ĐHBK TPHCM truyền nhiệt

The concepts• Cooling capacity of a refrigeration system that is therate of heat removal from the refrigerated space, interms of tons of refrigeration • Tons of refrigeration that is the

Lecture 9 Refrigeration cycles

The concepts

Apparatus described Jacob Perkins in the patent of 1834

The concepts

• Cooling capacity of a refrigeration system that is therate of heat removal from the refrigerated space, interms of tons of refrigeration

• Tons of refrigeration that is the rate of heat can freeze

1 𝑡𝑜𝑛 2000 𝑙𝑏𝑚 of liquid water at 0℃ 32℉ into ice

at 0℃ in 24 hours, this amount is said to be 1 𝑡𝑜𝑛

1 𝑡𝑜𝑛 𝑜𝑓 𝑟𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑖𝑜𝑛 = 211 𝑘𝐽 𝑚𝑖𝑛 = 200Τ 𝐵𝑡𝑢 𝑚𝑖𝑛Τ

= 3.5𝑘𝑊

Reversed Carnot cycle

isothermal

isothermal

isentropicisentropic Turbine

𝑇𝐻Condenser

1

2 Evaporator

𝑇𝐿

𝑄𝐻

𝑄𝐿

Compressor WARM medium at 𝑇𝐻

COLD medium at 𝑇𝐿

𝑇𝐻

𝑇𝐿 − 1

ideal vapor–comp refrigeration cycle

isentropicisobaric

𝑄𝐻

𝑄𝐿

WARM environment

compartment

Condenser coils

ideal vapor–comp refrigeration cycle

2

𝑊𝑖𝑛

𝑄𝐿

𝑄𝐻

real vapor–comp refrigeration cycle

Condenser

1 Evaporator

𝑄𝐻

𝑄𝐿

WARM environment

COLD refrigerated

space

Expansion valve

8

3 4

6 7

4 𝑇

𝑠

3

2

1 4

Decrease in compressor work

Increase in refrigeration capacity

𝑄𝐻

𝑄𝐿

5

6 7

8

6 7

5 8

𝐴

𝐵 𝐴

𝐵

Multistage compression refrigeration

8 𝑇

𝑠

7

2

1 8

9 6

𝐴

𝐵 3

3 𝐴

𝐵

8 5

3 2

9 6

7

4

7

5 𝑄

compressor

Refrigerant selection

• Refrigerant is a heat carrying medium which during theircycle in a refrigeration cycle absorbs heat from a lowtemperature system and delivers it to a highertemperature system

• Refrigerants: chlorofluorocarbons (CFCs), ammonia,hydrocarbons, carbon dioxide, air water…

Refrigerant selection

• Designation of refrigerants 𝐶𝐹𝐶, 𝐻𝐶𝐹𝐶 : 𝑅𝑥𝑦𝑧

Number of Carbon = 𝑥 + 1Number of Hydrogen = 𝑦 − 1Number of Fluorine = 𝑧

Number of Chlorine = 2𝑥 − 𝑦 − 𝑧 + 5For example:

Refrigerant selection

• Designation of refrigerants (other): 𝑅7𝑤

𝑤 ≅ molar weightFor example:

Refrigerant selection

• Requirement for refrigeration selection: temperatures ofcondenser and evaporator, then toxic, corrosive, chemical,latent heat and the cost

Reversed Brayton (gas) cycle

isobaric

isentropic

Heat exchanger

4

1 Heat

Reversed Brayton (gas) cycle

Reversed Brayton (gas) cycle

𝑇

𝑠 4

𝐴 𝐵

Refrigeration

cycles

Superheating & Subcooling

𝑃

ℎ

3

1 4

compression work

Superheating & Subcooling

Condenser

Evaporator

1 4