Useful cycles
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Otto – SIIC
Diesel – CIIC
Brayton – gas turbine
Rankine – steam turbine
Vapor compression – cooling &
heating
Example
 An ideal vapor compression refrigeration
system operates at steady state with
R134a. Superheated vapor enters the
compressor at 10psi & 0oF, and saturated
liquid leaves the condenser at 180psi.
Refrigeration capacity is 8 tons.
 Find: compressor power (hp), rate of heat
transfer at condenser, & COP
Intercooling
 Brayton: reject heat to lower
temperature surroundings
 VCC: use working fluid
 Flash chamber
| 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27
Example
 A vapor compression refrigeration system
uses two-stage compression with
intercooling. Saturated R134a vapor at 30oC enters the first stage. The flash
chamber and heat exchanger operate at 4
bar and the condenser pressure is 12 bar.
Saturated liquid streams at 12 & 4 bar
enter the high & low expansion valves
respectively. The compressor stages are
isentropic. The refrigeration capacity is 10
tons.
Example
 Find the power input to each
compressor (kW) & the COP
Absorption refrigeration
 Compressor requires more energy
than a pump to achieve ΔP.
 VCC adjust conditions to
avoid/minimize liquid into compressor
(dry compression cycle).
 Replace compressor with pump?
 Water-soluble working fluid
Ammonia
 Nonflammable
 Inexpensive
 COPR of ammonia system ~ 20%
R134a.
Absorption system
 Vapor side
 Liquid side
Cascade system
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Low temperatures: e.g. cryogenics
Liquefaction of gases
Storage of biological materials
Reduce fluid friction in gas pipelines
COPR
Example
 A two-stage cascade refrigeration
system operates between pressure
limits of 0.8 and 0.14 MPa. R134a is
the working fluid. The heat
exchanger is an adiabatic, counterflow where both streams enter at
0.32MPa. The mass flow of the upper
cycle is 0.05kg/s.
Example
 Find
• Mass flow rate in the lower cycle
• Rate of heat removal from the
refrigerated space
• Power to compressors
• COP of cascade
Heat pump
 Air source
 Ground coupled
 Central conditioning
Test 3
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4/8
VCC
Chapter 10
10.26 a&b
10.29
10.30 a,b,&c
10.33
10.42