Post on 22-Dec-2015
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PAL #21 Refrigeration
Refrigerator cycle where P = 120 kPa and x = 0.3 before the evaporator and 60 C after the compressor Start at point 4, P4 = 120 kPa, x = 0.3, look up h4 =
h3 = h4 = 86.83, for a saturated liquid this means P3 =
Since P2 = P3 and T2 = 60 , look up h for superheated vapor, h2 =
At point 1, P1 = P4 = 120 kPa, saturated vapor, h4 =
PAL #21 Refrigeration
Mass flow rate if W’in = 0.45 kW W’in = m’(h2-h1)
m’ = (0.45)/(298.87-236.97) =
Find COP from W’in and Q’L Q’L = m’(h1-h4) = (0.000727)(236.97-86.83) =
1.091 kW COP = Q’L/W’in = (1.091)/(0.45) =
Cascade Systems
For larger commercial systems, efficiency becomes more important
e.g., deep freeze
Called cascade cycles
Cascade Efficiency
The condenser of cycle B (points 1-4) is connected to the evaporator of cycle A (points 5-8) m’A(h5-h8) = m’B(h2-h3)
COPCascde = m’B(h1-h4)/[m’A(h6-h5)+m’B(h2-h1)]
Multistage Compression
Some fluid is vaporized and is sent back to the high pressure compressor
Can also use just one compressor and multiple throttle valves and evaporators for multiple temperatures
Gas Refrigeration
We can also us a reverse Brayton cycle
Isentropic compression Isentropic expansion in turbine
Gas Refrigeration Efficiency
wnet,in = wcomp – wturb = (h2-h1)-(h3-h4)
COP = qL/wnet,in = (h1-h4) / [(h2-h1)-(h3-h4)]
Heat Pumps
COPHP = QH/Wnet,in = QH / (QH – QL)COPHP,Carnot = 1 / (1 – TL/TH)
Often designed as dual heat pump/air conditioners
Low COP if the outside temperature is very cold Can also push the heat extraction underground
Joule-Thompson Expansion
Can be achieved by a pump circulating fluid through a pipe with an expansion valve in the middle
We know that in this case, hi = hf
What will be the final properties of the fluid?
Isenthalpic Curve
If the apparatus is changed a little, a new Pf and Tf are produced
The curve represents possible beginning and ending points for a throttling process
A series of isenthalpic curves can be produced for a
substance
Inversion Curve Each curve has two regions
Tf >Ti
Tf < Ti
In between, the slope, or Joule-Thompson
coefficient (), is zero:
For a series of isenthalpic curves, a curve connecting =0 points is the inversion curve
Liquefying Gasses In order to cool a gas, its temperature must
start below the maximum inversion temperature
TM.I. is near room temperature for many gasses
Some gasses have to be pre-cooled
Heat Exchanger How is gas liquefied?
Throttled and cooled Cold gas runs back through the heat exchanger
cooling the incoming gas Cycle starts over